When the allowable soil pressure is low, or the building loads are heavy, the use of spread footings would cover more than one-half of the area and it may prove more economical to use mat or raft foundation.

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 penetration resistance N values should be taken at 75 cm intervals for a depth equal to width of the raft, below the base of the raft.

The maximum bending moment should be adopted as the design value for the reinforced concrete rectangular footing, which should also be checked for maximum shear and bond etc.

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

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.

The strap used does not remain in contact with soil, and thus does not transfer any pressure to the soil.

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

In the pressure intensity equation q2 = a. qs, if a= 0 we have a triangle with xÌ… = L/3.