Question

A group of nine piles in a $3\times 3$ square pattern in 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 $126\text{cm}$. The size of pile group is $240\text{cm}\times 240\text{cm}$. The recently filled clay has undrained shear strength of $15\text{kPa}$ and unit weight of $16{\text{kN/m}}^3$.


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

• A. 472.32

Answer 1

The correct option is A 472.32
Negative skin friction by considering individual pile

$P_{n1}=n[\alpha C_u\times \text{Perimeter of single pile}\times L]$

According to the $C_u$ value of soil it is classified as soft clay. Adhesion factor for soft clay is 1.
So,
$P_{n1}=9\times [1\times 15\times 1.26\times 2]=340.2\text{kN}$

Negative friction load of pile group

$P_{n2}=C_u.P_g.L+\gamma \times \left[A_{\text{pile group}}\right]\times L$

$=15\times 4\times 2.4\times 2+16\times [2.4\times 2.4]\times 2$

$=472.32\text{kN}$

So, negative skin friction on pile group will be maximum of $P_{n1}$ and $P_{n2}$ and given by $472.32\text{k}N$