Compaction behaviour and definitions in soil mechanics In compaction testing for earthwork (e.g., Proctor compaction), consider the following statements: After the optimum moisture content (OMC) is reached, any further addition of water reduces the achieved dry density. The curve joining the peak points (maximum dry densities) of several moisture–density plots obtained using different compactive energies is called the “line of optimums”. Well-graded coarse-grained soils generally attain higher maximum dry density than fine-grained soils under comparable compactive effort. Which statements are correct?

Introduction / Context:
Compaction is the mechanical densification of soil by reducing air voids. The moisture–density relationship and the influence of gradation and energy are central to field control for embankments, subgrades, and earth dams. This question checks three foundational facts used daily by site engineers and geotechnical practitioners.

Given Data / Assumptions:

• Standard laboratory Proctor-type compaction behavior.
• Comparable compactive efforts unless otherwise stated.
• Well-graded indicates a wide particle size distribution with minimal gaps.

Concept / Approach:

The moisture–density curve exhibits a peak at the optimum moisture content (OMC). To the dry side, adding water improves lubrication and enables better particle rearrangement; to the wet side, excess water replaces air and adds to the total mass without increasing solid contact, reducing dry density. Across different energies, the locus of peak points defines the line of optimums. Gradation strongly influences achievable density; well-graded coarse soils interlock efficiently and can be compacted to higher dry densities than uniform fines.

Step-by-Step Solution:

Verification / Alternative check:

Field compaction data commonly show higher MDD and lower OMC with increased energy, and a clear decrease in dry density on the wet side. Specifications reflect these trends.

Why Other Options Are Wrong:

Choosing any single statement ignores the other true facts; the combination that is fully correct is “All of the above”.

Common Pitfalls:

Confusing wet density with dry density; assuming fines always compact better than coarse soils; forgetting that the line of optimums references peak points across compactive energies.

Final Answer:

All of the above.