CSLB C-27 Landscaping ContractorSite Preparation and DrainageMedium
A C-27 contractor is designing a French drain system for a sloped backyard with persistent surface water issues. The trench will be 18 inches wide and 24 inches deep. What is the minimum recommended depth of gravel that should be placed above the perforated pipe to ensure effective filtration and prevent soil infiltration?
- A8 inches
- B12 inches
- C4 inches
- D6 inches
Show answer & explanationAnswer & explanation
Correct answer: B. 12 inches
For effective filtration and to prevent soil from clogging the perforated pipe, a minimum of 12 inches of gravel (or 6 inches above and 6 inches below) is generally recommended above the pipe in a French drain system.
Why the other options are wrong
- A. 8 inches is better than 6, but 12 inches is typically the recommended minimum for robust performance.
- C. 4 inches of gravel above the pipe is insufficient for proper filtration and can lead to premature clogging.
- D. 6 inches of gravel above the pipe is often considered a minimum, but 12 inches provides better longevity and filtration.
French Drain Gravel Depth
The recommended thickness of gravel placed above the perforated pipe in a French drain to ensure proper filtration and prevent clogging.
- Minimum 12 inches of gravel above pipe is often recommended.
- Gravel should be clean, washed, and angular.
- Geotextile fabric is crucial to separate gravel from soil.
Memory trick: The DRAIN needs a deep GRAVEL bed to work.