The settlement is not a problem for raft on sands as allowable bearing capacity is computed on the basis of settlement criteria. However, settlement is of great concern when a raft rests on a deposit of normally consolidated clay. The net foundation pressure for computing settlement is taken as:
qns = (Q/A) – Î³Ã—Df ———– (1)
The pressure in the Eq. (1) shall not cause settlement in excess of permissible value. If settlement exceeds the permissible limits, the foundation pressure should be reduced either by increasing the area under the structures or by increasing the depth of raft or by providing one or more basement. The base area of raft cannot be increased due to space limitation and hence the only practical method to limit the settlement in order to obtain a required factor of safety is to lower the elevation of the raft. If this procedure does not bring the settlement within the permissible limits then a deep foundation should be provided.