Depth and Location of Foundation

In general a foundation must be at a depth in which the settlement and  bearing capability standards are met through the soil strata available. 

But there are certain recommendations in Indian Standard Code (IS: 1904-1986) for the depth of foundation which should be met. The code recommends the minimum depth as 50 cm below NSL. 

Also the foundation must be placed below the volume change zone i.e where the swelling and shrinkage of  the soil mass takes place because of rise and lowering of ground water table.

For Black cotton soil of India the zone of seasonal variation's ( resulting in lowering and rise of ground water table) thickness varies from 1.5 m to 3.5 m. For areas with exposure to lower temperature , in case of fine sand and silts the foundation must be placed below zone of frost heave.

For structures in stream or river the foundation has to be protected from the scouring action of the flowing stream. Therefore depth of structures in this case must be enough deep below the deepest scour depth.