Limitations of direct shear test

Limitations of the direct shear test:

1. Drainage cannot be controlled. Pore pressure cannot be measured, so the test cannot give true undrained or effective stress parameters for clays.
2. Failure plane is fixed (horizontal). The soil may not fail along its weakest plane.
3. Non-uniform stress and strain distribution over the failure plane, with stress concentration at the edges and progressive failure.
4. Area of contact decreases as the upper box moves, so the shear stress is not exactly $T/A$ and a correction is needed.
5. Principal planes rotate during shear and the major principal stress direction is not known; a full Mohr circle cannot be drawn.
6. Small specimen size limits use for coarse-grained soil and gives a small, non-representative sample.
7. Side friction between the box and the soil, and the effect of the rigid boundaries, introduce errors.
8. Difficult to apply other stress paths, and the volume change is hard to measure accurately.