---
title: "Limitations of Mohr-Coulomb theory"
subject: "Soil Mechanics"
chapter: 10
key: "semester_4/soil_mechanics:mohr-coulomb-limitations"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/mohr-coulomb-limitations
site: BCT Engineering
---

# Limitations of Mohr-Coulomb theory

Soil Mechanics · Chapter 10: Shear Strength of Soil

- Asked in: 2078 Poush, 2 marks
- Video: [#Limitations of #mohr's columb theory || Shear strength of soil || Soil mechanics](https://www.youtube.com/watch?v=U8n_027LIoA) — Ajaz Ali
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/mohr-coulomb-limitations
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-10

## Question

Point out the limitations of Mohr-Coulomb theory.

## Short answer

Limitations: ignores intermediate principal stress; uses a linear envelope (true envelope curved); c and φ are not constants (depend on drainage, density, stress path); does not model stress-strain behavior; neglects strain softening, anisotropy, time effects, and overestimates tensile strength.

## Answer

Limitations of the Mohr-Coulomb theory:

1. **Intermediate principal stress is ignored.** The criterion depends only on $\sigma_1$ and $\sigma_3$, but tests show $\sigma_2$ has some influence.
2. **Straight-line envelope is an approximation.** The true envelope is usually curved, especially at very low or very high stresses, so $c$ and $\phi$ are valid only for the stress range tested.
3. **$c$ and $\phi$ are not fundamental soil constants.** They depend on drainage condition, rate of loading, stress path, density, water content, and the test type.
4. **Does not model stress-strain behaviour.** It gives only the strength at failure, not the strains required to reach it.
5. **Strain softening and residual strength are not included.** Peak strength may not be available along an existing slip surface.
6. **Anisotropy, fissures and structure are neglected;** the soil is assumed isotropic and homogeneous.
7. **Time effects such as creep are not considered.**
8. **Tension is not covered properly.** The envelope extended to negative normal stress overestimates tensile strength.

## Check yourself

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

1. What does the criterion ignore?
   A. The minor principal stress
   B. The intermediate principal stress
   C. The major principal stress
   D. The cohesion
   Answer: B. It depends on only sigma1 and sigma3, though tests show sigma2 has some influence. (Key point: Intermediate stress)

2. Why are c and phi valid only for a limited range?
   A. The envelope is always exactly straight
   B. The real envelope is curved, so the straight line is an approximation over the tested stress range
   C. c and phi are fundamental constants
   D. The envelope depends only on soil colour
   Answer: B. Because of curvature, parameters obtained at one range of stress do not hold at very low or very high stresses. (Key point: Straight-line envelope)

3. Are c and phi true constants of a soil?
   A. Yes, they are fundamental constants
   B. Yes, they depend only on grain size
   C. No, they depend on drainage, loading rate, stress path, density and test type
   D. No, they depend only on temperature
   Answer: C. They vary with test and conditions, so they are not fundamental soil properties. (Key point: Soil parameters)

## Related in Soil Mechanics

- [Derive Mohr-Coulomb principal stress relation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Amohr-coulomb-derive-principal-stresses) — Soil Mechanics, chapter 10, asked 4×
- [State Mohr-Coulomb failure criterion](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Amohr-coulomb-criterion-state) — Soil Mechanics, chapter 10
- [Principal stresses; Mohr circle at failure](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aprincipal-stresses-mohr-circle-friction) — Soil Mechanics, chapter 10
- [Three triaxial tests, Mohr circles, φ](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Amohr-circle-three-triaxial-tests-friction) — Soil Mechanics, chapter 10
- [Limitations of Boussinesq and Westergaard](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Alimitations-boussinesq-westergaard) — Soil Mechanics, chapter 8, asked 5×
- [Terzaghi limitations; primary vs secondary](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aterzaghi-theory-limitations-primary-secondary) — Soil Mechanics, chapter 9

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