Which of the following statement(s) is/are true regarding constant head permeability test? (i) It is a laboratory method used to determine the coefficient of permeability of clayey soils. (ii) It uses Darcy’s Law to calculate hydraulic conductivity under a steady-state flow, where a constant hydraulic gradient is maintained across a sample length and area
- AOnly (i)
- BOnly (ii)Correct
- CBoth (i) and (ii)
- DNeither (i) nor (ii)
Solution
Concept
Constant head permeability test principles and soil suitability
Formula
k = (Q × L) / (A × h × t)
Step-by-step solution
1. Statement (i) is incorrect: The constant head permeability test is used for coarse-grained soils (gravels and sands) with high permeability (k > 10-3 cm/s); fine-grained clays require the falling head test. 2. Statement (ii) is correct: It applies Darcy’s law under steady flow with a constant hydraulic head difference maintained across the specimen. 3. Hydraulic conductivity is calculated as k = (Q × L) / (A × h × t). 4. Therefore, only statement (ii) is true. Answer: B
Explanation
The constant head permeability test is specifically suited for coarse-grained soils with high permeability (sands and gravels), not clayey soils (for which falling head test is used). It maintains a constant hydraulic gradient across the soil sample and applies Darcy’s law under steady-state flow conditions. Statement (i) is false and (ii) is true.
Why the other options are wrong
A) Incorrect because statement (i) erroneously specifies clayey soils. C) Incorrect because statement (i) is false. D) Incorrect because statement (ii) correctly describes the steady-state Darcy formulation.
Shortcut
Constant head = Coarse sands/gravels; Falling head = Fine clays/silts.
Common mistake
Assuming constant head permeameters are suitable for testing low-permeability clays.
Correct answer: B) Only (ii)