For a square foundation $B**B$ in plan, $D_f=1\mathrm{.}5m,$ vertical gross allowable load is $Q_{at}=2800kN,$ unit weight of the soil is $20k\frac{N}{m^3},$ the internal friction angle is $30$ degree and the cohesion of soil is zero. Assume a factor a safety of $4.$ Determine the size of the required foundation.

A mat foundation measuring $20m^{**}15m$ is to be constructed on saturated clay. The clay has a cu of $90k\frac{N}{m^3}$ and an internal friction angle of $0.$ The depth of the footing is $1\mathrm{.}5m.$ Determine the net ultimate bearing capacity.