ABSTRACT
The salient design feature of the subballast layer (sometimes called capping) is to protect the natural subgrade soil or embankment fill from excessive load that can lead to unacceptable settlement or bearing capacity failure under extreme conditions. The use of elastic theory (Chapter 2) to design the subballast layer (about 100 to 150mm thick) as a relatively stiff medium of compacted broadly graded granular fills is common practice. However, the key role of subballast is to act as a drainage layer and as an effective filtration medium. Drainage plays a significant role in the stability and safety of a track substructure. Saturated tracks can lead to a build up of excess pore water pressure under train loading. If the hydraulic conductivity of the substructure elements becomes excessively low, especially the subballast layer, the excess pore water pressure developed under axle loading often cannot dissipate completely before the next load is imposed. Thus, the residual pore pressures accumulate with increasing load cycles. After a few load cycles, the total excess pore water pressure becomes very high and often causes clay pumping [1]. Thus the subballast layer plays two major roles in track substructure, (a) act as a permeable medium to transmit water laterally into the drainage channels, and (b) dissipate excess pore water pressure from saturated subgrades by allowing upward flow. The subballast, therefore, must have greater permeability than the subgrade soils.
