UBxample $26\mathrm{.}20.$ A square group of $25$ piles extends between depth of $2m$ and $12m$ in a deposit of $20m$ thick stiff clay overlying rock. The piles are $0\mathrm{.}5m$ in diameter and are spaced at $1m$ centre to centre in the group. The undrained shear strength of the clay at the pile base level is $180$kPa and the average value of the undrained shear strength over the depth of the pile is $110$kPa. The adhesion coefficient $()$ is $0\mathrm{.}45.$ Estimate the capacity of the pile group considering an overall factor of safety equal to $3$ against shear failure. $N_c$ corresponding to ${}_u=0$ is $9.$

$($Gate Exam. $1994)$