The 'group efficiency' of pile groups is generally less than 1.0 in:
Choose the correct answer
Dense sand deposits where piles densify surrounding soil
Clay soils — overlapping stress/strain zones reduce group capacity per pile below individual pile capacity
Only for single piles not grouped
All soils equally
Correct Answer
B. Clay soils — overlapping stress/strain zones reduce group capacity per pile below individual pile capacity
Why is this the answer?
Pile group efficiency η = Q_group / (n × Q_single). In clays (cohesive soils): group efficiency < 1 due to overlapping stress zones → group settles more and has lower capacity per pile. In sands (cohesionless): group efficiency can be > 1 (densification effect). Converse-Labarre formula: η = 1 − φ[(n_cols−1)n_rows + (n_rows−1)n_cols] / (90 × n_cols × n_rows). Block failure also checked.
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