During the compaction test, the weight of compacted soil specimen along with mould is 38·2 N. The volume and weight of mould are 0·95 × 10⁻³ m³ and 20·5 N respectively and the water content is 12%. The dry unit weight of the compacted specimen will be nearly
- A16·6 kN/m³Correct
- B14·4 kN/m³
- C12·6 kN/m³
- D10·4 kN/m³
Solution
Concept
Bulk and dry unit weight
Formula
γd = γb / (1 + w)
Step-by-step solution
1. Weight of soil = 38.2 − 20.5 = 17.7 N. 2. γb = 17.7 / 0.95×10⁻³ = 18 632 N/m³ = 18.63 kN/m³. 3. γd = 18.63 / 1.12 = 16.64 kN/m³.
Explanation
The weight of the soil alone is the difference between the weight with the mould and the mould weight. The bulk unit weight is then converted into the dry unit weight.
Why the other options are wrong
The other values do not follow from 17.7/0.95 divided by 1.12.
Shortcut
γd = γb/(1 + w).
Common mistake
Using the total weight 38.2 N as the soil weight.
Correct answer: A) 16·6 kN/m³