Settlement of Foundations

By settlement we  mean the vertical downward movements of the foundation. The effect of settlement on the structure depends on its magnitude, both absolute as well as relative with respect to the different parts of the foundation. Settlement  may be caused by :

1. the weight of the structure and the superimposed loads, 
2. subsidence due to mining, 
3. shrinkage due to change in moisture content, and 
4. general earth movement. 

Settlements that take place when the static load is within the range of the safe bearing capacity of the soil, consists of the following elements :

1. Elastic deformation which takes places immediately on application of load, 
2. Primary consolidation of foundation soil resulting from expulsion of pore water, 
3. Secondary compression of foundation soil, and 
4. Creep of the foundation soil. 

If the structure settles uniformly, there will be no damage but if the settlement is excessive the underground service lines may be affected. In actual practice, as the soil is not a purely homogeneous material and superimposed loading are not equal, settlements are non- uniform, inducing corresponding stress in the structure. Depending upon the extent of these stresses, the settlements have to be limited by appropriate designs.

Based on the loading pattern and the soil characteristics the settlements can be calculated and if it exceeds the desirable limits, the foundation has to be redesigned.