Limitations of Mohr-Coulomb theory

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.