Soakaway Testing BRE 365 Infiltration Tests & Design
LLFA asking you to prove infiltration before they’ll accept your drainage? We arrange and witness BRE 365 soakaway tests, calculate the infiltration rate, and design compliant infiltration SuDS — or the attenuation fallback if infiltration fails.
BRE 365 Soakaway & Infiltration Testing
Infiltration sits at the top of the SuDS drainage hierarchy, and most LLFAs now require it to be properly tested — not assumed — before they will accept any drainage strategy. The recognised method is BRE Digest 365, which determines the soil infiltration rate from a series of full fill-and-empty trial pit tests measured over the effective storage depth.
We arrange and witness the BRE 365 testing, calculate the design infiltration rate from the worst-performing cycle, and use it to design soakaways, permeable paving or infiltration basins sized for events up to the 1 in 100-year plus climate change. Where infiltration is not viable — impermeable clay, Gault mudstone, or a high groundwater table — we evidence that clearly and move down the hierarchy to an attenuation-based strategy with a restricted discharge.
What’s Included
- ✓ BRE 365 testing arranged & witnessed — trial pits excavated and three full fill-and-empty cycles recorded per location
- ✓ Infiltration rate (f value) calculated — derived over the 75%–25% effective storage range from the slowest cycle
- ✓ Groundwater assessment — trial pit monitoring and review of the seasonal water table to confirm clearance below the soakaway base
- ✓ Soakaway & infiltration design — geocellular soakaways, permeable paving or basins sized to BRE 365 / CIRIA C753
- ✓ Half-drain time check — confirming the system empties within 24 hours ready for the next storm
- ✓ Attenuation fallback — if infiltration fails, a fully designed attenuation strategy with restricted discharge to watercourse or sewer
When Is Soakaway Testing Required?
Not sure if your ground will infiltrate? Tell us your site and soils and we’ll advise before you commit.
LLFA Asks You to Prove Infiltration
LLFAs routinely require infiltration to be tested and discounted before attenuation is accepted. A drainage strategy that assumes infiltration without BRE 365 data is usually rejected.
Discharging a Drainage Condition
Conditions frequently require infiltration testing to support the detailed drainage design. BRE 365 results are the evidence the LLFA needs to discharge them.
Designing Soakaways
Any soakaway, infiltration trench or permeable paving sub-base must be sized using a measured infiltration rate — a default value will not be accepted at detailed design.
New Dwellings & Extensions
Even smaller schemes draining to a soakaway need a test to BRE 365 to confirm the ground can take the runoff without surcharging.
Permeable Paving Schemes
Where permeable paving provides infiltration as well as treatment, the infiltration rate governs whether storage or an outfall is also required.
Brownfield & Contaminated Sites
Infiltration may be restricted where contamination or made ground is present; we assess suitability alongside the test results and the SuDS hierarchy.
Frequently Asked Questions
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