Question

What is the value of negative skin friction for a group of piles of 30 cm diameter, 5 m long and spaced at 80 cm c/c and having cohesive strength of soil as 25 kN/m${}^2$? (Neglect bottom contribution in bearing capacity)

• A. 925 kN
• B. 950 kN
• C. 975 kN
• D. 1000 kN

Answer 1

The correct option is A 925 kN

Diameter of pipe = 30 cm
Spacing 80 cm c/c
For group action of piles, minimum spacing required $=2.5D=2.5\times 30=75cm$
Hence negative skin friction of group is to be calculated
Assuming 4 number of piles
Neglecting point bearing capacity
$Q_g\left(u\right)=\alpha .C.P_g.D$
For cohesive soil, assuming $\alpha =1$
Given, $C=25kN/m^2$
$P_g=Perimeterofblock=4\times 110cm$
Depth of block = 5 m
$Q_g\left(u\right)=1.0\times 25\times (\frac{4\times 110}{100}\times 5)=550kN$