CSLB C-27 Landscaping ContractorSite Preparation and DrainageHard
A landscape contractor is planning to install a dry well as part of a stormwater management system for a residential property. Before construction, a percolation test is conducted to assess the soil's suitability. For a dry well to be effective, what is the generally recommended minimum soil percolation rate?
- A1 to 2 inches per hour
- BLess than 1 inch per hour
- C3 to 4 inches per hour
- DMore than 5 inches per hour
Show answer & explanationAnswer & explanation
Correct answer: A. 1 to 2 inches per hour
Dry wells rely on the soil's ability to absorb water. A minimum percolation rate of 1 to 2 inches per hour is generally recommended for a dry well to be effective and drain within a reasonable timeframe (e.g., 24-48 hours). Rates slower than this indicate unsuitable soil.
Why the other options are wrong
- B. Less than 1 inch per hour indicates very poor drainage (heavy clay) and is unsuitable for a dry well, which would remain full for too long.
- C. 3 to 4 inches per hour indicates good drainage and would be highly suitable for a dry well.
- D. More than 5 inches per hour indicates very rapid drainage (sandy soil) which is excellent for a dry well, but not the minimum requirement.
Dry Well Soil Permeability
For a dry well to be effective, the surrounding soil must have a minimum percolation rate, typically 1-2 inches per hour, to allow for timely absorption of stormwater.
- Percolation rate measures how quickly water drains into the soil.
- Dry wells are ineffective in highly impermeable soils (heavy clay).
- Faster rates are better, but a minimum is required.
- A percolation test is essential before dry well installation.
Memory trick: One to two inches an hour, for a dry well's power.