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

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

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.

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

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.

A raft may undergo large settlement without causing harmful differential settlement. For this reason, almost double settlement of that permitted for footing is acceptable for raft. Therefore if a maximum settlement of 50 mm (2 in) is permitted for a raft, the differential settlement is not likely to exceed 20 mm (0.75 in).

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.