What is the value of coefficient of permeability for coarse gravel ?
- AGreater than 100 mm/s
- BGreater than 10 mm/sCorrect
- CGreater than 1 mm/s
- DLess than 0·1 mm/s
Solution
Concept
Permeability of soils
Step-by-step solution
1. Clean gravels have k greater than 1 cm/s, i.e. above 10 mm/s.
Explanation
Coarse gravel is very highly permeable. Casagrande's classification gives k > 1 cm/s (10 mm/s) for clean gravel.
Why the other options are wrong
Less than 0.1 mm/s is for silts and clays.
Shortcut
Gravel > 1 cm/s; sand 10⁻² to 1 cm/s; silt 10⁻⁵ to 10⁻³ cm/s; clay < 10⁻⁷ cm/s.
Common mistake
Choosing 100 mm/s.
Correct answer: B) Greater than 10 mm/s