The minimum water content at which the soil just begins to crumble when rolled into threads 3 mm in diameter, is known as
- ALiquid limit
- BPlastic limitCorrect
- CShrinkage limit
- DPermeability limit
Solution
Concept
Plastic limit is the lowest water content at which soil can still be rolled into 3 mm threads without crumbling.
Formula
IP = wL - wP
Step-by-step solution
1. Roll the soil into threads on a glass plate. 2. Find the water content at which the thread just starts to crumble at 3 mm diameter. 3. This water content is the plastic limit (wP).
Explanation
The plastic limit is the boundary between the plastic state and the semi-solid state. At this water content, a 3 mm thread just begins to crumble.
Why the other options are wrong
A: Liquid limit is found with the Casagrande cup or cone penetrometer, not 3 mm threads. C: Shrinkage limit is the water content below which volume stops decreasing. D: There is no Atterberg limit called 'permeability limit'.
Shortcut
3 mm thread crumbles = plastic limit.
Common mistake
Confusing the plastic limit with the shrinkage limit.
Correct answer: B) Plastic limit