---
title: "Design pile group for 2000 kN"
subject: "Foundation Engineering"
chapter: 8
key: "semester_5/foundation_engineering:pile-group-design-2000kn"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/pile-group-design-2000kn
site: BCT Engineering
---

# Design pile group for 2000 kN

Foundation Engineering · Chapter 8: Pile Foundations

- Asked in: 2080 Baisakh, 6 marks
- Video: [Design of Group Pile Foundation Part 1 of 4](https://www.youtube.com/watch?v=9SPToXwBqow) — MathiBae
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/pile-group-design-2000kn
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-8

## Question

Design a pile group driven in a 16 m deep consolidated clay having unit cohesion of $30\ \text{kN/m}^2$ and unit weight of $22\ \text{kN/m}^3$ to carry the superstructure load of 2000 kN. Take adhesion factor as 0.6 and factor of safety of 3.

## Short answer

Using 0.40 m diameter,12 m long piles, single pile capacity = 305.4 kN.20 piles in5×4 arrangement at1.2 m spacing give group allowable load2036 kN (>2000 kN).

## Answer

**Given:** consolidated clay 16 m deep, $c_u = 30$ kN/m², $\gamma = 22$ kN/m³, load $Q = 2000$ kN, $\alpha = 0.6$, FS $= 3$.

**Trial pile:** diameter $D = 0.40$ m, length $L = 12$ m (the pile stays in the clay, with 4 m of clay below the tip).

#### Capacity of one pile
$$
\begin{aligned} Q_s &= \alpha c_u\pi DL = 0.6 \times 30 \times \pi(0.4)(12) = 271.4\ \text{kN} \\ Q_p &= 9c_uA_p = 9 \times 30 \times 0.1257 = 33.9\ \text{kN} \\ Q_u &= 305.4\ \text{kN} \end{aligned}
$$

#### Number of piles
$$
n = \frac{FS \times Q}{Q_u} = \frac{3 \times 2000}{305.4} = 19.6 \ \Rightarrow\ 20\ \text{piles}
$$

Arrange them in a 5 × 4 pattern at spacing $s = 3D = 1.2$ m.

#### Group check
- Sum of individual piles: $20 \times 305.4 = 6107$ kN.
- Block: $B_g = 3(1.2) + 0.4 = 4.0$ m, $L_g = 4(1.2) + 0.4 = 5.2$ m.
$$
Q_{block} = 2(B_g + L_g)Lc_u + 9c_uB_gL_g = 2(9.2)(12)(30) + 9(30)(4.0)(5.2) = 6624 + 5616 = 12\,240\ \text{kN}
$$
- Group capacity $=$ the smaller $= 6107$ kN, so $\eta = 1$.
- Allowable group load $= 6107/3 = 2036$ kN $> 2000$ kN. O.K.

**Answer: provide 20 piles of 0.40 m diameter and 12 m length in a 5 × 4 group at 1.2 m c/c** (allowable load 2036 kN > 2000 kN), joined by a pile cap of about 5.2 m × 4.0 m plus edge distance. Check the settlement of the group.

## Check yourself

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

1. The ultimate capacity of one 0.4 m, 12 m pile is made of:
   A. Only shaft adhesion
   B. Shaft adhesion 271.4 kN plus end bearing 33.9 kN
   C. Only end bearing
   D. Shaft 33.9 kN plus tip 271.4 kN
   Answer: B. Q_s = alpha c_u pi D L = 271.4 kN and Q_p = 9 c_u A_p = 33.9 kN. (Key point: Capacity of one pile)

2. Number of piles is found from:
   A. n = Q / (FS x Q_u)
   B. n = FS x Q / Q_u, rounded up
   C. n = Q_u / Q
   D. n = Q / Q_u
   Answer: B. n = 3 x 2000 / 305.4 = 19.6, so 20 piles. (Key point: Number of piles)

3. Block capacity (12240 kN) exceeds the sum of piles (6107 kN), so:
   A. Block failure governs
   B. Group capacity is 6107 kN and eta = 1
   C. Group capacity is 12240 kN
   D. The design fails
   Answer: B. The smaller value governs, so individual pile failure controls and eta = 1. (Key point: Group check)

4. The design is acceptable because:
   A. Allowable 6107 kN < 2000 kN
   B. Allowable 2036 kN > 2000 kN
   C. The spacing is 1D
   D. The pile is longer than the clay
   Answer: B. 6107/3 = 2036 kN exceeds the 2000 kN load; settlement should still be checked. (Key point: Final check)

## Related in Foundation Engineering

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- [Steel pipe pile group 3×4, layered clay](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-steel-pipe-layered-clay) — Foundation Engineering, chapter 8
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- [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×
- [Negative skin friction on 16-pile group](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aneg-skin-friction-group-16-piles) — Foundation Engineering, chapter 8
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