How can Water Table of Foundation Soil be Lowered?

There have many difficulties in foundation construction where water table is above the bottom level of foundation. To provide stability of foundation materials and to avoid difficulties in foundation construction, the only way is to lowering water table. The purpose is achieved by draining water to safe distance into galleries or ditches or pumping out from wells or sumps. This process is called gravity drainage as the gravity generates the force to flow the water to the designed drain.

This flow of water exerts a seepage pressure which forces the grains too with the water to the outlet. If there have silt or fine grained sand, finer constituents, they are washed out from the mix grained soil materials. They may clog drains, can result erosion tunnels or settlement to the drained soil. So this migration should be prevented. This purpose can be accomplished by using a filter that covers the drained materials with coarse materials of granular texture. The filter does not prevent the flow of water but the fine particle gets blocked by the small void of filter. 

For satisfactory result from the filter, particle-size distribution curve of it should be suitably related to the soil materials to be filtered.

To mathematically depict the relation, filter ration R are used as follows:

In addition to 50% and 15% sizes related requirements described by above expressions, the particle size distribution curve of filter material must have a smooth shape without sharp breaks and have to be parallel to the soil that have to be protected.

SI. NO.	Grading of filter material	R50	R50
1	Uniform where Cu ≤ 4	5-10	-----
2	Subrounded particles i.e. Nonuniform	12 -58	12-40
3	Angular Particles i.e. Nonuniform too	9-30	6-18
The particle size distribution curve of filter material must have a smooth shape without sharp breaks and have to be parallel to the soil that have to be protected.