Compaction MCQ with answer and solution

Concept

In sand, a small amount of water causes bulking, so dry density first decreases as water content increases.

Formula

γ_d = γ / (1 + w)

Step-by-step solution

1. In a compaction test on sand, dry density is high when the sand is fully dry. 2. When a little water is added, thin water films form around the grains. 3. Surface tension (capillary forces) in these films holds the grains apart. 4. This is called bulking of sand. The volume increases, so dry density decreases. 5. With more water (near saturation), bulking is lost and dry density increases again. 6. So the reason is bulking of sand, option D.

Explanation

Cohesionless soils show a decrease in dry density at low water content. Water films and surface tension keep the grains apart, so the sand bulks. Because of bulking, the same mass takes more volume, and the dry density falls.

Why the other options are wrong

A: specific gravity of solids does not change with water content. B: seepage is the flow of water through soil, not the cause of low density during compaction. C: capillary tension is the inner cause, but the standard name for this effect is bulking of sand.

Shortcut

Sand plus a little water gives bulking, which lowers dry density.

Common mistake

Choosing 'capillary rise'. Capillary rise means water rising in the soil above the water table. The effect here is bulking caused by surface tension.