GATE 2026 CE (CE1) – Question 58
A group of 25 circular piles is arranged in $5 \times 5$ uniform pattern in a soft clay soil with equal spacing in both the directions. These are friction piles with negligible end bearing.
Consider the following details:
Diameter of each pile = 1 m
Length of each pile = 15 m
Cohesion of the soil = 20 kN/m$^2$
Unit weight of the soil = 16 kN/m$^3$
Adhesion factor = 0.75
Considering the efficiency of the pile group as unity, the optimum value of the ratio of the pile spacing to pile diameter is ______________ (*rounded off to one decimal place*).
Practise this question in The GATE Grind →
Show answer and explanation
Correct answer: 3.38 to 3.42
Explanation
With an efficiency of 1, the capacity of the block of piles equals the sum of the capacities of the single piles. The block capacity from friction is $c\,(4B)\,L$, where the block width is $B = 4S + D$. The sum of the individual piles is $25 \times \alpha c\pi DL$. So $4(4S + 1) = 25 \times 0.75 \times \pi \times 1 = 58.9$, which gives $4S + 1 = 14.73$ and $S = 3.43$ m. The ratio $\frac{S}{D} = 3.4$.