BRE365 Infiltration Testing
We carry out BRE365 soakaway tests and percolation testing to determine infiltration rates, size soakaways, and demonstrate drainage hierarchy compliance. Our fieldwork follows BRE Digest 365 and CIRIA C753 methodology — delivering results that satisfy LLFAs and planning authorities.
Prove infiltration viability. Size soakaways. Satisfy the drainage hierarchy.
BRE365 infiltration testing — also known as percolation testing or soakaway testing — is the standard method for measuring how quickly water drains into the ground. The drainage hierarchy requires developers to discharge surface water to ground wherever practicable, and field-measured infiltration data is the evidence needed to prove whether that is feasible. We carry out BRE365 tests to determine the soil infiltration rate and establish whether soakaways, infiltration trenches, or permeable paving can form part of the SuDS strategy.
Where infiltration is viable, we use the test results to size soakaways and infiltration features to BRE Digest 365 and CIRIA C753 standards, feeding directly into the drainage design. Where ground conditions rule out infiltration, our test data provides the robust evidence needed to justify alternative discharge routes — moving down the drainage hierarchy with full LLFA and planning authority acceptance.
Rigorous field testing that feeds straight into drainage design.
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Desk study and geology review
We review BGS geology maps, published soil data, and groundwater records to assess infiltration potential before mobilising. We identify optimal test locations based on proposed soakaway positions, geological variability, and site constraints.
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Test pit excavation and soil logging
We excavate trial pits to the proposed soakaway invert depth, typically 1 to 3 metres below ground level. Each pit is logged to record the soil profile, identify impermeable strata, and note groundwater strikes or seepage.
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BRE365 infiltration testing
We fill each trial pit with water and record the drop in water level over time across three successive fill-and-drain cycles. The third cycle — the most conservative measurement — determines the infiltration rate in accordance with BRE Digest 365 methodology.
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Infiltration rate calculation
We calculate the soil infiltration coefficient from the field data, accounting for pit geometry and effective drainage area. The coefficient is the critical input for soakaway sizing and determines whether the site can rely on infiltration-based drainage.
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Soakaway design and reporting
We size soakaways and infiltration features using the BRE365 or CIRIA C753 method, and present full results — test logs, calculations, and design recommendations — in a report format accepted by LLFAs, SABs, and planning authorities.
Infiltration testing integrated with drainage design.
- In-house coordination with our SuDS and drainage design team — test results feed directly into MicroDrainage models.
- Fast turnaround from site visit to final report, with flexible scheduling to match your programme.
- Full CIRIA C753 and BRE Digest 365 compliance across all testing and reporting.
- Experienced fieldwork team with the equipment and methodology to handle variable ground conditions.
- Results presented in the format LLFAs and SABs require — no back-and-forth on technical queries.
Tools and methods
BRE Digest 365 methodology, soakaway sizing calculations (BRE365 and CIRIA C753), BGS geological mapping and borehole records, and calibrated permeability testing equipment for accurate field measurement.
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Frequently asked questions
- What is BRE365 testing?
- BRE365 testing is the standard UK method for measuring soil infiltration rates, set out in BRE Digest 365. It involves excavating a trial pit to the proposed soakaway depth, filling it with water three times, and recording how quickly the water level drops. The third cycle gives the design infiltration rate used for soakaway sizing.
- How many infiltration tests do I need?
- The number of tests depends on the site size and variability of ground conditions. As a minimum, one test per proposed soakaway location is recommended. For larger sites or sites with variable geology, additional tests are needed to characterise infiltration rates across the development area. We advise on the appropriate number during scoping.
- What is a good infiltration rate?
- An infiltration rate above 1 x 10^-6 m/s is generally considered viable for soakaway design, though the feasibility also depends on the available area and storage volume. Higher rates mean smaller soakaways. Rates below this threshold typically indicate that infiltration is not practicable and alternative discharge routes should be pursued.
- What is the difference between BRE365 and a percolation test?
- BRE365 tests are designed for soakaway sizing and use trial pits filled to the full proposed invert depth. Percolation tests — sometimes called Baermann tests — are smaller-scale tests typically used for septic tank drainage fields and follow BS 6297. The two methods serve different purposes and should not be used interchangeably.
- When is infiltration testing required?
- Infiltration testing is required whenever the drainage hierarchy demands that discharge to ground is considered first. Most LLFAs and planning authorities expect BRE365 test data to support or rule out soakaway drainage before alternative discharge routes are accepted. It is a standard requirement for almost all new development sites.