Select the correct option for the given statements. Statement 1: Consolidation is due to a reduction in the volume of air voids at a given water content. Statement 2: In compaction, volume reduction is due to expulsion of pore water from voids.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 15/11/2022 (Shift-III)
Detailed Explanation
This question tests whether you can tell compaction and consolidation apart. Read each statement carefully and compare it with the standard definitions:
- Compaction: densification by mechanical means, which reduces the volume of air voids at (almost) the same water content.
- Consolidation: gradual reduction in volume of a saturated soil under sustained load, caused by the expulsion of pore water as excess pore water pressure dissipates.
Why “Both statement 1 and statement 2 are false” Is Correct
Statement 1 gives the definition of compaction but calls it consolidation, so it is false. Statement 2 describes consolidation (expulsion of pore water) but calls it compaction, so it is also false. On the standard definitions, therefore, option (a) is correct.
Note on the official key: the SSC answer key for this paper marked option (c). That key does not match the textbook definitions used by every standard soil mechanics text. For your preparation, learn the definitions; if this exact question appears again, the definitions point to (a).
Why the Other Options Are Wrong
- B. Statement 1 is true but statement 2 is false: Statement 1 is not true: reducing air voids at a given water content is compaction, not consolidation.
- C. Both statements 1 and 2 are true but statement 2 is not the correct explanation of statement 1: Neither statement is true, so ‘both true’ cannot be correct (this is the option the official key marked, but it contradicts the definitions).
- D. Both statements 1 and 2 are true and statement 2 is the correct explanation of statement 1: Both statements are false, and in any case statement 2 does not explain statement 1.
Key Point
Swap test: if a statement says consolidation removes air or compaction removes water, it is false.
Key Concepts for Students
- Compaction: dynamic loading, air expelled, short time, all soils.
- Consolidation: static sustained loading, water expelled, long time, mainly saturated clays.
- In assertion–reason questions, first judge each statement on its own before checking whether one explains the other.
Practice more: this is question 4 in Soil Consolidation MCQs – Part 1. Try the whole set and check every answer instantly.