Why does a pile in a group behave differently from an isolated pile?

Group piles carry no skin friction

Group piles are driven only in clay

At the same load per pile, the settlement of the group compared with a single pile is:

Larger, so group settlement must be checked

Group capacity is taken as:

The smaller of the sum of individual capacities and block capacity

The larger of the sum of piles and block capacity

2075 Asoj · 4 marks

Single pile versus group action

Short answer

A single pile has a small, shallow stress zone and fails individually. In a group, stress zones overlap creating a wide, deep bulb; capacity is η×nQ_u (η<1 in clay due to overlap, >1 in loose sand from compaction), settlement is larger (S_g = S√(B_g/D)), and block failure may govern.

Elaborate the behavior of single pile differing in its group actions.

The behaviour of a pile in a group differs from that of an isolated single pile because the stress zones of neighbouring piles overlap.

Point	Single pile	Pile group
Stressed zone	Small, shallow bulb	Wide and deep bulb, about the size of the group
Capacity	$Q_u$ of the pile	$\eta\,nQ_u$ , can be smaller (efficiency < 1) or larger
Settlement	Small	Larger at the same load per pile, $S_g = S\sqrt{B_g/D}$
Failure mode	Individual shear failure	Individual or block failure (soil and piles as a block)
Clay	Full adhesion	Overlap of failure zones reduces capacity at close spacing
Loose sand	–	Driving compacts sand, capacity increases ( $\eta > 1$ )
Dense sand	–	Efficiency $\le 1$

Factors: spacing (generally 2.5 to 3D), number and arrangement, type of soil, method of installation and the length of pile. Group capacity is the smaller of the sum of the capacities of individual piles and the block capacity. Settlement must be checked for the group, not for the single pile.

✓ Answer checked by Claude Sonnet 5.5

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