A 0.5 m × 0.5 m square concrete pile is to be driven in a homogeneous clayey soil having undrained shear strength

Q. A 0.5 m × 0.5 m square concrete pile is to be driven in a homogeneous clayey soil having undrained shear strength, cu = 50 kPa and unit weight, γ = 18.0 kN/m³. The design capacity of the pile is 500 kN. The adhesion factor α is given as 0.75. The length of the pile required for the above design load with a factor of safety of 2.0 is

(A) 5.2 m                     (B) 5.8 m                   (C) 11.8 m                  (D) 12.5 m

Ans: 11.8 m   

Sol:

Given: 500 = Qu /2 ⇒ Qu = 1000 kN

c = 50 kN, Nc = 9.0 is generally used for the piles, a = 0.75, Ab = side2 (square base) = 0.5 × 0.5 = 0.25 m2

Fb = cNc = 50 × 9 = 450 KN

As = 4 × side × length (Surface area of square pile) = 4 × 0.5 × L = 2L

Fs = αC = 0.75 × 50 = 37.5 KN

1000 = 0.25 × 450 + 2L × 37.5

L = 11.833 m