Example $16\mathrm{.}8$: A square pile group of $9$ piles passes through a recently filled up material of $4\mathrm{.}5$

m depth. The diameter of the pile is $30$ cm and pile spacing is $90$ cm centre to centre. If the

unconfined compression strength of the cohesive material is $60k\frac{N}{m^2}$ and unit weight is $15$

$k\frac{N}{m^3},$ compute the negative skin friction of the pile group.