When compaction effort is increased in the standard compaction test, the optimum moisture content:
Try it first: pick an option, then read the explanation.
Correct answer: A. Decreases
Asked in SSC JE 2008
Detailed Explanation
If the same soil is compacted with more energy (heavier rammer, greater drop, more layers or blows), the particles can be pushed closer together with less water needed for lubrication.
Why “Decreases” Is Correct
The compaction curve moves upwards and to the left: the maximum dry density increases and the optimum moisture content decreases. Both peaks still lie below the zero-air-voids line. So the OMC decreases with increased effort.
Why the Other Options Are Wrong
- B. Increases: The OMC moves to the left (lower water content), not to the right.
- C. Does not change: The OMC depends on the compactive effort, so it does change.
- D. Is unpredictable: The trend is consistent and well established: higher effort, lower OMC.
Key Point
More effort → MDD ↑, OMC ↓. The line joining the peaks of curves for different efforts is roughly parallel to the zero-air-voids line.
Key Concepts for Students
- Heavy (modified) Proctor gives a higher MDD and a lower OMC than light Proctor for the same soil.
- Beyond a point, extra effort gives diminishing gains in dry density.
- In the field, more roller passes or heavier rollers act like more compactive effort.
Practice more: this is question 8 of Compaction of Soil MCQs – Part 2. Try the whole set and check every answer instantly.