---
title: "Single pile versus group action"
subject: "Foundation Engineering"
chapter: 8
key: "semester_5/foundation_engineering:single-vs-group-pile-behaviour"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/single-vs-group-pile-behaviour
site: BCT Engineering
---

# Single pile versus group action

Foundation Engineering · Chapter 8: Pile Foundations

- Asked in: 2075 Asoj, 4 marks
- Video: [Group Action Of Pile And Group Efficiency Of Pile In Hindi🔥🔥🔥](https://www.youtube.com/watch?v=7fiigYCgxkY) — Chaityendra Singh
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/single-vs-group-pile-behaviour
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-8

## Question

Elaborate the behavior of single pile differing in its group actions.

## Short answer

A single pile has a small, shallow stress zone and fails individually. In a group, stress zones overlap creating a wide, deep bulb; capacity is η×nQ_u (η<1 in clay due to overlap, >1 in loose sand from compaction), settlement is larger (S_g = S√(B_g/D)), and block failure may govern.

## Answer

The behaviour of a pile in a group differs from that of an isolated single pile because the stress zones of neighbouring piles overlap.

| Point | Single pile | Pile group |
|---|---|---|
| Stressed zone | Small, shallow bulb | Wide and deep bulb, about the size of the group |
| Capacity | $Q_u$ of the pile | $\eta\,nQ_u$, can be smaller (efficiency < 1) or larger |
| Settlement | Small | Larger at the same load per pile, $S_g = S\sqrt{B_g/D}$ |
| Failure mode | Individual shear failure | Individual or **block failure** (soil and piles as a block) |
| Clay | Full adhesion | Overlap of failure zones reduces capacity at close spacing |
| Loose sand | – | Driving compacts sand, capacity increases ($\eta > 1$) |
| Dense sand | – | Efficiency $\le 1$ |

**Factors:** spacing (generally 2.5 to 3D), number and arrangement, type of soil, method of installation and the length of pile. Group capacity is the smaller of the sum of the capacities of individual piles and the block capacity. Settlement must be checked for the group, not for the single pile.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does a pile in a group behave differently from an isolated pile?
   A. Group piles are always longer
   B. Stress zones of neighbouring piles overlap
   C. Group piles carry no skin friction
   D. Group piles are driven only in clay
   Answer: B. Overlapping stressed zones create a wide, deep bulb of stress for the group. (Key point: Why group differs)

2. At the same load per pile, the settlement of the group compared with a single pile is:
   A. Smaller
   B. Equal
   C. Larger, so group settlement must be checked
   D. Zero in sand
   Answer: C. The deeper stress bulb makes group settlement larger, S_g = S sqrt(B_g/D). (Key point: Group settlement)

3. Group capacity is taken as:
   A. The larger of the sum of piles and block capacity
   B. The smaller of the sum of individual capacities and block capacity
   C. Always n times Q_u
   D. Always the block capacity
   Answer: B. The group may fail individually or as a block; the smaller value governs. (Key point: Failure modes and capacity)

4. Why can efficiency exceed 1 for driven piles in loose sand?
   A. Sand has cohesion
   B. Driving compacts the sand
   C. Piles are closer than 2D
   D. Block failure never occurs
   Answer: B. Driving densifies loose sand around the piles, raising capacity (eta > 1). (Key point: Efficiency in loose sand)

## Related in Foundation Engineering

- [Negative skin friction on single pile and group](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Ansf-calculation-single-group) — Foundation Engineering, chapter 8, asked 2×
- [Pile group 4×4, two-layer clay, 2.4 m spacing](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-4x4-2-4m-layered) — Foundation Engineering, chapter 8, asked 2×
- [Pile capacity in sand, group effect, load test](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-capacity-sand-group-load-test) — Foundation Engineering, chapter 8
- [Settlement of pile group in sand](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-settlement-sand) — Foundation Engineering, chapter 8
- [Allowable load of 4×4 pile group, layered sand](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-4x4-figure-dcr) — Foundation Engineering, chapter 8
- [Design pile group for 2000 kN](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-design-2000kn) — Foundation Engineering, chapter 8

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- [Double ring versus single ring infiltrometer](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Adouble-ring-advantages) — Engineering Hydrology, chapter 3