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

# Negative skin friction on single pile and group

Foundation Engineering · Chapter 8: Pile Foundations

- Asked in: 2078 Kartik, 3 marks; 2076 Asoj, 3 marks (asked 2 times)
- 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/nsf-calculation-single-group
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-8

## Question

How is negative skin friction calculated for a single pile and a group of piles in clay?

## Short answer

Single pile: Q_NSF = perimeter × length × α c_u. For a group: equal to the smaller of total individual drag or the weight of the settling block plus its perimeter shear: Q_ng = min( n Q_nsf, γ_f H_f B_g L_g + c_u P_g L_n).

## Answer

#### Single pile
The negative skin friction acts over the length $L_n$ of the pile that lies in the settling clay (down to the neutral point).

Unit negative skin friction: $f_n = \alpha c_u$ (total stress method, $\alpha \approx 1$ for soft clay), or $f_n = \beta\sigma'_v$ (effective stress method, Bjerrum and Burland, $\beta = 0.2$ to $0.25$ for clay).

$$
Q_{nsf} = P \times L_n \times f_n = \pi D\,L_n\,c_u \quad (\alpha = 1)
$$

where $P$ is the perimeter of the pile and $D$ the diameter.

#### Group of piles
A group in clay moves with the soil mass between the piles, so the drag is the **lesser** of:

1. The sum of the drag on all piles, $Q_{ng} = n \times Q_{nsf}$.
2. The weight of the fill carried by the group, together with the shear along the perimeter of the block of settling clay:

$$
Q_{ng} = \gamma_f H_f B_g L_g + c_u\,P_g\,L_n
$$

where $B_g \times L_g$ is the plan of the group, $P_g = 2(B_g + L_g)$ its perimeter, $H_f$ and $\gamma_f$ the thickness and unit weight of the fill (surcharge) that causes the settlement, $c_u$ the undrained cohesion of the clay and $L_n$ the thickness of the settling clay along the piles.

The calculated drag is added to the structural load when checking the pile capacity, since it acts in the same direction.

## Check yourself

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

1. For a single pile in clay with alpha = 1, Qnsf =
   A. pi D Ln cu
   B. pi D^2/4 x cu
   C. 9 cu Ap
   D. gamma D Ln
   Answer: A. Perimeter x length in settling clay x unit drag (cu). (Key point: Single pile)

2. For a pile group the design drag is:
   A. The greater of the two values
   B. The lesser of n x Qnsf and the block value
   C. Always n x Qnsf
   D. Always block value
   Answer: B. The group drag cannot exceed the weight and perimeter shear of the enclosed soil block. (Key point: Group rule)

3. The block drag expression is:
   A. gamma Bg Lg Hf + cu Pg Hf
   B. cu Bg Lg
   C. n pi D cu
   D. gamma Hf
   Answer: A. Weight of settling soil within the group plus shear on the block perimeter. (Key point: Block drag)

4. How is the calculated drag used?
   A. Subtracted from structural load
   B. Added to the structural load when checking pile capacity
   C. Ignored
   D. Used to reduce FS to 1
   Answer: B. It acts in the same direction as the load. (Key point: Use of drag)

## Related in Foundation Engineering

- [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
- [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
- [Negative skin friction: causes, effects, remedies](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Anegative-skin-friction-effects) — Foundation Engineering, chapter 8, asked 4×
- [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 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×
- [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