Infiltration testing to BRE 365 and reading the result
A BRE Digest 365 infiltration test needs a trial pit at the soakaway location, dug to full depth, filled three times. Here is how the rate is calculated, why the slowest cycle governs, and what to present in a drainage strategy.
A soakaway infiltration test tells you one number: how fast water leaves a hole in the ground at the depth you intend to build the soakaway. Get that number wrong and you either build a soakaway that floods the garden in the first wet winter, or you pay for attenuation tanks you never needed. BRE Digest 365 is the method almost every lead local flood authority in England expects, and the test is simple enough that the errors are nearly always in how it was done, not in the arithmetic afterwards.
The short version: dig a trial pit at the proposed soakaway location, fill it three times, time how long the water level takes to fall from 75 per cent to 25 per cent of the effective storage depth, and design on the slowest of the three results.
What the test measures and why the method matters
The test measures the rate at which water passes from the pit into the surrounding soil, expressed in metres per second. It is not a soil classification, and it is not a percolation test of the kind used for septic tank drainage fields, which uses a different pit size and a different calculation.
That distinction matters when you submit. Turning up with a percolation test result from the foul drainage designer and presenting it as infiltration data is one of the more common reasons a drainage strategy gets a holding objection.
Three fill cycles are specified because the first is always the least representative. A dry soil takes water quickly at first, then slows as the surrounding ground wets up and reaches something closer to steady state. Design on the first cycle and you will overestimate the rate, sometimes by a wide margin.
Trial pit position, depth and the three fill cycles
The pit has to represent the soakaway, which means position and depth are not negotiable details.
- Dig the trial pit at, or immediately adjacent to, the proposed soakaway location. Testing in a convenient corner of the site and applying the result 40 metres away is not evidence about the soakaway.
- Take the pit to the full proposed depth of the soakaway. Ground conditions change with depth, and a clay layer at 1.8 metres will not show in a pit dug to 1.2 metres.
- Record the pit dimensions accurately, because the internal surface area up to half the effective storage depth is a term in the calculation.
- Fill the pit to at least 75 per cent of its effective depth and record the water level at intervals as it falls.
- Repeat so that you have three complete fill and empty cycles, ideally on the same day or consecutive days.
- Log the depth to groundwater if it is encountered, and note the date, the weather in the preceding days and the soil profile.
Where the scheme has more than one soakaway, test at each location. Ground varies across a site far more than most self-builders expect, particularly on made ground or where a former field boundary crosses the plot.
Seasonal groundwater and why winter testing differs
This is the part that catches people out. A test in August on a well-drained site can give an excellent rate, and the same pit in February can be standing in water.
The relevant figure for design is the highest expected groundwater level, and BRE Digest 365 expects a clear separation between the base of the soakaway and that level. A metre of unsaturated soil beneath the base is the conventional requirement, and lead local flood authorities routinely apply it.
If you test in summer, say so, and either allow for seasonal variation with a proper factor of safety or commit to groundwater monitoring over the winter. A standpipe with monthly readings from October to March is a low-cost insurance policy on a site where the water table is uncertain. Deep soakaways on sites within a groundwater source protection zone raise further questions, and the Environment Agency may need to be consulted.
Calculating the infiltration rate
The calculation divides the volume of water lost between the 75 per cent and 25 per cent levels by two quantities: the internal surface area of the pit up to half the effective storage depth, and the time taken for that fall.
A worked illustration, clearly illustrative rather than a real site: a trial pit 3.0 m long, 0.9 m wide and 2.0 m deep, tested with an effective storage depth of 2.0 m. The water falls from 1.5 m to 0.5 m above the base in 5,400 seconds. The volume lost is 3.0 × 0.9 × 1.0, which is 2.7 m³. The infiltration area is the pit sides to the 1.0 m mid-level, so 2 × (3.0 + 0.9) × 1.0, which is 7.8 m². The rate is 2.7 divided by (7.8 × 5,400), giving roughly 6.4 × 10⁻⁵ m/s. That is a sandy result and comfortably workable.
Run the same arithmetic for each cycle and take the slowest. Then apply the factor of safety appropriate to the consequence of failure and the area drained, following CIRIA C753 or whatever your lead local flood authority specifies.
When the result rules out soakaways
As a rough guide, rates faster than about 1 × 10⁻⁵ m/s are good, rates around 1 × 10⁻⁶ m/s make soakaways large but feasible, and anything slower generally means infiltration is not viable for anything beyond a small area. Clay soils commonly sit well below that figure.
A poor rate is not a failure of the test. It is a design input, and finding it at the outline design stage costs a fraction of finding it after the foundations are in. The lead local flood authority will not accept an assertion that infiltration is unsuitable without the data behind it, and our note on what the lead local flood authority checks in a drainage strategy sets out the rest of what they look for.
Alternatives where infiltration fails
The drainage hierarchy runs infiltration first, then discharge to a surface water body, then a surface water sewer, then a combined sewer as the last resort. Each step down requires evidence that the step above was properly considered.
Where infiltration is out, the usual routes are:
- attenuation storage with a restricted discharge to a watercourse, typically at greenfield runoff rate or a rate agreed with the lead local flood authority
- attenuation with a restricted discharge to a surface water sewer, with the water company's consent to connect
- permeable paving over a lined sub-base acting as storage rather than infiltration
- partial infiltration, using a shallow system in a more permeable upper layer with an overflow
Long-term maintenance of whatever you build is a planning issue in its own right, and our piece on SuDS adoption and maintenance explains who ends up holding it.
Presenting the data in a drainage strategy
A Drainage Strategy that includes infiltration testing should contain, as a minimum:
- a site plan showing every trial pit position against the proposed drainage layout
- pit dimensions, depth and a soil log for each pit
- dates, times and antecedent weather
- the raw water level readings for all three cycles, not just the derived rate
- the calculation shown in full
- the groundwater level observed, or a statement that none was encountered to the depth tested
- the factor of safety applied and the source of it
Include the raw readings. An officer who can see the three cycles converging has a reason to believe the number. A single quoted rate with no working behind it invites a request for further information, and that costs weeks.
Where the site also sits in a flood zone, the infiltration data feeds the Flood Risk Assessment as well, and the two documents should quote the same figures.
Enter your postcode on PlanScope to see which drainage and flood documents your site is likely to need before you book the digger.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
How many times does the trial pit have to be filled?
Three complete fill and empty cycles. The first cycle is the least representative because dry soil absorbs water quickly before the surrounding ground wets up, so designing on it overestimates the rate. Use the slowest of the three results for design, then apply a factor of safety appropriate to the area drained and the consequence of failure.
Is a percolation test the same as an infiltration test?
No. A percolation test for a septic tank drainage field uses a different pit size, a different procedure and a different calculation, and produces a value in seconds per millimetre rather than metres per second. Submitting percolation data in place of BRE Digest 365 infiltration results is a common reason for a holding objection from the lead local flood authority.
What infiltration rate is too slow for a soakaway?
As a general guide, rates faster than about 1 x 10 to the minus 5 metres per second work well, and around 1 x 10 to the minus 6 metres per second soakaways become large but feasible. Below that, infiltration is usually impractical for anything beyond a very small area, which is typical of clay soils.
Does the season affect the test result?
Considerably. A summer test on a site with a shallow winter water table can give a rate that is unachievable in February. Design uses the highest expected groundwater level, with a metre of unsaturated soil conventionally required beneath the soakaway base. If you test in summer, state so and either apply a proper factor of safety or monitor groundwater over winter.
What should the drainage strategy include about the test?
Pit positions on a plan against the drainage layout, pit dimensions and soil logs, test dates and antecedent weather, the raw water level readings for all three cycles, the full calculation, the groundwater level observed and the factor of safety applied with its source. Raw readings matter, because a single quoted rate invites a request for further information.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.