CSLB C-27 Landscaping ContractorSite Preparation and DrainageMedium
A landscape contractor is excavating for a new retaining wall. The backfill material behind the wall needs to be free-draining to prevent hydrostatic pressure buildup. Which type of material is best suited for this purpose?
- ACrushed gravel or clean sand
- BNative clay soil
- CSilty loam
- DOrganic topsoil
Show answer & explanationAnswer & explanation
Correct answer: A. Crushed gravel or clean sand
Crushed gravel or clean sand are ideal free-draining materials for backfilling behind a retaining wall. Their coarse particle size allows water to pass through easily, preventing the accumulation of hydrostatic pressure that could otherwise cause the wall to fail.
Why the other options are wrong
- B. Native clay soil is not free-draining; it retains water and would contribute to hydrostatic pressure.
- C. Silty loam has moderate drainage but is not considered free-draining enough to prevent hydrostatic pressure buildup behind a retaining wall.
- D. Organic topsoil retains water and would not be suitable for free-draining backfill behind a retaining wall.
Retaining Wall Backfill
The material placed behind a retaining wall to support the soil mass and allow for proper drainage.
- Must be free-draining.
- Prevents hydrostatic pressure buildup.
- Commonly crushed gravel, clean sand, or drain rock.
- Typically separated from native soil by filter fabric.
Memory trick: Gravel Guards Walls, Guiding Groundwater Gracefully.