Site Investigations

The following data should be collected for determining the type of treatment to be given to the formation.

History of the affected section This includes information about the period of construction, method of construction, date of opening to traffic, sub-soil, bank settlement slips, speed restrictions, etc.

Site details These include various details of the site such as bank heights, depth of cuttings, nature of existing slopes, drainage conditions, stagnation of water, condition and proximity of borrow pits, signs of movement and bulging of the slopes, and groundwater level and its position during rains.

Number of visits to track The particulars of the number of visits to the track should be obtained from the gang charts for the last five years to get an idea of track maintainability. Man-days utilized for maintenance per kilometre should also be determined vis-a-vis the total number of men required for normal maintenance.

Ballast penetration profiles Ballast penetration profiles should be obtained at regular intervals from one telegraph post to indicate the extent of ballast penetration and the condition of the ballast.

Exact nature of present problem The exact nature of the present problem should be identified.

Remedial measures

The relevant remedial measures should be formulated based on the site investigations and soil testing. Some of the suggested remedial measures for the formation problems generally encountered are listed in Table 9.2.

Table 9.2 Remedial measures for foundation problems

Nature of problem

Remedial measure

Track level variations due to

Inadequate drainage due to high cess,

Improve side drainage by lowering the

dirty ballast

cess and screening the ballast

Weakening of soil at the top of the

Cationic bituminous emulsion below

formation upon contact with rain water,

ballast; provision of a moorum/sand

resulting in mud pumping

blanket of depth 20-30 cm below ballast; laying of geotextiles

Strength failure below ballast or

Provision of a 30-60-cm-deep blanket

between sleepers causing heaving of cess

below ballast; provision of sub-ballast

Seasonal variations in the moisture

Treatment with lime slurry pressure

content of the top of the formation in

injection; Use of a macadam blanket of

expansive soils, causing alternate heaving and shrinkage of formation

depth 30-45 cm with macadam lining

Gradual subsidence of the bank core

Cement grouting of ballast pockets if

under live loads due to inadequate initial

ballast pockets are permeable; sand or

compaction/consolidation of embankment

boulder drains

Gradual consolidation of earth below

Lime pilling in subsoil; sand drains in

embankment

subsoil

Creeping away of formation soil

Easing of side slopes

Coal ash pockets due to treatment of

Sand drains below deepest level of coal

previous slips

ash; cement pressure grouting

Instability of bank/cutting slopes due to Inadequate side slopes causing bank slips

Flattening of slopes and provision of

after prolonged rains

berms; improvement in drainage

Consolidation/settlement of subsoil

Providing of sand drains to expedite

causing bank slips

consolidation

Hydrostatic pressure built up because of

Draining out ballast pockets by sand or

live loads in ballast pockets containing

boulder drains; Cement sand pressure

water, causing bank slips

grouting of ballast pockets

Creeping of soil

Reducing stresses by the provision of side berms or by flattening the slopes

Swelling of over-consolidated clay slopes in cuttings causing loss of shear strength and slipping

Flattening side slopes

Erosion of banks

Providing turfing, mats, etc.

Summary

Subgrade is the natural soil which is prepared and compacted at its maximum density to receive the ballast and the track. It is also called the formation. The track should be so designed that the stresses transmitted to the formation do not exceed the permissible limits. The bearing capacity of the soil depends upon the type of soil and the degree of compaction. The formation may fail in different modes, and remedial measures should be taken in time to safeguard the track.

Review Questions

1. Briefly describe the normal and special measures adopted to ensure the stability of railway embankments.

2. Define Good-soil and other-than-good soil. Give the specifications of embankment in good soil and other-than-good soil.

3. Prepare a neat sketch of a typical cross section of an embankment with the ballast section for a double-line broad gauge track and indicate its dimensions and salient features.

4. Discuss the requirements for the stability of an embankment and the precautionary measures commonly adopted against failure. What are the requirements of the subgrade material for a railway track?

5. What are the main causes of failure of a railway embankment? Discuss the remedial means for each one of them.

6. What is formation width? Give the standard formation width for a BG track in cutting and embankments. Illustrate your answer with suitable sketches.

7. What are the main causes of failure of a railway embankment? Suggest remedial measures for failure of formation.

8. What are the main functions of formation? Give the width of embankment and cutting for BG and MG tracks. If concrete sleepers are provided, what will their dimensions be?

9. Write short notes on the following.

(i) Blanket and blanketing material

(ii) Stabilization by geotextiles

(iii) Black cotton soil

(iv) Sand piling

CHAPTER

10

Failure of Railway Embankment | RAILWAY ENGINEERING | Track Fittings and Fastenings