Design of Ballast Section
The design of the ballast section includes the determination of the depth of the ballast cushion below the sleeper and its profile. These aspects are discussed below.
1 Minimum Depth of Ballast Cushion
The load on the sleeper is transferred through the medium of the ballast to the formation. The pressure distribution in the ballast section depends upon the size and shape of the ballast and the degree of consolidation. Though the lines of equal pressure are in the shape of a bulb, yet for simplicity, the dispersion of load can be assumed to be roughly 45 o to the vertical. In order to ensure that the load is transferred evenly on the formation, the depth of the ballast should be such that the dispersion lines do not overlap each other.
For the even distribution of load on the formation, the depth of the ballast is determined by the following formula (refer to Fig. 8.1):
Fig. 8.1 Minimum depth of ballast cushion (dimensions in mm)
Sleeper spacing = width of the sleeper + 2 × depth of ballast (8.1) If a BG track is laid with wooden sleepers with a sleeper density of N + 6, then the sleeper spacing would be 68.4 cm. If the width of the sleeper is 25.4 cm, then the depth of the ballast cushion would be
A minimum cushion of 15-20 cm of ballast below the sleeper bed is normally prescribed on Indian Railways.
2 Ballast Profile for Fish-plated Track
The ballast profile for a fish-plated track is shown in Fig. 8.2. The requirements of ballast for different groups of railway lines as adopted by Indian Railways are given in Table 8.2.
* In the case of ordinary fish-plated tracks, to be increased on the outside of the curves to 400 mm in the case of sharper curves of a radius more than 600 m. In short welded panel tracks, it is to be increased to 400 mm on the outside of all curves flatter than 875 m and to 450 mm in the case of sharper curves with a radius more than 875 m. To be increased to 550 m on the outside of the turn on curves of turnouts in passenger yards. In the case of a short welded rail (SWR) track, the minimum depth of cushion should be 200 mm.
3 Ballast Profile for Long Welded Rail Tracks
The ballast profile for a long welded rail (LWR) track is shown in Fig. 8.3. The requirements of ballast for different types of sleepers on a BG railway line are given in Table 8.3.
The minimum clean stone ballast cushion below the bottom of sleeper (A) is 250 mm. For routes where speeds are to be more than 130 kmph, A is 300 mm- 200 mm along with 150 mm of sub-ballast. Suitable dwarf walls should be provided in the case of cuttings, if necessary, for retaining the ballast.
* On the outer side of the curves only. Cess may be widened where required depending on local conditions and the outer ends of the curves.
† 200 mm over 150 mm sub-ballast.