Guidelines for Planning and Construction of Roads in cyclone Prone Areas

Guidelines for Planning and Construction of

Roads in Cyclone Prone Areas

CRRI Report – July 2013

Sponsored by

National Disaster Management Authority

New Delhi

Geotechnical Engineering Division

Central Road Research Institute is an ISO 9001 Institution

DISCLAIMER

All the data and technical information furnished in this report are based on the literature review and

discussions held with expert members/field engineers and site visits undertaken by CSIR-Central

Road Research Institute (CSIR-CRRI) team. The responsibility of CSIR-CRRI is limited to the

technical and scientific matters contained in this report. All the procedural/ legal/ operational

matters would be responsibility of implementing agencies who would be using this report.

FOREWORD

India has long coastline of about 7500 km including its island territories. Thriving cities and ports

have been built on our coasts. Road network is very vital for providing connectivity to these

population centres. However, road infrastructure in coastal region faces constant threat due to

tropical cyclones. With the technological advancement, many new products and techniques are

now available for civil engineers to provide protection to road infrastructure against cyclone impact.

Keeping in view these issues, National Disaster Management Authority (NDMA) approached CSIR-

CRRI to prepare the ‘Guidelines for Planning and Construction of Roads in Cyclone Prone Areas’.

This task was jointly undertaken by a team from Geotechnical Engg Division and Bridges and

Structures Division of CSIR-CRRI.

The project team is grateful to NDMA for giving us an opportunity to work on this task. Special

thanks are due to Prof Prem Krishna, Prof D.K.Paul and Dr.S.Arunachalam who reviewed the draft

many times and provided valuable suggestions and comments. Acknowledgements are due to

Hon’ble members of ‘Disaster Management Committee’ and ‘Earthwork, Embankment and Ground

Improvement Committee’ of Indian Roads Congress, New Delhi and also to Prof.M.R.Madhav,

Member, Research Council, CSIR-CRRI for many useful comments/suggestions received from

them. The draft report was presented in three workshops held at Visakhapatnam, Bhubaneswar

and at New Delhi and received suggestions/ comments from various officers and engineers of state

disaster management authorities and Public Works Departments. These reviews/comments/

suggestions immensely helped in improving the draft. CSIR-CRRI Team expresses special thanks

to all of them.

(Dr.S.Gangopadhyay) Director, CSIR- CRRI

Draft Preparation Team at CSIR-CRRI

Dr.S.GangopadhyayDirector, CSIR-CRRI

Shri Sudhir MathurChief Scientist & Advisor

Shri U.K.Guru VittalHead, Geotechnical Engg Division (Project Leader)

Dr. Lakshmy Parameswaran Chief Scientist

Dr. Rajeev GargHead, Bridges and Structures Division

(Technical Assistance: Shri J.Ganesh and Dr.Pankaj Gupta)

Expert Committee for Review

Dr.Prem Krishna, FNAEHonorary Visiting Professor, Department of Civil Engineering

Indian Institute of Technology, Roorkee – 247667 (Uttarakhand)

Dr.D.K.PaulDean of Faculty Affairs & Professor, Department of Earth Quake Engg& Head, Centre for Excellence in Disaster Mitigation and Management

Indian Institute of Technology, Roorkee – 247667 (Uttrakhand)

Dr.S.ArunachalamFormerly Advisor (M), SERC, Chennai

Director, Wind Engineering Application CentreJaypee University of Engineering & Technology

A.B. Road, P.B. No. 1, Raghogarh, Dist: Guna (M.P.) - 473226

CONTENTS

Page No

Chapter – 1 Introduction 1

Chapter – 2 Destructions Caused by Cyclones 4

Chapter – 3 Planning of Road Network in Cyclone Prone Areas 8

Chapter – 4 Construction of Road Embankments 11

Chapter – 5 Sea Erosion Control Techniques & River Bank Protection 19

Chapter – 6 Road Pavements in Cyclone Prone Areas 37

Chapter – 7 Mitigation Measures for Culverts and Bridges 42

Chapter – 8 Road Traffic Operations During Evacuation 55

References 59

Annexure – I (Technical Specifications for Geotextile Tubes)

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LIST OF FIGURES

Figure No. Title Page No1.1 Wind and Cyclone Zones in India (Ref: NDMA) 21.2 Cyclone Hazard and PMSS Map (Ref: BMPTC) 32.1 Effect of Cyclone ‘Aila’ on Embankment 72.2 Another View of Eroded Embankment due to Cyclone ‘Aila’ in West Bengal 74.1 Rill Erosion in Road Embankment 134.2 Deep Cut in Road Embankment Due to Erosion 134.3 Severe Erosion of Road Embankment 134.4 A Type of Polymeric Vertical Drain (Band Drain) 175.1 Provision of Sea Wall and Concrete Tetrapods for Sea Erosion Protection at

Mumbai 21

5.2 Protection Against Sea Erosion by Retaining Wall and Gabions – Puri Konark Road

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5.3 Another View of Protection Works – Puri Konark Road 215.4 Beach Protection Using Boulder Revetment at Paradip, Odisha 225.5 Another View of Boulder Revetment at Paradip, Odisha 225.6 Masonry Sea Wall Constructed at Sea Coast at Koteshwar, Kori Creek, Gujarat 225.7 Typical Components of a Geotextile Tube 255.8 Geotextile Tube Application for Coastal Protection 265.9 Applications of Geotextile Tubes 275.10 Typical Use of Multiple Geotextile Tubes for Coastal Embankment Construction 275.11 Geotextile Tubes with Gabions as Armour Protection layer 275.12 View of geotextile Tubes Covered with Armour Protection Layer of Gabions 285.13 Shore Reclamation Using Geotextile Bags 295.14 Geotextile Bag 295.15 Use of Gabions for River Bank Protection 306.1 Construction of Roller Concrete Pavement for Rural Roads 386.2 Problem of Sand Dunes Encroaching Road Pavement 416.3 Close up View of Black Top Pavement Abraded by Sand 417.1 Cable Restrainer 497.2 Cable Restrainer Installed in Longitudinal Direction 497.3 Examples of Connecting the Beam Ends of Adjacent Spans 497.4 Connection of Deck to the Substructure 517.5 Cable restrainer between superstructure and substructure 517.6 Typical Details of a Restrainer 527.7 Tying the Restrainer from the Girders Around the Pier 537.8 Reaction Block/Stopper 547.9 Seat Extension to Accommodate Large Longitudinal Displacements 54

LIST OF TABLES

Table No.

Title Page No

1.1 Classification of Cyclones 2

2.1 Storm Intensity and Expected Damages 5

4.1 Property Requirements for 3-D Mat 16

5.1 Basic Engineering Parameters for Geotextile Tubes filled with Sand 26

5.2 Geotextile Hydraulic Property Requirements under Different Regimes 31

7.1 Hourly Mean Wind Speed and Pressure at 10m Level for Cyclone Resistant Design of Bridges Situated Within 60 km off the Coast

44

7.2 Transverse Wind Forces Due to Cyclone Acting on Unit Exposed

Frontal Area of Bridge Deck at 10m Level (Plain Terrain)

45

7.3 Qualitative Damage State Descriptions for Typical Cyclone Induced Bridge Damage (FEMA, 2003)

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LIST OF ABBREVIATIONS

AOS Apparent Opening Size of Geotextile (Also known as O95)

BIS Bureau of Indian Standards

CBP Concrete Block Pavement

CDO Central Dense Overcast (Area immediately surrounding eye region of cyclone)

CRZ Coastal Regulation Zone

DPR Detailed Project Report

ICBP Interlocking Concrete Block Pavement

JGT Jute Geotextile

MDD Maximum Dry Density

MDR Major District Road

MORD Ministry of Rural Development, Government of India

MoRTH Ministry of Road Transport and Highways, Government of India

NH National Highways

NRRDA National Rural Roads Development Agency

ODR Other District Roads

OH Organic Soil having High Liquid Limit

OI Organic Soil having Medium Liquid Limit

OL Organic Soil having Low Liquid Limit

OMC Optimum Moisture Content

PCMS Portable Changeable Message Signs

PMSS Probable Maximum Storm Surge

Pt Peat

PVD Polymeric Vertical Drain/ Prefabricated Vertical Drain

RCCP Roller Compacted Concrete Pavement

RECP Rolled Erosion Control Product

SH State Highways

TRM Turf Reinforcement Mats

VR Village Roads

WMO World Meteorological Organisation

WPS Wireless Priority Service

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Chapter – 1

INTRODUCTION

A tropical cyclone is a storm system characterised by a large low pressure centre and numerous thunderstorms that produce strong winds and flooding rain. Tropical cyclones feed on heat released when moist air rises, resulting in condensation of water vapour contained in the moist air. The term ‘tropical’ refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in maritime tropical air masses. The term ‘cyclone’ refers to such storms’ cyclonic nature, with counter clockwise rotation in Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Depending on its location and strength, a tropical cyclone is called by many other names, such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression and simply cyclone. While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surges. They develop over large bodies of warm water, and lose their strength if they move over land. This is the reason for coastal regions receiving a significant damage from a tropical cyclone, while inland regions are relatively safe from their effect. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions. They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which make them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the earth’s troposphere, and to maintain a relatively stable and warm temperature worldwide.

A strong tropical cyclone usually harbours an area of sinking air at the centre of circulation. This area is called ‘eye of the cyclone’. Weather in the eye is normally calm and free of clouds, although sea may be extremely violent. The eye is normally circular in shape, and may vary in size from 3 km to 370 km in diameter. Surrounding the eye is the region called ‘Central Dense Overcast (CDO)’, a concentrated area of strong thunderstorm activity. Curved bands of clouds and thunderstorms trail away from the eye in a spiral fashion. These bands are capable of producing heavy bursts of rain and wind, as well as tornadoes. If one were to travel between the outer edge of a hurricane to its centre, one would normally progress from light rain and wind, to dry and weak breeze, then back to increasingly heavier rainfall and stronger wind, over and over again with each period of rainfall and wind being more intense and lasting longer.

1.1 Classification of Tropical Cyclones

Tropical cyclones with an organised system of clouds and thunderstorms with a defined circulation, and maximum sustained winds of 61 kmph or less are called ‘tropical depressions’. Once the tropical cyclone reaches wind speed of more than 61 kmph, they are typically called a ‘tropical storm’ and assigned a name. When maximum sustained winds reach a speed of 119 kmph, such a cyclone is called a ‘severe cyclonic storm’. The criteria followed by the Meteorological Department of India to classify the low pressure systems in the Bay of Bengal and in the Arabian Sea as adopted by the World Meteorological Organisation (WMO) are given in Table 1.1. Cyclones affect both Bay of Bengal and the Arabian Sea. The areas affected by cyclone in India are shown in Fig 1.1 and 1.2.

1.2 Scope of These Guidelines

These guidelines cover various aspects related to planning and construction of road infrastructure in cyclone prone areas, mainly dealing about preparedness in the eventuality of a cyclone disaster.

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Table – 1.1 Classification of Cyclones

Type of Disturbances Associated Wind Speed in the CirculationLow Pressure Area Less than 17 knots (< 31 kmph)Depression 17 to 27 knots (31 to 49 kmph)Deep Depression 28 to 33 knots (50 to 61 kmph)Cyclonic Storm 34 to 47 knots (62 to 88 kmph)Severe Cyclonic Storm 48 to 63 knots (89 to 118 kmph)Very Severe Cyclonic Storm 64 to 119 knots (119 to 221 kmph)Super Cyclonic Storm 120 knots and above (222 kmph and above)

Source: India Meteorological Department

Fig – 1.1 Wind and Cyclone Zones in India(Ref: NDMA)

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Fig – 1.2 Cyclone Hazard and PMSS Map(Ref: BMPTC)

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Chapter – 2

DESTRUCTIONS CAUSED BY CYCLONES

There are three elements associated with a cyclone, which cause destruction. These have been described below:

2.1 Storm Surge

Cyclones are associated with high-pressure gradients and consequent strong winds. These, in turn, lead to storm surges. A storm surge can be defined as an abnormal rise of sea level near the coast caused by a severe tropical cyclone; as a result of which, sea water inundates low lying areas of coastal regions drowning human beings and live-stock, eroding beaches and embankments, destroying vegetation and reducing soil fertility. Storm surge is the single major cause of devastation from tropical storms. Storm surge is formed due to pushing of sea water towards shore by the force of the winds swirling around the storm. In addition, wind driven waves are superimposed on the storm tide. This advancing surge may happen to combine with the high tides to create the hurricane storm tide, which can increase the average water level to 4.5 m or more. The level of surge in a particular area is also determined by the slope of the continental shelf. Storm surge is inversely proportional to the depth of sea water. A shallow slope off the coast will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf will not see as much surge inundation, although large breaking waves can still present major problems.

Vulnerability to storm surges is not uniform along Indian coasts. The following segments of Indian coast are most vulnerable to high surges:

a) North Odisha and West Bengal Coastsb) Andhra Pradesh coast between Ongole and Machilipatnamc) Tamilnadu Coast, south of Nagapattinam

The west coast of India is less vulnerable to storm surges than the east coast of India in terms of height of storm surge as well as frequency of occurrence. However, the following segments of western coast are vulnerable to significant surges:

a) Maharashtra coast, north of Harnai and adjoining south Gujarat coast and the coastal belt around the Gulf of Mumbai

b) The coastal belt around the Gulf of Kutch

The world’s highest recorded storm surge was about 12.5m (about 41 ft) and it was associated with the Backergunj cyclone in 1876 near the Meghna estuary in present-day Bangladesh. The Probable Maximum Storm Surge (PMSS) is the highest along the West Bengal coast where it ranges from 9 m to 12.5 m. It reduces to about 3.8 m in Khurda district, Orissa, increasing again to about 8.2 m along the south Andhra Pradesh coast in Krishna, Guntur and Prakasam districts. A small region in south Tamil Nadu around Nagapattinam coast also has higher PMSS of about 8.4 m. Along the west coast; the PMSS varies from about 2 m near Thiruvananthapuram to around 5 m near the Gulf of Khambat in the Saurashtra region of Gujarat. Expected storm surge height in metres along India’s coastline is shown in Fig 1.2.

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2.2 Strong Winds/ Squall

Cyclones are known to cause severe damage to infrastructure through high speed winds and gusts. Very strong winds which accompany a cyclonic storm, damage installations, dwellings, communication systems, trees, etc., resulting in loss of life and property. A tropical cyclone damages and destroys structures in two ways. First, many homes are damaged or destroyed when the high speed wind simply lifts the roof of the dwellings. High speed wind moving over the top of the roof creates lower pressure on the exposed side of the roof relative to the attic side. The higher pressure in the attic lifts the roof. Once lifted, the roof acts as a sail and is blown clear of the dwelling. With the roof gone, the walls are much easier to be blown down by the hurricane wind. The second way that wind destroys buildings can also be a result of the roof becoming airborne. The wind picks up the debris (i.e. wood, metal siding, toys, trash cans, tree branches, etc.) and sends them hurling at high speeds into other structures. Based on observations made during damage investigations, researchers have concluded that much of the damage in windstorms is caused by flying debris. Brief details about damages caused by winds of different speed are given in Table 2.1 (Ref: Saffir-Simpson Hurricane Scale – Management of Cyclones: NDMA Guidelines – http://nidm.gov.in/PDF/guidelines/cyclones.pdf)

Table – 2.1 Storm Intensity and Expected Damages

Scale No (Category)

Sustained winds (kmph)

DamageStorm Surge

in m1 119 – 153 Minimal: Unanchored mobile homes,

vegetation and signs1.2 to 1.5

2 154 – 177 Moderate: All mobile homes, roofs, small craft and flooding

1.6 to 2.4

3 178 – 209 Extensive: Small buildings, low lying roads cut-off

2.5 to 3.6

4 210 – 249 Extreme: Roofs destroyed, trees down, roads cut-off, mobile homes destroyed, beach homes flooded

3.7 to 5.5

5 250 or more Catastrophic: Most buildings destroyed, vegetation destroyed, major toads cut-off, homes flooded

More than 5.5

The vertical wind shear in a tropical cyclone environment is also important. Wind shear is defined as the amount of change in the wind velocity direction or speed with increasing altitude. The damages produced by winds are extensive and cover areas occasionally greater than the areas of heavy rains and storm surges which are in general localised in nature. The impact of the passage of the cyclone eye, directly over a place is quite different from that of a cyclone that does not hit the place directly. The latter affects the location with relatively unidirectional winds i.e. winds blowing from only one side, and the lee side is somewhat protected. An eye passage brings with it rapid changes in wind direction, which imposes torques and can twist the vegetation or even structures. Part of structures that were loosened or weakened by the winds from one direction are subsequently severely damaged or blown down when hit upon by the strong winds from the opposite direction. A partial eye passage can also do considerable damage, but damage would be less than a total eye passage.

2.3 Torrential Rains and Inland Flooding

Torrential rainfall (more than 30 cm/hour) associated with cyclones is another major cause of damage. It also creates problems in post cyclone relief operation. Unabated rain gives rise to unprecedented

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floods. Rainwater on the top of the storm surge may add to the fury of the storm. Rain is the serious problem for the people who become shelterless due to a cyclone. Heavy rainfall resulting from a cyclone would be usually spread over a wide area. As a result, soil erosion also occurs on a large scale. Heavy rains inundate the low-lying ground and cause softening of the soil due to soaking. This contributes to weakening of the embankments, leaning of utility poles or even collapse of pole type structure. Heavy and prolonged rains due to cyclones cause river floods and submergence of low lying areas. River floods occur when the runoff from torrential rains, brought on by landfall of cyclones reach the rivers. Even after the wind has diminished, the flooding potential of cyclonic storms remains for several days. Most of the fatalities due to flooding occur because people underestimate the power of moving water and purposely walk or drive into flooding conditions. It is common to think that stronger the storm the greater the potential for flooding. However, this is not always the case. A weak, slow moving tropical storm can cause more damage due to flooding than a more powerful fast moving hurricane. In addition to the storm surge, tropical cyclones usually cause flash flooding. Flash floods are rapidly occurring events. This type of flood can begin within a few minutes or hours of excessive rainfall. The rapidly rising water can reach heights of 10 m or more and can roll boulders, rip trees from the ground, and destroy buildings and bridges. Urban area floods are also rapid events although not quite as severe as a flash flood. Still, streets can become swift-moving rivers and basements can become death traps as they fill with water. The primary cause is due to the conversion of fields or woodlands to roads and paved parking lots.

It may be mentioned that all the three factors mentioned above occur simultaneously and, the rescue and relief operations for distress mitigation become difficult. So, it is imperative that advance action is to be initiated for relief measures before commencement of adverse weather conditions due to cyclones.

2.4 Effect of Cyclones on Road Infrastructure

From the above discussion, it becomes apparent that cyclonic storms affect human habitations and infrastructure in multiple ways. Providing road connectivity in cyclone prone areas emerges as a vital tool for undertaking rescue and rehabilitation operations. It is also obvious that road infrastructure created in cyclone prone areas need to be designed and constructed to withstand the onslaught of cyclonic storms. The first step would be to identify the road stretches which are vulnerable to effect of cyclonic storm. Principle mode of destruction of road embankments and pavements due to cyclonic storms would be through erosion caused due to storm surge and flooding. The storm water causes damage to the road pavement surface, washes away portions of road at many locations, even breaching portions of embankment. Because of these problems, such road stretches in cyclonic areas become bottle neck and hamper relief operations.

Embankments and road pavements are not much susceptible to damage due to winds. However wind forces affect design of bridge structures. Wind forces also affect certain road furniture like sign boards, electric and telephone poles and trees planted along roadside causing disruptions to traffic flow by uprooting of trees, falling branches of trees, electric/ telephone poles falling in roadway, etc. Keeping these points in view, identified vulnerable road stretches in cyclone prone areas would have to be designed/ constructed to ensure that damage due to storm surge/flooding/winds are minimised or totally alleviated. Impact of a cyclone on road infrastructure may lead to (a) Damage to the roads due to storm surge / flooding (b) Hindrance to traffic movement due to deposition of debris left on roadway (c) Unseating / drifting of bridge superstructure due to storm surge / flooding (d) damages to the bridges due to debris impact. Impact damages can occur due to barge impact, boats, oil rigs, uprooted trees, boulders etc. The impact damage is manifested in the form of span misalignment, damage to fascia girder, fender, pier or pile damage. During cyclone occurrence, bridges may fail due to unseating of individual span, depending on the connection between the bridge deck and pier. The bridge decks with

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low elevation are likely to fail as a result of excessive longitudinal or transverse movement of bridge deck. Under the storm surge, the bridge decks are subjected to buoyant forces and pounding action of waves. Bearings also suffer damages due to the unseating/drifting of bridge deck. In some bridges, shifting of span due to lateral wave and wind forces often causes damages to the abutments, pier caps, or girders. Damage to parapets on bridge decks, scouring of bridge foundations, erosion of abutment, etc are seen after cyclone disaster. Further details regarding damages caused to bridges due to cyclone are given in Chapter 7. While designing road structure in cyclone prone areas, the above mentioned factors are to be considered and suitable remedial measures described in subsequent chapters are to be provided. Further it is to be noted that usually a package of remedial or protection works are usually fashioned to suit individual site conditions.

CHAPTER – 3

Fig 2.1 – Effect of Cyclone ‘Aila’ on Embankment

Fig 2.2 – Another View of Eroded Embankment due to Cyclone ‘Aila’ in West Bengal

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Chapter – 3

PLANNING OF ROAD NETWORK IN CYCLONE PRONE AREAS

As in any other part of the country, in a similar manner, the hierarchy of road infrastructure in cyclone prone area would comprise of various categories like National Highways (NH), State Highways (SH), District Roads and Rural Roads. National and State Highways are the arterial roads connecting major cities, ports, state capitals, industrial centres, etc. These roads should provide uninterrupted road communication throughout their length. District roads are intended to act as important roads within a district serving areas of production and markets and connecting with each other or with the main highways of a district. They are further sub-divided into two categories – Major District Roads (MDR) and Other District Roads (ODR). Village Roads (VR) provide connectivity between villages or habitations and District Roads. The term ‘Rural Roads’ is used to denote both ODR and VR. All these types of roads are very important in the overall road network of cyclone affected areas. Effective road connectivity ensures fast deployment of men, materials and machinery to cyclone affected areas and also ensures speedy evacuation of people from vulnerable places to safer areas in the face of an impending disaster threat. Hence the need for development of a reliable road network in the vulnerable areas is very vital to ensure coordination of relief and response in the event of a cyclone. Designation of arterial roads like National Highways and State Highways are based on traffic volume and importance of cities/ towns/ ports to be connected by them.

Planning a rural road network in a district is carried out in our country based on guidelines provided by Ministry of Rural Development (MORD) and the Indian Roads Congress (IRC). As per the MORD Guidelines, all-weather road access is to be provided to all villages/habitations of population greater than 500 people. The Operations Manual of MORD states that an all weather road is defined as one which is negotiable during all weathers, with some permitted interruptions. Essentially this means that at cross-drainage structures, the duration of overflow or interruption at one stretch shall not exceed 12 hours for ODRs and 24 hours for VRs in hilly terrain, and 3 days in the case of roads in plain terrain. The total period of interruption during the year should not exceed 10 days for ODRs and 15 days for VRs. As per MORD Guidelines, population criteria for providing all weather connectivity has been kept equal to 250 in case of hill States (North-Eastern states, Sikkim, Himachal Pradesh, Jammu & Kashmir and Uttarakhand), desert areas and tribal areas.

The most important issue with the road construction is the alignment of the road. Many issues of drainage, inundation and breaching of road embankment can be tackled at planning stage by choosing the best possible alignment. But in many projects it may not be possible to change the alignment of existing track due to problems like land acquisition, forest area, etc. These are important issues but such issues need to be taken care of by respective state Governments. Moreover, road projects involve huge investment. Hence, it is crucial to give adequate attention at the planning and design stage itself so as to achieve better and economical alignment. For planning road network in cyclone prone areas, following additional points need to be considered:

a) For planning higher category of roads like NH, SH and MDRs, ‘20 Year Road Development Plan’ and ‘Vision 2020 – Road Development Document’ published by IRC/ MoRTH are to be considered. While such arterial roads are necessary to connect main cities and towns, considerations of traffic to be catered and trade/economic importance of cities being connected are also equally essential.Additionally in case of cyclone prone areas, arterial roads required for evacuation in the event of cyclone occurrence need to be identified. Upgradation of such arterial evacuation routes to NH/SH/MDR category depending upon their importance and population being catered by such routes needs to be considered. Width of the important arterial evacuation routes (SH or MDR)

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should be preferably Two-Lane or atleast they should be of intermediate lane width. While planning the alignment of the rural link roads, it is imperative to connect existing and proposed cyclone shelters in addition to providing connectivity to habitation/ village.

b) Rural link roads in cyclone prone areas are very crucial for evacuation and rescuing of people. Similar to hilly areas and tribal regions, in areas prone for severe cyclone impact (coastal belt of 25 km from the sea), population criteria for providing all weather connectivity can be kept equal to 250 for a habitation to be connected by an all weather road. All weather roads, as already pointed out, may experience interruptions to the traffic due to submergence of a bridge for periods extending from 24 hours to 72 hours. However, roads built in cyclone prone areas need to be designed to reduce the duration of traffic interruption due to flooding. This duration of disruption for roads identified for evacuation (belonging to ODR and VR category) should be preferably not more than 3 hours even after highest flooding expected in that region.

c) The geometric design standards for rural roads are to be followed as per IRC SP – 20, ‘Rural Roads Manual. Roads are always associated with culverts and bridges as the terrain demands, to make them fit throughout the year. Geometric design of NH and SH are to be carried out as per IRC: 73. While selecting the bridge site, factors like (i) permanency of the channel, (ii) presence of high and stable banks (iii) narrowness of the channel and average depth compared to maximum depth, straight reach of the stream, freedom from islands in both upstream side and downstream side, possibility of right angled crossings, good approaches, etc., are to be given adequate attention so as to keep them functional in the event of any disaster.

d) Concerned state agencies who are in charge of the road project should prepare beforehand ‘Hazard Zonation Maps’ of suitable scale, showing extent of cyclone/ flood hazard expected in that region. These maps should indicate vulnerable roads and bridges and risk assessment is to be carried out. In case of failure of bridge/road pavement during a cyclone event, alternate routes should be identified for evacuation/ rescue and relief on these maps. Missing links/ additional infrastructure needs should also be marked on these maps so that they can be attended to during planning process. These state agencies may take assistance from State Disaster Management Authorities and local bodies for preparation and validation of such maps.

e) Construction of roads is to be taken up in such a manner that roads are atleast 500 m away from seashores / coastal regulation zones (CRZ). Intensive protective works to prevent erosion towards seashore side of the road should be planned.

f) The most important consideration for construction of road in the cyclone prone area would be its alignment avoiding inundation of the road under cyclonic rain. Adequate cross drainage works should be provided to prevent such occurrence. Therefore, a survey along the most probable route is needed to ascertain the highest flood level that had occurred during its past history. The free board allowance for different categories of roads is indicated in section 5.6. Provision of minimum free board as per section 5.6 will ensure connectivity even after the cyclonic storm.

g) Rigid pavements are preferable over flexible pavements as there is no appreciable variation in temperature to cause significant thermal stress and resulting distress. Cement concrete pavements withstand flooding/ waterlogging in a better manner than bituminous pavements. Techno-economics of adopting cement concrete pavements vis-a-vis flexible pavement needs to be undertaken before making a final choice.

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h) Road top level/ alignment are to be decided after taking into account high flood levels and flooding pattern. While fixing the road top levels, special care will have to be taken to cater for rapid changes in underground water table and consequent movement of the soil moisture. This can be achieved by designing and constructing an efficient drainage system. Keeping the road levels above the high flood levels and highest water table need to be ensured. For provision of storm water drainage for roads in urban areas, IRC SP: 50, ‘Guidelines on Urban Drainage’, can be referred to.

i) Preparation of a Detailed Project Report (DPR) for each of the proposed road is a pre requisite for proper evaluation of the project and it ensures timely completion and avoids time and cost over runs.

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CHAPTER – 4

CONSTRUCTION OF ROAD EMBANKMENTS

Successful performance of an embankment depends as much on adopting standards of good compaction in construction as on careful pre investigations leading to selection of appropriate borrow material and design features of the embankment. Soil is the primary construction material for embankment and also for road subgrade. So soil and construction material survey forms the basic step for preparation of DPR for any road project. While carrying out soil survey along proposed road alignment, representative samples should be collected wherever there is a visible change in soil type. In case the same type of soil continues, at least three representative samples from each kilometre length of road alignment should be collected for laboratory testing.

4.1 Material Specifications for Embankment and Subgrade

The material used in embankments, subgrades, earthen shoulders and miscellaneous backfills shall be soil, moorum, gravel, a mixture of these or any other suitable material approved by the engineer. Such material shall be free from organic materials like logs, stumps, roots, rubbish or any other ingredient likely to deteriorate or affect the stability of the embankment/subgrade. The following material shall be considered unsuitable for embankment: Materials from swamps, marshes and bogs, peat, log, stump and perishable material, any soil

classified as OL, OI, OH or Pt in accordance with IS: 1498 The fill soil to be used should have liquid limit less than 70 and plasticity index less than 45 Materials having salts which may result in leaching in the embankment

Expansive clay exhibiting marked swell and shrinkage properties (‘free swell index’ exceeding 50 when tested as per IS: 2720–Part 40) shall not be used as fill material. Where expansive clay with free swell index less than 50 is used as a fill material, subgrade and top 500 mm portion of the embankment below subgrade shall be non-expansive in nature. The soil to be used as embankment fill or subgrade should also meet maximum dry density and other requirements as specified in MoRTH Specifications (in case of NH/SH works) or MORD Specifications (in case of rural roads).

4.2 Design of Embankments

The cyclone impact occurs in the form of erosion of road embankments. Apart from preventing erosion, the designer has to ensure stability of road embankments. For details regarding design of road embankment IRC: 75 can be referred to. Failure of embankments may be due to either inadequate bearing capacity or due to deep seated shear failure. The objective of the stability analysis is to ensure that embankment does not face any risk of shear failure. Generally in the slip circle method failure plane is assumed to be circular. A particular circle gives the minimum factor of safety. Calculation of factor of safety of different circles until the critical circle is located is a very time consuming process. Available software may provide quick solutions.

4.3 Important Considerations for Embankment Construction

(a) Fill material should conform to MoRTH/ MORD Specifications depending on road classification. Borrow pit excavation should be located at a distance atleast 5 m away from the toe of embankment. Top soil should not be used as fill material. Top soil should be spread back on the excavated land or used for covering the side slopes of the embankment.

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(b) After clearing the site, limits of the embankment are to be marked by fixing batter pegs and marking toe lines on both sides at regular intervals as guides. Where ever feasible, stagnant water, if any, from the roadway (embankment foundation area) should be removed.

(c) After removing the topsoil / unsuitable material, the ground surface should be loosened upto a minimum depth of 150 mm by ploughing or scarifying and compacted to the specified density. For embankment construction over ground not capable of supporting equipment, successive loads of embankment fill material should be spread in a uniformly distributed layer of adequate thickness to support equipment and to construct the lower portion of the embankment. In case of soft sub-soil areas (marine clay sub-soil), ground improvement measures may be necessary to prevent failure of embankment. Expert advice should be obtained in such cases and specified foundation treatment should be carried out in a manner and to the depth as specified. Brief details of ground improvement techniques are given in section 4.5.

(d) The soil should be spread over the entire width of the embankment in layers not exceeding required loose layer thickness. The moisture content of the fill material spread for compaction should be within ±2 per cent of the optimum moisture content of the soil. Clayey soils should be compacted at moisture content slightly higher than OMC (upto 2 per cent above OMC).

(e) Each layer of fill material should be compacted using rollers to meet the specified compaction requirements. Adequate quality control and field tests as per MoRTH/ MoRD specifications are needed to ensure this.

(f) The top 50 cm of the embankment (in case of NH and SH) or 30 cm (in case of rural roads) which forms the subgrade should be built to specification requirements of the subgrade.

For further details/ specifications, reference may please be made to ‘MoRTH Specifications for Road and Bridge Works’ or ‘MoRD Specifications for Rural Roads’ and ‘IRC 36 – Recommended Practice for Construction of Earth Embankment and Subgrade for Road Works’.

4.4 Embankment Slope Protection against Soil Erosion

Road embankments experience a high degree of damage due to erosion from torrential rains which accompany cyclones and hence erosion protection of embankment slopes should receive special attention in such areas. Soil erosion is the process of detachment and transportation of soil particles by wind or water. Cohesionless soil particles may get blown away by wind (Aeolian) erosion. However erosion due to surface run-off would be the principal cause for failure of road embankments in the aftermath of a cyclone disaster. The kinetic energy of falling raindrops causes detachment of soil particles which are subsequently carried away by surface run-off. Nature of soil and impact of rain drops are determinant factors in the erosion process. Silty and sandy types of soils are more susceptible to erosion than clayey soils. Distress in the form of rills to gullies and finally to erosion ditches develop when intensity of rainfall is high and the slope is steep. These problems will impair slope stability if not controlled with proper protective measures. The surface protection of embankment against action of rain and wind is usually achieved by promoting vegetation growth. Whenembankments are constructed using non-cohesive material, cover of 0.3 to 0.6 m thick cohesive material can be given. In case of high embankments, a system of kerb channel and median drains coupled with chutes should be provided to drain off the rain water from the road embankments. Different engineering measures which may be adopted for erosion protection of roads built in cyclone prone areas are briefly described below. For more details, IRC: 56, ‘Recommended Practices for Treatment of Embankment and Roadside Slopes for Erosion Control’ can be referred to.

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Fig 4.1 – Rill Erosion in Road Embankment

Fig 4.2 – Deep Cut in Road Embankment Due to Erosion

Fig 4.3 – Severe Erosion of Road Embankment

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4.4.1 Slope protection by simple vegetative turfing

Vegetation is ideal for erosion control because it is relatively inexpensive to establish and maintain and it presents aesthetically appealing look. Vegetation on the embankment side slopes provides adequate canopy interception to the falling rain drops and saves the soil from splash erosion, while the mass of litter and Rhizomes act as speed breakers for running water on the slope. Mechanical function of plant is to reinforce the soil by binding the loose soil particles with its fibrous root system.

However, planting of tree species which grow considerably big/tall should not be permitted alongside the road in cyclone prone areas. During cyclones, such trees may get uprooted/ braches may snapwhich may cause obstruction to movement of traffic and may even lead to accidents. Moreover, roots of big trees may tend to loosen the structure of the embankment when shaken by wind storm which would cause cracks in the embankment. Shrubs, thorny bushes and short grass growing on the slope of embankments provide good protection against erosion and such vegetation should be promoted. Tree plantation should be carried out in areas beyond road land (Right of way) width. Generally the side slopes and unpaved shoulders in the top portion of the embankment should be turfed with grass sods and this turfing should extend beyond the toe on the country-side and the river side by 6.0 meters and 3.0 meters respectively. This is as per existing practices of some cyclone prone states.

Simple vegetative turfing method should be adopted where the soil has enough nutrients and the environmental conditions are conducive to promote vegetation growth. The density of sowing is of great importance. In general, while sowing a mixture of grass and legume plants, seed rate would be normally 15 gm/m2. Prior to sowing, the soil surface should be adequately prepared. On highly erodible slopes where seeding or sprigging is liable to be washed down before they have had time to take root. In such circumstances, it is advisable to go for special techniques such as the ones recommended in the succeeding paragraphs.

4.4.2 Transplantation of readymade turfs of grass

‘Sodding’ technique which involves bodily transplantation of blocks of turfs of grass (with 5-8 cm of soil covering the grass roots) from the original site to the barren slopes to be treated can be adopted in locations where ensuring grass growth would require considerable time. The sod to be used for transplantation should consist of dense, well-rooted growth of permanent and desirable grasses, indigenous to the locality where it is to be used, and it should be practically free from weeds or other undesirable matter. Thickness of the sod should be as uniform as possible, with some 50-80 mm or so of soil covering the grass roots depending on the nature of the sod, so that practically all the dense root system of the grasses is retained in the sod strip. The completed embankment side slopes should be scarified to a depth of about 25 mm and application of fertiliser/ manure should be carried out. After the sods have been laid in position, the surface shall be cleaned of loose sod, a thin layer of top soil shall be scattered over the surface of top dressing and the area thoroughly moistened by sprinkling with water. For further details MoRTH Specifications for Road and Bridge Works, Clause 307 and 308 can be referred to.

4.4.3 Application of mulch

The term ‘mulch’ refers to any loose or soft organic material, e.g. straw with cowdung or wood shavings mixed with cowdung or saw dust and dung mixture, etc laid down on the slopes to protect the roots of plants. In the case of embankments which are less than 3 m high, where the severity of the erosion problem is not of a high order, the mulch application would be very helpful for vegetation growth even in

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infertile slopes. The approximate thickness of mulch cover should be about 2.5 cm. The organic mulch covering the soil slopes can be held in place and made resistant to being washed downhill or being blown away by pegging them down with bamboos, at suitable intervals, in a grid pattern. Cellulose based fibrous mulches can be hydraulically spray applied with the seed. These ‘spray-on’ mulch systems (also called Hydro-mulching or Hydro-seeding) are somewhat more resistant to erosion than dry applied systems but they are relatively costlier also.

4.4.4 Promotion of vegetative turfing by using jute/ coir netting

Growth of appropriate vegetation on exposed soil surface is facilitated by use of natural (agro based) geotextiles such as open weave jute geotextiles (JGT) or coir netting. Such nettings laid on slopes provides a cover over exposed soil lessening the probability of soil detachment and at the same time reduces the velocity of run-off, the main agent of soil erosion. Natural geotextiles bio-degrade within one to three years. In spite of this, agro based geotextiles facilitate rapid growth of dense vegetationduring its service life. Once dense vegetation develops on the slope, plant cover would prevent erosion and it would be self sustaining. Hence biodegradability of jute/ coir nettings cannot be considered as a drawback in areas which experience adequate precipitation to ensure green vegetation cover throughout the year. For more details and specifications of this technique, IS: 14986 ‘Guidelines for application of Jute Geotextile for rain water erosion control in road and railway embankments and hill slopes’, IS: 15869 ‘Open weave coir Bhoovastra-Specification’ and IS 15872 ‘Application of coir geotextiles (coir woven Bhoovastra) for rain water erosion control in roads, railway embankments and hill slopes-Guidelines’ may be referred to.

4.4.5 Erosion control using two dimensional (2–D) synthetic geogrids/ Geosynthetic nettings

Geosynthetic nettings/ geogrids can be used for promoting vegetation growth on barren slopes in a manner similar to biodegradable nettings. Under erratic weather conditions, successful vegetation growth and its sustenance depends on un-seasonal rainfall and hence longer life of reinforcing material would be required for ensuring vegetation growth apart from contribution from the mesh towards reduction in velocity of surface runoff. Agro based nettings may fail to provide erosion prevention in areas which experience repetitive change in climate, prolonged drought in particular. Use of polymer geogrid mesh provides a permanent protection as it is not biodegradable, long lasting and has almost unfailing success rate for vegetation growth, year after year.

4.4.6 Three dimensional erosion control mat / Rolled erosion control products

Relying upon vegetation growth alone may be sometimes very unpredictable and unreliable as it may be extremely difficult to achieve 100 per cent vegetation coverage, leaving exposed areas vulnerable to erosion. Furthermore, vegetation may sometimes dry up or become diseased, reducing its erosion control capability. Reinforced vegetation (or reinforced grass) is a better method that can be adopted for enhancing slope stability and erosion control. Such erosion control products are usually three dimensional mats, having multi-filamented materials of specified thickness. Such materials are known as Rolled Erosion Control Products (RECPs)/ 3-D Mats and also as ‘Turf Reinforcement Mats (TRM). While mats made using natural fibres last for one to two years, polymeric mats are used in situations where such products are required to last for a longer time. 3-D mats having a wide ranging variety of strength are available. The material used for manufacturing these mats also varies. Hence following general specifications are given (Table 4.1) for guidance. However, field conditions like harsh areas/ high survivability requirements may warrant use of 3-D mats with tensile strength as high as 35 kN/m or even more.

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Table – 4.1 Property Requirements for 3-D Mat

3-D Mat Property Specified value* Test MethodMinimum Tensile Strength 2 kN/m ASTM D 5035UV Stability (Min % tensile strength

retention)80% ASTM D 4335 (500 hour

exposure)Minimum thickness 6.5 mm ASTM D 6525Mass per unit area (Minimum) 250 gm/ m2 ASTM D 3776

* Minimum Average Roll Values, machine direction only for tensile strength test

4.4.7 Preformed polymer geosynthetic cells or webs

Often, embankments are to be constructed in areas where vegetation may be difficult to establish and erosion problem might be severe due to water bodies. It may also be not possible to mitigate potential erosive forces that are likely to overcome the strength of the root system. In such cases ‘Geosynthetic Cells’ can be adopted. However, geosynthetic cells would be relatively more costly than all other techniques outlined above.

4.5 Ground Improvement Techniques

Often problems like slip failure of road embankment or high degree of unevenness of road pavements which occur in coastal roads can be traced to inadequate consolidation of clayey sub-soil found in such locations. Such problems in the coastal and delta areas arise due to low shear strength and high compressibility of soft clay sub-soils which are commonly referred to as marine clays. In severe cases, road embankments may even fail or pavement surface may experience unacceptable levels of settlements stretching over considerable period of time. Improvement of the load response behaviour of such soft sub-soil becomes necessary if the embankments are to be built economically and serviceability levels are to be kept high. Accelerating the consolidation process by providing vertical drains has been widely adopted for road embankment construction in such marine clay areas.

4.5.1 Ground improvement using vertical drains

Vertical drains have been in use for more than half a century to promote rapid consolidation of thick soft clay deposits like marine clays, where preloading alone will be insufficient. Sand drains were the earliest type of vertical drains used for consolidation of soft clay layer. Installation of sand drains is usually done by drilling boreholes in soft clay and back filling the borehole using sand of specified gradation. The major problem in this case would be formation of cavities due to bulking of sand. Polymeric vertical drains (PVD) which are also known as ‘Band drains’ have now virtually replaced sand drains/ sand wick technique for ground improvement.

4.5.2 Band drains (PVD)

Band drains consists of a plastic/polymeric core formed to create channels or paths which are surrounded by a thin geotextile filter jacket. Typically the size of band drains is 10 cm in width and 3 to 9 mm in thickness. The primary use of band drains is to accelerate consolidation and to greatly decrease the settlement time of embankments over soft soils. By doing so, band drains also accelerate the rate of strength gain of the in-situ soils. Band drains are used in consolidation situations where soil to be treated is a moderate to highly compressible soil with low permeability and fully saturated in its natural state. The soil should be either normally consolidated or under consolidated prior to loading. The loading should exceed maximum past consolidation pressure for the band drains to be beneficial.

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Band drains are generally installed by displacement methods. The mandrels used with band drains are hollow and normally rectangular or trapezoidal in cross section. The mandrel covers and protects the band drain material during installation. All installation methods employ some form of anchoring system (generally using a disposable end shoe) to hold the drain in place when mandrel is withdrawn. Commonly used methods employ an installation mast (called ‘Stitcher’) which contains the material reels, mandrel and provision for providing installation force. Added to this is a carrier, which is a crawler excavator or crawler crane, depending somewhat on the depth of installation. Usually for drain installation depth upto 20 m, the mast can be mounted on a crawler excavator. Drains requiring depth greater than 20 m most often require an installation mast mounted to a crane to provide stability. The most important criteria for method of installation is the size of the installing mandrel. The mandrel should be kept to a minimum size, usually not greater than 80 cm2 unless larger size is required for penetrating to greater depth. Although equipment is available to work over slopes, a level granular surface containing no large obstructions is ideally required for band drain installation. Sufficient head room is also required for its installation. A thumb rule for head room required would be 3 m longer than depth of installation. Band drains have been installed upto 60 m depth, by using specialised equipment.

It is essential to recognise that band drains serve no structural function. By providing a shorter drainage path, it provides a faster release of excess pore pressure, thereby resulting in faster settlement and quicker strength gain through consolidation. For sites with a stability problem, the soil will initially have the same strength with or without the band drains installed. Further band drains do not play any role in secondary consolidation. Therefore in cases where secondary consolidation is expected to be significant, it is necessary to provide excess surcharge and/or extended waiting periods prior to final construction. It is not recommended to install band drains where pre-drilling is necessary for installation. A drainage layer of coarse sand or gravel is provided above the ground to drain off water from band drains. Generally sand layer is provided for a thickness of 0.5 to 1.0 m.

4.5.3 Stone columns

Stone columns comprise of boreholes of designed diameter made at specified distance apart in the soft soil, which are then back filled using stone aggregates and compacted. The diameter of stone columns varies from about 0.4 m to 0.7 m and their spacing varies from 1.5 m to 3.5 m. This method is used in soft subsurface soils to both accelerate settlement and provide sufficient increase in strength to

Fig – 4.4 A Type of Polymeric Vertical Drain (Band Drain)

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minimise settlement and prevent deep seated shear failure. However stone column technique would be comparatively costlier than providing polymeric vertical drains. Hence stone column technique is selectively adopted to support structures which are sensitive to large amount of settlement or in cases where it is also required to increase the bearing capacity of the sub-soil. At locations where undisturbed shear strength of clayey soil (Su) is lower than 15 kPa, providing stone columns may result in considerable wastage of stone aggregates and Geosynthetic encased stone columns may be adopted in such places. IS 15284 (Part 1) provides guidelines for design and construction of stone columns.

4.5.4 Instrumentation and monitoring

Field instrumentation such as piezometers, settlement platform, settlement gauges and inclinometers are used to monitor performance of band drains and possibly control the rate of embankment construction and/or surcharge. It is important that both the designer and the instrumentation personnel have a full appreciation of the instrumentation being installed. Generally settlement measuring devices of different types like settlement platforms, deep settlement points or horizontal deflection devices are used to measure only the rate and total amount of consolidation. An inclinometer is used to measure horizontal deflection with depth and as a warning device against potential failure. The pore pressure devices (piezometers) are used for both calculation of achieved consolidation rate and excessive build up of pore pressure which are an indication of potential failure. Proper selection of instrumentation devices and the frequency of monitoring a project are important. For simple projects where stability is of no concern, and time is not the critical factor, only surface settlement platforms, which are relatively easy to install, are needed. In situations where stability is critical, pore pressure measurements and measurements of horizontal deformations (using inclinometer) are also necessary. The monitoring can be done daily or once in two/three days during loading period depending on rate of loading. The periodicity of taking the readings from the instruments can then be reduced to once a week or ten days gradually after loading is over. The design of PVD system and monitoring of consolidation using instrumentation are a specialised job and hence advice of geotechnical consultants is to be obtained in these tasks.

4.6 Embankment Construction in Waterlogged Areas

When embankment construction is to be undertaken through an existing pond, dewatering and slush removal should be taken up before placing the embankment fill. In case dewatering is not considered to be feasible and embankment is to be constructed under water, only acceptable granular material shall be used. Acceptable granular material should consist of well graded, hard durable particles with maximum particle size not exceeding 75 mm. The material should be non plastic having coefficient of uniformity not less than 10. The material placed in standing water shall be deposited by end tipping without compaction.

Other methods which can be adopted in water logged areas include – Depressing the water table by using geotextile wrapped aggregate drains (also known as trench drains), raising the embankment height and providing a capillary cut off. Custom made synthetic drains made of polymeric materials are also available which can be used in place of aggregate trench drains. For more details regarding embankment construction in waterlogged/ salt infested areas or in areas where ground water table is very high, IRC: 34, ‘Recommendations for road construction in areas affected by waterlogging, flooding and/or salts infestation’ may be referred.

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Chapter – 5

SEA EROSION CONTROL TECHNIQUES & RIVER BANK PROTECTION

Coastal beach erosion occurs in various forms around the world. This phenomenon gets more acute during cyclones and in-turn causes damage to infrastructure facilities including roads. This is due to severity of waves and storm surge which result in coastal erosion. The basic approach to mitigate coastal erosion related problems is to provide suitable cover to the soil. The measures to control coastal erosion can be categorised as structural and soft/ non-structural. These can be taken up together or separately also. Structural measures used for arresting coastal erosion are sea wall, revetment (rock armour, gabion mattress or precast concrete block revetment systems), offshore breakwater, groynes, etc. Soft measures generally adopted to prevent coastal erosion are artificial nourishment of beaches, vegetative cover such as mangrove plantation, etc. Instead of providing rock armour layer, latest and environmental friendly technologies which make use of geosynthetics for construction of armour protection layer can also be adopted.

5.1 Wave Generation in Sea

Waves are caused by a disturbance of the water surface. Such disturbances become more prominent during cyclones because of wave surge and high speed winds. Most waves are generated by wind. After waves are formed, they can propagate across the surface of the sea for thousands of miles. When waves break on a shoreline or coastal structure, they have fluid velocities and accelerations that can impart significant forces. The wave period of individual waves remains constant through the transformations until breaking but the direction of propagation and the wave height can change significantly. As a wave moves into shallower water the wavelength decreases and the wave height increases. Waves break at two general limits:

In deepwater, waves can become too steep and break when the wave steepness defined as, H/L, approaches 1/7 (where H = Height of the wave i.e., distance between crest of the wave and water surface, L = Wave length defined as distance between two successive wave crests).

In shallow water, waves break when they reach a limiting depth (d) of water.

This depth-limited breaking is important in the design of coastal revetments protecting highways. For an individual wave, the limiting depth is roughly equal to the wave height and lies in the range given below:

0.8 < Maxd

H

> 1.2 ........... Equation 5.1

A practical value of wave height which can be considered when there is mild sandy slope offshore is:

MaxdH

≈ 0.8 ........... Equation 5.2

5.2 Systems for Protection of Coastline Against Sea Erosion

The systems adopted for protection against water erosion comprise of two different parts – the outer revetment or armour layer to absorb the hydraulic energy of velocity of water flow and/or the wave energy; and the inner part of filter layer. Revetment systems in the form of rip-rap blocks, prefabricated concrete elements or gabion mattresses or RCC/stone masonry walls are most commonly used as

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armour layer. The function of inner filter layer is to prevent soil particles from being eroded and to allow free escape of internal water simultaneously. Conventionally several layers of granular material with well designed grain size distribution and thicknesses are used for this purpose. Geotextiles can be successfully adopted to replace such granular filter material. They are now being increasingly adopted owing to various technical advantages, cost benefits, ease of installation, faster completion of the project and superior long term performance of the system. Fig 5.1 to 5.6 show photos of protection measures adopted at various locations in India for protection of sea coast.

5.2.1 Bulkheads and revetments

The distinction between revetments, seawalls, and bulkheads is one of functional purpose. Revetments are layers of protection on the top of a sloped surface to protect the underlying soil. Seawalls aredesigned to protect beach against large wave forces. Bulkheads are designed primarily to retain the soil behind a vertical wall in locations with less wave action. Bulkheads are mostly adopted where wave heights are very small. Seawalls are more common where wave heights are quite large. Revetments are often common in intermediate situations such as on bay or lake shorelines. Seawalls can be rigid structures or rubble-mound structures specifically designed to withstand large waves. Vertical sheet pile seawalls with concrete caps are common but require extensive marine structural design. A more common seawall design type is a rubble-mound that looks very much like a revetment with larger stones to withstand the design wave height. Thus, the two terms, seawalls and revetments, can be used interchangeably with the former typically used for the larger wave environments.

5.2.2 Seawall

Seawall is useful in case of protection of specific area from erosion due to waves and storm surges. Seawalls are constructed along the coast adopting stone masonry technique or using reinforced cement concrete. Seawall can be constructed using gabions also when wave heights are low, typically less than about 1.0 m. Seawalls constructed using gabions are permeable and flexible; thereby they would be able to withstand differential settlement without loss of its structural integrity. Provision of filter layer behind the seawall is essential to prevent piping of sand and subsequent destabilisation of structure. Sometimes a combination of sea wall constructed using masonry or reinforced cement concrete is further protected on sea side using gabions or concrete blocks/ tetrapods. Design of the masonry or gabion seawalls is to be carried out in a manner similar to design of retaining walls, to ensure stability against overturning, sliding, excessive foundation pressure (bearing capacity failure) and water uplift. Additionally ‘Wave flume studies’ may also have to be adopted to arrive at satisfactory design of stone, rock and/or concrete armour units.

5.2.3 Breakwater

Breakwaters are coastal structures constructed to protect an area from the effects of waves. Breakwaters are adopted to protect a ship berthing area, to train and prevent silting of the entrance of river mouths or to prevent erosion of coastlines. However, adverse effects are observed on down drift side and it should be avoided unless their main purpose is to protect a specific area at the cost of adjoining areas. An off-shore breakwater may be constructed to prevent beaches or coastlines from erosion by wave activity. The off-shore breakwaters are submerged structures located at certain distance offshore in order to dissipate wave energy before they reach shoreline. The broken waves would not be having the energy to erode the beach or coastline and the coastline may even increase in extent as a result of accretion. It is an expensive option and needs regular maintenance.

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Fig – 5.1 Provision of Sea Wall and Concrete Tetrapods for Sea Erosion Protection at Mumbai

Fig – 5.2 Protection Against Sea Erosion by Retaining Wall and Gabions – Puri Konark Road

Fig – 5.3 Another View of Protection Works – Puri Konark Road

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Fig – 5.4 Beach Protection Using Boulder Revetment at Paradip, Odisha

Fig – 5.5 Another View of Boulder Revetment at Paradip, Odisha

Fig – 5.6 Masonry Sea Wall Constructed at Sea Coast at Koteshwar, Kori Creek, Gujarat

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5.2.4 Soft Structural/ Non-structural measures – Artificial nourishment of beaches

Beach nourishment may be adopted for protection and beach development. Combination of nourishment of beaches with seawall/ groynes will create beach in front of protected area and eliminate leeside erosion.

5.2.5 Vegetation cover

Plantation of mangroves and palm trees can be taken up for beach protection. Vegetation cover can restrict sand movement and erosion.

5.2.6 Artificial reef balls

A reef ball is a designed artificial reef used to restore ailing coral reefs and to create new fishing and scuba diving sites. Reef balls are the only type of artificial reefs that can be floated and towed behind a boat. Reef balls are made of a special grade marine environment resistant concrete and are designed to mimic natural reef systems. They are also used widely to create habitats for fish and other marine and fresh water species. Reef balls are made in many sizes to best match the natural reef system which is being mimicked.

Out of these measures, depending on the techno-economic viability, any suitable measure can be adopted, while a combination of these measures usually gives optimum results.

5.3 Design of Coastal Rip-rap Against Wave Attack

Rip-rap can be used for protection from four different types of hydraulic situations: direct rainfall impacts, overland flow, stream or river currents, and waves. This section addresses only wave attack. IRC:89 provides procedures for the design of riprap revetments for channel bank protection on larger streams and rivers where the active force of the flowing water exceeds the bank material’s ability to resist movement. Brief details of the same are provided in section 5.6. Flow in a stream or river is unidirectional and typically aligned parallel to the banks. Waves produce oscillatory velocities and accelerations that can be in almost any direction on a revetment. In such situations, it is recommended that ‘Hudson’s equation’ be used to estimate stone size for revetments subject to wave action. This involves determining the design wave height (as per equation 5.1) and using Hudson’s equation to size the stones to be used for rip-rap. This approach can lead to designs with larger stones and narrower stone gradations than designs for non-wave situations. The difference is due to the higher forces caused by waves. Situations where riverine and wave flows are also significant, the design engineer should consider both design approaches and develop a conservative design. A simplified version of Hudson’s equation for calculating required median weight for the outer, or armour layer, stones is:

θcot 280H

W3

50 ........... Equation 5.3

Where, W50 = Median weight of armour stone in kgsH = Design wave height in mΘ = Slope

The range of recommended slopes for revetments is up to 2:1 (horizontal:vertical) or flatter. Hence cotΘwould be equal to 2 for a 2:1 slope and cotΘ=3 for a 3:1 slope. Apart from armour stone, either graded aggregate filter layers or preferably geotextile needs to be placed below armour to prevent piping

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failure. Selection of geotextile can be carried out as per IRC SP:59. A typical rip-rap gradation for coastal revetment with a median weight W50 = 350 kgs, will have 50 per cent of stones weighing between 100 kgs to 350 kgs, 30 per cent weighing between 350 kgs to 700 kgs, and 20 per cent weighing between 700 kgs to 1350 kgs. Thus, the recommended coastal revetment gradation precludes the smaller stones and allows for some larger stones as compared to gradation adopted for river bank protection. These smaller stones are typically not included in coastal revetments because of their tendency to move in response to wave action. Further it may be noted that the construction of a revetment, while it protects the upland, does not address the underlying cause of erosion. The depths at the toe of the revetment will likely increase if the erosion process continues. The presence of a revetment or seawall can increase the vertical erosion at its base. A common practice to overcome toe erosion is to extend revetment beyond the slope inside water and provide toe protection. A commonly proposed alternative to rubble mound revetments is a concrete block revetment, which are also known as 'Tetrapods'. Some of these have physical interlocking between individual blocks also. Many such concrete blocks which have a patented shape are also available. These are essentially unreinforced concrete objects designed to resist the wave action. If the intensity of wave action is severe, then additional layers of armour protection would be required. In such cases, ‘Tetrapods’ can be placed over stone blocks.

5.4 Use of Geosynthetic Products as Revetment

Coastal and waterways protection applications comprised the earliest use of geosynthetics. Over the last 40 years, there have been numerous coastal and waterway protection projects that have utilised geosynthetics. Geosynthetics can be used as components of coastal and waterway protection measures in two different ways – they can be used as filters within coastal and waterway protection structures and they can also be used to create revetment systems (containers) to act as mass-gravity protection works. During the construction of structural measures to control sea erosion, problem generally faced is the non availability of construction materials like big size boulders, sand, etc., within reasonable and cost effective distance. This problem can be sorted to a great extent by using geosynthetic revetment systems. The most universal and widely used geotextile containers are well-known, ubiquitous sand bags which are seen world over for shoring up flood defences in times of natural calamity. The dominant geosynthetic material used for making revetment systems is geotextiles, which are robust and permeable materials. Three types of geoetextile revetment systems differentiated by geometrical shape are available. They are geotextile tubes, geotextile containers and geotextile bags. Geotextile tubes are tubular containers that are filled in-situ on land or in water. Geotextile containers are large volume containers that are filled above water and then deposited into the submarine environment. Geotextile bags are small volume containers that are filled on land or above water and then pattern-placed either near water or below water level. The geotextile revetment systemshave the following advantages:1. They are resistant to chemical attacks occurring in usage, especially to alkalies and acids.2. Geotextiles are quite durable when exposed to elements of nature like – Sun, precipitation, etc.

However, ultraviolet radiation reduces their strength in long term. Hence they need to be treated to enhance their ultraviolet resistance if they are going to be exposed to sun during their service life.

3. They are resistant to organic attacks like bacteria and fungus and are not attractive to rats or termites.

Geotextile containers behave as mass-gravity elements that can resist hydraulic forces. For these applications, the geotextile skin should have specific mechanical, hydraulic and durability requirements.Distinction must be made between those applications where the geotextile containment is required for only temporary use and those applications that require long term performance. For temporary works the requirements of the geotextile container itself is fairly basic as it only has a short life over which it has to

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perform, however for long term applications, the performance requirements of the geotextile container are more severe. With regard to long term performance, distinction must also be made according to type of hydraulic environment acting on the geotextile container. For example, the action of still water, or intermittent water flows, will have a different effect on the geotextile container than the action of breaking waves.

5.4.1 Geotextile tubes

Geotextile tubes are large cylindrical structures made using high strength woven geotextile material which are then filled with dredged material in-situ. Geotextile tubes may be used for a range of coastal and waterway protection applications where barrier type, mass-gravity, structures are required. Thedredged material is usually pumped in a slurry form from nearby area and consists of a mixture of sandy soil and water. The geotextile tube, being permeable, enables the excess water to pass the geotextile skin while the fill attains a compacted, stable mass within the tube. For coastal and waterway applications the type of fill used is sand, or a significant percentage would be sand. The reasons being – sand can be compacted to a good density by hydraulic means, sand has good internal shear strength which gets further improved by the presence of confining geotextile tube skin, and this type of fill once compacted, will not undergo settlement, which would change the shape of the filled up geotextile tube The tube is filled by direct coupling to a hydraulic pumping system conveying dredged material. Designed with appropriately sized openings called ‘Filling Ports’, the geosynthetic tubes retains fill material while allowing water to permeate through tube wall. After dewatering typically very little consolidation will occur in case of pure sands while it may be as much as 70 per cent in case of tube that has been filled with fine grained organic material. Openings called, fill ports are provided in geotextile tubes at a spacing of about 8 to 10 m for filling dredged material. Special high strength seaming techniques are adopted in their manufacturing process to resist pressure during pumping action. Geotextile tubes permanently trap granular material in both dry and underground construction. Geotextile tubes are generally about 1 m to 3 m in diameter, though they can be custom made to any size depending on their application. Geotextile tubes ranging in diameters from 1.5 m to 5.0 m are available for coastal and waterway protection applications. Stacking of geotextile tubes one over other can also be made to construct structures of higher heights. Geotextile tubes may be used for a range of coastal and waterway protection applications where barrier type, mass gravity structures are required. Geotextile tubes can be used for construction of groynes, off-shore breakwater, etc. When geotextile tubes are used as off-shore breakwater structures, the dimensions of geotextile tubes are to be chosen in such a way that waves break over the geotextile tubes.

Geotextile tubes are normally described in terms of either a theoretical diameter, D or a circumference, C. While these two properties represent the fundamental characteristics of geotextile tubes they are not of direct interest when it comes to engineering parameters for coastal and waterway protection

Fig – 5.7 Typical Components of a Geotextile Tube

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applications where the geotextile tube in its filled condition is of prime importance. When the geotextile tube has been filled with sand, it assumes an oval shape. The width of the oval tube and its height are of importance from engineering performance point of view. Table 5.1 lists relationships between the fundamental geotextile tube characteristics and engineering parameters. The relationships are applicable to geotextile tubes that have a maximum strain of about 15 per cent, low unconfined creep, and are filled to maximum capacity with sand. It is also assumed that the foundation beneath the tube is a flat, solid surface.

Geotextile tubes are used for revetments where their contained fill is used to provide stability. They have been used for both submerged as well as exposed revetments (Fig 5.8). For submerged revetments, the geotextile tube is covered by local soil and is only required to provide protection when the soil cover has been eroded during the periods of intermittent storm activity. Once the storm is over, the revetment is covered by soil again either naturally or by maintenance filling. For exposed revetments, the geotextile tube is exposed throughout its required design life. To prevent erosion of the foundation soil in its vicinity, and undermining of geotextile tube revetment, it is common practice to install a scour apron. This scour apron usually consists of a geotextile filter layer that passes beneath the geotextile tube and is anchored at the extremity by a smaller sized geotextile tube, called anchor tube.

Table – 5.1 Basic Engineering Parameters for Geotextile Tubes filled with Sand

Engineering parameter In terms of theoretical diameter, D In terms of tube circumference, CMaximum filled height, H H ≈ 0.6 D H ≈ 0.19 CFilled width, W W ≈ 1.4 D W ≈ 0.45 CBase contact width, b b ≈ 0.9 D b ≈ 0.29 CCross sectional area, A A ≈ 0.65 D2 A ≈ 0.07 C2

Average vertical stress at base, σ

σv ≈ 0.72 γ D(γ = Density of the fill)

σv ≈ 0.24 γ C(γ = Density of the fill)

Revetments using multiple-height geotextile tubes are also constructed. Here the geotextile tubes are staggered horizontally to achieve the required stability. Considerable care should be exercised during construction of these types of revetments to ensure the water emanating from hydraulic filling of upper geotextile tubes does not erode the soil and undermine the lower geotextile tubes in the multiple-height revetment structure. In a similar manner, geotextile tubes can be used for constructing offshore breakwaters, protection dykes, containment dykes and groynes as shown in Fig 5.9.

Fig 5.8 – Geotextile Tube Application for Coastal Protection

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Fig 5.9 – Applications of Geotextile Tubes

Fig 5.10 – Typical Use of Multiple Geotextile Tubes for Coastal Embankment Construction

Fig 5.11 – Geotextile Tubes with Gabions as Armour Protection layer

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5.4.2 Geotextile containers

Geotextile containers, as their name may imply, are large volume containers that are filled above water and then positioned and placed at reasonable water depth. Geotextile containers are made from high strength woven geotextiles or a combination of woven and non woven geotextiles (depending on the fill characteristics) which are filled with sand/ dredged material. The volumes of these containers more commonly range from 100 m3 to 700 m3, although containers as large as 1000 m3 have also been installed. To facilitate the installation of geotextile containers, an efficient and practical installation system is required. To date, this has been achieved by using split bottom barges. This entails, the filling of the geotextile container in a split bottom barge. The container is then sealed and the barge is positioned at the correct dumping location. The split bottom of the barge is then opened and the container is deposited on the seabed. Geotextile containers are used as mass-gravity structural components in coastal and waterway protection applications such as offshore breakwaters, containment dykes, artificial reefs, slope buttressing, etc in a manner similar to geotextile tubes.

5.4.3 Geotextile bags

Geotextile bags are made from high strength woven and nonwoven geotextiles or a combination of these can be used (depending on the fill characteristics) which are filled with sand/ dredged material.Geotextile bags are used at sea shores or bunds adjacent to rivers which are to be protected from erosion, especially during emergency situations. Geotextile bags have also been used as revetments, breakwaters, etc to build structural erosion protection measures during normal periods also (Fig 5.13). Geotextile bags provide stability and prevent erosion. Geotextile bags are filled off-site and then installed to the geometry required in a similar manner to geotextile containers. For best performance, they have to be filled to maximum volume and density with sand in an identical manner to geotextile tubes. However, geotextile bags have two major differences to other geotextile containment techniques – they can be manufactured in a range of shapes, and they are installed in a pattern-placed arrangement that greatly improves their overall stability and performance. Geotextile bags ranging in volume from 0.05 m3 to about 5 m3, which are pillow shaped or box shaped or mattress shaped are available, depending on the required application. Filling geotextile bags with dry sand becomes more difficult as the volume of the bag increases, but filling task can be efficiently done by using sand+watermixture (hydraulically filling the sand into a bag). Filled density and volume are important from the view point of maximising the stability, but it is also important from the view point of minimising the effects of

Fig 5.12 – View of geotextile Tubes Covered with Armour Protection Layer of Gabions

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fill liquefaction and loss of shape of the geotextile bags. To ensure that the contained fill is maintained in its dense state, the geotextile skin should have adequate tensile strength.

One major advantage of geotextile bags is that these small volume units can be used to construct hydraulic and marine structures that require adherence to designed geometrical shape accurately. This makes them preferable to large volume units such as geotextile containers when specific slope and height tolerances are to be attained. Another advantage of small volume units of geotextile bags is that maintenance and remedial works can be carried out easily by replacing the failed bags. This is much simpler than carrying out remedial works on large volume containment units.

5.4.4 Gabions

Gabions, which are mesh like structures filled with relatively small size stones, are an attractive alternative to large boulder stones for various erosion control and scour protection applications. Gabions by holding the small stones together, function like large boulders but at the same time facilitates easy construction and offer a flexible structure. Thereby gabions provide a technically

Fig 5.14 – Geotextile Bag

Fig 5.13 – Shore Reclamation Using Geotextile Bags

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satisfactory and cost effective solution. Gabions can be made from either polymeric material or double twisted steel wires having zinc+polymer coating. Gabions are generally available in a prefabricated collapsible form with the bottom and four sides held together by appropriate binding and with a flip open top lid. Filled with stones, the gabion becomes a large, flexible and permeable building block usingwhich a broad range of structures can be built. Because of their inherent flexibility, gabion structure can yield to earth movement and retain their full efficiency while remaining structurally sound. They are quite unlike rigid or semi-rigid structures, which may suffer complete failure when even slight changes occur in their foundation. Besides the above, gabions can be easily lifted by cranes, they are suitable for underwater construction and several gabions can be tied together to create continuous, integral structures. The pervious structure of gabions gradually absorbs the heavy wave impact than an impervious structure. IS 16014 provides specifications for zinc+polymer coated steel wire gabions. Compared to steel wire gabions, polymeric gabions have advantages like superior corrosion resistance, ability to withstand acidic and alkaline environment, excellent durability, excellent flexibility to take the shape of ground contour, etc. However, these gabions due to their very high flexibility, may not be as much amenable to construction of retaining structures as compared to steel wire rope gabions.

5.5 Geotextiles as Filters

Below the revetments (either stone/rock or concrete armour units), filters are invariably required to prevent soil washout. Traditional granular filters usually consist of several layers of stone aggregates. If the water forces are strong enough and the soil to be protected is fine grained, then upto four layers of granular materials may be required to satisfy the hydraulic design requirements. Hence, this kind of relatively complex structures can be expensive and difficult to construct. Furthermore, granular/ aggregate filters are difficult to place on steep slopes, cannot always be installed in tidal zones and laying process demands reliable and expert supervision. Geotextiles can be used as substitutes for one or more granular under layer materials below revetments. Geotextiles offer many advantages over granular filter materials:

They enable design flexibility with regard to the choice of the size of the granular material in the layer immediately adjacent to the geotextile filter.

They are easier to install to specific geometrical configurations than granular materials – in many cases below water level.

In general, in-situ quality control test requirements for geotextiles are nominal.

Where geotextiles are used as filters for coastal and waterway protection, their primary function is to prevent the erosion of soil through the protection structure and thus prevent instability. In case of geotextile filters, hydraulic characteristics like apparent opening size and permittivity are most

Fig 5.15 – Use of Gabions for River Bank Protection

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important. The selection of filter fabric with correct opening size depends on the percentage of finer material available in bed material. In order to fulfil its function, the geotextile material has to be robust enough to resist mechanical stresses applied to it during installation. Secondly, the geotextile material must have required hydraulic properties in order to perform as a filter material. Thirdly, the geotextile must have adequate durability to maintain its mechanical and hydraulic properties throughout the design life of revetment. The criteria for selection of filter fabric can be based on IRC SP: 59. As the weight of the stones/ drop height increases, thicker geotextile having greater mass per unit area would be required. Another important property would be trapezoidal tear strength. Normally, geotextiles having trapezoidal tear strength varying from 200 to 600 N (ASTM D 4533) are used in coastal works. When determining the appropriate hydraulic properties for the geotextile revetment filter consideration needs to be given to the critical hydraulic regime that will act on the revetment structure over its design life. Table 5.2 list the geotextile filter hydraulic properties requirements according to the type of hydraulic regime. When several different hydraulic regimes occur at the same location then the most critical hydraulic regime (1 being the least critical and 3 being the most critical in Table 5.2) should be chosen for design. While installing geotextile filter, it is to be first laid out on the soil surface prior to placing stones and rocks. For good long term performance, the geotextile filter should be covered with an adequate thickness of granular material to ensure that it remains protected from the effects of long term exposure to ultra-voilet (UV) rays. The minimum thickness of stone coverage above the geotextile filter to protect against UV radiation should be atleast two times the maximum stone size in the rock armour layer above the geotextile filter. During installation, it may be inevitable that the geotextile filter would be exposed to UV rays and this condition may extend, depending upon pace of construction. To cover such eventualities, the UV stability of requirement of geotextile to be used should meet the specification requirements as per IRC SP:59. The geotextile filter coverage beneath the revetment armour layer should extend beyond the zone of erosion. This would ensure that revetment structure will remain stable throughout the life of the structure.

Table – 5.2 Geotextile Hydraulic Property Requirements under Different Regimes

1 Water current flows parallel to revetment faceNon-dispersive soil O95 ≤ 0.35 mm

Dispersive soil d15 ≤ O95 ≤ d85

2 Gradual reversing water flows d15 ≤ O95 ≤ d85

3 Impacting wave activity d15 ≤ O95 ≤ d50

1. d15, d50, and d85 are percentile particle size fractions to be protected2. O95 is apparent opening size (AOS) of the geotextile filter (ASTM D 4751)

5.5.1 Geotextile filters for breakwaters

For rubble mound and caisson wall breakwaters geotextile filters are placed on top of the sea bed prior to construction of the breakwater. In this location, the primary role of geotextile filter is to prevent erosion of sea bed and the undermining of the breakwater. To facilitate installation on the sea bed, the geotextile filter is usually prefabricated onsite into a fascine mattress structure. This technique involves the fabrication of geotextile filter into large sheets on land and attaching an interconnecting grid of fascines, bamboo or timber. The resulting mattress is then pulled into the water and floated into place and sunk on the sea bed. This technique has proved to be an efficient and cost effective means of installing geotextile filters on the sea bed. The tensile stresses imposed on the geotextile filter during fascine mattress installation procedure are relatively high. Consequently, woven geotextiles with wide-width tensile strengths ranging from 80 kN/m to 200 kN/m are normally used for this type of application.

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Offshore breakwaters also may be constructed to protect beaches or coastlines from erosion by wave activity. In such cases, the breakwaters would be submerged structures that force the waves to break when passing thus, expending much of their wave energy. The broken waves would not have the energy to erode the beach or coastline and the coastline may even extend outwards into the sea as a result.

5.5.2 Geotextile filters for containment dykes

To reclaim land from sea, it is common to first construct a containment dyke around the extremity of the reclamation area. Soil or sand fill is then dry dumped or hydraulically pumped into the containment area to form dry land. The function of the containment dyke is to prevent loss of the placed soil or sand fill into the surrounding water. The nature of the containment dyke is slightly different depending on whether the reclamation occurs in relatively deep water or in shallow water. Where land reclamation occurs in relatively deep water, the size of the containment dyke is fairly large and may require two or more stages to complete the structure. Commonly, the dyke consists of a rubble mound of dumped rock with a geotextile filter placed across the base of the dyke. The role of the geotextile filter is to prevent the loss of reclamation fill through the rubble mound dyke and the erosion of the sea bed beneath the rubble mound. The geotextile filter across the base of the dyke can also prevent the loss of the rubble mound material into the sea bed if the foundation is soft. For permanent protection, a rock armour layer may be placed on the outside of the rubble mound depending on the water forces acting on the structure.

Where land reclamation occurs in relatively shallow water, the containment dyke is normally constructed in a single stage. Commonly, the bund consists of a rubble mound with geotextile filter placed across the base of the dyke. Again for permanent protection, a rock armour layer may be placed on the outside of the rubble mound depending on the water forces acting on the structure.

It is not uncommon for the base geotextile filter beneath the containment dyke to have different properties on different faces. Normally, the base geotextile filter is installed in a manner similar to the breakwater structure which may require a fascine mattress approach to installation. This imparts relatively high tensile stresses on the geotextile filter during installation, and consequently woven geotextile filters with wide width tensile strengths between 80 kN/m and 200 kN/m are usually used for this purpose.

5.6 Mangrove Cultivation

Among soft/ non-structural measures for coastal protection, mangrove cultivation is one of the most effective techniques. Mangrove is a group of typical tropical and specialised trees growing in the saline and brackish water system. The mangrove trees are highly productive, economical and most importantly they protect the shoreline from erosion and cyclonic impact. The mangroves are angiosperms, with about 45 species found in India. They have special characters like viviparous germination, pneumatophores, prop or knee roots and salt glands. These trees form a thick forest belt on the deltas, along major estuaries, and fringe the estuarine banks, as well as backwaters. This unique tree resource is useful for tannin extraction, paper and pulp, firewood, timber, charcoal, fodder and several other by-products. The mangrove swamps are rich in the larvae of many economically important fishes, prawns, crabs and bivalves. These are the most suitable area for feeding, breeding and nursery grounds of these marine organisms and hence important for aquaculture purposes. Afforestation of coastal areas suitable for mangrove cultivation would go a long way for preventing soil erosion. Mangrove trees generally prefer soft, clay mud for their growth. These species show different salinity tolerance limits. The expanse of mangrove forest depends on the intertidal

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expanse, substratum and salinity of soil as well as water. Out of 45 mangrove species occurring in India, some are true mangrove while others are considered as 'associated' flora. The most dominant mangrove species found along the east and west coast of India are listed below:

Rhizophora mucronata R. apiculata Bruguiera gymnorrhiza B. parviflora Sonneratia alba S. caseolaris Cariops tagal Heretiera littoralis Xylocarpus granatum X. molluscensis Excoecaria agallocha Lumnitzera racemosa Avicennia officinalis A. marina

The species mentioned above are available easily and their seedlings (propagules) or seeds are also available in considerable quantity in mangrove forest. Mangrove seeds (fruits and seedlings) are always available in small quantity throughout the year. The main fruiting or seedling season, however, start from June to September, when plenty of seedlings of all the Rhizophoraceae, Avicennia and other types can be collected. Only mature seedlings of these mangrove species should be collected for afforestation or nursery purpose. The seedlings of rhizophoracious trees have a podlike structure with tapering end of varying sizes and with typical morphological characters. Avicennia fruits are triangular in shape while Sonneratia is globular. It is however, always advisable to store these seedlings partially immersed (pointed end in water) in seawater. There are two ways of planting the mangrove seedlings

Direct planting in the swamp Raising seedling in the nursery

Seedlings which are healthy, non-infected and fully matured should only be used for planting. Any intertidal area (between the high tide and low tide) where mangroves are absent and the substratum is of soft clay or mud and is inundated by regular tidal waters every day, are suitable for direct mangrove planting. Along the Gujarat coast and West Bengal, where intertidal expanse is very large with highest tidal amplitude of 6 to 8 m, the upper limit of 1 m tidal water level has to be selected. After selecting the area to be planted, planting of seedlings may be undertaken according to the length of the propagules. Rhizophora mucronata or Rhizophora apiculata whose seedlings are the longest should always be planted towards the waterfront, these can be followed by Kandelia, Ceriops, Bruguiera, Avicennia, Lumnitzera, etc. Species with smallest seeds like Sonneratia should come to the landward side of the intertidal expanse, followed by species of grasses. Direct planting method has to be used in open areas. Nursery technique method is useful where the mangrove species are not available in plenty. This also has advantages like selected species can be grown in large numbers. Mangrove nurseries can be developed in the upper part of the intertidal region where seedlings can be grown in polyethylene bags supported with bamboos. The mangrove nursery may be located near the estuary or sea where seawater or estuarine water is available. The nursery may be on the open ground or in the low lying protected areas where seawater reaches. The collected and selected seedlings are inserted in the polyethylene bags filled with mangrove soil. If the nursery is on the raised ground then the perforations in the bags are not needed, but the nurseries in the low lying area need the perforations in the polyethylene bags. Care should be taken to cut open the polythene bags at the base before

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planting them at desired locations. Once the plantation is established, then there is not much to be done. Only these plantations are to be protected from the grazing cattle. Periodical checking is helpful in finding out whether there is any need to replacement. But this would be very minor. There is no need of any insecticide, pesticide, fertilizer, or anything but only protection from grazing.

5.7 River Erosion Control

Many of the states which are subjected to cyclone effect have large coastal river delta regions. Often, roads constructed in cyclone prone areas would be skirting river banks. Rivers may experience flooding due to incessant rains during cyclone. In such cases, it would be necessary to protect the guide bunds (near the bridges) and road embankments skirting river banks from erosion due to flooding in rivers. River embankments are to be designed taking into consideration flood records for a period of 25 yearsin predominately agricultural areas and 50 years frequency for works pertaining to protection of towns, important industrial and other vital installations. Generally, a minimum free board of 1.5 m above HFL is provided at subgrade bottom level for NH and SH skirting major rivers. In case of rural roads, this value may be suitably reduced but a minimum free board of 0.6 m is required. These embankments can be designed as either homogeneous embankments (comprising of practically uniform type material with coarser material being placed at the slopes away from river side) or zoned embankments (comprising of a core of impervious material). A side slope of 1 V:2 H or 1 V:3 H can be adopted provided adequate factor of safety is obtained when checked for slope stability analysis. Where space constraints do not permit adoption of flatter slopes, retaining walls can be provided at embankment toe portion so that flatter side slopes can be adopted above the toe walls.

Additional care is to be exercised in case of city roads located in urban areas prone for cyclones. In such urban areas, plinth level of houses was historically defined with reference to the adjacent road level. However, a recent trend that has emerged in many cities is that the roads are being resurfaced without removing the older layer. As a consequence, over the years, the new road levels, in many instances, are now much higher than the approved plinth level of the adjacent properties. This prevents drainage of the houses and during periods of rainfall, storm water runoff causes flooding of these properties even during rainfall of low intensity. All road strengthening/ overlay works in urban areas prone for flooding/ waterlogging should be carried out by milling the existing layers of the road and recycling of materials obtained as a result of milling so that road levels are not allowed to increase.

5.6.1 Hard armour for river erosion control

Soil banks or slopes exposed to constant concentrated flows, currents, or waves cannot support vegetation and thus need to be protected by hard armour systems. Hard armour systems include stone rip-rap, fabric formed revetments, geocellular confinement systems, gabions/ revet mattresses, articulating concrete blocks, etc to act as energy absorbing armour. Similar type of protection may be required for slopes of guide bunds also. The term ‘Rip-Rap’ is used to connote placing of stones of different sizes (conforming to specified gradation) against the river bank to deflect the force of the water hitting the banks. The term ‘Rip-Rap’ is used to connote placing of stones of different sizes (conforming to specified gradation) against the river bank to deflect the force of the water hitting the banks. Stone pitching refers to placement of approximately single sized rocks along the slope to serve the same purpose. The size of the stone to be used for pitching can be calculated using the equation:

d = Kv2 ........... Equation 5.4Where K = 0.0282 for 2(H):1(V) slope and 0.0216 for 3(H):1(V) slope

d = equivalent diameter of stone in mv = mean design velocity of flow in metre/sec

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However, no stone weighing less than 40 kg should be used. Where the required size stones are not available within economic leads, cement concrete blocks or gabions can be used in place of isolated stones of equivalent weight. The thickness of stone pitching (t) may be determined using the equation:

t = 0.06 Q1/3 ........... Equation 5.5

where Q = design discharge in m3/ sec

The thickness of stone pitching shall be limited to an upper limit of 1.0 m and lower limit of 0.3 m. Quarry stones are preferable to round boulders, since angular stones have better interlock and do not roll off the slope easily. Further, in states like West Bengal, where supply of good quality stones is difficult armour for slope protection is prepared using bricks. Since weight of each brick is considerably less than 40 kg, a bigger block of several bricks is prepared using cement mortar and individual bricks and these blocks are used as armour. Use of fly ash bricks to prepare such brick blocks can also be adopted. The size of the brick blocks usually used would be 53 cm x 53 cm x 25 cm.

5.6.2 Design of granular filter

When a hard armour system is in place, water can seep in and out of the bank or slope, but the force of water is resisted by the armour. As the water seeps, it can gradually carry soil particles along with it. The resulting voids cause armour support to be lost after some time. This process is called ‘piping’which can culminate in shifting, rolling, sinking or other instability in the hard armour system. Typical solutions include placing a granular filter layer or geotextile layer between the bank soil and the armour to prevent piping. The material for filter shall consist of sand, gravel, stones or coarse sand. Provision of a suitably designed filter is necessary under the slope pitching to prevent escape of underlying embankment material through the voids of stone pitching/ cement concrete slabs as well as to allow free movement of water without creating any uplift head on the pitching when subjected to the attack of flowing water and wave action. The same criteria can be adopted for granular filter design below sea coast armour system also. The gradation of filter material to be placed over soil slope (base) may be designed using the following criteria:

( ) ( ) < 5 ........... Equation 5.6

4 < ( ) ( ) < 20 ........... Equation 5.7

( ) ( ) < 25 ........... Equation 5.8

(a) Filter design may not be required if embankment fill consists of CH or CL type of soils with liquid limit greater than 30 (resistant to surface erosion). In such a case, if a layer of material is used as bedding for pitching, it shall be well graded and its D85 size shall be atleast twice the maximum void size in pitching.

(b) In the above equations, D15 means the size of that sieve which allows 15 per cent by weight of the filter material to pass through it and similar is the meaning of D50 and D85

(c) If more than one filter layer is required, the same requirement as above shall be followed for each layer. The finer filter shall be considered as base material for selection of coarser filter.

(d) Where brick bats are used as filter material, normally the grading is not possible, and in such cases, a layer of graded gravel shall be provided below the brick bats.

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(e) The filter shall be compacted firmly. The thickness of filter is generally of the order of 200 to 300 mm. Where filter is provided in two layers, thickness of each layer shall be 150 mm.

For more details regarding stone pitching, filter material and river training works, ‘IRC:89, Guidelines for Design and Construction of River Training and Control Works for Road Bridges’ and MoRTH Specifications for Road and Bridge Works can be referred to. Geotextiles can be used as alternate filter layers beneath hard armour systems for river bank protection also. Specifications and other details about provision of geotextile layer below armour protection system can be obtained from IRC SP:59, ‘Guidelines for Use of Geotextiles in Road Pavements and Associated Works’.

5.6.3 Selection of river erosion control technique

While choosing the most effective solution, the approach should start from the analysis of the problem, viz., site condition, causes and effects, etc., collecting the site data and design information and deciding on the desired performance of the solution to be adopted. The solution may range from pitching or revetment, toe walls, apron protection or a gravity retaining structure. Emphasis should be on a speedier construction, best use of locally available resources, least damage to the environment and cost effectiveness. In many cases, the most predominant factor is speedier construction and easy availability of raw materials (which indirectly means cost effectiveness). Geo fabric forms are chosen over stone filled gabions if there is a scarcity of stones. In some cases, a combination of the two may also be adopted such as use of geotextile bags filled with sand, which are in turn put inside the gabions. While designing a river training solution, efforts should be made to maintain the natural geometry of the river and at the same time providing an environmentally friendly solution. Proper attention should be given to following aspects during design/ installation stage:

(a) Provision of scour protection measures to retaining structures – To be ensured by either providing scour protection aprons or deep foundations resting on hard non erodible strata.

(b) Proper selection of geotextile filter fabric – Depending upon the requirements of soil particle size and permittivity required, the geotextile filter should be appropriately designed and provided. The selected geotextile fabric should also be capable of providing abrasion resistance and puncture resistance during installation and service life.

(c) Keying of the gabions into the banks at the end of the structure – Inadequate key-ins may cause failures in river bank protection works as the scouring and erosion of soil starts taking place from the ends. Hence it is advisable to key the gabions into the existing banks to minimise the chances of scouring around the structure.

5.6.4 Bioengineering erosion control

Stream bank and embankment side slope erosion can be severe, in cases where shorelines are composed of erodible soil and where the road runs along the stream. Traditional methods of controlling stream flow and wave induced erosion have relied on structural practices like rip-rap, retaining walls and sheet piles. In many cases these methods are expensive or may be ineffective. An alternative approach is bioengineering, a method of construction using live plants alone or combined with dead or inorganic materials, to produce living, functioning systems to prevent erosion, control sediment and provide habitat. Bioengineering involves the use of live plants to add structural strength to soil. For details about Bioengineering techniques reference can be made to IRC: 56, ‘Recommended Practices for Treatment of Embankment and Roadside / Hill Slopes for Erosion Control’.

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Chapter – 6

ROAD PAVEMENTS IN CYCLONE PRONE AREAS

The road pavement constitutes nearly one-third to one-half of the total cost of the road. Therefore, very careful consideration should be made for the choice of the type of pavement and its design. The factors, which govern the selection of the type of the pavement, are initial construction cost, quality of locally available materials, maintenance cost and technology of construction required and its availability. The options available are, flexible pavement, cement concrete pavement, composite pavement with semi-rigid base with suitable bituminous surfacing, semi-rigid base with surfacing of inter connected concrete paving blocks, and roller compacted concrete.

The road including the wearing course and base course as well as subgrade should be properly designed to withstand not only traffic induced load during its design life but also to withstand stresses caused by the temperature variations and changes in sub-soil conditions. Large scale run-off from heavy rains, resultant humidity, tidal variations and rapid changes in underground water table are causes for concern in so far as construction of roads in coastal areas are concerned.

6.1 Preparation of Subgrade

This is the single most important aspect to be taken care of and may involve removal of undesirable soil/ modifying the soil properties to get adequate strength. Settlement of soil may be reduced by increasing its bearing capacity with adequate compaction or by removing air voids. The following measures are advocated:

Adequate density and compaction can be achieved using proper type of compacting equipment (such as static, vibratory and sheep foot roller) and by giving desired number of passes

Geoetxtiles can be used to enhance subgrade strength where subgrade CBR values are very low. Subgrade soil stabilisation techniques can be adopted using admixtures like cement, lime, fly ash, or

a combination of these materials selected judiciously.

Following are the causes for moisture changes in subgrade soil of coastal roads which need special consideration:

Rise and fall of water table with tidal movements – The changes in the moisture content of the soil in such cases depend upon capillary action. Proper attention should be paid for determining the height of the embankment.

Percolation of water through the surface of the road – In case of rigid pavements, water may percolate through joints and cracks which are not adequately sealed, while bituminous surface with open graded mix are known to be porous. Special care is to be taken in coastal areas to avoid such percolations through the road surface.

6.2 Choice of Pavement Type for Rural Roads

In case of rural roads, keeping in view advantages of stage development strategy and the initial cost, flexible pavement may be the appropriate choice. Generally the choice of pavement will be further guided by several other factors, such as, rainfall, temperature variations and strength of soil along the alignment. However, in cyclone prone areas where waterlogging is expected or in some rural road projects where the ground conditions and material availability may pose restriction for use of flexible

pavement, rigid pavement (cement concrete) should be considered as appropriate type of pavement. Further, cement concrete pavement can be coast (belt of severe cyclonic impact). Acompacted concrete, interlocking concrete cost effective, can also be adoptedsalts, usage of sulphate resistant cement/ Portland pozzolana cement/ Portland slag cement are advocated when road construction activitie

6.3 Rigid (Concrete) Pavements for Rural R

For rigid and semi- rigid pavements tensile stress to prevent fracture of the cementpavements, vertical subgrade strain is the critical criterion to limit rut depth due to traffic loading.following guidelines published by Indian Roads Congress (IRC)/ Agency (NRRDA) can be referredrural roads.IRC SP:62 – Guidelines for design and construction of cement concrete pavement for rural roadsMinistry of Rural Development – Specifications for Rural RoadsMinistry of Rural Development – Quality Assurance Handbook for Rural Roads

6.3.1 Construction of roller compacted concrete pavement (RCCP)

RCCP is a technique, which makes use of uniformly layingand compacting with vibratory or static road roller of always kept as minimum as possible during construction. The most apparent difference betweePortland cement concrete is the lower water content of RCCresults in higher strength than similar blends of conventional several advantages like speed of construction, lower cost, elimination of expansion joints, and most importantly use of fly ash to replace 30smaller than sand particles, and hence it fills the pores to produce denser concleached calcium hydroxide to form gel of calcium silicate hydrate. Because fly ash particles are spherical in shape, it creates adequate workability in spite of lower w/c ratio. RCCsuitable surface roughness adequatthere is drainage problem. Like other types of concrete roads, tadopting adequate quality controldesign life of RCCP road is likely to be greatly affected. Construction of RCCPthan the construction of plain cement concrete using conventional method of compaction, i.e., screed vibrator/needle vibrator.

Fig 6.1 – Construction of Roller Concrete Pavement for Rural Roads

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rigid pavement (cement concrete) should be considered as appropriate type of pavement. rete pavement can be a better option for rural roads falling within 25 km from sea

coast (belt of severe cyclonic impact). Alternate forms of concrete pavement interlocking concrete block pavements and composite pave

, can also be adopted. Further, keeping in view, corrosion aspects due to presence of salts, usage of sulphate resistant cement/ Portland pozzolana cement/ Portland slag cement are advocated when road construction activities are taken up nearer to the sea coast.

) Pavements for Rural Roads

rigid pavements tensile stress due to vehicular loading is taken as the design criteria to prevent fracture of the cemented layer within the design period. In case of concrete block pavements, vertical subgrade strain is the critical criterion to limit rut depth due to traffic loading.following guidelines published by Indian Roads Congress (IRC)/ National Rural Roads Development

referred for more details about materials and construction methodology of

Guidelines for design and construction of cement concrete pavement for rural roadsSpecifications for Rural RoadsQuality Assurance Handbook for Rural Roads

Construction of roller compacted concrete pavement (RCCP)

makes use of uniformly laying very low or zero slump concrete manually tory or static road roller of 8 to 10 tonnes capacity. The slump of concrete is

always kept as minimum as possible (zero slump) so that RCCP can take the load of moving roller even The most apparent difference between roller compacted concrete

cement concrete is the lower water content of RCCP. The lower water/cement results in higher strength than similar blends of conventional Portland cement concrete. RCC

eed of construction, lower cost, elimination of expansion joints, and most importantly use of fly ash to replace 30-50 per cent of cement. The size of the fly ash particles is smaller than sand particles, and hence it fills the pores to produce denser concrete and reacts with the leached calcium hydroxide to form gel of calcium silicate hydrate. Because fly ash particles are spherical in shape, it creates adequate workability in spite of lower w/c ratio. RCCsuitable surface roughness adequate for moderate traffic speeds. RCCP can be used in areas where

Like other types of concrete roads, these roads too must be constructed quality control checks. In case if there is any deficiency in design

is likely to be greatly affected. Construction of RCCPthan the construction of plain cement concrete using conventional method of compaction, i.e., screed

Construction of Roller Concrete Pavement for Rural Roads

rigid pavement (cement concrete) should be considered as appropriate type of pavement. rural roads falling within 25 km from sea

pavement options like roller block pavements and composite pavements which may be

Further, keeping in view, corrosion aspects due to presence of salts, usage of sulphate resistant cement/ Portland pozzolana cement/ Portland slag cement are

is taken as the design criteria n period. In case of concrete block

pavements, vertical subgrade strain is the critical criterion to limit rut depth due to traffic loading. The National Rural Roads Development ls and construction methodology of

Guidelines for design and construction of cement concrete pavement for rural roads

zero slump concrete manually The slump of concrete is

RCCP can take the load of moving roller even compacted concrete and conventional . The lower water/cement (w/c) ratio

cement concrete. RCCP has eed of construction, lower cost, elimination of expansion joints, and most

50 per cent of cement. The size of the fly ash particles is rete and reacts with the

leached calcium hydroxide to form gel of calcium silicate hydrate. Because fly ash particles are spherical in shape, it creates adequate workability in spite of lower w/c ratio. RCCP can provide a

be used in areas where must be constructed by design or construction,

is a faster technique than the construction of plain cement concrete using conventional method of compaction, i.e., screed

Construction of Roller Concrete Pavement for Rural Roads

39

The mix design for RCCP shall be based on flexural strength. Flexural strength achieved usually exceeds those for conventional concrete with similar cement contents. Flexural strengths of even 4.5 MPa can be achieved using fly ash based RCCP technology. Initial rate of strength gain of RCCP made by admixing fly ash may be slower when compared to the same mix of plain cement concrete. Consequently, designs based on more than 28 day period day strengths would be appropriate depending on the available lead time prior to opening to traffic. Mechanically saw cut contraction joints shall be provided in RCCP in a manner similar to PQC pavement. Following guidelines published by Indian Roads Congress (IRC)/ NRRDA can be followed for construction of roller compacted concrete pavements for rural roads:

IRC SP:68 – Guidelines for construction of roller compacted concrete pavementsMinistry of Rural Development – Specifications for Rural RoadsMinistry of Rural Development – Quality Assurance Handbook for Rural Roads

6.3.2 Construction of concrete block pavement / Interlocking concrete block pavement

Rectangular Concrete block pavement (CBP) or Interlocking Concrete Block Pavement (ICBP) is alabour intensive technology, which can be used in areas where conventional types of construction are less durable due to many technical and environmental constraints. CBP/ ICBP have many advantages as detailed below:

o No need for heavy construction equipmento Factory production facilitates centralised and efficient quality controlo Labour intensive constructiono Instant opening to traffico No need for expansion and contraction joints o Accommodates higher elastic deflection without failureo Unlike bituminous pavements, CBP/ICBP is not damaged due to fuel and oil spillageo High salvage value – almost all blocks can be recycled / reusedo Least life cycle cost due to low maintenance costo Environment friendly technology

The following guidelines published by Indian Roads Congress (IRC)/ NRRDA or Bureau of Indian Standards can be followed for more details about construction methodology and quality control.

IRC SP: 63 – Guidelines for the use of Interlocking Concrete Block PavementMinistry of Rural Development – Specifications for Rural RoadsMinistry of Rural Development – Quality Assurance Handbook for Rural RoadsBureau of Indian standards, IS: 15658, ‘Precast Paving Blocks – Specifications’

6.4 Flexible (Bituminous) Pavements for Rural Roads

Flexible pavements comprise of different layers of granular material compacted over subgrade soil and it may be provided with a black top (bituminous) wearing course. In cyclone prone areas, where erosion/ water-logging is not expected and the roads are situated beyond 25 km away from sea (cyclone severe impact belt), provision of black top pavement (flexible pavement) can be considered. Flexible pavements are amenable to stage development and their initial construction cost is also lesser than cement concrete pavements. Considering the intensity of annual rainfall in cyclone prone coastal

40

areas, it is suggested that wearing surface of premix carpet with liquid seal coat or mix seal surfacing can be adopted for these roads.

The principal criterion for determining the thickness of a flexible pavement with a thin bituminous surfacing is the vertical compressive strain on top of the subgrade imposed by a standard axle load of magnitude 8.17 kN (8170 kg). Excessive vertical subgrade strain causes permanent deformation in the subgrade, which is manifested in the form of rutting on the pavement surface. Since the width of rural roads will be single lane, design traffic should be based on total number of commercial vehicles per day in both directions. Bullock carts with iron rims are still in use in rural areas and the total weight including the payload of a bullock cart may range from 1.0 tonne to 1.5 tonne. Though the designed pavement as a whole will be safe from shear failure, the iron rims damage the top layer of the pavement because of high concentration of stress. Thus the wearing course must be made up of good quality aggregates with aggregate impact value not exceeding 30 to reduce degradation of the aggregates by crushing. For the purpose of structural design, only the number of commercial vehicles of laden weight 3 tonnes or more should be considered. To obtain a realistic estimate of design traffic, due consideration should be given to the existing traffic and its rate of growth.

In case of rural roads, with low volume of traffic, structural layer of bituminous mix need not be provided. Use of bituminous emulsion for such work may give good surfacing because of processing of material at ambient temperature. Maintaining the right mixing temperature of the hot mix is not easy when the dampness of aggregates stacked at the sites varies. The thin wearing course shall not be counted towards the total designed thickness of pavements. The minimum recommended flexible pavement thickness should not be less than 150 mm. In case of unsealed roads, aggregates are often lost due to traffic action as well as erosion by rains, material lost must be replenished periodically to maintain the riding quality. It is considered appropriate that roads in rural areas should be designed for a life of 10 years. The thin bituminous surfacing that is commonly provided on the low volume roads has a life span of about five years. The following guidelines published by Indian Roads Congress (IRC)/ NRRDA can be followed for more details about construction methodology and quality control.

IRC SP: 72 – Guidelines for design of flexible pavement for low volume rural roadsMinistry of Rural Development – Specifications for Rural RoadsMinistry of Rural Development – Quality Assurance Handbook for Rural Roads

6.5 Road Pavements for Highway works

The major cause of distress to roads in coastal areas is rainfall of high intensity for varying durations, which cause flooding and damage to wearing course and underlying layers of flexible pavements. Owing to a weak sandy stratum at different locations, the roads do not get a proper subgrade support. In order to overcome such problems following suggestions are made:

(a) Ensure proper transverse slopes (camber) on the road surface and a well planned and efficient drainage system to drain out rainwater quickly.

(b) Use of geosynthetic products and soil stabilisation techniques for improving the subgrade soil strength before laying subsequent pavement layers in poor soil strata locations and reclaimed lands

(c) In case of roads constructed very near to sea coast, sea sand may get deposited on the road pavements due to winds. This in turn may lead to abrasion of black top pavements. Frequent cleaning of road pavements may become necessary in such circumstances.

(d) Use of modified bitumen in wearing course of flexible pavements – such modified bitumen has improved properties which give flexible pavements longer maintenance free life when

41

compared to normal bitumen. For details regarding use of modified bitumen, IRC: SP 53 may be referred to. Other new materials like water repellent chemicals; etc which have been accredited by IRC can also be tried in road construction depending upon the requirement of the situation.

Further, as envisaged in case of rural roads, provision of cement concrete pavement needs to be considered as the preferred choice. Construction of road pavement for NH/ SH needs to be carried out as per provisions of MoRTH Specifications for Road and Bridge Works.

Fig 6.2 – Problem of Sand Dunes Encroaching Road Pavement

Fig 6.3 – Close up View of Black Top Pavement Abraded by Sand

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Chapter – 7

MITIGATION MEASURES FOR CULVERTS AND BRIDGES

7.1 Culverts and Minor Bridges

A culvert is a cross drainage structure having a total length of 6 m or less between the inner faces of dirt walls or extreme vent way boundaries measured at right angles. A bridge having a total length up to 60 m is considered as a minor bridge. When the overall length of a bridge between dirt walls is up to 30 m and where the individual span is not more than 10 m, it is called a small bridge. Culverts carry the stream flow through a roadway embankment. In India, Culverts are generally constructed using stone or brick masonry and concrete (both plain concrete and reinforced concrete) and also by using concrete/ polymeric pipes. The geometrical shapes and configurations of the culverts vary, it can be rectangular box or slab or even circular. The criteria for the selection of a particular type of a culvert depends on the (i) roadway profile, (ii) channel characteristics (iii) flood damage evaluations (iv) service life and (v) construction and maintenance cost. The most commonly used shapes of the culvert are circular or rectangular. The shape of the culvert depends on the (i) cost of construction (ii) limitation of upstream water level, (iii) roadway embankment height and (iv) hydraulic performance. The selection of the construction material depends on the (i) structural strength, (ii) hydraulic roughness (iii) durability and (iv) corrosion and abrasion resistance.

7.2 Culvert Hydraulics

Flow of water in a culvert may vary depending on the upstream and downstream conditions, inlet geometry and barrel characteristics. The upstream condition of the culvert is denoted by (i) head water elevation, which determines the energy required to force the flow through a culvert. The hydraulic capacity of a culvert can be improved by the selection of an appropriate inlet configuration. Since, at the upstream, the width of the stream is more than the inlet width of the culvert, the flow is contracted at the inlet. Therefore, there is a need to provide a more gradual energy flow transition to minimise the energy loss and thus to create a more efficient inlet condition. The examples of the different inlet configurations are (i) bevelled edges (ii) side tapered inlet and (iii) slope tapered inlet. The barrel characteristics which affect the performance of the culvert are roughness, area, shape, length and slope. The tail water level denotes the downstream condition of the culvert. The hydraulic design of the culvert is carried out in such a way that there is an adequate performance of the culvert even under the least hydraulic conditions. It should be ensured that proper water way is available at cross drainage structures to ensure that storm water is drained quickly. Inundation problem can be tackled by providing proper water way. The common problems associated with the culvert are (i) scour at the inlet and outlet (ii) sedimentation in the barrel and (iii) clogging of the barrel with debris.

7.2.1 Scour protection techniques at the inlet

A culvert generally constricts the channel or the stream, thereby forcing the water to flow through a reduced opening. Due to this high velocity flow, vortices impinge the embankment at the upstream adjacent to the culvert. In some cases, scour holes are also formed at the upstream of the culvert floor, due to the acceleration of the flow where it leaves the stream and enters the culvert. Therefore, measures to protect the slopes of the embankment and channel bed such as upstream slope paving, construction of headwall, wing wall and cut-off wall and channel paving help in minimising the scour.

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7.2.2 Scour protection techniques at outlet

As the culvert usually constricts the available stream area, the flow velocities are higher than the channel. The increased exit velocities can cause streambed scour and bank erosion in the vicinity of the culvert outlet. At the outlet, where the flow expands to the natural stream, turbulent and erosive eddies are formed. The scour at the outlet is a function of the velocity and depth of flow at the culvert outlet, velocity distribution of the flow upon the re-entering the natural stream, material characteristics of the channel bed and embankment and the amount of debris and sediment in the flow. Scour in the culvert outlet are of two types, i.e., local scour and general stream degradation. The local scour is normally identified by a scour hole, which is formed due to high velocities and the effects are extended to a limited distance in the downstream. The dimension of the scour hole change due to the sedimentation during the low flows and varying erosive nature of the storm event. The scour hole is generally deepest during the passage of the peak flow. Protection against scour at the culvert inlet varies from the rip rap placement to the complex energy dissipaters and outlet protection devices. Also, the velocity of flow in the culvert can be minimised by increasing the barrel roughness of by flattening the barrel slope. For more details IRC:89 may be referred to.

7.2.3 Protection against sedimentation

Most of the streams tend to carry sediments which are likely to get deposited when the velocity of the flow reduces. Therefore, barrel characteristics such as slope and roughness are very important parameters which indicate the possibility of sedimentation apart from the magnitude of discharge and the characteristics of the channel material. Also, in culverts with multiple number of pipes or boxes, sedimentation is more likely to occur. For more details, IRC: SP: 13 may be referred.

7.2.4 Debris accumulation and control

Debris may include any material moved by a flowing stream such as floating material, suspended sediment and so on. Debris can accumulate at the culvert inlet or inside the barrel and under such situations the culvert fail to perform as designed. Flooding may occur due to flow obstruction. Also, the debris accumulation may lead to roadway overtopping which may cause inconvenience to the traffic flow and subsequently result in roadway/culvert wash outs. The debris control system can be of three types, (i) Debris interceptors which include debris rack and debris boom, whose function is to intercept the debris at the culvert entrance, (ii) Debris deflectors may be installed at the culvert entrance which will deflect the debris from the entrance to a holding area for eventual removal. These may vary from simple inclined steel bar or rail placed in front of the inlet to a more complex V-shaped debris deflector (iii) Debris fins which are provided inside the barrel which helps to align the floating debris with the axis of the culvert to assist the passage of the debris. However, prior to the design of debris control system, field investigations on the following aspects need to be carried out:

(i) Stream velocity, slope and alignment(ii) Presence of trees/ shrubs on the eroding banks(iii) Storage of materials such as solid waste or logs within the flood plain (iv) Stream susceptibility to flash flooding/water logging(v) Study on land use such as cultivation, construction

7.3 Wind Load for Design

Wind load is one of the important environmental effects to be considered in the design of bridges. It is to be considered for design of superstructure, appurtenances, substructure and foundations. The parameters influencing the response of a bridge under the action of wind are cross sectional shape of

44

components, stiffness, natural frequencies of vibration and damping besides the turbulent characteristics of approaching wind. The wind pressure acting on a bridge depends on the geographical locations, the terrain of surrounding area, the fetch of terrain upwind of the site location, the local topography, the height of bridge above the ground, horizontal dimensions and cross section of the bridge or its element under consideration. The maximum pressure is due to gusts that cause local and transient fluctuations about the mean wind pressure. Bridge structures shall be designed for the wind forces as per IRC: 6, depending upon its location in the wind map of India as given in IS: 875(Part-3) and IRC: 6. Wind load calculation (given in IRC:6) shall be applicable to normal span bridges with individual span length up to 150 m or for bridges with height up to 100 m. For all other bridges including cable stayed bridges, suspension bridges and ribbon bridges specialist literature shall be used for computation of design wind load.

Wind forces shall be considered to act in such a direction that the resultant stresses in the member under consideration are maximum. In addition to applying the prescribed loads, stability against overturning, uplift and sliding due to wind shall be considered.

7.3.1 Calculation of wind pressure

In IRC:6, the hourly mean wind speed and wind pressure for a height up to 100 m above the mean retarding surface have been given corresponding to the six basic wind speed values (33m/s, 39m/s, 44m/s, 47 m/s, 50m/s and 55m/s) as indicated in wind map of India, for a return period of 100 years and for two terrain categories, i.e., plain terrain and terrain with obstruction. Plain terrain refers to open terrain with no obstruction or very well scattered obstructions having height up to 10m. Terrain with obstruction refers to a terrain with numerous closely spaced structures, forests up to 10m in height with a few isolated tall structures or terrain with large number of closely spaced obstructions like structures, trees, forest etc. If the topography (hill, ridge, escarpment or cliff) at the bridge site can cause acceleration or funnelling of wind, the wind pressure given in IRC: 6 shall be increased by 20%. For construction stages, the hourly mean wind pressure shall be taken as 70% of the value calculated.As these guidelines are meant for cyclonic conditions, recourse is taken to IS: 15498, whereby the hourly mean wind speed values given in IRC: 6 are to be multiplied by 1.3 for bridges of importance for post cyclone activities (for e.g., on a road leading to the cyclone shelter) and situated within 60km inland from the coast. Thus, for the bridges located in plain terrain and corresponding to a basic wind speed of 50m/s and 55m/s as per wind map of India, the hourly mean wind speed and wind pressure at10m level above the mean retarding surface is given in Table 7.1. For other deck levels, similar calculation needs to be carried out using the Table 4 of IRC.6.

Table – 7.1 Hourly Mean Wind Speed and Pressure at 10m Level for Cyclone Resistant Design of Bridges Situated Within 60 km off the Coast

Basic wind speed as per Wind speed map (m/s)

Bridge Situated in Plain TerrainVz (m/s) Pz (N/m2)

50 54.75 1798.5055 60.20 2174.40

7.3.2 Design wind force on superstructure

The superstructure shall be designed for wind induced horizontal forces (acting in the transverse and longitudinal direction) and vertical loads acting simultaneously. The assumed wind direction shall be perpendicular to longitudinal axis for a straight structure or to an axis chosen to maximise the wind induced effects for a structure curved in plan.

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The transverse wind force on a bridge superstructure shall be computed and considered to be acting on the area calculated as given below. The area at all stages of construction shall be the appropriate unshielded solid area of the structure. The transverse wind force FT (in N) shall be taken as acting horizontally at the centroid of the appropriate areas and shall be estimated from:

D1zT CG x x A x PF ........... Equation 7.1

Where, Pz is the hourly mean wind pressure in N/m2 at a height H( H, is the average height in metres of exposed surface above the mean retarding surface) A1 is the solid area in m2, G is the gust factor and CD is the drag coefficient depending on the geometric shape of the bridge deck. For highway bridges up to span of 150 m, which are generally not sensitive to dynamic action of wind, gust factor shall be taken as 2.0. The transverse wind load per unit exposed frontal area of the live load shall be computed using the expression FT given above except that CD shall be taken as 1.2. The exposed frontal area of live load shall be the entire length of the superstructure seen in elevation in the direction of wind or any part of that length producing critical response, multiplied by a height of 3 m above the road way surface. Areas below the top of a solid barrier shall be neglected. However, the bridge shall not be considered to be carrying any live load when the design wind speed at deck level exceeds 36 m/s. As an example, transverse wind force computed using Equation 7.1 due to wind forces acting on a unit exposed frontal area of bridge deck situated in plain terrain are given in Table 7.2, for three different configurations of bridge deck.

The longitudinal wind force shall be taken as 25% of transverse wind load as calculated from Eq. 7.1. Therefore, for bridges in locations prone to cyclone, the longitudinal wind force may be taken as 25% of the value given in Table 7.2, for the bridge decks covered therein.

The vertical wind load (FV) acting upward and downward at the centroid of the appropriate areas, for all superstructures, shall be estimated using Eq.7.2.

LCG x x A x PF 3zV ........... Equation 7.2

where, A3 is the plan area of deck in m2, CL is the lift coefficient, which shall be taken as± 0.75 for normal type of slab, box and I-girder bridges.

Table – 7.2 Transverse Wind Forces Due to Cyclone Acting on Unit ExposedFrontal Area of Bridge Deck at 10m Level (Plain Terrain)

DescriptionDrag

Coefficient(CD)

FT (N/m2) corresponding to a basic wind speed of

50m/s 55m/sFor a slab bridge b/d≥10 1.1 3955.60 4783.6

Bridge deck supported by single box beam

b/d=2 1.5 5395.50 6523.2b/d≥6* 1.3 4676.10 5653.4

Bridge deck supported by two or more

beams/box and c**/d<7

b/d=2 2.25 8091.00 9784.8

b/d≥6 1.95 7012.10 8480.2

b is the breadth of deck and d is the depth of deck* for intermediate b/d values the force can be interpolated.

** c/d, ratio of clear distance between beam or box to its depth

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Under the action of cyclones, for bridges located in terrain with obstructions (or in urban situation), the transverse wind forces on a unit exposed frontal area of bridge deck up to a level of 10m from mean retarding surface, shall be taken as 0.41 times the values given in Table 7.2 for the bridge deck types and wind speed covered therein.

7. 3. 3 Design wind force on bridge substructure

The substructure of the bridge shall be designed for wind induced forces transmitted to it from the superstructure and for the wind loads which are acting directly on the substructure. Loads for wind directions both normal and skewed to the longitudinal central line of the superstructure need to be considered for the design. The transverse wind load acting directly on the substructure can be computed using Eq 7.1, with A1 taken as the solid area in normal projected elevation of pier, with no allowance to be made for shielding. Drag coefficient (CD), for different plan shapes of pier and various heights to breadth ratio shall be taken from Table 5 of IRC: 6.

7.4 Design Water Level

The selection of design water level can be one of the most critical decisions for designs of bridges in coastal areas. The design water level often controls the design wave height, stone size and extent of armouring on embankments. Also wave load on elevated bridge decks are sensitive to water levels. Essentially the water level dictates where waves can reach and attack. For the storm surge levels and associated wind speed, Table 2.1 may be referred. Design water level decisions should be addressed using the traditional, risk based approach of a ‘design return period’. For example the ‘100 year storm surge level’ is the surge level elevation with a 1 per cent annual risk of exceeding. Each year, there is a 1 per cent chance that a storm surge of this magnitude (or greater) will occur. Some designs may justify a lower return period (e.g. 25 year or 50 year) in certain areas – balancing the greater risk affiliated like such design with engineering and economic considerations. Generally in predominantly agricultural areas, surge data for a period of 25 years is to be considered while in case of heavily populated areas and near vital installations, surge return period of 100 years is to be considered for design. Additionally in case of culverts and bridges nearer to sea coast, tidal flow if any for culverts and bridges to be built should be determined. In such bridges/ culverts in addition to the highest flood level, ‘high tidal levels’ should also be established. 7.5 Aspects to be considered for Bridge / Culvert Design

Bridges are vital structures for any transportation network and their damage can pose a threat to emergency response and recovery efforts resulting in severe economic loss. Bridge structures experience damage due to cyclones in a multiple way. Keeping this in view, following general recommendations are made with regard to design and construction of bridges/ culverts: 1. Bridges and culverts shall be designed (including hydraulic calculations and floor protection works)

as per following IRC Codes(a) IRC:5, Standard specifications and code of practice for road bridges, Section I – General

features of design (b) IRC:6, Standard specifications and code of practice for road bridges, Section II – Loads and

Stresses(c) IRC:112,Code of practice for concrete road bridges(d) IRC:78, Standard specifications and code of practice for road bridges, Section IV –

Foundations and Sub-structure (e) IRC:83 (Part I), Standard specifications and code of practice for road bridges

47

(f) IRC:83 (Part II), Standard specifications and code of practice for road bridges, Section IX, Bearings, Part II: Elastomeric Bearings

(g) IRC:83 (Part II), Standard specifications and code of practice for road bridges, Section IX, Bearings, Part III: Pot, Pot cum PTFE, Pin and Metallic Guide Bearings

(h) IRC SP:13, Guidelines for design of small bridges and culverts

2. The minimum width of the culvert should be for two lane traffic even if the road width is one lane.3. Minimum M 30 grade concrete and Fe-415 grade reinforcement steel should be used in all bridge/

culvert works (except for prestressing). Cement to be used should be preferably suphate resistant type or Portland slag cement or Portland pozzolana cement.

4. Major bridges in the coastal regions prone to cyclonic storms are to be designed for storm surge and wind/wave induced loading. Wind loads may be computed as per IRC: 6, brief details of the same are given in section 7.3.1. Expert advice may be obtained for computing forces induced by waves on bridges and for storm surge levels Table 2.1 may be referred.

5. Provide an open railing system instead of solid railing system in bridges and culverts.6. Addition of fairing plates to bridge deck cross sections, so as to achieve a streamlined cross –

section will help in reducing the wind forces.7. Pier and abutment caps shall be generously dimensioned, to prevent the dislodgement of deck,

when subjected to cyclonic wind forces and associated wave action. The minimum dimension of the bearing support shall be computed as N (in mm) =305+2.5L+10HP, where ‘L’ is the span in metres and HP is the average pier height in metres.

8. Bridge deck should be constructed at a higher elevation, so that the deck is not susceptible to wave/ storm surge action.

7.5.1 Damages due to storm surge and flooding

Major mode of damage due to storm surge would be through impact of debris/ storm surge waves on the bridge structure. Impact damages can occur due to barge impact, boats, oil rigs, uprooted trees, boulders, etc. The impact damage is manifested in the form of span misalignment, damage to fascia girder, fender, pier or pile damage. The damage to fascia girder includes spalling of concrete or exposure of reinforcement, breaking of reinforcement/ prestressing strands. The most common failure mode of bridge is the unseating of individual span, depending on the connection between the bridge deck and pier. This type of damage can occur in both simply supported and continuous span bridges. The bridge decks with low elevation are likely to fail as a result of excessive longitudinal or transverse movement of bridge deck. Under the storm surge, the bridge decks are subjected to buoyant forces and pounding action of waves. Bearings may also suffer damages due to the unseating/drifting of the bridge deck. In some bridges, shifting of the span due to lateral wave and wind forces often causes damages to the abutments, pier caps, or girders. Damage to parapets on bridge decks is likely to occur due to storm surge coupled with wave and wind loading. Flooding affects the bridges in multiple ways. Flooding leads to scouring of bridge foundations. It may also results in impact damage arising from debris being carried by flood waters. Scour damage to bridges may lead to erosion of abutment, slope failure and undermining of the approach. During inspection of existing bridges after a cyclonic storm, Table 7.3 may be used for description of qualitative damage states.

Table – 7.3 Qualitative Damage State Descriptions for Typical Cyclone Induced Bridge Damage (FEMA, 2003)

Damage Status Description

Slight Minor cracking and spalling in the abutment, Cracks in shear keys at abutments,Minor spalling and cracking at hinges, pier and in the deck

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Moderate Pier experiencing moderate cracking and spalling, moderate movement of abutment (Less than 50mm), extensive cracking and spalling of shear keys, rocker bearing failure, moderate settlement of approaches, damages to railing or parapet

Extensive Degradation of pier without collapse, shear failure, significant residual movement in the connections, major settlement of approaches, vertical offset of the abutment, differential settlement at connections, shear key failure at abutment, extensive scour of abutments

Complete Collapse of any pier, tilting of substructure due to foundation failure

7.5.2 City bridges

Increasing road networks for ever increasing urban population has resulted in the construction of a large number of flyovers and bridges. In many instances, due to the shortage of land, the piers of roads and railway bridges are located in major storm water drains and/or rivers in cities. These are known to cause backwater effects as much as 1 m high and as far away as 5 km upstream thereby resulting in flooding of the upstream catchments. All future road and rail bridges in cities crossing drains should be designed such that they do not block the flows resulting in backwater effects.

Further, it is observed that in many cities, rainwater causes flooding on the road despite the existence of the underground drainage system. It is seen that the inlets to drain the water from the roads into the road side drains are either not properly aligned or non-existent leading to severe water logging on the roads. The provision of a simple connecting element namely, the drainage inlet through which the water can flow from the road side drain into the underground drain can significantly reduce the water logging on the roads. Inlets should be provided on the roads to drain water to the roadside drains and these should be designed based on current national and international practices. IS 5691 provides the design details of cast iron gratings for drainage. For more details in this regard, ‘IRC SP:42, Guidelines on Road Drainage’ and ‘IRC SP:50, Guidelines on Urban Drainage’ may be referred to.

7.6 Mitigation Measures

As the storm surge and wave action induced by the cyclonic storm are extreme events, design of bridges or culverts for higher forces against these events would not be economical. Therefore, there is a need to adopt appropriate innovative design features, so that the loading is rationalised. Some of the mitigation measures which can be adopted during construction of new bridges as well as retro fitting measures for existing bridges constructed in cyclone prone areas are given in following sections:

7.6.1 Cable restrainers

Cable restrainers prevent unseating of bridge superstructure by inhibiting movement of superstructure. They are useful in restraining the wave induced deck movements as well as for reducing the damage to the bridge sub-structure. Restrainers are add–on structural devices which do not participate in resisting effects other than due to wind, wave or seismic effects. Three types of restrainers- longitudinal, transverse and vertical are generally used. Longitudinal and transverse restrainers help to prevent unseating of superstructure, whereas the vertical restrainers prevent the vertical separation of superstructure. Wire rope or other type of cables can be used to provide both vertical and lateral restraint. These can be looped around the pier cap and threaded through either the end blocks of the precast beams, holes drilled in the bottom flanges of steel beams, or through the end diaphragms of either steel or concrete beams (Fig. 7.1, Fig. 7.2 and Fig. 7.7). For additional lateral capacity, the cable could utilise the pile to pier cap intersection as a reaction point.

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(a) For I-girder Bridge (b) For Steel Superstructure Using Straight Cables

Fig. 7.1 Cable Restrainer

Fig. 7.2 Cable Restrainer Installed in Longitudinal Direction

Cable restrainers are to be connected to the bridge pier using steel bent plates, angles, and undercut anchors embedded in the concrete as specified by typical bridge retrofit plans. One of the most common retrofit strategies is to provide cable restrainers at the intermediate hinges and abutments inorder to reduce the likelihood of collapse due to unseating. Model studies can be adopted to evaluate the force-displacement behaviour of the cable restrainer retrofits for design purposes.

Fig – 7.3 Examples of Connecting the Beam Ends of Adjacent Spans

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7.6.2 Connecting beam webs at pier bents

This retrofit measure consists of using restraint cables to connect the ends of the beam in adjacent spans (Fig. 7.3). Holes would be drilled in the end blocks of precast beams as well as through the end diaphragm, and a cable would be looped through and spliced. Significant vertical displacement between the two ends would be required to engage the restraining cable.

Consideration should be given to the potentially large impact forces arising from the large unstrained vertical displacements. It may be desirable to place neoprene or other cushioning materials on the beam ends in contact with each other. Additionally, it may be advantageous to design the size of the vertical bearing to limit damage to the beams and pier bents.

7.6.3 Connecting diaphragms at pier bents

Provision of vertical shear continuity among spans can also be accomplished by connecting diaphragms across expansion joints at pier bents. In this retrofitting concept, a steel pipe anchored to one diaphragm and free to slide in a hole in the other would provide the linkage. The pipe would behave as a cantilever beam, carrying the load in bending. Consideration of clearances in this retrofit concept is important, as the annular space around the pipe in the expansion diaphragm must be sufficient to allow for much smaller differential vertical displacements before engaging than the cable restrainers attached to the two beam end, so that the collateral damage is reduced.

7.6.4 Connecting of adjacent spans

Observation of surge and wave damage have shown that continuous spans may experience less shifting (lateral and longitudinal displacement) during storm surge and wave events than do simple spans. This may be due to effect of wave impact along the length of the bridge. When one span of a bridge is experiencing maximum wave/surge loading, adjacent spans will be experiencing loads with a smaller magnitude due to the small probability that a wave will impact the different spans at the same time. Connection of adjacent simple spans can be accomplished by connecting the webs of the beam ends or by connecting the end diaphragms. The purpose of the connection is to share dead load under uplift conditions, and not to transmit loads under normal operating conditions. A connection that requires some limited amount of movement before the connection is engaged would prove to be effective.

Connecting adjacent spans as a retrofit for wave loads will generally only be suitable when the surge/wave loads on a span are not significantly higher than the dead load resistance of the span. If the surge/wave loads are significantly higher, adequate reserve capacity of adjacent spans may not be available and spans may be shifted or lost. Knowledge of the sea status (spacing of waves, direction of waves, variation of wave height perpendicular to the direction of wave travel, etc) and engineering judgment will be required to determine the likelihood of simultaneous loading of adjacent spans and how much additional resistance adjacent spans will provide.

7.6.5 Strengthening the connection between superstructure and substructure

Surge and wave damage to highway bridges in past events have consisted of displacement of superstructures, both laterally and longitudinally, on the pier bents and in some cases the displacements were large enough to completely dislodge the entire span. In some cases spans appear to have been lifted above the shear blocks or angles that were provided to supply lateral restraint and then displaced laterally. Tying the superstructure to the substructure may provide suitable means of

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preventing the shifting of spans due to uplift and lateral loads during surge/wave events (Fig 7.4 and Fig. 7.5). Any connection retrofit measure should account for the normal displacements that occur between the superstructure and substructure. Movement due to the following should be provided for:

Thermal expansion and contraction of girders Rotation of girders due to live load Vertical deformation of elastomeric bearing pads under live loads

Fig. 7.4 Connection of Deck to the Substructure

Fig. 7.5 Cable restrainer between superstructure and substructure

In addition to these displacements, the normal maintenance needs of a bridge should also be provided for, jacking access for bearing replacement being one example. As with any structural modification, the loads need to be followed through the structural load path to the ground and this aspect needs to be ascertained. When utilising the retrofit approach, the connection of the superstructure to the bent cap may create additional failure modes such as: Negative bending in the superstructure at mid span due to the vertical uplift forces along the span

and the restraint reaction. Shear at the ends of the girder due to the same Reduction in bending capacity of the piles due to decreased compression, or even tensile forces Increased lateral loads on the substructure

In case of coastal bridges with the largest exposure vulnerabilities, existing substructures will not be capable of resisting the large horizontal loads expected. Depending on the magnitude of the wave/surge forces expected to act on an individual bridge, utilisation of this approach in isolation may

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not be sufficient and may in fact result in more damage to the bridge than if not retrofitted. Utilization of fuse elements in the restraints system, in conjunction with a realistic expectation of damage may provide the best overall retrofit strategy.

7.6.6 Minimum requirements of the restrainer

Restrainers are designed normally to resist the expected forces within the elastic range. Contrary to the dominant resistance to be provided under a seismic event which is in the longitudinal or transverse direction of the bridge, the cyclone induced forces are predominantly exerted by the wind in the transverse direction. Therefore, the number of the restrainer cables and their location are to be chosen so as to resist the forces mainly in the transverse direction of the bridge. However, the steel cables should be at least of 20 mm in diameter conforming to IS: 2266. There should be threaded stud of diameter 25 mm and cold-swaged fittings on the ends, as shown in Fig. 7.6. Appropriate locking arrangement with the washer, nuts (glued using anaerobic adhesive) and the yield indicator should be adopted.

Fig – 7.6 Typical Details of a Restrainer

While designing the restrainers, it may be appropriate to keep the radius of the bent radius of the cable more than 100 mm. Attempt should be made to place the restrainer symmetrical to the pier to avoid the possibility of inducing eccentricity during the load transfer mechanism. Special care is needed for using the restrainer in the box-girder bridges in which the thin concrete walls may be exposed to the large punching shear stress in the vicinity of the bearing plates. Anchorages and other attachments should be designed to resist 1.25 times the nominal breaking strength of the cable. There should be sufficient width of the pier cap or the pier itself in the lateral direction so as to resist the forces transferred from the restrainer. For effective application of the restrainer, it would be imperative to tie them with the substructure. One method for covering the cable in a typical beam-girder bridge with the pier is shown in Fig. 7.7. In the case of slab-girder Bridge, sufficient resistance in the lateral direction may be achieved by covering each of the girders (from spans on either side) in the longitudinal direction with the pier (Fig. 7.7).

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Fig – 7.7 Tying the Restrainer from the Girders Around the Pier

7.6.7 Cable restraints to piles

The pier cap may not have sufficient capacity to resist the loads applied by the cable restrainers, especially due to the vertical uplift forces. One measure that circumvents this issue is to tie the beams or diaphragms directly to the piles, bypassing the pier cap. Holes would be drilled in the piles to pass the cable loops through. Weakening of the piles by the presence of the holes, as well as the ability of the piles to carry the uplift and lateral loads should be considered.

New foundation element may be attached to the existing substructure through the pier cap to augment the load capacity of the existing foundation. When the superstructure is being restrained by both existing and auxiliary foundations, the amount of movement required to engage each element should be investigated to ensure that the two foundations will be engaged simultaneously as desired.

7.6.8 Provision of sacrificial shear keys or reaction blocks to prevent the damage to bridge substructure

Shear keys and ductile bracings which are installed on ends of a bridge, are formed based on the concept of the sacrifice means to a lateral force. The shear keys are devices which function to support horizontal force generated in a direction perpendicular to a bridge axis (bridge extending direction). The shear keys cause horizontal load (either due to earthquake or cyclone) to be concentrated in them, and thereby prevent abutments and piers from being damaged (Fig. 7.8).The shear keys are divided into different categories depending upon the shape of the internal keys which are installed inside the abutment below the super structure and external shear keys which are installed on the sides of the superstructure.

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Fig – 7.8 Reaction Block/Stopper

7.6.9 Seat Extenders

Another means of preventing the unseating of bridge deck is by extending the seat width. This can be achieved with the help of seat extenders as shown in Fig. 7.9. Seat extenders are generally comparatively simple and inexpensive to install. Support length at abutments or piers of simply supported spans can be increased by corbels or brackets added to the sides of abutments or piers as the case may be, as shown in the Figure. For the design of retrofit measure, there is a need to determine the possible physical movement/opening of the joint so that unseating of the deck may occur.

(a) Concrete Corbels (b) Steel Bracket

Fig. 7.9 Seat Extension to Accommodate Large Longitudinal Displacements

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Chapter – 8

ROAD TRAFFIC OPERATIONS DURING EVACUATION

When cyclonic impact is predicted over densely populated areas, it may become necessary to evacuate population to safer inlands. Adequate planning about roads which are to be chosen in such an eventuality, population to be evacuated, the locations where the population can be temporarily shifted, etc need to be planned in advance. The infrequent nature of cyclone evacuation means that evacuation plan elements are infrequently tested. Therefore opportunity to refine them is also limited. Continued improvement in cyclone evacuation traffic operations will require reassessments after each evacuation.

One of the major features of emergency evacuation is to ensure some form of traffic contraflow. An evacuation plan usually necessitates operating four lane divided highway in such a way that traffic in all the four lanes is travelling away from the coast towards inland destinations where the dangers posed by approaching cyclone are significantly reduced. The terminology for this kind of traffic operation varies, the most common term being ‘Contraflow’. Other terms include ‘lane reversal’ and ‘reverse laning’. Most of the 4 lane divided highways in India are without access control. In roads identified for usage during emergency evacuation, provision of ‘flip down traffic signs’ would be necessary at intersecting streets to direct approaching traffic from cross streets to merge into the direction of evacuating traffic.

8.1 Precautionary Measures Before the Onset of Cyclone

In a cyclonic storm disaster situation most important mitigation measure would be to ensure traffic flow operations on the roads. In spite of providing adequate embankment protection measures, breaches may occur at vulnerable locations which were not anticipated earlier. Hence to maintain roads in operation condition, there should be arrangements to plug the breach in the embankment or quickly rebuilding the portion of the washed road. To meet this challenge, men and material should be identified and made available at the time of need. Various equipment like earth moving machine, truck, etc should be kept at strategic locations and made available immediately after cyclonic storm. Sometimes such operations may need to be undertaken during cyclonic storms also. The men and equipment should be in sufficient numbers. It is also important to identify good earth / sand in nearby areas which can be used for repairing the road breaches. It would be a good practice to keep filled up sand bags ready in adequate numbers in advance, to tackle any initiation of breach before it gets out of control and becomes unmanageable. Geotextile bags (section 5.4.3) or even empty cement bags can be used for this purpose.

8.2 Ensuring Uninterrupted Communication Among Critical Staff

An important issue during evacuation/ rescue and rehabilitation would be good coordination among administration and the engineers entrusted with maintaining road traffic operations/ rescue and relief. Help of local people would also need to be mobilised during such events. Communication between staff responsible for operation of road system and other critical agencies is extremely essential during cyclone events. During a disaster event like cyclones, the communication systems (Landline and cellular phone systems) are found to be overloaded with high demand. The situation may persist even during rescue and recovery phase (after the cyclone), since cellular telephone towers/ poles may be damaged and electricity outages would occur. To ensure that communication lines remain operational during such events, following recommendations are made:

(1) Ensuring availability of satellite phones with staff of roads department who have been vested with the task of maintaining road communication network: satellite phones do not require

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cellular towers for communication. Hence they can be used at any location but they require a clear line of sight and so these phones are not reliable for indoor usage.

(2) Providing ‘Wireless Priority Service (WPS)’ to critical phones: With technological advancement, it is now possible to provide WPS to identified critical landline and cellular phones. WPS are designed to push emergency service providers call through when communication networks are congested.

(3) Making use of ‘Walkie Talkie’ radio communication units: These ‘walkie talkie’ radios provide excellent means of communication. But often their range would be considerably limited.

(4) Text Messaging: When familiar communication networks (landline and cellphone) can no longer handle the demand for voice calls, very often text messaging can still get through because of lower band width requirements. Consequently, critical personnel should be trained in use of text messaging.

8.3 Emergency Shoulder Lanes / Overtaking/ Passing Areas

Identified sections of Highways, where evacuation traffic is to be plied, can be provided with additional widths of paved shoulders so that these can be operated as emergency shoulder lanes. Normally, in four laning projects, paved shoulders are provided. However, the width of paved shoulder is kept as 1.5 m. To ensure that an additional lane is available for evacuation operations, paved shoulder width may be increased to 3 m at selected stretches during construction. This additional width can even be paved with low cost pavement options like lean concrete or roller compacted concrete. If availability of additional emergency shoulder lane can be ensured, there may not be any need to enforce contraflow regulations and hence the need to flush the lanes before contraflow can be avoided.

In certain roads identified for evacuation, roadway width available may be restricted and such roads may be having single lane only. In such cases, provision of passing/ overtaking areas should be taken up. These overtaking/ passing areas would be having two lane wide pavement and they should be located at strategic locations (between 500 m to 1.0 km). Such passing areas would help fast moving traffic to overtake slow moving vehicles/ cattle and also provide space for halting traffic coming from opposite direction during evacuation.

8.4 Traffic Signage

Motorists communications can be handled by using ‘Portable Changeable Message Signs (PCMS)’ and deploying sufficient number of highway patrol officers/ law enforcement personnel. PCMSs are used to inform contraflow motorists that they are approaching an exit. Mile markers on contraflow routes should be painted on both front and back sides to provide necessary information to motorists. It would be necessary to ensure that kilometre and other markers in the roads in which contraflow traffic is to be plied, are marked in both front and back sides. Traffic signage helps motorists immensely during evacuation operations. Hence for critical roads which have been earmarked for evacuation operations, care must be taken to ensure that traffic signages are adequate. It is recommended that number of signages on such roads be suitably increased in consultation with traffic engineering experts. Traffic signs should also indicate the location of the nearest emergency shelters (such as schools, hospitals with emergency care facility) for the benefit of stranded motorists.

8.5 Measures for Removal of Obstacles

After a cyclonic storm, traffic operations would be invariably affected due to falling trees, damaged electric or telephone poles, advertisement bill boards, even debris created due to high speed winds. Such debris/ obstacles block the roadway making it unserviceable to traffic. Adequate men and

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equipment like diesel operated saw, cranes, trucks, etc should be made available to clear the roadway from such debris. Further as a precautionary measure well before onset of cyclone, trees, electric and telephone poles which are located adjacent to the road can be shifted farther if feasible. It would also be necessary to take into consideration, wind loads, while designing such structure.

8.6 Motorist Information

Typically, evacuation routes extend into rural areas where there may not be adequate provision of traffic signs and markings. In such a scenario, it would be necessary to keep the motorists updated about traffic conditions and evacuation routes. In addition to suggestions made with regard to ‘Traffic signage, the following proposals need attention:

(a) Creating an emergency information services telephone system whereby callers can receive real-time traveller information, evacuation routes, etc

(b) Providing/Disseminating motorist information via press releases, postures, websites, TV channels, FM radio, and also at cyclone shelters, etc

8.7 Restrictions on Vehicle Types

An evacuation is a trying time for motorists. When motorists are directed to use unfamiliar lanes (e.g., contraflow or shoulders), the experience introduces unusual features to drivers. In the case of contraflow, there are no or few traffic signs, entrance ramps may be used as exit ramps, motorists encounter ‘wrong way’ messages, etc. To ease traffic flow, trucks and trailers should be directed to the normal lanes and be restricted from the contraflow or emergency shoulder lanes. At the same time, when contraflow restrictions have been imposed, the roads department personnel and other emergency services operators should be aware about which alternate routes are available for accessing coastal communities. Additionally, sufficient provisions of fuelling stations needs to be ensured for contraflow traffic route. When evacuation has been completed and contraflow restrictions are to be lifted and normal traffic flow is to be enforced, the operations personnel should once again ensure that there are no vehicles in the lanes which were carrying contraflow traffic. 8.8 Access Control Issues

Contraflow operations may necessitate closure of some of the access points. Suitable gates may be constructed at such identified locations (coast bound lane entrance points). Additionally, such gates would have to be manned by law enforcement officers during closure. Before closing or opening such gates (before and after the evacuation exercise), law enforcement officers should ensure that all vehicles have left the lanes (including stalled vehicles).

8.9 Time of the Day Operations

Because of the unusual nature of the traffic flow in contraflow situations, there is some concern about minimal number of traffic control devices available to these motorists, the limited or non-existent intermediate opportunities for motorists to exit the highway, and other related problems. Therefore to the extent possible, contraflow operations should be taken up only during day light hours. But it is also to be noted that, if such a measure (contraflow only during day light hours) is to be adopted as a policy during evacuation, then the call for evacuation may have to be made earlier and prediction of sites vulnerable to cyclone effect may change due to increased chances of cyclone changing its course.

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8.10 Inland Terminus

One of the most critical features of a contaflow lane is its inland terminus. It is important to ensure that the traffic on the highway, which would comprise of lane traffic (instead of usual two lane traffic) is distributed in a way that minimises congestion. This can be achieved by terminating contraflow traffic at a grade separated interchange with another major road (NH or SH). Beyond this interchange traffic would be operating in normal manner. This can be possible if the proposed contraflow terminus has sufficient capacity to cater such a traffic scenario. Another approach would be to use multiple terminals, for example, directing two lanes of contraflow traffic flow to a particular inland exit onto the intersecting highway. The other two lanes of contraflow traffic can be directed to another interchange at a sufficient distance downstream. In that case, it would be necessary to ensure that these evacuation routes do not cross or merge as otherwise traffic congestion would occur at such points.

8.11 Traffic Patrols

In-house and contract patrols with communications equipment should be operated during evacuation/ rescue operations. Adequate number of ‘Tow-trucks’ would be necessary to remove stalled vehicles and any debris which might have fallen on roadway.

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REFERENCES

1. C.R.Lawson, ‘Geosynthetics for Coastal and Waterway Protection’, Published in the Conference –Geosynthetics – New Horizons, New Delhi, 2006 (M/s Asian Books Private Ltd., New Delhi)

2. Fokke Sathoff, Hocine Oumeracibi, Simon Restall, ‘Australian and German Experiences on the Use of Geotextile Containers’, Published in Geotextiles and Geomembranes, June 2007

3. E.C.Shin, Y.I.Oh, ‘Coastal Erosion Prevention by Geotextile Tube Technology’, Geotextiles and Geomembranes, April 2007

4. IRC:5, Standard specifications and code of practice for road bridges, Section I – General features of design, Published by Indian Roads Congress

5. IRC:6, Standard specifications and code of practice for road bridges, Section II – Loads and Stresses

6. IRC:18, Design criteria for Prestressed concrete bridges (Post tensioned concrete)

7. IRC:21, Standard specifications and code of practice for road bridges, Section III – Cement concrete (Plain and Reinforced)

8. IRC:78, Standard specifications and code of practice for road bridges, Section IV – Foundations and Sub-structure

9. IRC:83 (Part I), Standard specifications and code of practice for road bridges

10. IRC:83 (Part II), Standard specifications and code of practice for road bridges, Section IX, Bearings, Part II: Elastomeric Bearings

11. IRC:83 (Part II), Standard specifications and code of practice for road bridges, Section IX, Bearings, Part III: Pot, Pot cum PTFE, Pin and Metallic Guide Bearings

12. IRC SP:13, Guidelines for design of small bridges and culverts

13. National Disaster Management Guidelines – Management of Cyclones, Published by National Disaster Management Authority, Government of India (http://nidm.gov.in/PDF/guidelines/cyclones.pdf)

14. Rural Road Development Plan Vision 2025, Ministry of Rural Development, Government of India

15. Road Development Plan – Vision 2021, Published by Ministry of Road Transport and Highways & Government of India

16. IS 2720, Tests on soils, Published by Bureau of Indian Standards, New Delhi

17. IRC 75, Guidelines for design of high embankments, Indian Roads Congress, New Delhi

18. Specifications for Road and Bridge Works, Ministry of Road Transport & Highways, Published by Indian Roads Congress, New Delhi

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19. Specifications for Rural Roads, Ministry of Rural Roads, Published by Indian Roads Congress, New Delhi

20. Quality Assurance Handbook for Rural Roads, Ministry of Rural Development, Government of India, Published by NRRDA, New Delhi

21. IRC 56, Recommended Practices for Treatment of Embankment and Roadside slopes for Erosion Control, Published by Indian Roads Congress, New Delhi

22. Highways in Coastal Environment, US Department of Transportation, Federal Highway Administration, June 2008 (http://www.fhwa.dot.gov/engineering/hydraulics/pubs/07096/07096.pdf)

23. http://www.niobioinformatics.in/mangroves/MANGCD/gro.htm

24. IRC SP:59, ‘Guidelines for Use of Geotextiles in Road Pavements and Associated Works’, Published by Indian Roads Congress, New Delhi

25. IRC SP:62, ‘Guidelines for design and construction of cement concrete pavement for rural roads’, Published by Indian Roads Congress, New Delhi

26. IRC SP:68, ‘Guidelines for construction of roller compacted concrete pavements’, Published by Indian Roads Congress, New Delhi

27. IRC SP:63, ‘Guidelines for the use of Interlocking Concrete Block Pavement’, Published by Indian Roads Congress, New Delhi

28. IS 15658, ‘Precast Paving Blocks – Specifications’ Published by Bureau of Indian standards, New Delhi

29. Brown C, W. White, C.V. Slyke and J.D.Benson, ‘Development of Strategic Hurricane Evacuation Model Using Dynamic Traffic Assignment for Greater Houston Region, Published in Transportation Research Record, Journal of Transportation Research Board, No, 2629, Transportation Research Board of National Academies, Washington D.C, 2009

30. Murray-Tuite, P.M and Hani S Mahmassani, ‘Transportation Network evacuation planning with household interactions, Published in Transportation Research Record, Journal of Transportation Research Board, No, 1894, Transportation Research Board of National Academies, Washington D.C, 2007

31. IRC:6, Standard specifications and code of practice for road bridges, Section II – Loads and Stresses, Published by Indian Roads Congress, New Delhi

32. IRC SP:42, Guidelines on Road Drainage’, Published by Indian Roads Congress, New Delhi

33. IRC SP:50, Guidelines on Urban Drainage, Published by Indian Roads Congress, New Delhi

34. Federal Emergency Management Agency (http://www.fema.gov/)

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Annexure – I

Technical Specifications for Geotextile Tubes

Material: Geotextile tube should be composed of high-tenacity polypropylene/ polyester yarns which are woven into a rip-stop stable network such that the yarns retain their relative position. These Geotextile Tubes are often filled hydraulically with slurry of sand and water, although many other fill materials have been used. The length of the Geotextile tube should not be less than 10m, but not to exceed 30m, for the ease in placement and handling. The specifications of Geotextile tubes differ for aggressive, non aggressive and very harsh coastal conditions. Geotextile Tubes composed of Geocomposite consisting of double layer Geotextile made of a polyester high-tenacity woven Geotextile coupled to non woven polypropylene Geotextile by needle punching method are suitable for very harsh coastal conditions. The Geotextile tubes shall be constructed to meet the properties mentioned in Table 1 for aggressive condition and Table 2 for non aggressive (Typical) conditions.

Table – 1 Properties of Geotextile Tube for aggressive conditionProperty Test Method Units ValuesTube Circumference Measured m 2.3 to 25 m (Specified

dimensions may vary by +5%)Fill Port (diameter) Measured cm 30 to 45

Tensile Strength (MD) ISO 10319/ D 4595 (kN/m) 175 (Minimum)

Tensile Strength (CMD) ISO 10319 / D 4595 (kN/m) 175 (Minimum)Elongation – MD ISO 10319 / D 4595 % 15 (Maximum)Trapezoidal Tear Strength D 4533 kN 2.7 x 2.7 (Minimum)Puncture Strength D 4833 kN 1.8 (Minimum)Seam Strength (Factory) D 4884 kN/m 105 (Minimum)Apparent Opening Size (AOS),O₉₅ ISO 12956 / D 4751 (mm)

0.425 (Maximum)(No. 40 US Sieve)

Water Flow Rate D 4491 240 l/minute/m2 (Minimum)Accelerated UV Resistance D 4355 65 per cent (Minimum) Length m 10 to 30 m

Table – 2 Properties of Geotextile Tube for non-aggressive (typical) conditionProperty Test Method Units Values

Tube Circumference Measured m 2.3 to 25 m (Specified dimensions may vary by +5%)

Fill Port (diameter) Measured cm 30 to 45Tensile Strength (MD) ISO 10319/ D 4595 (kN/m) 70 (Minimum)Tensile Strength (CMD) ISO 10319 /D 4595 (kN/m) 95 (Minimum)

Elongation – MD ISO 10319 / D 4595 % 20 (Maximum)

Trapezoidal Tear Strength D 4533 kN 1.8 x 1.2 (Minimum)Puncture Strength D 4833 kN 1.2 (Minimum)Seam Strength (Factory) D 4884 kN/m 60 (Minimum)Apparent Opening Size (AOS),O₉₅ ISO 12956 / D 4751 (mm) 0.425 (Maximum)Water Flow Rate D 4491 240 l/minute/m2 (Minimum)Accelerated UV Resistance D 4355 65 per cent (Minimum) Length m 10 to 30 m

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NB: since the maximum circumferential stress is at the side of the filled Geotextile, the seam position should be in such a way that, it does not coincide with the maximum stress position

Installation guidelines Geotextile tubes used in coastal applications are most often filled hydraulically with slurry of

sand and water. A scour apron may be necessary to provide with the Geotextile tube to prevent the undermining

effects of scour. Scour apron is made of high strength woven Geotextile designed to protect the foundation of the main Geotextile tube from undermining effects of scour. Scour aprons are typically anchored by small tubes at the water’s edge called scour tubes.

The foundation for the placement of the Geotextile tube and its scour aprons shall be smooth and free of protrusions which could damage the Geotextile.

Suitable material for filling the tubes should not contain more than 15% fines (percent by weight by passing the No 200 sieve) to minimise subsidence of the tubes after filling. Gradation testing of hydraulic fill materials shall be conducted at a minimum, one gradation test shall be performed for each 1000 linear feet (300 m) of fill tube. Discharge pressure at geotextile tube fill port shall not exceed 35 kPa.

The Geotextile tubes require an alignment within + 600mm of the base line. The filled tubes shall have an effective height of ± 0.5 feet (150 mm) of the specified elevation.

The main Geotextile tube and the scour apron shall be deployed along the alignment and secured in place as necessary to assure proper alignment after filling. No portion of the tube shall be filled until the entire tube segment has been fully anchored to the foundation along the correct alignment.

The ends of tubes can be overlapped or butted together. An overlap of 1.5 m is recommended to ensure proper terminal connection. Beneath the Geotextile tube, the ends of each Geotextile scour apron shall be overlapped a minimum of 1.5 m. The effective height of the tube structureat the overlap is typically 80 per cent of the specified height.

The underwater alignment of tube can be achieved by placing temporary guides on either side of the Geotextile tube. Before filling the tube with the sand slurry mix, the alignment correctionshould be carried out by filling the tube with water. In case there are scour tubes along with the main tube, the scour tubes should be filled prior to the main tube.

After completing the deployment and anchorage of the Geotextile tube, the filling with sand water mixture should be started. The mixture shall contain 5 to 15 per cent of sand. The inlet port pressure should be limited to 35 kPa.

After filling the tube, the port sleeves shall be closed and attached to the main tube. Closing of the fill ports can be done by sewing or knotting by rope or nylon cables.