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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