Percolation Testing
A BRE 365 soakaway test evaluates how quickly surface water can drain into the ground, ensuring safe and effective stormwater management. Typically conducted on-site, it measures the soil’s infiltration rate by filling a test pit or trial soakaway with water and monitoring how long it takes to drain. The results help determine the size and type of soakaway or drainage system required for a property. This test is crucial for preventing flooding, waterlogging, and structural damage. BRE 365 is a recognised standard in the UK, providing reliable guidance for designing sustainable drainage solutions.
Why Undertake a Percolation Test?
Undertaking a BRE 365 soakaway test provides clear, reliable evidence of how well a site can manage surface water through natural infiltration. By measuring the ground’s true drainage performance, the test helps ensure soakaways are designed to the correct size, preventing flooding, waterlogging, and long-term structural damage.
It also supports compliance with planning and building regulations, giving engineers, developers, and homeowners confidence that their drainage solution is safe, efficient, and sustainable. Ultimately, a BRE 365 test reduces the risk of costly design errors, avoids unnecessary oversizing, and promotes environmentally responsible stormwater management tailored to real site conditions.
How to Undertake a Percolation Test
Key Steps for a BRE‑365 Soakaway Test
1. Select and excavate trial pit(s):
- Choose a location representative of where the soakaway will go (soil type, depth, proximity to building, etc.).
- Pit dimensions: typically 0.5m wide and 1.0 m long, with depth of 1.0m or to match the proposed soakaway’s invert level under the drain outlet.
- Ensure vertical sides (square sides) — if ground stability is poor, backfill with granular material and use a perforated observation pipe to measure water levels.
2. Rapidly fill the pit with clean water:
- Use a water bowser or pump rather than a slow hose, you want to fill the pit quickly to simulate real storm run‑off conditions.
3. Measure drainage (infiltration) rate of three repeated tests:
- For each test, record the time taken for water to drop from 75% full to 25% full of the effective water depth.
- Run at least three tests (often repeating in same pit, or ideally across at least two pits) to capture reliable data.
- We use the lowest infiltration rate among the tests for designing the soakaway (safe/prudent approach).
4. Document conditions and context:
- Record soil type, groundwater level, pit dimensions, water‑level vs time data and any site constraints (e.g. proximity to buildings, boundaries, water table).
- Add dated photos to support test validity.
5. We analyse and interpret results:
- We use BRE‑365 methods to convert infiltration rate into required soakaway volume/area.