BRE Digest 365 • Infiltration Testing • SuDS Design

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.

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BRE 365 testing
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Infiltration rate calculation
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Soakaway & SuDS design
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Fallback strategy if infiltration fails
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England & Wales

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
Ground works and excavation Drainage pipework laid in the ground Infiltration and green drainage

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.

How BRE 365 Soakaway Testing Works

The site test that decides whether infiltration drainage is viable.

Infiltration — soaking rainwater into the ground — sits at the top of the SuDS hierarchy, so LLFAs expect it to be considered first. BRE Digest 365 is the recognised method for testing whether your ground can actually accept the water. A trial pit is excavated, filled and monitored to measure the soil infiltration rate, and the result feeds directly into the design (or rules infiltration out).

The test itself

A trial pit is dug to the proposed soakaway depth, filled with water and the fall in level timed. The test is normally repeated three times to give a reliable infiltration rate.

What the result is used for

The infiltration rate sizes the soakaway or infiltration feature for the 1 in 100 year event plus climate change, and confirms the half-drain time meets the standard. Low rates, clay, or a high water table can rule infiltration out.

Design constraints

Soakaways must be at least 5 m from buildings and 2.5 m from boundaries, and clear of the groundwater table — we check these before recommending infiltration.

Frequently Asked Questions

What is BRE 365?
BRE Digest 365 “Soakaway Design” is the industry-standard method for measuring soil infiltration. A trial pit is filled with water and the rate at which it drains is recorded over three cycles; the soil infiltration rate is calculated from the time taken to fall across the effective storage depth.
How many tests do I need?
At least three full fill-and-empty cycles per trial pit, and usually more than one trial pit across the site or in the footprint of each proposed soakaway, so the design uses a representative — and conservative — infiltration rate.
What if infiltration fails?
That’s a normal outcome on clay, mudstone or high water table sites. We document the failed test as evidence and design an attenuation-based strategy instead — geocellular storage or a basin with a restricted discharge to watercourse or sewer at greenfield or agreed rates.
Do you carry out the digging?
We arrange and witness the testing with a groundworks contractor, or work with results from your own contractor provided they follow BRE 365. Either way we produce the calculations and the design that the LLFA will accept.
How much does it cost and how long does it take?
Testing is usually completed in a single site visit, with the infiltration rate and design following within a few days. Cost depends on the number of trial pits and ground conditions — we’ll give you a fixed-fee quote within 24 hours.
How many BRE 365 tests are needed?
Normally three fills of the same trial pit to give a consistent rate, and on larger sites more than one pit to reflect ground variation. We advise the number for your site and the LLFA's expectations.
What if the soakaway test fails?
A low or zero infiltration rate is a legitimate finding — it demonstrates to the LLFA that infiltration is not viable and justifies moving down the hierarchy to an attenuated discharge to a watercourse or sewer. The drainage strategy is then designed on that basis.
Do I have to do the test before planning?
Not always — many LLFAs accept a strategy that identifies infiltration as the intended approach with testing 'before detailed design'. For condition discharge and detailed design, however, the actual test results are required.

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