Bulking of sand is caused due to:
Correct Answer: A. Surface moisture
📚 Detailed Explanation: Cause of Sand Bulking
Why A (Surface moisture) is correct:
Surface moisture = thin films of free water coating the outer surface of sand grains (not absorbed inside the pore). These films create surface tension (capillary tension) that acts as a repulsive force between adjacent particles, holding them slightly apart and causing the apparent volume to increase. The key word is surface moisture — not absorbed moisture (which goes inside pores and does NOT cause bulking).
Surface moisture = thin films of free water coating the outer surface of sand grains (not absorbed inside the pore). These films create surface tension (capillary tension) that acts as a repulsive force between adjacent particles, holding them slightly apart and causing the apparent volume to increase. The key word is surface moisture — not absorbed moisture (which goes inside pores and does NOT cause bulking).
Why the Other Options Are Wrong
| Option | Verdict | Reason |
|---|---|---|
| A. Surface moisture | Correct | Thin water films create surface tension → particle separation → bulking |
| B. Air voids | Wrong | Air voids are a result, not a cause, of bulking; they are between the separated particles |
| C. Viscosity | Wrong | Viscosity of water has no role in the bulking mechanism |
| D. Clay contents | Wrong | Clay increases water demand but causes slaking/swelling, not bulking of sand grains |
- Bulking is caused by surface moisture (free water on grain surfaces creating inter-particle surface tension).