SUB-SURFACE DRAINAGE
Changes in moisture content of subgrade are caused by fluctuations in ground water table seepage tlow, percolation of rain water and movement of capillary water and even water vapour. In sub-surface drainage of highways, it is attempted to keep the variation of moisture in subgrade soil to a minimum. However only the gravitational water 1s
drained by the usual drainage systems.
Lowering of Water Table
The highest level of water table should be fairly below the level.of subgrade, in orderbthat the subgrade and pavement layers are not subjected to excessive moisture. From practical considerations it is suggested that the water table should be kept atleast 1.0 to
1.2 m below the subgrade. In places where water table is high (almost at ground level at times) the best remedy is to take the road formation on embankment of height not less than 1.0 to 1.2 metre. When the formation is to be at or below the general ground level, it would be necessary to lower the water table.
If the soil is relatively permeable, it may be possible to lower the high water table by merely construction of longitudinal drainage trenches with drain pipe and filter sand. The depth of the trench would on the required lowering of water table, distance between the
drainage trenches and soil types. See Fig. 11.5
If the soil is relatively less permeable, the lowering of the ground water level may not be adequate at the centre of the pavement or in between the two longitudinal drainage trenches. Hence in addition, transeverse drains may have to be provided in order to efectively drain off the water and thus to lower the water table upto the level of transverse drains. A typical sketch of sub-surface drainage system with longitudinal and transverse drains for effective lowering of water table is shown in Fig. 11.6.
