---
title: "Skempton's findings on clay bearing capacity"
subject: "Foundation Engineering"
chapter: 6
key: "semester_5/foundation_engineering:skempton-net-safe-bc"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/skempton-net-safe-bc
site: BCT Engineering
---

# Skempton's findings on clay bearing capacity

Foundation Engineering · Chapter 6: Bearing Capacity and Settlement of Shallow Foundations

- Asked in: 2078 Kartik, 2 marks
- Video: [Skempton Bearing Capacity Theory Explained | Clay Soil (ϕ=0) | Foundation Engineering](https://www.youtube.com/watch?v=_ojftqgIwIA) — Digitech education
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/skempton-net-safe-bc
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-6

## Question

Discuss the findings of Skempton on clayey soil regarding net safe bearing capacity.

## Short answer

Skempton showed that for saturated clay (φu=0), Nc increases with Df/B. For strip footings, Nc = 5(1+0.2 Df/B) ≤ 7.5; for square/circular, Nc = 6(1+0.2 Df/B) ≤ 9. Net ultimate capacity is c Nc, and net safe bearing capacity is c Nc / F, with F=3 and c = qu/2.

## Answer

Skempton (1951) studied the bearing capacity of saturated clay ($\phi_u = 0$) and showed that the bearing capacity factor $N_c$ is not constant. It increases with depth of foundation relative to width.

- For a **strip footing**: $N_c = 5\left(1 + 0.2\dfrac{D_f}{B}\right)$ for $D_f/B \le 2.5$, and $N_c = 7.5$ for larger depths.
- For a **square or circular footing**: $N_c = 6\left(1 + 0.2\dfrac{D_f}{B}\right)$ up to $D_f/B = 2.5$, and $N_c = 9$ beyond that.
- For a **rectangular footing**: $N_c(\text{rect}) = \left(0.84 + 0.16\dfrac{B}{L}\right)N_c(\text{square})$.

The **net ultimate** capacity is $q_{nu} = cN_c$ and the **net safe bearing capacity** is

$$
q_{ns} = \frac{q_{nu}}{F} = \frac{cN_c}{F}
$$

with $F$ usually 3, where $c = q_u/2$ is the undrained cohesion. The gross safe pressure is $q_{ns} + \gamma D_f$.

## Check yourself

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

1. What did Skempton show about Nc for saturated clay (phi_u = 0)?
   A. It is constant at 5.7
   B. It decreases with depth
   C. It increases with Df/B
   D. It depends on phi
   Answer: C. Nc rises with relative depth until a limiting value. (Key point: Nc variation)

2. What is the limiting Nc for a strip footing at large Df/B (> 2.5)?
   A. 5
   B. 7.5
   C. 9
   D. 5.7
   Answer: B. Strip Nc = 5(1+0.2Df/B) up to 2.5, then 7.5. (Key point: Strip Nc)

3. What is the net safe bearing capacity?
   A. (cNc + gamma*Df)/F
   B. cNc*F
   C. qu / (F*gamma*Df)
   D. cNc / F
   Answer: D. Net ultimate qnu = cNc and qns = qnu/F, with F usually 3. (Key point: Net safe capacity)

## Related in Foundation Engineering

- [Mat 8 m × 10 m, Skempton net safe pressure](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Amat-8x10-skempton) — Foundation Engineering, chapter 7
- [Factors affecting bearing capacity; water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abearing-capacity-factors-water-table) — Foundation Engineering, chapter 6, asked 5×
- [Circular footing on stiff clay, water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Acircular-footing-stiff-clay) — Foundation Engineering, chapter 6
- [Mat bearing capacity: cohesive vs cohesionless](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Amat-bc-cohesive-vs-cohesionless) — Foundation Engineering, chapter 7, asked 2×
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- [Development of classical bearing capacity theories](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Aclassical-bearing-theories-development) — Foundation Engineering, chapter 6, asked 2×

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