Which of the following properties should the subgrade soil have in order to be used as highway material?
- AStability, incompressibility, good drainageCorrect
- BLarge changes in volume under adverse conditions of weather, stability
- CLarge changes in volume under adverse conditions of weather, compressibility, good drainage
- DEase of compaction, compressibility, stability
Solution
Concept
Good subgrade soil must be stable, incompressible, well drained, easy to compact, and show little volume change with weather.
Step-by-step solution
1. List the desirable properties of subgrade soil: stability, incompressibility, permanency of strength, minimum volume change, good drainage, ease of compaction. 2. Check each option against this list. 3. Options B and C have large volume change, which is bad. 4. Option D has compressibility, which is bad. 5. Only option A has all good properties. Answer is A.
Explanation
Subgrade soil supports the pavement, so it must stay stable and must not compress under traffic load. Good drainage keeps water out and keeps the strength of soil. So stability, incompressibility and good drainage are the correct properties.
Why the other options are wrong
B: Large volume change with weather causes cracks and failure of pavement. C: Large volume change and compressibility are both undesirable. D: Compressibility is undesirable because it causes settlement of pavement.
Shortcut
Reject any option that has 'large volume change' or 'compressibility'.
Common mistake
Choosing D because 'ease of compaction' is a good property, while missing that 'compressibility' in the same option is bad.
Correct answer: A) Stability, incompressibility, good drainage