9.3 DRAINAGE AND WATER CONTROL
Drainage of water is an effective method of increasing the stability of a slope. Water in a slope may come from two primary sources: surface water and groundwater. Water control is generally maintained through installation of surface and subsurface drainage devices within and adjacent to potentially unstable slopes. Runoff and infiltration of water along a slope or over a bluff face can often be reduced by planting vegetation on top of the slope or bluff to reduce erosion.
Surface Drainage Systems: Surface drains and landscape design are used to direct water away from the head and toe of cut slopes and potential landslides and to reduce infiltration and erosion in and along a potentially unstable mass.
Subsurface Drainage Systems: The main functions of subdrains are to remove subsurface water directly from an unstable slope, to redirect adjacent groundwater sources away from the subject property and to reduce hydrostatic pressure beneath and adjacent to engineered structures. Control of subsurface drainage is generally attained by installing a network of horizontal and/or vertical subdrains.
Drainages are the most frequently used means of stabilizing slopes. Slope failures are very often precipitated by a rise in the groundwater level and increased pore pressures. Therefore, lowering groundwater levels and reducing pore pressures is a logical means of improving stability. In addition, improving drainage is often less expensive than other methods of stabilization, and a large volume of ground can frequently be stabilized at relatively low cost. Once a system of drainage has been established, it must be maintained to keep it functional. Various methods of drainage for this purpose are:
(a) Application of external load to the soil mass: its aim is to squeeze out pore water by applying the external loads.
(b) Drainage of pore water by gravity and/or pumping: Well-points are used to drain pore water either by gravity and/or pumping. Vertical sand drains or sand piles are used to expedite drainage of a soil stratum. A proper design of sand-drain installation involves determination of the diameter and the spacing of sand drains, the thickness of the drainage blanket and the amount and duration of surcharge fill loading.
(c) Application of electrical gradient or electro-osmosis: When a direct electric current is passed through a saturated soil, water moves towards the cathode. If this is removed, the soil undergoes consolidation. This phenomenon is called ‘‘electro– osmosis’’. In addition to electro-osmotic consolidation, passage of electric current can cause ion exchange, alteration in arrangement of the particles, and electro-chemical decomposition of the electrodes. The combination of these changes brought about in the soil is called ‘electrical stabilisation’. This procedure has been successfully employed to increase skin friction of piles.
(d) Application of thermal gradient: Heating or cooling a soil can cause significant changes in its properties. Even a slight increase in temperature can reduce the electric repulsion between clay particles and can cause slight increase in strength. The main drawback of thermal stabilisation is the cost involved, which makes it seldom cost-competitive with other techniques.