Compaction MCQ with answer and solution

Concept

Optimum water content is the water content at which a soil reaches its maximum dry density for a given compactive effort.

Formula

γ_d = γ / (1 + w)

Step-by-step solution

1. In a compaction test, plot dry density against water content. 2. The curve rises, reaches a peak, and then falls. 3. The peak point gives the maximum dry density. 4. The water content at this peak is the optimum water content (OMC/OWC).

Explanation

When water is added, it lubricates the soil particles and dry density increases. After a certain water content, extra water pushes the particles apart and dry density falls. The water content at the peak is the optimum water content.

Why the other options are wrong

A: At OWC the soil is not fully saturated; some air voids remain. C: OWC is not the maximum water content; water can go higher, but density then falls. D: At OWC the dry density is maximum, not minimum.

Shortcut

OMC is the peak of the compaction curve, which means maximum dry density.

Common mistake

Thinking that soil is fully saturated at OMC. Actually, the compaction curve always lies below the zero air voids line.