Gravity methods, such as well points and deep wells are not much effective in the fine-grained soils with permeability in the range of 0.1 –10 x 10 ^-3 mm/s. Such soils can be dewatered satisfactorily by applying a vacuum to the piping system. A vacuum dewatering system requires that the well-point screens, and rise a pipe be surrounded with filter sand extending to within a few metres of the ground surface. This method is most suitable in layered or stratified soils with coefficient of permeability of the range 0.11 - 0 x 10 ^-4 cm/s.

Figure 6.50 Vacuum Dewatering Systems

When an external electro motive force is applied across a soild liquid interface the movable diffuse double layer is displaced tangentially with respect to the fixed layer . this is electro osmosis. As the surface of fine grained soil particles causes negative charge, the positive ions in solution are attracted towards the soil particles and concentrate near the surfaces. Upon application of the electro motive force between two electrodes in a soil medium the positive ions adjacent to the soil particles and the water molecules attached to the ions are attracted to the cathode and are repelled by the anode. The free water in the interior of the void spaces is carried along to the cathode by viscous flow. By making the cathode a well, water can be collected in the well and then pumped out.