Which one of the following statements is not correct regarding soil sampling ?
- AIn open drive sampler, the most widely used sample tube has an internal diameter of 100 mm and a length of 450 mm. The area ratio is approximately 30%.
- BIn thin-walled sampler, the internal diameter may range from 35 to 100 mm. The area ratio is approximately 100%.Correct
- CIn split-barrel sampler, the internal and external diameters are 35 and 50 mm, respectively. The area ratio is approximately 100%.
- DContinuous sampler is a highly specialized type of sampler which is capable of obtaining undisturbed samples up to 25 m in length. The sampler is used mainly in soft clays.
Solution
Concept
Area ratio of samplers
Formula
Area ratio Ar = (Do² − Di²) / Di²
Step-by-step solution
1. Thin-walled (Shelby) samplers have very thin walls, so their area ratio is about 10% (or less), not 100%. 2. Split-barrel: Di = 35 mm, Do = 50 mm: Ar = (50² − 35²)/35² = 104% ≈ 100% (disturbed sampler) ✓. 3. Open drive sampler: Ar ≈ 30% ✓.
Explanation
The area ratio measures the volume of soil displaced by the sampler walls. Undisturbed sampling needs a low value (below about 10–15%), which is achieved by thin-walled samplers. The statement giving 100% for a thin-walled sampler is therefore incorrect.
Why the other options are wrong
Statements (a), (c) and (d) are consistent with standard values.
Shortcut
Thin-walled: Ar ≤ 10%. Split spoon: ~100%.
Common mistake
Mixing up the split-barrel and thin-walled area ratios.
Correct answer: B) In thin-walled sampler, the internal diameter may range from 35 to 100 mm. The area ratio is approximately 100%.