Which one of the following tests cannot be done without undisturbed sampling?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2015
Detailed Explanation
Some properties depend only on what the soil is made of (grain size, plasticity); others depend on how it is arranged in the ground (structure, density, stress history). The second kind needs undisturbed samples.
Why “Shear strength of clay” Is Correct
Disturbing a clay breaks down its structure and greatly reduces its strength (especially in sensitive clays). So the in-situ shear strength of clay can only be measured on undisturbed samples (or in situ). Sand strength can be measured on samples rebuilt to the field density, and compaction and Atterberg limits use disturbed soil anyway.
| Test | Type of sample required |
|---|---|
| Natural water content | Undisturbed or SPT sample |
| Specific gravity, grain size, Atterberg limits | Representative (disturbed) is enough |
| Compaction (Proctor) | Disturbed (soil is remoulded in the test) |
| Density, permeability, consolidation | Undisturbed |
| Shear strength of clay | Undisturbed |
Why the Other Options Are Wrong
- A. Shear strength of sand: Undisturbed sand samples are almost impossible to take; sand is tested on specimens re-made at the measured field density.
- C. Compaction parameters: Compaction tests deliberately remould the soil, so disturbed samples are used.
- D. Atterberg limits: Atterberg limits are found on remoulded soil, so a disturbed sample is fine.
Key Point
Structure-dependent properties (strength of clay, consolidation, permeability, density) need undisturbed samples.
Key Concepts for Students
- Thin-walled tubes (area ratio ≤ 10–15%) are used for undisturbed clay samples.
- Sensitivity = undisturbed strength ÷ remoulded strength.
- In-situ tests (vane, SPT, CPT) avoid sampling disturbance.
Practice more: this is question 33 in Shear Strength of Soils MCQs – Part 3. Try the whole set and check every answer instantly.