The net ultimate bearing capacity may be determined from both Skempton and Terzaghi’s equation as given below:

Mat footing is used where the soil contains compressible lenses or the soil is sufficiently erratic so that the differential settlement would be difficult to control.

If N is less than 5, sand should be compacted by artificial means to rise N above 10, or else piles or piers should be used.

The designing of strap or cantilever footing is based on trial and error method, it begins with a trial value of e so that the length L1 is known. The calculations are repeated with another value of e till values of the reactions R1 and R2 calculated by both the procedure, coincide.

The raft is subdivided into a series of continuous beams (strips) centered on the appropriate column rows to establish shear failure and moment diagram.

The weight of the raft is not considered in the structural design because it is assumed to be carried by directly by the subsoil.

If the resultant of the soil pressure coincides with the resultant of the loads and the center of gravity of the footing, the soil pressure is assumed to be uniformly distributed.

In strap footing, two independent columns are supported by as trap or beam at the bottom.

If the Center of gravity of the load coincides with the centroid of the raft, the upward load is regarded as a uniform pressure, which will be equal to the downward load divided by the area of the raft.

For a rectangular combined footing, xÌ… = L/2 and for a trapezoidal combined footing solutions lies between the limits L/3 < xÌ… < L/2.