Question

A group of nine piles 12 m long and 250 mm in diameter is arranged in a square form in clay having undrained cohesion of 30 kN/m2. Neglecting bearing at tip of the piles and taking adhesion factor as 0.9, ultimate capacity of all piles in individual action is:
1. 2045 kN
2. 2290 kN
3. 2545 kN
4. 2690 kN

Answer 1

Correct Answer - Option 2 : 2290 kN

Concept:

The load-carrying capacity(Friction) of a single pile driven in clay is given by

QF = α × CU × AF

⇒ QF = α × CU × π × D × L

The load-carrying capacity(Friction), when piles act as a group is given by

QFG = α × CU × AFG

⇒ QFG = 1 × CU × (2S + D) × 4L

Where, QF = capacity of a single pile, QFG = Capacity when piles act as a group,

AF = Area for friction of single pile, CU = Cohesion

α = Adhesion factor, D = Diameter, L = Length of the pile,

AFG = Area under friction for a group of piles

Calculation:

Given,

Diameter of pile, D = 250 mm = 0.25 m, Length of pile, L = 12 m

Undrained cohesion, Cu = 30 kN/m²

Adhesion factor, α = 0.9

The load-carrying capacity(Friction) of a single pile driven in clay is given by;

QF = α × CU × π × D × L

Q_F = 0.9 × 30 × π × 0.25 × 12 = 254.469 kN

For 9 piles,

Q_F = 9 × 254.469 = 2290.22 kN

Q_F ≈ 2290 kN