A group of nine piles in a 3 × 3 square pattern is embedded in a soil strata comprising dense sand

Q. A group of nine piles in a 3 × 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 an individual pile is 126 cm. The size of pile group is 240 cm × 240 cm. The recently filled clay has undrained shear strength of 15 kPa and unit weight of 16 kN/m³.

The negative frictional load (in kN, up to two decimal places) acting on the pile group is

Ans: 472.32

Sol:

Negative skin friction by considering individual pile

Pn1=n[αCu×Perimeter of single pile×L]

According to the Cu value of soil it is classified as soft clay. Adhesion factor for soft clay is 1.
So,
Pn1=9×[1×15×1.26×2]=340.2 kN

Negative friction load of pile group

Pn2=Cu.Pg.L+γ×[Apile group]×L

=15×4×2.4×2+16×[2.4×2.4]×2

=472.32 kN

So, negative skin friction on pile group will be maximum of Pn1 and Pn2 and given by 472.32 kN