Pile Foundations MCQ with answer and solution

Concept

Pile group efficiency in plastic cohesive clays

Formula

η_g = Q_ug / (n × Q_up) < 1.0

Step-by-step solution

1. Pile group efficiency is the ratio of group capacity to the sum of individual single pile capacities: η_g = Q_ug / (n × Q_up). 2. In plastic cohesive clays, installing a closely spaced pile cluster remolds the clay and causes extensive overlap of shear stress zones. 3. Block failure of the soil mass often controls over individual perimeter friction, resulting in η_g < 1.0 (typically 0.7 to 0.85). 4. In loose sand, compaction from driving increases relative density, making η_g > 1.0. Answer: C

Explanation

In cohesive plastic clays, driving or boring a pile group causes stress zone overlaps and remolding of the sensitive clay matrix. As a result, the group capacity is lower than the sum of the individual pile capacities, leading to a pile group efficiency (η_g) of less than 1.0 (100%).

Why the other options are wrong

A) Driven piles in loose sand compact the surrounding granular soil, yielding η_g > 1.0. B) Piles in dense sand maintain efficiencies near or above 1.0. D) End-bearing piles on rock transfer load directly to bedrock without stress overlap reduction.

Shortcut

Piles in loose sand: η_g > 1.0 (compaction); Piles in plastic clay: η_g < 1.0 (stress overlap).

Common mistake

Believing group efficiency is less than 1.0 in loose sand where pile driving actually densifies the deposit.