In calculation of bearing capacity of soils beneath foundation, it is very important to set adequate factor of safety. In case of plastic silts or clays, it is more important to predict reliably the value of differential settlement that a foundation may suffer even soil pressure does not reach the safe load. As we know differential settlement is very harmful resulting cracks in floors, foundation and many structural members and utility elements may be fractured. The fracture in utility element especially plumbing fixtures leads to leaks. Any leak results the situation worse as moisture introduction results swelling or shrinkage.
If the evaluated differential settlement is above the safe limit, the foundation designers look for a new type or layout of foundation to compensate excessive settlement. But in many situations the degree of accuracy is not guaranteed.
The steps for evaluating the settlement due to consolidation of thin clay layer of particular point on the foundation base are:
a. The first step is to estimate the effective pressure at midpoint of layer’s thickness.
b. The next step is to estimate the increment of pressure at midpoint of layer as described above and just below given point. The pressure increment is due to construction process of the structure above.
c. The next step is to compute the compressibility of the required clay laying in estimated layer.
d. The final step is consists of utilization of above estimated quantities to compute the decreasing thickness of the required stratum.
If there have several layers underlying the foundation, the subsidence of surface are computed by summation of changes in thickness of various layers i.e. these four steps are performed for each layer and their summation produce the final result which is used in foundation design.