Based on the settlement criterion for raft, the net pressure can be calculated from the following equation having its width greater than 6m.

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

Ordinarily, rafts are designed as reinforced concrete flat slabs using heavy beams.

It is assumed that the combined footings are rigid and rests on a homogeneous soil, so as to give rise to a linear stress distribution on the bottom of the footing.

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.

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

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.

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

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.