The direct shear test suffers from the following disadvantage:
Try it first: pick an option, then read the explanation.
Correct answer: D. All of the above
Asked in SSC JE 2007
Detailed Explanation
The direct shear test is simple and quick, but it has several limitations compared with the triaxial test.
Why “All of the above” Is Correct
(a) There is no way to stop or control drainage from the box. (b) There is no arrangement to measure pore water pressure. (c) Stress on the shear plane is concentrated at the edges, so it is not uniform. In addition the failure plane is forced to be horizontal. All the listed options are disadvantages.
| Feature | Direct shear test | Triaxial test |
|---|---|---|
| Failure plane | Forced: horizontal | Free to form on the weakest plane |
| Stress distribution | Non-uniform (higher at edges) | Uniform on the failure plane |
| Drainage control | None | Full control (UU, CU, CD) |
| Pore pressure measurement | Not possible | Possible |
| Stress state | Only on the failure plane | Known at every stage (σ₁, σ₃) |
| Best for | Sands, quick drained tests | All soils |
Why the Other Options Are Wrong
- A. Drainage conditions cannot be controlled: True, but not the only disadvantage.
- B. Pore water pressure cannot be measured: True, but not the only disadvantage.
- C. Shear stress on the failure plane is not uniform: True, but not the only disadvantage.
Key Point
Direct shear: no drainage control, no pore pressure, non-uniform stress, forced failure plane.
Key Concepts for Students
- Still widely used for sands and for interface friction tests.
- Can be run slowly to give drained parameters of clays.
- The contact area reduces during shearing.
Practice more: this is question 47 in Shear Strength of Soils MCQs – Part 4. Try the whole set and check every answer instantly.