A pile group does not carry N times a single pile's load: group action, overlapping stress zones and possible block failure mean the group capacity and (especially) the group settlement differ from single-pile values. Minimum centre-to-centre spacing limits the efficiency loss, and group settlement — typically larger than a single pile's — often governs design.
Key Requirements
•Do not take group capacity as N × single-pile capacity without checking group efficiency and block (group) failure
•Maintain minimum pile spacing (commonly ≈ 2.5–3 × pile diameter, per IS 2911 / project specification) to limit efficiency loss
•Check group settlement separately — it is generally larger than single-pile settlement and frequently governs
•Consider the equivalent-raft / block approach for group settlement in cohesive soils
•Account for negative skin friction acting on the group where applicable
Formulas
Q_group = η · N · Q_single (efficiency check) / block-failure capacity (whichever is lower)
Group capacity is the lower of the efficiency-reduced sum and the block (group) failure capacity
Q_group = pile-group capacityη = group efficiency factor (≤ 1 for closely-spaced groups in clay)N = number of pilesQ_single = single-pile capacity
Practical Notes
✓Group settlement, not group capacity, is the usual governing check for pile groups in clay — and it is the one most often skipped.
✓Closer spacing increases pile-cap economy but reduces group efficiency and increases settlement — spacing is a balance set by IS 2911 minimums.
Common Mistakes
⚠Taking group capacity as simply N × single-pile capacity.
⚠Checking group capacity but never group settlement (often the governing criterion).
⚠Spacing piles below the IS 2911 minimum to shrink the cap, losing group efficiency.