The dry density decreases in cohesionless soil with an increase in water content due to which of the following reasons?

Asked in SSC JE 10/10/2023 (Morning)

Detailed Explanation

Clean sands behave differently from clays when compacted. Completely dry sand compacts well, and so does saturated sand. In between, at small water contents (roughly 4–6%), a thin film of water forms around each grain. Surface tension in these films pulls on the grains and resists them sliding into a denser arrangement, so the sand occupies a larger volume. This swelling of moist sand is called bulking.

Why “Bulking of sand” Is Correct

Because the same mass of solids now fills a larger volume, the dry density decreases as water content increases in this low range. That is why the compaction curve of a cohesionless soil often dips at low moisture contents instead of rising smoothly like a clay curve, and why sands are often compacted either dry or flooded with water.

Why the Other Options Are Wrong

• A. Capillary rise: Capillary rise is the upward movement of water in soil pores above the water table; the surface-tension effect in bulking is related, but the volume increase itself is called bulking.
• C. Seepage: Seepage is the flow of water through soil under a head difference; it does not explain the drop in dry density during compaction.
• D. Specific gravity: Specific gravity of the solids does not change with water content.

Key Concepts for Students

• Bulking matters in concrete mix design too: volume batching of moist sand must be corrected for bulking.
• Vibratory rollers, often with flooding, are the effective field method for sands.
• For cohesionless soils, relative density (density index) is often a better compaction control than Proctor MDD.

Practice more: this is question 5 of Compaction of Soil MCQs – Part 1. Try the whole set and check every answer instantly.