Execution of Earthwork in Embankments and Cuttings
The stability of the formation depends, apart from other factors, upon the subgrade material and the methods of construction. Experience has shown that many of the problems in the maintenance of the track are due to incorrect methods of execution of earthwork. In order to have a certain uniformity in practices, guidelines have been laid down by Indian Railways for the execution of earthwork in embankments and cuttings in new constructions, doubling, and conversion projects. These guidelines, given briefly in the following sections, are required to be modified to suit local conditions and prevailing circumstances.
Mechanical compaction of earthwork
For mechanical compaction, earthwork should be done in layers not exceeding a thickness of 300 mm to 650 mm in the loose state using static and vibratory rollers, respectively. The layers should be compacted preferably at or near the optimum moisture content with suitable rollers so as to achieve the dry density 98% for laboratory density.
The top of the formation should be finished to a slope of 1 in 30 away from the centre. An extra wide bank of 50 cm should be rolled on either side and then dressed to size to avoid any loose earth at the shoulder.
Proper quality control should be exercised during mechanical compaction. Coarse-grained soil which contains fines up to 5% passing through a 75-micron sieve should be compacted to get the relative density of a minimum of 70%. However, all other types of soil, when compacted, should normally have at least 98% of the maximum dry density determined by using Proctor's compaction.
1 Soil Classification
Soil exploratory surveys are carried out in the beginning by taking soil samples from the site. The soil is then classified as 'good' or 'other-than-good' depending upon its grain size and consistency limits. Broadly speaking, coarse-grained soils come under the category of good soils. Fine-grained soils such as inorganic clay, silts, sandy soils, and clayey soils are grouped under the category of other-than-good soils. However, the Indian Standard method of soil classification, which is also based on grain size distribution and consistency limits of the soil, is more scientific and elaborate. The grain size is determined by mechanical analysis of the soil. The soil is screened through a set of sieves, and based on the sieve analysis, the soil is classified as gravel (coarser than 2.00 mm), coarse sand (2.00-0.60 mm), medium sand (0.60-0.20 mm), fine sand (0.20-0.60 mm), silt (0.06-0.002 mm), or clay (finer than 0.002 mm).
Black cotton soil
Black cotton soil is a type of shrinkable soil which changes its properties considerably with change in moisture content. With the addition of water, this type of soil swells, thereby losing its strength. Loss of moisture may result in cracks in the soil. During dry seasons, the ballast penetrates into these cracks causing the track to sink. The situation worsens during the rainy season, when water entering these cracks makes the soil soft, and with the hydrostatic pressure and impact of moving loads, deeper ballast pockets are formed. This undesirable property of swelling and shrinkage of black cotton soil presents a lot of problems in the maintenance of proper levels of subgrade.
Remedial Measures The suggested remedies for the problems discussed are as follows.
(a) Treating the top layer of the soil with quick lime so as to reduce the harmful effects of the soil.
(b) Providing a blanket of a graded inverted filter at the top of the embankment.
(c) Consolidating the soil at optimum moisture content.
(d) Providing a bituminous carpet or other similar intercepting material such as polythene sheets to intercept the surface water getting into the formation.
(e) Improving the drainage conditions of the formation at surface and sub-surface levels.
2 Specifications for Embankments in Good Soil
The following guidelines are followed by Indian Railways for embankment construction on good soil.
For embankments up to 6 m high
The earthwork should be carried out manually in layers not exceeding 30 cm in thickness. All clods of earth should be broken. Earthwork should be carried out in this manner for a height of up to 1 m below the formation. The earthwork is then to be exposed to rains for one season before taking up the remaining work. The remaining earthwork is carried out by mechanical compaction of the soil in layers not exceeding 30 cm at optimum moisture content in order to obtain at least 90% of the maximum dry density.
For embankments more than 6 m high
In the first working season, up to 6 m or less of earthwork should be done and exposed to the rains. In the second working season, earthwork should be progressed further up to a distance of 1 m lower than the formation level and exposed to rains. The remaining earthwork should be done in the third working season by mechanical compaction. The work can also be completed in the second working season if mechanical compaction is used.
On high-speed and heavy-density routes, a blanket of suitable material or a sub-ballast of 30 cm thickness may be provided. The formation should be given a cross slope of 1 in 40 or 1 in 30 from the centre towards the cess.
3 Specifications for Cuttings in a Good Soil
The following guidelines are followed by Indian Railways for cuttings in good soil.
(a) If the normal dry density of the top 30 cm of soil is less than 90% of the maximum dry density, the formation should be rolled to obtain the desired density.
(b) The road bed should be given a cross slope of 1 in 40 or 1 in 30 from the centre towards the drains on either side.
4 Embankments in Other-than-good Soils
The guidelines adopted by Indian Railways for embankment construction in soil that is not categorized as good are given below.
(a) The earthwork should be compacted to full height at optimum moisture content in layers not exceeding 30 cm in thickness in order to obtain 90% of the maximum dry density.
(b) A blanket of suitable material of height not less than 30 cm should be provided on the road bed and should be compacted.
(c) A cross slope of 1 in 30 should be provided from the centre towards the cess.
5 Cuttings in Other-than-good Soils
The guidelines listed here are followed by Indian Railways for digging cuttings in soil that is not categorized as good.
(a) The cutting should be provided with drainage.
(b) A 30-cm-thick blanket of suitable material should be provided in two layers at optimum moisture content and duly compacted.
(c) A cross slope of 1 in 30 should be provided.
Track drainage is defined as the interception, collection, and disposal of water from upon or under the track. It is accomplished by a surface and sub-surface drainage system. Proper drainage of the subgrade is very vital, as excess water reduces the bearing capacity of the soil as well as its resistance to shear. The full details about track drainage can be obtained from Chapter 19 where this subject is dealt with in depth.