Which of the following laboratory methods is more suitable to determine the permeability of less permeable soils?
- AUnconfined flow pumping test
- BConstant head test
- CFalling head testCorrect
- DConfined flow pumping test
Solution
Concept
The falling head test is suitable for less permeable soils (fine sand, silt, clay). The constant head test is suitable for highly permeable coarse soils.
Formula
k = 2.303 × (a L / (A t)) × log10(h1 / h2)
Step-by-step solution
1. Laboratory permeability tests are the constant head test and the falling head test. 2. Pumping tests (confined or unconfined) are field tests, not laboratory tests. 3. In soils of low permeability, the flow is very small, so the constant head test cannot measure the discharge accurately. 4. The falling head test measures the fall of water in a thin standpipe, which works well for low flow. 5. So the answer is the falling head test, option (C).
Explanation
In less permeable soils very little water flows, so collecting discharge in a constant head test is not accurate. The falling head test uses a small standpipe and measures the drop in head with time, which is suitable for fine soils like silt and clay.
Why the other options are wrong
A and D: pumping tests are field methods, not laboratory methods. B: the constant head test suits coarse, highly permeable soils like sand and gravel.
Shortcut
Low k: falling head. High k: constant head. Pumping tests: field.
Common mistake
Choosing the pumping test without noticing that the question asks for a laboratory method.
Correct answer: C) Falling head test