Formation - Introduction
Subgrade is the naturally occuring soil which is prepared to receive the ballast. The prepared flat surface, which is ready to receive the ballast, sleepers, and rails, is called the formation. The formation is an important constituent of the track, as it supports the entire track structure. It has the following functions.
(a) To provide a smooth and uniform bed for laying the track.
(b) To bear the load transmitted to it from the moving load through the ballast.
(c) To facilitate drainage.
(d) To provide stability to the track.
The formation can be in the shape of an embankment or a cutting. When the formation is in the shape of a raised bank constructed above the natural ground, it is called an embankment. The formation at a level below the natural ground is called a cutting. Normally a cutting or excavation is made through a hilly or natural ground for providing the railway line at the required level below the ground level.
The formation (Fig. 9.1) is prepared either by providing additional earthwork over the existing ground to make an embankment or by excavating the existing ground surface to make a cutting. The formation can thus be in the shape of either an embankment or a cutting. The height of the formation depends upon the ground contours and the gradients adopted. The side slope of the embankment depends upon the shearing strength of the soil and its angle of repose. The width of the formation depends upon the number of tracks to be laid, the gauge, and such other factors. The recommended widths of formation as adopted on Indian Railways for BG, MG, and NG are given in Table 9.1.
The following points are relevant with respect to the dimensions given in Table 9.1.
(a) The widths have been calculated for a minimum width of 900 mm in banks and 600 mm in cuttings and a ballast profile slope of about 1:1.
(b) The width of a double-line section has been calculated with a track centre of 5.30 m on BG and 3.96 m on MG. These dimensions are based on a ballast cushion of 300 mm.
(c) The side drain should have a minimum of 0.30 m horizontal berm on the side (i.e., on other than the track side) in order to be fully effective.
Table 9.1 Width of formation for different tracks