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

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