Question

A 16-pile group has to be proportioned in a uniform pattern in a soft clay with equal spacing in both directions. Assuming any value of cohesion, taking α = 0.7, neglecting the end bearing effect and assuming pile circular of diameter ‘d’, the optimum spacing (s) of piles in group will be
1. 1.6d
2. 2.6d
3. 3.3d
4. 4d

Answer 1

Correct Answer - Option 2 : 2.6d

Concept:

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

Q_F = α × C_U × A_F

⇒ Q_F = α × C_U × π × D × L

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

QFG = α × CU × AFG

⇒ Q_FG = 1 × C_U × (3S + D) × 4L

Where, Q_F = capacity of a single pile, Q_FG = Capacity when piles act as a group,

A_F = Area for friction of single pile, C_U = Unconfined compressive strength of soil, 

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

A_FG = Area under friction for a group of piles

Calculation:

Given:

α = 0.7, N = 16

For finding the optimum spacing between the pile, we can say that

Capacity of single pile × Number of piles = Group capacity of piles

⇒ N × Q_F = Q_FG

⇒ N ×  α × CU × π × D × L = 1 × CU × (3S + D) × 4L

⇒ 16 × 0.7 × π × D = (3S + D) × 4

⇒ 8.796 × D = 3S + D

⇒ S = 2.5988D ≈ 2.6D

∴ The optimum spacing between the piles is 2.6D