CSLB C-27 Landscaping ContractorSite Preparation and DrainageEasy
A landscape contractor is installing a subsurface drainage system using perforated pipe. To prevent soil particles from entering and clogging the pipe, which material should be placed directly around the pipe before backfilling?
- AGeotextile fabric
- BFine sand
- CNative clay soil
- DCoarse gravel or drain rock
Show answer & explanationAnswer & explanation
Correct answer: D. Coarse gravel or drain rock
Coarse gravel or drain rock (typically 3/4 inch to 1 1/2 inch) creates a permeable layer around the perforated pipe, allowing water to enter while filtering out larger soil particles and preventing the pipe from becoming clogged. This is a fundamental component of effective subsurface drainage.
Why the other options are wrong
- A. While geotextile fabric can be used as a filter wrap around the drain rock or pipe in certain situations, coarse gravel is the primary and most direct material placed around the pipe for immediate filtration and water flow.
- B. Fine sand can still migrate into pipe perforations over time, leading to clogging, and does not provide as much void space for water movement as gravel.
- C. Native clay soil would impede water flow into the pipe and eventually clog the perforations due to its fine particle size and low permeability.
Perforated Drain Pipe Backfill
The specific material used directly around a perforated drainage pipe to facilitate water entry while preventing soil infiltration and clogging.
- Must be highly permeable.
- Filters out fine soil particles.
- Typically coarse aggregate (gravel/drain rock).
Memory trick: Pipe in rock, water's clock, keeps the flow, from soil's slow.