UNIT III PART A - Top Engineering Colleges in Chennai, …...... Sumps and ditches b) Deep well sumps c) WellPoint systems d) Deep well drainage e) Horizontal drainage f) Vacuum –

UNIT III

PART A

1. What is an under-reamed pile? (N/D 16)

It is a pile with one or more bulbs in its vertical shaft .These bulbs are known as under-reams and it

increases the bearing capacity of the soil.

2. What is a shoring? (N/D 16)

Shoring is a temporary structure used to support tilted or endangered walls .The walls might have been

endangered due to unequal settlement of foundation, removal of adjoining structures or making large

opening in the walls

3. What is meant by coffer dam? (M/J 16)

Cofferdam is a temporary structure constructed to exclude water from the site to construct a permanent

sub-structure, without the interface of water. It is used when the well foundation is to be carried in

running water.

4. What is meant by pipe jacking? (M/J 16)

Pipe jacking is a method of installing a pipe under roadway , railway or highways without using an open

cut trench .The pipe jacking procedure uses a casing pipe of sheet or reinforced concrete ie, jacked

through the soil.

5. State the uses of Box jacking. (M/J 12)

Box jacking is a method of constructing a Box like concrete structure for transportation purposes –

namely below railway, roadway. Box is jacked following the procedure of Pipe jacking.

6. What are sheet piles? (M/J 12)

Sheet piles are installed in sequence to design depth along the planned excavation perimeter or seawall

alignment. The interlocked sheet piles form a wall for permanent or temporary lateral earth support with

reduced groundwater inflow. Anchors can be included to provide additional lateral support, if required.

7. What is called Caissons? (N/D 11)

It is a special type of foundation used for the construction of bridge piers in v prevent ery deep water,

where it is either difficult to construct a cofferdam or to prevent its leakage. Types: a) Box caisson b)

Open caisson c) Pneumatic caisson.

8. How will you increase the frictional resistance of piles? (N/D 11).

A box pile displaced a large volume of soil. In such a pile, the frictional resistance is rapidly developed.

In clayey strata, both H pile and box piles from a plug of clay between the flanges of H pile and between

the walls of the box pile. The plug of clay is as hard mass acts along with the pile and cause additional

friction.

9. What are the methods used for tunnel driving? (N/D 12).

Following are the methods generally used for driving a tunnel, a) Full face heading b) Heading

and bench method c) Drifts method d) Pilot tunneling

10. What is mucking? (N/D 14).

The operation of removal of excavated material in tunneling operation is called mucking.

11 What are the advantages of drift method? (N/D 15).

Drift method of tunnel excavation has the following advantages: a) It helps to determine the

region of bad rock or excessive ground water before actually taking up the full excavation, so as to

enable to take up the corrective measures. b) The drift provides ventilation while driving the main

tunnel. c) It reduces the consumption of explosives.

12 Explain about cement grouting .Uses. (N/D 16).

In this method, cement grout which is a mixture of cement, sand and water is used. The process

consists of making a number of holes in ground and then filling these holes by cement grout under

pressure. This process is continued till no grout is coming up through the hole. Uses: a) The

grouting procedure can be used in stopping leakages from rock. b) It can also be used to fill the

voids in soil so as to strengthen the soil and to make the rock or soil water tight.

13 Write the situations under which pile foundation is recommended. (N/D 12).

The pile foundation is recommended for the following situations: a) When spread footing, raft and

grillage foundations are uneconomical. b) When heavy concentrated loads are to be transmitted

by the foundations. c) Where there is scouring in the soil near the foundations. c) Where the soil is

made up and of a compressible nature.

14 Write the essential features of a pump to be used for dewatering. (N/D 15).

The pump to be used for dewatering process should have the following features: a)The pump

should be portable so that it can be easily moved as and when required. b) The pump should be

capable of handling water mixed with impurities such as sand, earth, etc., c) The pump should be

of strong make. 20) What is the equipment used for driving a pre-cast pile in a sandy soil? The

equipment used for driving a pre-cast pile in a sandy soil is a hammer. Hence maximum stresses

are developed at the top due to direct strokes and at the point in overcoming the resistance to

penetration. Therefore additional reinforcement is provided.

15 What are the various methods to dewater deep excavations?

The various methods of dewater deep excavations are, a) Sumps and ditches

b) Deep well sumps

c) WellPoint systems

d) Deep well drainage

e) Horizontal drainage

f) Vacuum – dewatering system

g) Drainage by electro-osmosis

PART B

1. Explain the various types of sheet piles (N/D 16) (M/J 15)

Sheet piles may he made up of wood, concrete or steel. Steel piles are driven side by side into the

ground to form a continuous vertical wall for retaining soil. The alignment and resistance or thrusts are

normally provided by horizontal wallers, braces or tiebacks. Factors affecting the choice of a particular

type of pile include nature of ground, cost, ease of installation, availability of material, ability to withstand

driving, lateral strength and ease of making connections. Depending upon the material used in their

manufacture, some of the types of sheet piles are,

1. Wooden sheet piles

2. Precast concrete Sheet piles

3. Prestressed concrete sheet piles

4. Steel sheet piles

1. Wooden sheet piles:

Wooden sheet piles are made in various sizes and forms. The nature of site conditions determine, the

choice of a particular type, In places where excavation is small and the ground water problem is not

serious, 5 cm x 30 cm to 10 cm x 30 cm wooden planks arranged in a simple row will serve the purpose.

If the water-tightness is required to a great extent, lapped sheet piling is used. In this case, each pile is

made up of two planks, either spiked or bolted to one another. Thus if only earthen banks of small

height are to be supported, a single or double row of planks properly erected will perform the function

of sheet piling. If complete water tightness is desired or pressure of the retained material wakefield or

tongue and grooved sheeting is generally used. To facilitate the driving of the piles, they are usually

bevelled at foot. This not only assists in driving but also prevents bruising, if the piles encounter hard

stratum.

Wakefield piles:

This type of pile is made with three planks, 5 cm, 8 cm or 10 cm in thickness. The planks are nailed

together with the middle plank offset forming a tongue on one edge and a groove on the other. The

planks are connected by using a pair of staggered bolts at 80 cm centre to centre at intermediate points.

The triple lap piles prove stronger in driving. There is no wastage in forming the tongue and groove

joints and the piles have less tendency to warp. Timber sheet piles have light weight and as such the

equipment required for pile driving is also light. This is considered to be an important advantage timber

piles have over piles of other materials.

Sectional Plan of a Wakefield Pile

2. Precast concrete sheet piles:

Precast concrete piles are made in square or rectangular cross-section and are driven similar to wooden

piles to form a continuous wall. The interlock between two piles is normally provided with the help of

tongue and groove joint. The tongue and groove extend to the full length of the piles in most of the

cases.

An alternative method of providing joint between two piles is shown below. In this method, after the piles

are driven to the required. depth, the joint is grouted with cement mortar 1: 2 (1 cement : 2 sand).

Sectional Plan of Different Types of Precast Concrete Piles

The piles are reinforced to avoid formation of cracks due to rough handling or shrinkage stresses. In

order to reduce the possibility of damage due to driving impact, the stirrups should be spaced closely

near the top and bottom of the piles. The piles are normally bevelled at their feet to facilitate tightly close

driving of a pile against the already driven one. Reinforced concrete sheet piles are bulky and heavy

and as such they are gradually being superseded by prestressed concrete piles.

3. Prestressed concrete sheet piles:

On account of the numerous advantages the prestressed concrete members have over the conventional

type of reinforced concrete members, prestressed concrete sheet piles are commonly used for sheet

piling jobs. Similar to concrete sheet piles, they are reinforced on both the faces so that they could be

handled from either side. They are comparatively lighter in weight, more durable and economical in the

long run. They are advantageously used in sea water, since the danger of cracking of concrete is

negligible and also the corresponding danger of corrosion of pile reinforcement is reduced.

4. Steel sheet piles:

Steel sheet pile is a rolled steel section consisting of a plate called the web with integral interlocks on

each edge. The interlocks consist of a groove, one of whose legs has been suitably flattened. This

flattening forms the tongue which fits into the groove of the second sheet. Commonly used sheet piles

can be broadly divided into the following three categories,

Straight-web type

Shallow or deep arched-web type

Z web type

Special shapes and sizes of steel sheet piles are manufactured for meeting the requirement of

junctions and other similar situations. Each of the above mentioned type of piles is manufactured in

varying widths and lengths. The selection of the type of pile and the section to be adopted depend upon

the depths up to which the pile is to be driven, the nature of soil to be penetrated the elevation of the

earthen embankment, ground water level etc.

In general, Straight web type of piles are used where the piles are liable to he subjected to tensile forces

and interlocking strength is of prime importance (Cellular cofferdam etc); Arched-web type are used

where the piles are required to resist bending stresses (in cantilever retaining walls etc,) and Z-web type

of piles arc used where the piles are required to resist bending stresses of very large magnitude.

Steel Sheet Piles

Steel sheet piles are driven with the help of pile drivers which may be of drop hammer type or single or

double acting hammer driven by steam or compressed air. The outstanding feature of steel sheet piles

is that they can be used for greater depths. The continuous interlocking arrangement of the piles gives

strength and rigidity to the supported structure. A wall made from properly driven sheet piles leaks very

little, hence steel sheet piling is used with advantage in the construction of deep cofferdams. They are

commonly used in coastal defence works which are likely to be subjected to tidal action

2. Explain in detail about tunneling techniques (N/D 16) (M/J 12)

Methods of Tunnel Construction

The method of Tunnel construction adopted for a project depends on various

factors. Tunnel construction and Tunnel Engineering is considered to be one of the most sophisticated

and specialized art in the field of Civil Engineering. Unpredictable ground conditions, environmental

requirements and geological factors makes Tunneling a challenging job.

Portal Structure for Cumberland Gap Tunnel

Like immersed tunnels are used for crossing water bodies, it is important to plan and do a complete

feasibility study on which tunnel is appropriate for the project. The type of tunnel and method used

depends on various factors, some of them are listed below.

1. Geometrical configuration;

2. Geology

3. Ground condition;

4. Type of crossing;

5. Project Requirement;

6. Environmental requirements.

Complete List of Different Methods of Tunnel Construction

Cut-and-cover tunnels

In this type of tunnels, the tunnel structure is cast-in-situ or precast in an excavation. After construction,

the structure is back-filled with new or excavated soil. Cut and cover construction is adopted when the

depth of tunnel is shallow and the safe excavation is possible from the surface with out collapsing the

walls of excavation and when it is economical and acceptable. This methodology is usually used for the

construction of underpasses, approach sections of other tunnels & tunnels in flat terrain or shallow

depth. The tunnels may be constructed in place or by using precast sections. Two types of cut and

cover construction are; bottom-up and top-down.

Cut and Cover Construction using Side Slopes Excavation- Ft McHenry Tunnel

Bored or Mined Tunnels

These tunnels are built without excavating the ground surface. These tunnels are named according to

the type of material through which the tunnel is being excavated. When a tunnel passes through

different types of material it is known as mixed face construction. In bored tunneling, the excavation

takes place at the portal or at a shaft, thus the is a minimum impact on usual traffic, air & noise quality,

and utilities. Linings are the most important component of these kind of tunnels. For depths 10 m to 12

m, cut-and-cover is usually more economical and practical than mined tunneling.

A Typical Horseshoe Section for a Two-lane Tunnel

Rock Tunnels

Rock tunnels are excavated through the rocks either by drilling or by blasting. The tunneling method

utilizes mechanized excavators in case of soft rocks or rock tunnel boring machines (TBM). Sequential

Excavation Method (SEM) is also used in some cases. The behavior of rocks can change place to place

and type to type so depending on this stabilization measures ranging from no support at all to anchor

bolts to steel sets to even heavily reinforced concrete lining and combination of all these are used. It is

one of the most challenging tunneling geology.

Unlined Rock Tunnel in Zion National Park, Utah

Soft Ground Tunnels

When tunnels are excavated in soil using a shield or pressurized face TBM, or by mining methods

commonly known as sequential excavation method (SEM) are used they are called soft ground tunnels.

Soft ground includes cohesive soils as well as cohesionless soils and silty sands. Very soft ground

tunnels when excavated sequentially by small drifts and openings, it is known as New Austrian

Tunneling Method (NATM).

Immersed Tunnels

Immersed tunnels usually consist of very large pre-cast concrete or concrete filled steel tunnel elements

which are fabricated in the land and later installed under water. After installation, these tunnels are

backfilled. There are lot of immersed tunnels around the world used for road or rail connections.

Immersed tunnels are fabricated in required lengths in dry docks or in improvised floodable basins or on

shipways. The ends of the elements are sealed with bulkheads at each end, and then floated out and

towed to the installation location.

Immersed Tunnel

Jacked Box Tunnels

In these types of tunnels, prefabricated box structures are jacked horizontally through the soil against a

thrust wall using methods to reduce surface friction, like bentonite slurry. These are often used for

construction beneath runways or railroads embankments where surfaces are shallow but the must not

be disturbed since it can disrupt their the normal services. The method was developed from pipe jacking

technology. The Jacked box tunneling is used in soft ground and for short lengths of tunnels.

Jacked Box Tunnel Structure Construction Operation

3. Describe the procedure involved in underwater construction of diaphragm walls and basement. (M/J

16) (N/D 14)

In structural engineering, a diaphragm is a structural system used to transfer lateral loads to shear walls

or frames primarily through in-plane shear stress . These lateral loads are usually wind and earthquake

loads, but other lateral loads such as lateral earth pressure or hydrostatic pressure can also be resisted

by diaphragm action. The diaphragm of a structure often does double duty as the floor system or roof

system in a building or the deck of abridge, which simultaneously supports gravity loads.

Diaphragms are usually constructed of plywood or oriented stand board in timber construction; Metal

deck or composite metal deck in steel construction; or concrete slab in concrete construction. The two

primary types of diaphragm are flexible and rigid. Flexible diaphragms resist lateral forces depending on

the tributary area, irrespective of the flexibility of the members that they are transferring force to . On the

other hand, rigid diaphragms transfer load to frames or shear walls depending on their flexibility and

their location in the structure.

Parts of a diaphragm include:

the membrane, used as a shear panel to carry in-plane shear

the drag strut member, used to transfer the load to the shear walls or frames

The chord, used to resist the tension and compression forces that develop in the diaphragm, since

the membrane is usually incapable of handling these loads alone.

Diaphragm wall construction methods

Diaphragm wall construction requires that a proper sequence of works is followed. Specialized

excavating equipment has to be used. This equipment requires more available space when compared to

other construction methods.

1. Guide wall installation for diaphragm walls

Guide walls are constructed in-situ typically as lighly reinforced concrete elements. Guide walls maintain

the horizontal allignment and wall continuity of a diaphragm wall while the provide support for the upper

soils depth during panel excavation. This temporary support is important as the slurry levels vary during

construction and the upper few feet or one meter of the wall tends to be unstable. Equally important,

guide walls help guide the diaphragm wall grabs vertically and aid in the positioning of the final

structure.

2. Pre-excavation for diaphragm wall installation

Prior to the diaphragm wall grabs starting excavation, the slurry pump must be fully submerged in

bentonite slurry. To achieve this, a small initial excavation by the grab is carried out that is filled with

slurry. Occassionally, some preexcavation might be required before guide walls are installed to remove

certain obstructions.

3. Primary panel excavation for diaphragm wall construction

The primary panels are excavated first. The minimum length of a panel depends on the grab equipment

size and is generally in the order of 3.0m (15ft). If soils are stable, the primary panels can be

constructed in multiple bites. In such a case, a panel can be subdivided into three bites with the left and

right panels excavated first while the middle bite is excavated last. With this approach, diaphragm wall

panels in the order of 6.5m to 8.0m are achieved. Multiple bites are also required when corner or T

panels are constructed.

4. Slurry cleaning and desanding for diaphragm wall construction

Prior to tremieying the concrete, and while the panel is excavated, the supporting slurry fluid must be

cleaned so that it's properties are within acceptable levels (density, sand content, viscocity, PH). Slurry

is circulated at regular intervals throughout the construction period through the regeneration plant.

Otherwise, fresh slurry fluid can also be used although this approach is not the most economical.

5. Joint constuction methods for diaphragm wall construction

Diaphragm wall joints need to receive special attention do detail. Various joint types are available for

diaphragm walls. Joint selection depends on the excavating equipment as much as contractor

preference Joints can be flat, circular, with steel beams, or special grooved type with water stops.

Grooved type joints with water stops are typically preferred while in the US it is also very common to use

steel I beams for water stops. Flat panels and circular joints are generally avoided.

6. Reinforcement cage lowering and concrete tremieing

Once the bottom of the panel is reached (and cleaned), the reinforcement cage can be lowered into

position. The reinforcement cage is typically suspended from the guide wall panels, and must have

enough transverse and diagonal reinforcement to permit it to be properly lifted and lowered into place.

Sufficient space must be left for at least two or three tremie pipes so that tremieing can take place.

Concrete tremieing refers to the process of replacing the supporting slurry with the permanent concrete

with the use of vertical pipes called tremies. With the tremies, concreting of a diaphragm wall starts from

the bottom and the tremies are lifted progressively as the concrete level rises. During this process the

tremies are maintained within the freshly poured concrete for a minimum depth of 2ft or (0.6m).

Overpouring might be required to make sure that all slurry is displaced from the panel by concrete. Poor

tremieing can result in slurry pockets getting entraped within the diaphragm wall concrete. These

pockets can then lead to excessive and costly groundwater leaks or even blowouts. This has been the

case in certain portions of the Central Artery Project in Boston, MA (Big dig) and has led to costly

repairs and delays.

7. Secondary panel excavation for diaphragm wall construction

Secondary panels are constructed between primary diaphragm wall panels. When trench cutters are

used, the primary panel is formed with a single bite excavation. With trench cutters a flat panel joint is

typically used, but the trench cutter eats into unreinforced concrete of the adjacent primary panels. After

the specified depth is reached, the reinforcement cage is lowered into position and concrete is tremied

with tremie pipes from the bottom up.

4. What is dewatering? And briefly explain the various dewatering techniques. (M/J

16) (N/D 14)

DEFINITION

When water table exists at a shallow depth below ground surface, it is essential to lower the water so as

to carry out construction of foundation, basement, and metro tunnels etc. This is achieved by pumping

out water from multiple wells installed at the site. The process is called as dewatering.

Types of dewatering method

Dewatering can be done by adopting one of the following four strategies Dewatering of soil by

temporary lowering of water table using wells and pumps prior excavation as depleted .Allowing water

to reap into excavation area, collecting it in sumps and pumping it out. Before that adequate steps have

to be taken to support the soil on sides of the excavated area, to prevent washing away of fines and

have sufficient space for the work area. Making the soil around excavated zone impermeable by

technique such as grouting are freezing so that inflow of water is stop are minimized.

INSTALATION TECHNIQUE

Sufficient size and capacity of dewatering system is necessary to lower and maintain ground water

table and to allow material to be excavated in a reasonable dry condition. Excavation slopes to be

stabilized where sheeting is not required Dewatering system is to be operated continuously until backfill

work has been completed. Then, the structure to be constructed at the excavated area has to be

finished The complete stand by have to be available for immediate operation as may be required, to

adequately maintain dewatering on continuous basis and in the event that all or any other part of the

system may become inadequate or fail The water removed from the excavation to be disposed in such

a manner as will not endanger portions of work under construction or completed. For dewatering

purpose, well points deep well, caissons and tunnels are used.

WELL POINTS DEWATERING

When construction operation have to be excited below the ground water table level. Dewatering of soil

can be done by the following methods Collecting water in sumps and pumping it out. Installing well

points small or deep wells and pumping out ground water Using special technique in fine grained soils

such as vaccum dewatering and electro osmosis

WELL POINTS

To pump out the ground water small sized wells called well points are used for a more dry working area

the two methods used most often for lowering water table below the excavation level are the well point

method and the deep well method.

WELL POINT METHOD :

This is economical and useful for lowering the water table by 15m or less. Incase of well point method

or deep well method it is based on the fact that removal of water by continuous pumping from a well

causes the water table level to become depressed and result in the formation of draw down. When a

series of wells are placed close to each other, the overall effect is lowering of the water table level. Well

points, being smaller, are easy to install. Well points, can lower the water table by only 6.7m because

the pump, is located at the ground surface and connected to group of well points through a pipe, cannot

lift water from greater depth. Beyond 7m, multistage well points are used. DEEP WELL METHOD This

method is useful for lowering the water table by more than 15m. Deep wells have larger diameter more

depth and greater spacing. The pump is located at the bottom of well and hence can pump out water

from greater depth. Deep wells become more economical if more points are required.

5. What are Caissons and cofferdam. (M/J 12) (N/D 15)

A caisson foundation also called as pier foundation is a watertight retaining structure used as a bridge

pier, in the construction of a concrete dam, or for the repair of ships. It is a prefabricated hollow box or

cylinder sunk into the ground to some desired depth and then filled with concrete thus forming a

foundation.

Caisson foundation is Most often used in the construction of bridge piers & other structures that require

foundation beneath rivers & other bodies of water. This is because caissons can be floated to the job

site and sunk into place.

Caisson foundations are similar in form to pile foundations, but are installed using a different method. It

is used when soil of adequate bearing strength is found below surface layers of weak materials such as

fill or peat. It is a form of deep foundation which are constructed above ground level, then sunk to the

required level by excavating or dredging material from within the caisson.

Caissons (also sometimes called “piers”) are created by auguring a deep hole into the ground, and then

filling it with concrete. Steel reinforcement is sometimes utilized for a portion of the length of the caisson.

Caissons are drilled either to bedrock (called “rock caissons”) or deep into the underlying soil strata if a

geotechnical engineer finds the soil suitable to carry the building load. When caissons rest on soil, they

are generally “belled” at the bottom to spread the load over a wider area. Special drilling bits are used to

remove the soil for these “belled caissons”.

The caisson foundations carry the building loads at their lower ends, which are often bell-shaped.

Functions of Caisson Foundation

The foundation system of and the soils beneath the building prevent the complex from moving vertically.

When a load is placed on soil, most soils settle. This creates a problem when the building settles but the

utilities do not. Even more critical than settlement is differential settlement. This occurs when parts of

your building settle at different rates, resulting in cracks, some of which may affect the structural integrity

of the building. Conversely, in some rare instances soils may swell, pushing your building upwards and

resulting in similar problems. Therefore, the foundation system must work in tandem with the soils to

support the building.

Types of Caissons:

Box Caissons

Excavated Caissons

Floating Caissons

Open Caissons

Pneumatic Caissons

Sheeted Caissons

Box caissons are watertight boxes that are constructed of heavy timbers and open at the top. They are

generally floated to the appropriate location and then sunk into place with a masonry pier within it.

Excavated caissons are just as the name suggests, caissons that are placed within an excavated site.

These are usually cylindrical in shape and then back filled with concrete.

Floating caissons are also known as floating docks and are prefabricated boxes that have cylindrical

cavities.

Open caissons are small cofferdams that are placed and then pumped dry and filled with concrete.

These are generally used in the formation of a pier.

Pneumatic caissons are large watertight boxes or cylinders that are mainly used for under water

construction.

Advantages and Disadvantages of Caissons:

Advantages of Caissons:

Economics

Minimizes pile cap needs

Slightly less noise and reduced vibrations

Easily adaptable to varying site conditions

High axial and lateral loading capacity

Disadvantages of Caissons:

Extremely sensitive to construction procedures

Not good for contaminated sites

Lack of construction expertise

Lack of Qualified Inspectors

Drilled Pier Foundations

A drilled pier is a deep foundation system that is constructed by placing fresh concrete and reinforcing

steel into a drilled shaft. The shaft is constructed by rotary methods using either a self-contained drill

unit or a crane mounted drill unit. The hole is advanced through soil or rock to the desired bearing

stratum. Temporary or permanent steel casings may be used to maintain the sides of the drilled

excavation if caving soils or water infiltration becomes a problem.

Drilled shafts can be used to sustain high axial and lateral loads. Typical shaft diameters range from 18

to 144 inches. Drilled shafts (also called caissons, drilled piers or bored piles) have proven to be a cost

effective, excellent performing, deep foundation system, that is utilized world-wide. Typically they are

used for bridges and large structures, where large loads and lateral resistance are major factors.

Concrete Caissons:

A 10″ or 12″ diameter holes are drilled into the earth and embedded into bedrock 3 to 4 feet. Usually

used for the structural support for a type of foundation wall, porch, patio, monopost, or other structure.

Two or more “sticks” of reinforcing bars (rebar) are inserted into and run the full length of the hole and

then concrete is poured into the caisson hole. A caisson is designed to rest on an underlying stratum of

rock or satisfactory soil and is used when unsatisfactory soil exists

Caisson Construction Process:

After some initial form work and concrete pours, the cutting edge is floated to the breakwater by towboat and

fastened to the caisson guide. Concrete is placed (poured) into steel forms built up along the perimeter of the

box. With every concrete placement, the box becomes heavier and sinks into the water along the caisson guide.

Forms are also built inside the box around the air domes and concrete is placed in between. The resulting open

tubes above the air domes are called dredge wells.

When the caisson finally touches the river bottom, the air domes are removed and earth is excavated through the

long dredge well tubes, as shown in the animation below. The caisson sinks into the river bottom. Excavation

continues until the caisson sinks to its predetermined depth.

As a final step, concrete is placed (poured) into the bottom 30 feet of the hollow dredge wells and the tops are

sealed.

Coffer Dam

There are various types of cofferdams used for construction of structures in water. Construction details

of these cofferdams are provided in this article.

The basic needs of human being are food, air, water, shelter and transport. To fulfill the basic needs of

shelter and transport every inch of the earth land is being used for the construction of roads, building or

other structures.

Nowadays even structure on water are being constructed. But the construction in water is a very tedious

job. As the structure is hard to build in water as concrete doesn’t set in water. Many methods are being

used to overcome this problem. One the methods used for this purpose are Cofferdams.

Cofferdam can be defined as the temporary structure that is built to keep the water away from the

execution site, so that the structure can be built on the dry surface.

The cofferdams should have walls that exclude water from building site. For this the walls must be water

proof and the height of the wall must be more than the maximum water level. These types of cofferdams

are preferred where the area of building site is large and the dry soil bed is at reasonable depth

Types of Cofferdams and Their Construction Details

Coffer dams can be classified into many types depending upon the depth, soil conditions, and

fluctuations in the water level and type of material used.

Types of Cofferdams

Considering the material used in their construction, cofferdams may be divided into the following

categories.

Earthen cofferdam

Rockfill cofferdam

Single-walled cofferdam

Double-walled cofferdam

Braced cofferdam

Cellular cofferdam (Circular or diaphragm type)

Earthen Cofferdam

Earthen cofferdams are constructed at the place where the height of the water is less say 3m and the

current velocity is low. These dams are built using the local available material such as clay, fine sand or

even soil.

The height of the dam is kept 1m more than that of max water level. Freeboard of the dam or the top of

the dam is kept 1m so that the water doesn’t enter the other side even when waves arise.

The slope is usually given but 1:1 or 1:2. The slope of the water side is pitched with rubble stones so the

water action doesn’t score the embankment. Even sheet piles are driven in the center of the dam to

resist water seepage. After the construction of earthen cofferdam, the water from the other site is

pumped out and construction is executed.

Fig: Cross-Section of an Earthen Cofferdam

Rockfill Cofferdam

Rock-fill cofferdams are better than that of earthen dams. These dams are preferred when the rock is

available easily at the construction site. These dams are very pervious, to prevent water from seeping

an impervious membrane of soil is provided in the dam.

The height of the dam is can be up to 3m. The slope can be maintained at 1:1.5 to 1:125. The slope on

the water side is pitched so as to protect dam from wave action.

Fig: Cross-Section of Rockfill Cofferdam

Single-Walled Cofferdam

This type of cofferdam is preferred when the depth of the water is more than 6m and area of

construction is less. Usually this is used in construction of bridges.

Wooden or timber sheets are driven into the river bed on the perimeter of the area of construction. On

the inside steel or iron sheets are driven into the river bed. This inside sheets are placed at equal

distance with the help of wales which are bolted to both sheets for either sides.

To improve the stability of this types of dam, half-filled bags of sand are placed on the both side of the

walls. The water from the inside is pumped out and the construction process is undertaken.

Fig: Construction Details of Single Walled Cofferdam

Double-Walled Cofferdam

Double-walled types of cofferdams are used when the area of construction site is large and depth of

water is high. In this place use of single walled cofferdam becomes uneconomical as the supports are to

be increased. So double walled cofferdam is used.

The difference in one wall and double wall dam is that her it has two walls instead of walls for extra

stability. This type of dams can hold water up to 12m high.

Two piles are driven inside the water bed with a space in between and attached each other with wales

with bolted connection. As the water depth increases the space between the walls increases.

The space between the walls are filled with soil. To prevent the leakage from the ground below, the

sheet piles are driven to a good depth in the bed.

Fig: Construction Details of Single Walled Cofferdam

Braced Cofferdam

When it’s difficult to drive piles inside the bed in the water, then this type of cofferdam is used. In braced

cofferdam two piles are driven into the bed and they are laterally supported with the help of wooden

cribs installed in alternate courses to form pockets.

The empty pockets here are filled with stone and earth. The framework of the cofferdam (made from,

logs of wood) is prepared on ground and then floated to the site where the cofferdam is to be

constructed.

The layers of sand and the other loose material overlying the impervious hard bed is dredged out. Crib

is then sunk to the position, the bottom of each crib is given a shape to fit in the variation in the surface

of bedrock. After the pit is dewatered, the structure is concreted. When concreting has been completed

above the water level, the cofferdam is removed.

Fig: Braced Cofferdam Construction Details

Cellular Cofferdam

When the water layer is more than 20m, common types of cofferdams are uneconomical to use. In this

situations cellular cofferdams are used. This type of dam is used in construction of dams, locks, weirs

etc.

Cellular cofferdam is made by driving straight web steel sheet piles, arranged to form a series of

interconnected cells. The cells are constructed in various shapes and styles to suit the requirements of

site.

Finally, the cells are filled with clay, sand or gravel to make them stable against the various forces to

which they are likely to be subjected to.

The two common shapes of the cellular cofferdam are,

(i) Circular type cellular cofferdam.

(ii) Diaphragm type cellular cofferdam.

(i) Circular Type Cellular Cofferdam

This type of cellular cofferdam consists of circular arcs on the inner and outer sides which are

connected by straight diaphragm walls. The connection between the curved parts and the diaphragms

are made by means of a specially fabricated Y-element.

The cofferdam is thus made from interconnected steel sheet piles. The empty spaces are filled with non

pervious materials like clay or sand. Due to the filling material the self weight of the membrane

increases and leakage is reduced.

One advantage of the diaphragm type is that the effective length of the cofferdam may be increased

easily by lengthening the diaphragm. Hence in case, from design consideration it is necessary to have

effective width of the cofferdam more than 21 meter, diaphragm type of cofferdam must be used.

Fig: Plan and Section Details of Circular Type Cellular Cofferdam

(ii) Diaphragm Type Cellular Cofferdam

It consists of a set of large diameter main circular cells interconnected by arcs of smaller cells. The walls

of the connecting cells are perpendicular to the walls of the main circular cells of large diameter.

The segmental arcs are joined by special T-piles to the main cells. The circular type cellular cofferdams

are self-sustaining, and therefore independent of the adjacent circular cells. Each cell can be filled

independently.

The stability of such cells is much greater as compared with that of the diaphragm type. However, the

circular cells are more expensive than the diaphragm type, as these require more sheet piles and

greater skill in setting and driving the piles.

Because the diameter of circular cells is limited by interlock tension, their ability to resist lateral pressure

due to high heads is limited.

Fig: Plan and Section Details of Diaphragm type Cellular Cofferdam

Straight Shaft Drilled Piers (Caissons)

Used in moderate to high swelling soils. (This is one of the most effective foundation designs for use in sites

that contain expansive soils.)

Purpose is to attain required penetration into zone where there is little or no seasonal moisture variation. Current

standard of care in the area is a minimum penetration of 6 feet into bedrock and minimum length of 16 feet. Dead

loads should be as high as practical. This design requires relatively long spans between piers and more

reinforcing in grade beam.

Caissons into bedrock

Friction Piers into stiff clays

End Bearing Belled Piers

Appropriate Voiding – Should be constructed with void material of appropriate strength and thickness

Fig: A series of 1.2-metre thick diaphragm wall panels were joined to form a 24-metre diameter caisson

shaft. Four of these caissons were built to provide a sound base for the foundation of the main structure

of the building tower. The photo shows the excavation work using typical excavating machines inside

one of the caisson shafts.

6. Describe the procedure involved in underwater construction of diaphragm walls. (M/J 12) (N/D 15)

Diaphragm wall Construction

Diaphragm wall is a continuous wall constructed in ground in to facilitate certain construction activities,

such as:

a) As a retaining wall

b) As a cut-off provision to support deep excavation

c) As the final wall for basement or other underground structure (e.g. tunnel and shaft) c) As a

separating structure between major underground facilities

d) As a form of foundation (barrette pile – rectangular pile)

Diaphragm wall

Diaphragm wall is a reinforced concrete structure constructed in-situ panel by panel. The wall is usually

designed to reach very great depth, sometimes up to 50m, mechanical excavating method is thus

employed.

Typical sequence of work includes:

a) Construct the guide wall

b) Excavation to form the diaphragm wall trench

c) Support the trench cutting using bentonite slurry

d) Inert reinforcement and placing of concrete to form the wall panel

Guide wall – guide wall is two parallel concrete beams constructed along the side of the wall as a guide

to the clamshell which is used for the excavation of the diaphragm wall trenches.

Trench excavation – In normal soil condition excavation is done using a clamshell or grab suspended by

cables to a crane. The grab can easily cut through soft ground. In case of encountering boulders, a

gravity hammer (chisel) will be used to break the rock and then take the spoil out using the grab.

Excavation support – the sides inside the trench cut can collapse easily. Bentonite slurry is used to

protect the sides of soil. Bentonite is a specially selected fine clay, when added to water, forms an

impervious cakelike slurry with very large viscosity. The slurry will produce a great lateral pressure

sufficient enough to retain the vertical soil.

Reinforcement – reinforcement is inserted in the form of a steel cage, but may be required to lap a few

sections in order to reach the required length.

Concreting – placing of oncrete is done using tremie pipes to avoid the segregation of concrete. As

Concrete being poured down, bontonite will be displaced due to its lower density than concrete.

Bontonite is then collected and reused.

Joining for the diaphragm wall panel – Diaphragm wall cannot be constructed continually for a very long

section due to limitation and size of the mechanical plant. The wall is usually constructed in alternative

section. Two stop end tubes will be placed at the ends of the excavated trench before concreting. The

tubes are withdrawn at the same time of concreting so that a semi-circular end section is formed. Wall

sections are formed alternatively leaving an intermediate section in between. The in-between sections

are built similarly afterward but without the end tube. At the end a continual diaphragm wall is

constructed with the panel sections tightly joined by the semi-circular groove.

Using hydrofraise (reverse circulation trench cutter) to form diaphragm wall panel.

Bored piles of square section can be installed using the Hydrofraise or similar drilling techniques. The

bore hole is stabilised by drilling mud. The "Hydrofraise" is a drilling machine powered by three down-

the-hole motors, operating with reverse circulation. A heavy metal frame, serving as a guide, is fitted at

its base with two cutter drums carrying tungsten carbide tipped cutters. These rotate in opposite

directions and break up the soil. A pump is placed just above the drums and evacuates the loosened

soil, which is carried up to the surface by the drilling mud. The mud with cuttings is continuously filtered

(desander unit) and then poured back into the trench. A heavy crawler crane supports and manipulates

the machine. It carries the power pack supplying the hydraulic power, which is conveyed through hoses

to the three down-the-hole motors, two of them driving the cutter drums and the third driving the pump.

The hydraulic cutting device is designed to give the cutter drums a high torque at low speed of rotation.

The guide frame is suspended from the cable-operated crane. A hydraulic feed cylinder is used to

achieve a constant rate of advance or to maintain a constant weight on the cutter drums. Another

important advantage is that the drilling mud is constantly screened and desanded during excavation.

Thus the reinforcement can be placed and concreting carried out as soon as the required depth has

been reached. This excavation system makes it possible to drill piles panels or diaphragm wall elements

in a very wide range of soils, from cohesion less soils to hard rock.

7. What are the advantages of belt conveyors? (M/J 12) (N/D 11)

Belt conveyors have many advantages over other types of bulk material handling equipment.

Some of the advantages are:

Belt conveyors are capable of handling a wide range of bulk materials from very fine to large lump sizes.

Very fine materials such as portland cement are loaded at terminals using belt conveyors. Large lump

size materials such as coal are transported from mines using belt conveyors.

Belt conveyors can be designed to handle capacities for any operation. It is common for belt conveyors

to unload ships at capacities up to 10,000 tons per hour. Belt conveyors can also be designed for

batching operations or to convey a small amount of material between processes.

Belt conveyors can be configured to fit almost any application. A belt conveyor can convey material

horizontally, on an incline or a combination of both. It is common to use a single belt conveyor to

transport material horizontally a certain distance, then elevate the material on an inclined section of belt

conveyor and then horizontally again.

Belt conveyors can be used to stock-pile or reclaim bulk materials. Radial stackers are used for creating

large piles of materials such as wood chips, coal or ore. Reclaim belt conveyors are located under the

piles to carry the materials into the plant for processing.

Belt conveyors require less horsepower to operate than other types of conveyors. Bulk materials are

carried on top of the belt and remain static, therefore requiring much less energy to move.

Belt conveyors have proven to be a reliable method of conveying bulk materials. Industry standards for

the design of belt conveyors have been developed by the Conveyor Equipment Manufacturer’s

Association (CEMA).