Formation and Its Drainage

LECTURE 2

Formation and Its Drainage

Elements of Railway Track

Roadbed BallastSleeperFasteningsRails

What is a roadbed?

A railway roadbed is a regular, prepared subgrade on which are laid the ballast section, ties and rails.

Purpose and Basic functions of roadbed

Weights of tracks + ballast + train loadings are supported as uniformly as possible and transmitted with diminishing uniform pressures to the supporting natural ground beneath.

Drainage is facilitated.Smooth, regular surface is provided on which

the ballast section and track structure can be laid.

Design of Roadbed section

Three important factors have to be considered:

Width of subgrade (or base of cut)Depth of cut or fillSide slopes of the sut or fill

Width of roadbed

It is determined in part by width of ballast section which depends on several variables like subgrade and ballast material, weather, size of ties, weight of rail, volume and speed of traffic and axle loadings.

Design of ballast section & width of subgrade is based on expected traffic at least 10 to 15 years in future.

The cost of extra width at the time of construction should be balanced against estimated future cost of roadbed widening.

Separate standards of roadbed & ballast are used for mainline and branch line.

Road bed shoulders should extent a minimum of 18” beyond the toe of ballast slope to give adequate support to ballast section.

More widening of subgrade is required where soils are susceptible to wind / water erosion.

In case of settlement, final top subgrade width must be maintained. Otherwise with successive ballast layers (also called as lifts) to hold the established gradient, top of fill becomes too narrow for it’s height and for the width of ballast section.

The most important factor affecting width of cuts is side ditches. Base width of 3’ – 6’ with side slopes of 1:1 are standards in common use

for width of cuts. If gradient is steep, shoulder must be protected against erosion.

Depth of Fill (Embankment)

Allowance for settlement & erosion must be given.

High water levels must be considered.

Side Slopes

Depends primarily upon shearing strength & angle of repose of the materials forming the slope.

A slope of 1 : 1 is commonly used in railway design for both cuts and fills and gives reasonable stability for most materials.

Sands / Clays may require 2 : 1 or even 3 : 1Solid rock cuts may stand on : 1 or : 1A final decision on rate of slope must depend on type of

soil used rather than on rule of thumb.A slope of 2 : 1 is approximately one third longer than

1 : 1 and receives, approx. one third more rainfall. Therefore effects of erosion must be considered while deciding whether to make slope wider or flatter.

Slope Protection

Paving: To pave means to make hard, durable and permanent. It gives pleasing

appearance. There are options of stone paving, asphalt paving, etc. Decision must be made according to budget.

Rip Rap: Loose placement of stones along slope with toe wall. Protects saturated fills along rivers and lakes. Grouting between stones could be used for greater stability. Planting: Choice of plantings is determined by conditions of local soil, climate and

rainfall. Native plants are likely to give best results. Cinder Blankets: Used for clay embankments. Retaining Walls: Provided when angle of slope is considerably greater than angle of repose of

slope materials.

Roadbed Materials

Functions of soilsSoils have a dual function for roadbed. 1) Soils

constitute the foundation on which railways are constructed. 2) Soils are also the materials of which the roadbed is constructed.

An engineer can’t always select soils entirely by his own choice because a railway track is hundreds of miles long and a variety of soils are encountered in the field. Proper knowledge of soils & their functions can help creating a stable and purpose fulfilling track. This knowledge is primarily based on classification of soils and their properties.

Testing of Soils for Selection

Classification by Grain Size Gravel Coarser than 2 mm Coarse Sand 2 – 0.6 Medium Sand 0.6 – 0.2 Fine Sand 0.2 – 0.06 Silt 0.06 – 0.002 Clay 0.002mm and finer

BrainStorming

What soils can be used for railway formation?How can you drain Water away from railway

track/ formation?What could be the causes of failure of

formation?

Atterberg Limits

It compares the differences in physical properties of clays at various water contents

Plastic limit Pw is that lowest water content at which the soil begins to crumble when rolled into threads.

Liquid limit Lw is that highest water content at which the soil will not flow under standard conditions.

Some highway departments prohibit soils as fill materials having a Lw greater than 65%

With consistency limits, we are able to know about the swelling and shrinking potential of clays & silts.

Effect of Soil Properties On Roadbed / Subgrade

Properties of good roadbed materialsTo fulfill the functions of subgrade, the greatest

uniformity and permanency is obtained when subgrade material is

1. free of excess moisture and has physical characteristics providing

2. high internal friction3. high cohesion and density4. low compressibility5. low elasticity6. low capillarity

Why High Internal Friction and Cohesion?

To hold the soil firmly in placeSoil with less cohesion, such as wet sand , is

likely to slump and slide in addition to being susceptible to surface erosion.

Why High Density?

A dense soil tends to exclude moisture since the volume is largely filled with soil particles and little room is left for moisture to intrude

Why low compressibility?

A highly compressible soil is slow in consolidating. If not fully compacted at the beginning, it continues to compress under traffic, causing the top of subgrade to settle.

Why low elasticity?

The unfavourable reaction of compressibility is intensified if the soil is highly elastic.

The compressed soil rebounds when the load is removed, and the process of consolidation is prolonged.

Effect of Soil Properties On Roadbed / Subgrade

Adverse PropertiesTendency to flow or run because of rounded

shapes of sand & silts. Flow occurs due to low internal friction and cohesion.

Swelling and frost action.Lateral flow under pressure

Use of soils

Gravel is the only natural soil which does not require an admixture to make it suitable.

Theoretically ideal soil is one in which several constituents are equally proportioned. Greater percentage of gravel is desirable due to its hardness and structural strength. To this should be added sand to “bed” larger gravel particles, silt to act as filler & clay to fill remaining voids and provide a water film for cohesion.

Soil Proportioning

First MethodBring from scattered pits the desired soils in

proper amounts and mix the several types in spreading and compacting operation.

Second MethodEngineer may obtain helpful selection at pit. If

several grades and soil types are available in one or more pits, the loading and unloading can be performed to place the individual soils in proper relation to each other in fill.

Control Tests

Standard tests are done on actual construction samples and curves, charts & other guides are provided, against which to measure & obtain the desired degree of compaction. These tests are

Compaction TestMoisture Content – for use in compaction

test.

Inspection Tests

These tests are done to insure that the standards established by the control tests are being secured. These tests are:

Dry density test – to determine adequacy of compaction

Soil-Volume Relations – determination of unit weights before and after excavation to establish change in volume.

Moisture-density relations – amount of water required per unit of borrow material to bring the moisture content of borrow material upto that of compacted material.

Formation(Track) Drainage

What is Track Drainage?

Drainage is defined as interception, collection and disposal of water away from track.

If you intercept someone or something that is travelling from one place to another, you stop them before they get to their destination

You have to intercept the evil water before it reaches its destination which is your very own railway formation.

Sources of Water Entering Track

Why Drainage is so important to study?

Because:There is scarcely any item of maintenance cost which

is not increased by effects of poor drainage.A poorly drained subgrade permits cinders, stone dust

and dirt to accumulate in and foul the ballast, leading to pumping joints.

Pumping joints cause excessive rail end batter, joint bar wear & tie deterioration.

Washing out of tracks, poor line surface of gauge, accumulation of ice & snow, obstructing tracks in tunnels.

Poor drainage may lead to a low or zero shear strength

Drainage Systems

Track Drainage

Subsurface Drainage

Surface Drainage

Surface Drainage

Most important factor in embankment design and maintenance

Generally, provision of 1 in 30 cross slope on top of formation towards cess, side and catch water drains, culverts and bridges comes under this category.

Types of Surface Drainage

Side Drains

Catch Water Drains

Side Drains

Normally not needed for embankmentRequired if blanket is below ground level due

to height of embankmentIn case of cuttings, properly designed side

drains of required water carrying capacity are to be provided

Catch Water Drains

Surface water flowing from top of hill slope towards the track is controlled by provision of catch water drains

Providing side drains for the same purpose is not feasible

Catch water drains are provided running almost parallel to the track upto a point where the water can be safely discharged off

Discharge Capacity of Catch Water Drains

Channel Capacity =

Where:- Q = discharge in cum/sec V = mean velocity in m/sec n = Manning’s roughness coefficient R = A/P where, R is Hydraulic radius in m, A = area of the flow cross- section in square metre P = wetted perimeter of cross-section S = energy slope of the channel, which is roughly taken as slope of drain

bed

Subsurface Drainage

Main objectives of sub surface drains are to lower the level of water table and to intercept or drain out underground water

The sub-surface drains may consist of perforated pipe or open jointed solid pipe in a trench with backfill around it or it may simply be free draining material in the trench without any pipe

The subsurface drains can also be provided with geotextile either along the trench or around the pipe or both

Backfilling

Backfilled with excavated soil and thoroughly compacted so as to stop water directly percolating from backfill material around the pipe.

Use of Free draining materials in subsurface drains

When only free draining material is used in trench, the main drain may be constructed without any pipe. The trench may be filled with material such as gravel or stone aggregate free from organic and deleterious substances.

Types of Subsurface Drainage

Boulder Drains

French Drains

Horizontal Drans

Failure of Railway Embankments

Failure of Railway Embankments

Failure of Natural Ground

Failure of Fill material

Failure of Formation top

Symptoms of Failure

Slips in slopesLoss of BallastVariation in Cross LevelsUpheaval of the ground beyond toe of

embankment

Failure of Natural Ground

Shear Failure during or just after construction

Shear Planes

Failure of

Natural Ground

Failure of Fill material

Failure of Formation top

Failure of Natural Ground

Excessive settlement due to Upheaval of Ground beyond toe of embankment

Failure of

Natural Ground

Failure of Fill material

Failure of Formation top

Remedial Measures for preventing Failure of Natural Ground

Sand Drains

Remedial Measures for preventing Failure of Natural Ground

Balancing Embankment

BALANCING EMBANKMEN

T

BALANCING EMBANKMEN

T

Remedial Measures for preventing Failure of Natural Ground

Sheet piles / ordinary piles

Failure of Fill Material of Embankment

Following reasons cause excessive settlement and shear failure mainly due to failure of fill material:

Heavy TrafficInadequate slopes to EmbankmentsPercolation of water in Embankment (low

shear strength)

Failure of Natural Ground

Failure of

Fill material

Failure of Formation top

Failure Types

Failure of Natural Ground

Failure of

Fill material

Failure of Formation top

Base Failure

Toe Failure

Slope Failure

Remedial Measures for preventing Failure due to fill material of

embankment

Careful selection of fill materialBetter construction techniquesProper designFlatter side slopesReduced Embankment HeightStone pitching of side slopesProper drainageBalancing embankmentsVertical Piles

Material Control