The grouping of piles will not reduce the load-carrying capacity in the case of:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2008
Detailed Explanation
When piles are placed close together in a group, the zones of soil they stress overlap. Whether this reduces capacity depends on how the piles carry load.
Why “End bearing piles” Is Correct
End-bearing piles resting on rock or a very hard stratum pass their load to that stratum at the tips. The hard layer can take the combined load, so group capacity equals the number of piles × single pile capacity — no reduction.
Friction piles, especially in clay, share the same surrounding soil. The overlapping stress zones mean the group fails as a block, and its capacity is usually less than the sum of individual piles (group efficiency < 1). So grouping does reduce capacity for friction piles, and the correct answer is (b).
Disputed answer: the book's printed key gives (c) ‘Both’, which contradicts its own explanation and standard soil mechanics. We give (b), which matches standard pile group theory.
| Pile type | How it works / where it is used |
|---|---|
| End-bearing (point-bearing) pile | Passes through soft soil and rests on a hard stratum; carries load at its tip like a column |
| Friction (floating) pile | No hard stratum within reach; carries load by skin friction along its length |
| Compaction pile | Driven into loose sand to densify it and raise its bearing capacity; not meant to carry load itself |
| Tension (uplift) pile | Resists uplift, e.g. under towers, basements below the water table |
| Batter pile | Driven at an angle to resist inclined or horizontal loads |
| Sheet pile | Forms a wall to retain soil or water; not for vertical load |
Why the Other Options Are Wrong
- A. Friction piles: Friction piles in clay do lose capacity in a group (efficiency < 1).
- C. Both (a) and (b): Printed in the book's key, but grouping reduces the capacity of friction piles, so ‘both’ is not correct.
- D. None of the above: End-bearing piles do not lose capacity, so (b) is correct.
Key Point
Group efficiency ≈ 1 for end-bearing piles on rock; < 1 for friction piles in clay.
Key Concepts for Students
- Group capacity of friction piles in clay = smaller of n × Q_single and block failure capacity.
- Driven piles in loose sand can show group efficiency greater than 1 because driving densifies the sand.
- Minimum pile spacing is usually 2.5 to 3 times the pile diameter.
Practice more: this is question 43 in Shallow and Deep Foundation MCQs – Part 3. Try the whole set and check every answer instantly.