Over-excavation is performed when soils are unstable or contaminated.

Over-excavation is performed when soils are unstable or contaminated.

• A. Remove topsoil only
• B. Excavating below subgrade to remove unsuitable soils to provide a stable foundation
• C. Excavate only for drainage trenches
• D. Compact the surface layers

Answer: (B)

Over-excavation is the process of removing soils deeper than the planned subgrade where instability or contamination exists, so the foundation sits on a stable, clean layer. By digging down to remove unsuitable material and then backfilling with engineered fill that is properly compacted, you achieve reliable bearing capacity and reduce the risk of excessive or differential settlement. The goal is to eliminate problematic soils rather than just treating the surface. Removing only the topsoil won’t reach the unstable or contaminated layers, so it doesn’t guarantee a stable foundation. Excavating only for drainage trenches targets a different purpose and doesn’t address foundation stability. Compacting the surface layers alone doesn’t remove the deeper troublesome soils either.

Over-excavation is the process of removing soils deeper than the planned subgrade where instability or contamination exists, so the foundation sits on a stable, clean layer. By digging down to remove unsuitable material and then backfilling with engineered fill that is properly compacted, you achieve reliable bearing capacity and reduce the risk of excessive or differential settlement. The goal is to eliminate problematic soils rather than just treating the surface.

Removing only the topsoil won’t reach the unstable or contaminated layers, so it doesn’t guarantee a stable foundation. Excavating only for drainage trenches targets a different purpose and doesn’t address foundation stability. Compacting the surface layers alone doesn’t remove the deeper troublesome soils either.