A group of nine piles in a $3 \times 3$ square pattern is embedded in a soil strata comprising dense sand underlying recently filled clay layer, as shown in the figure. The perimeter of a individual pile is $126$ cm. The size of pile group is $240$ cm $\times \: 240$ cm. The recently filled clay has undrained shear strength of $15$ kPa and unit weight of $16 \: kN/m^3$.