---
title: "Negative skin friction on 16-pile group"
subject: "Foundation Engineering"
chapter: 8
key: "semester_5/foundation_engineering:neg-skin-friction-group-16-piles"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/neg-skin-friction-group-16-piles
site: BCT Engineering
---

# Negative skin friction on 16-pile group

Foundation Engineering · Chapter 8: Pile Foundations

- Asked in: 2075 Asoj, 8 marks
- Video: [Numerical on Negative Skin Friction in Pile Group](https://www.youtube.com/watch?v=yb4EasDGfaA) — Curious Civil Engineer
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/neg-skin-friction-group-16-piles
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-8

## Question

A circular pile group of 16 piles penetrates through an unconsolidated soil of 3.5 m depth. The diameter of circular pile is 60 cm and pile spacing of 800 cm (as printed). The average unconfined compressive strength of material is $60\ \text{kN/m}^2$ and the unit weight of soil is $16\ \text{kN/m}^3$. Compute the negative skin friction on the group. Take adhesion factor = 1.

## Short answer

Negative skin friction is the smaller of sum of individual pile NSF (3167 kN) and block NSF (1764 kN). Block failure governs, so NSF on group is 1764 kN.

## Answer

**Given:** 16 piles (taken as a 4 × 4 group), $d = 0.60$ m, through $H = 3.5$ m of unconsolidated (settling) soil, $q_u = 60$ kN/m² so $c_u = 30$ kN/m², $\gamma = 16$ kN/m³, $\alpha = 1$.

The printed spacing of "800 cm" is taken as **800 mm = 0.80 m**; a spacing of 8 m is not reasonable for a 16-pile group.

The negative skin friction (NSF) on the group is the **smaller** of (1) the sum of the NSF on individual piles and (2) the NSF on the block of soil.

#### 1. Individual piles
$$
\begin{aligned} Q_{nsf} &= \alpha c_u\pi dH = 1 \times 30 \times \pi \times 0.6 \times 3.5 = 197.9\ \text{kN} \\ 16Q_{nsf} &= 3166.7\ \text{kN} \end{aligned}
$$

#### 2. Block
$$
\begin{aligned} B_g &= 3(0.8) + 0.6 = 3.0\ \text{m},\quad P_g = 4(3.0) = 12\ \text{m} \\ Q_{ng} &= \gamma B_g^2H + P_gHc_u \\ &= 16 \times 3.0^2 \times 3.5 + 12 \times 3.5 \times 30 \\ &= 504.0 + 1260.0 = 1764.0\ \text{kN} \end{aligned}
$$

**Answer: negative skin friction on the group $\approx 1764$ kN** (the block value governs). If the spacing were really 8 m, the piles would act separately and the NSF would be $16Q_{nsf} \approx 3167$ kN.

## Check yourself

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

1. Negative skin friction on the group is the:
   A. Larger of individual and block values
   B. Smaller of sum of individual NSF and block NSF
   C. Individual value only
   D. Average of both
   Answer: B. The group takes the smaller of the two. (Key point: What governs NSF on a group)

2. NSF on one pile is:
   A. gamma B^2 H
   B. 9 c_u A_p
   C. alpha c_u pi d H = 197.9 kN
   D. c_u H
   Answer: C. 1 x 30 x pi x 0.6 x 3.5 = 197.9 kN, so 16 piles give 3167 kN. (Key point: Individual NSF)

3. The block NSF includes:
   A. Only perimeter shear
   B. Only soil weight
   C. 9 c_u B_g^2
   D. gamma B_g^2 H plus perimeter shear P_g H c_u
   Answer: D. Soil weight inside the block 504 kN plus perimeter shear 1260 kN = 1764 kN. (Key point: Block NSF)

4. Why was 800 cm read as 0.80 m?
   A. Piles were 8 m long
   B. 8 m spacing is unreasonable for a 16 pile group
   C. Cohesion needed it
   D. Alpha required it
   Answer: B. At 0.8 m the piles act as a block and the block value 1764 kN governs. (Key point: Spacing assumption)

## 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 16 piles of 50 cm, efficiency](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-16-piles-50cm) — Foundation Engineering, chapter 8
- [Pile spacing for 100% efficiency, 16 piles](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-spacing-100-efficiency-square-30cm) — Foundation Engineering, chapter 8, asked 2×
- [12 piles, spacing and diameter, 100% efficiency](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-12-circular-spacing-diameter) — Foundation Engineering, chapter 8
- [Spacing for 4×5 pile group, block failure](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-20-piles-block-failure) — Foundation Engineering, chapter 8
- [Pile group 16 piles, 0.40 m, soft clay](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-16-soft-clay-040) — Foundation Engineering, chapter 8

## Related in other subjects

- [Skin depth and power in dissipative medium](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Askin-depth-time-average-power-density) — Electromagnetism, chapter 4