Pile foundations – understanding pile group efficiency The efficiency of a pile group (ratio of group capacity to the sum of individual pile capacities) depends on spacing, layout, and soil behavior. Which statement best reflects reality across soils and arrangements?

Introduction:
Pile group efficiency reflects how piles interact through the surrounding soil. Unlike isolated piles, a group may experience overlapping stress bulbs, block failure, or beneficial densification—making the overall capacity differ from the simple sum of single-pile capacities. This question assesses conceptual understanding of when efficiency can rise above or fall below 100%.

Given Data / Assumptions:

• Group of similar vertical piles carrying axial load.
• Soil can be cohesive (clays) or cohesionless (sands), possibly layered.
• Typical group spacings and cap configurations.

Concept / Approach:

If stress zones overlap significantly (tight spacing), group behaves like a block whose capacity per pile is reduced—efficiency < 100%. Conversely, in certain cohesionless soils, driving can densify sand around the group, increasing shaft resistance and sometimes raising efficiency above 100%. Overall, efficiency is not fixed; it depends on geometry, installation method, and soil response.

Step-by-Step Solution:

Verification / Alternative check:

Group tests and design methods (e.g., block capacity checks in clay, empirical spacing factors in sand) show that efficiencies can range on either side of 100%, not strictly below or above.

Why Other Options Are Wrong:

Always less/greater than 100% ignores observed variability. The soil-specific rule in Option D is oversimplified and not universally true. Exactly 100% (Option E) holds only under special conditions (large spacing, minimal interaction).

Common Pitfalls:

Assuming linear additivity of pile capacities; ignoring installation effects; neglecting block failure checks in soft clays.

Final Answer:

may be less than 100% or more than 100%