CSLB C-27 Landscaping ContractorSite Preparation and DrainageMedium
A landscape contractor is installing a retaining wall on a sloped property to create a level planting area. To ensure the long-term stability and drainage of the wall, which material is most appropriate for backfilling the area immediately behind the wall?
- AFree-draining granular material (e.g., crushed gravel)
- BSilt-rich soil from the excavation
- CNative clay soil
- DCompacted organic-rich topsoil
Show answer & explanationAnswer & explanation
Correct answer: A. Free-draining granular material (e.g., crushed gravel)
Free-draining granular material, such as crushed gravel or clean aggregate, is crucial for backfilling behind retaining walls. It allows water to pass through easily, reducing hydrostatic pressure behind the wall and preventing saturation of the soil, which can lead to instability.
Why the other options are wrong
- B. Silt-rich soil has poor drainage characteristics and would contribute to hydrostatic pressure behind the wall, similar to clay.
- C. Native clay soil would retain water, leading to hydrostatic pressure build-up behind the wall and potential failure.
- D. Compacted organic-rich topsoil would also retain water and potentially decompose, leading to settlement and instability.
Retaining Wall Backfill
The area immediately behind a retaining wall must be backfilled with free-draining granular material (e.g., crushed gravel) to prevent hydrostatic pressure buildup.
- Allows water to pass through, reducing pressure.
- Prevents soil saturation and wall instability.
- Typically includes a drainage pipe at the base.
- Should be separated from native soil by filter fabric.
Memory trick: Gravel behind the wall, so water won't stall!