Clayey soils are best compacted by:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2015
Detailed Explanation
Clays are cohesive and plastic. Their particles stick together and form lumps, so smooth static pressure only compresses the surface and vibration has little effect.
Why “Sheep foot rollers” Is Correct
The feet of a sheep-foot roller penetrate the loose clay layer, apply high concentrated pressure and knead the soil, breaking up clods and expelling air. As the layer densifies, the feet ‘walk out’ of the soil. This makes the sheep-foot roller the best choice for clayey soils; it is not used for gravel and sand.
| 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
- B. Vibratory rollers: Vibratory rollers are best for sands and gravels, not cohesive clays.
- C. Heavy drum rollers: Heavy drum (smooth) rollers compact only the upper part of a clay layer and seal its surface.
- D. Ramming and pneumatic tampering: Rammers are used for small, confined areas, not for efficient compaction of large clay fills.
Key Point
Clay → sheep-foot; sand → vibratory; finishing → smooth-wheeled; mixed soils → pneumatic.
Key Concepts for Students
- Clay layers are compacted near OMC (often slightly wet of optimum for impervious cores).
- Padfoot (tamping-foot) rollers are a modern version of the sheep-foot roller.
- Number of passes is fixed by trial compaction on site.
Practice more: this is question 10 of Compaction of Soil MCQs – Part 2. Try the whole set and check every answer instantly.