1. The supporting soil should be safe from shear failure due to the loads imposed on it by the superstructure, 2. The settlement of the foundation should be within permissible limits.
Hence, we have to deal with two types of bearing pressures. They are,
1. A pressure that is safe from shear failure criteria,
2. A pressure that is safe from settlement criteria.
For the sake of convenience, let us call the first the allowable bearing pressure and the second the safe bearing pressure.
In all our design, we use only the net bearing pressure and as such we call qna the net allowable bearing pressure and qs the net safe bearing pressure. In designing a foundation, we use the least of the two bearing pressures. We learnt that qna is obtained by applying a suitable factor of safety (normally 3) to the net ultimate bearing capacity of soil. In this chapter we will learn how to obtain qs. Even without knowing the values of qna and qs, it is possible to say from experience which of the two values should be used in design based upon the composition and density of soil and the size of the footing. The composition and density of the soil and the size of the footing decide the relative values of qna and qs.
The ultimate bearing capacity of footings on sand increases with an increase in the width, and in the same way the settlement of the footing increases with increases in the width. In other words for a given settlement 5p the corresponding unit soil pressure decreases with an increase in the width of the footing. It is therefore, essential to consider that settlement will be the criterion for the design of footings in sand beyond a particular size. Experimental evidence indicates that for footings smaller than about 1.20 m, the allowable bearing pressure q is the criterion for the design of footings, whereas settlement is the criterion for footings greater than 1.2 m width.
The bearing capacity of footings on clay is independent of the size of the footings and as such the unit bearing pressure remains theoretically constant in a particular environment. However, the settlement of the footing increases with an increase in the size. It is essential to take into consideration both the shear failure and the settlement criteria together to decide the safe bearing
However, footings on stiff clay, hard clay, and other firm soils generally require no settlement analysis if the design provides a minimum factor of safety of 3 on the net ultimate bearing capacity of the soil. Soft clay, compressible silt, and other weak soils will settle even under moderate pressure and therefore settlement analysis is necessary.