Which one of the following is not correct regarding the typical values of the coefficient of earth pressure at rest ?
- AFor dense sand: 0·35
- BFor loose sand: 0·6
- CNormally consolidated clay: 0·55
- DOverconsolidated clay: 0·25Correct
Solution
Concept
Typical K₀ values
Formula
K₀ = 1 − sin φ (NC soils); K₀(OC) = K₀(NC) √OCR
Step-by-step solution
1. Dense sand: about 0.35; loose sand: about 0.6. 2. Normally consolidated clay: about 0.5–0.6. 3. Overconsolidated clays have K₀ larger than normally consolidated ones, and K₀ can even exceed 1 for heavily overconsolidated clay. 4. So a value of 0.25 for overconsolidated clay is not correct.
Explanation
Overconsolidation locks in horizontal stress, so K₀ increases with OCR. A value of 0.25 would be lower than that of a normally consolidated clay, which is not possible.
Why the other options are wrong
Values in (a), (b) and (c) are the usual textbook values.
Shortcut
K₀ rises with OCR: K₀(OC) = K₀(NC)√OCR.
Common mistake
Assuming overconsolidated clay has lower K₀.
Correct answer: D) Overconsolidated clay: 0·25