For field compaction of cohesive soil, the combination of tamping action and kneading action on the soil is achieved by:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 04/12/2023 (Morning)
Detailed Explanation
A sheep-foot roller is a steel drum with many projecting feet (lugs). As it rolls, each foot pushes into the loose layer and applies a high concentrated pressure (tamping), and the feet also work the soil sideways as they enter and leave it (kneading). This combination breaks down clay lumps and squeezes out air, which is exactly what cohesive soils need.
Why “sheep foot rollers” Is Correct
Clays have cohesion, so pressure alone from a smooth surface does not rearrange the particles well. The feet of a sheep-foot roller penetrate the layer and compact it from the bottom upwards; once the feet ‘walk out’ and stop sinking in, the layer is properly compacted. Typically about 8 passes are needed for good compaction.
| 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-tyred rollers: Pneumatic-tyred rollers apply pressure with some kneading through flexible tyres, but no tamping; they suit a wide range of soils rather than specifically clays.
- C. vibro flotation: Vibro-flotation is a deep densification method for loose sands below the ground, using a vibrating probe and water jets.
- D. frog rammers: Frog rammers deliver impact (tamping) in confined spaces such as trenches; they do not provide kneading.
Key Point
Clay → sheep-foot roller (tamping + kneading). Sand/gravel → vibratory roller (vibration).
Key Concepts for Students
- Sheep-foot rollers are not suitable for gravels and sands; the feet just disturb the loose grains.
- Lift thickness for sheep-foot rollers is usually kept slightly less than the length of the feet.
- Kneading action helps produce a uniform, dense layer in clays at or near OMC.
Practice more: this is question 3 of Compaction of Soil MCQs – Part 1. Try the whole set and check every answer instantly.