Undisturbed samples are obtained by:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2010
Detailed Explanation
An undisturbed sample keeps the natural structure, density and water content of the soil. It is needed for tests on shear strength, consolidation and permeability. Disturbance comes mainly from the volume of soil pushed aside by the sampler walls.
Why “Thin walled samplers” Is Correct
Disturbance is measured by the area ratio, Ar = (Do² − Di²)/Di² × 100. A thin-walled sampler keeps this ratio low (10% or less), so the soil entering the tube is hardly disturbed. That is why undisturbed samples are taken with thin-walled tubes, pushed in smoothly rather than hammered.
| Sampler | Area ratio | Sample quality |
|---|---|---|
| Thin-walled tube (Shelby) | ≤ 10% | Undisturbed (soft to stiff clays) |
| Split spoon (SPT) | ≈ 110% | Disturbed (for classification) |
| Piston sampler | Low | Undisturbed (very soft clays, loose sands) |
Why the Other Options Are Wrong
- A. Direct excavations: Excavation only gives block samples carefully cut by hand; by itself it does not give undisturbed samples.
- C. Thick walled samplers: Thick walls push aside a lot of soil and give disturbed samples (like the split spoon).
- D. None of the above: Thin-walled samplers are the correct answer.
Key Point
Undisturbed sampling: thin-walled tubes with area ratio ≤ 10%.
Key Concepts for Students
- Inside clearance (1–3%) reduces friction on the sample inside the tube.
- Outside clearance helps the tube withdraw easily.
- Hand-cut block samples from pits are the best quality undisturbed samples.
Practice more: this is question 5 in Soil Exploration MCQs – Part 1. Try the whole set and check every answer instantly.