The degree of compaction of a soil is characterised by its:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 11/12/2020 (Evening)
Detailed Explanation
Compaction packs soil particles closer together by expelling air. What really changes is how much soil solid sits in each cubic metre, which is exactly what dry density (dry unit weight) measures. Water content does not count, because water can be added or removed without the soil being any more compact.
Why “dry density” Is Correct
Specifications therefore state the degree of compaction as a ratio of dry densities: \( \text{Relative compaction} = \gamma_{d,\text{field}} / \gamma_{d,\max} \times 100\% \). The factors that control it are the dry unit weight achieved, the compactive effort (rammer or roller weight, number of passes) and the water content.
Why the Other Options Are Wrong
- A. consistency: Consistency (soft, firm, stiff; Atterberg limits) describes how a fine-grained soil behaves with water content, not how well it was compacted.
- B. compressibility: Compressibility is a consequence of compaction (it reduces), not the measure of it.
- D. saturated unit weight: Saturated unit weight includes water filling all voids; it is not used to express the state of compaction.
Key Point
Compaction is judged by dry density; consolidation is judged by change in void ratio over time.
Key Concepts for Students
- Relative compaction of 95% (standard Proctor) is a common embankment specification.
- For sands, relative density (density index) is also used.
- Field dry density = bulk density/(1 + w).
Practice more: this is question 1 of Compaction of Soil MCQs – Part 2. Try the whole set and check every answer instantly.