Which statement best describes subbase compaction for different soils?

Which statement best describes subbase compaction for different soils?

• A. All subbases must be compacted with vibratory compactors
• B. Granular subbases and sandy soils can be compacted with vibratory compactors; clay or silty soils require kneading action of a sheepsfoot roller
• C. Clay soils should be compacted with vibratory compactors
• D. Subbase compaction is not necessary for slabs on grade

Answer: (B)

Achieving the right density in the subbase depends on how the soil particles interact. Granular and sandy subbases are non-cohesive, so vibration lets the particles rearrange and pack more tightly, making vibratory compactors effective when moisture is near the optimum for maximum density. In contrast, clay and silty soils are cohesive; vibration can cause pumping and surface flushing rather than uniform compaction. The kneading action of a sheepsfoot roller gently presses and shears the soil, breaking bonds and reorganizing the particles to reach higher density without creating surface pumping. So, granular and sandy subbases respond well to vibratory compaction, while clay or silty soils require kneading action for proper compaction. Subbase compaction is essential under slabs on grade to provide a stable, even support and prevent settlement.

Achieving the right density in the subbase depends on how the soil particles interact. Granular and sandy subbases are non-cohesive, so vibration lets the particles rearrange and pack more tightly, making vibratory compactors effective when moisture is near the optimum for maximum density. In contrast, clay and silty soils are cohesive; vibration can cause pumping and surface flushing rather than uniform compaction. The kneading action of a sheepsfoot roller gently presses and shears the soil, breaking bonds and reorganizing the particles to reach higher density without creating surface pumping. So, granular and sandy subbases respond well to vibratory compaction, while clay or silty soils require kneading action for proper compaction. Subbase compaction is essential under slabs on grade to provide a stable, even support and prevent settlement.