Which type of rollers are most effective for the compaction of sands?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 30/10/2020 (Morning)
Detailed Explanation
Sands and gravels have no cohesion. Their grains do not stick together, but friction between them resists rearrangement under static pressure. Vibration momentarily reduces that friction, letting the grains settle into a much denser packing under their own weight and the roller’s weight.
Why “Vibratory rollers” Is Correct
Vibratory rollers have an eccentric mass inside the drum that vibrates it at about 2000–5000 cycles per minute. For maximum efficiency the vibration frequency is kept close to the natural (resonant) frequency of the soil–roller system. This makes them the most effective compaction equipment for sands, gravels and rockfill.
| Equipment | Best suited for | How it compacts |
|---|---|---|
| Smooth-wheeled (static) roller | Crushed rock, gravel; finishing surfaces; proof rolling subgrades | Static pressure |
| Pneumatic-tyred roller | Sands, gravels, silts and clayey soils (not uniformly graded soils) | Pressure + kneading |
| Sheep-foot roller | Clays and silty clays (cohesive soils) | Tamping + kneading |
| Vibratory roller | Sands and gravels (cohesionless soils) | Vibration |
| Rammers / frog hammer | Confined areas, trenches, near structures | Impact |
Why the Other Options Are Wrong
- A. Pneumatic rollers: Pneumatic rollers suit a wide range of soils but are not the most effective for clean sands.
- B. Smooth wheeled rollers: Smooth wheeled rollers apply static pressure, which densifies only a thin surface layer of loose sand.
- D. Sheep foot rollers: Sheep-foot rollers are for clays; their feet only disturb loose sand.
Key Point
Cohesionless → vibration. Cohesive → tamping and kneading (sheep-foot).
Key Concepts for Students
- Deep sand deposits are densified with vibro-flotation or dynamic compaction.
- Flooding sands with water while vibrating also helps grains slide into place.
- Compaction control in sands often uses relative density.
Practice more: this is question 2 of Compaction of Soil MCQs – Part 2. Try the whole set and check every answer instantly.