Soil Exploration MCQ with answer and solution

Concept

Shelby tube is a thin-walled tube sampler used to take undisturbed samples of soft cohesive soil.

Formula

Area ratio A_r = (D₂² - D₁²) / D₁² × 100

Step-by-step solution

1. A Shelby tube is a thin-walled seamless steel tube. 2. It is pushed into soft clay or silt with a steady push, not by hammering. 3. Because the wall is thin, the area ratio is low (about 10% or less), so soil disturbance is small. 4. So it is used to collect undisturbed soil samples. Answer is (A).

Explanation

A Shelby tube has a very thin wall, so it disturbs the soil very little. It is pushed into soft cohesive soil to take undisturbed samples. These samples are used for lab tests like consolidation and shear strength.

Why the other options are wrong

B: Wash boring uses water jet and chopping bit, not a Shelby tube. C: Rock drilling needs core barrels with diamond or carbide bits. D: Effective stress and the Mohr envelope are found from lab tests like triaxial, not from a sampler.

Shortcut

Thin wall + low area ratio = undisturbed sample.

Common mistake

Confusing the Shelby tube with the split spoon sampler, which gives disturbed samples in the SPT.