Example $16\mathrm{.}5$: A square group of $9$ piles was driven into soft clay extending to a large depth.

The diameter and length of the piles were $30$ cm and $9$ m respectively. If the unconfined

compression strength of the clay is $90k\frac{N}{m^2},$ and the pile spacing is $90$ cm centre to centre,

what is the capacity of the group? Assume a factor of safety of $2\mathrm{.}5$ and adhesion factor of $0\mathrm{.}75.$