Infiltration testing to BRE 365 and when it is worth doing

BRE 365 infiltration testing tells you one thing: how fast water actually soaks into the ground at the depth you intend to build a soakaway. It is a low cost test with an expensive consequence, because a drainage strategy built on an assumed infiltration rate collapses the moment the real figure comes back and the whole surface water design has to change.

For a self-builder the sequence that works is to test early, before the layout is fixed, and to test at the depth and position the soakaway will occupy rather than wherever the digger can reach.

What the test measures

The method is straightforward. Excavate a trial pit of known dimensions to at least the depth of the proposed soakaway, fill it with water to at least three quarters of the effective storage depth, and record the time taken for the water level to fall from 75 per cent to 25 per cent of that depth.

The infiltration rate is then calculated from the volume of water lost between those two levels, the internal surface area of the pit measured to half the effective storage depth, and the time taken. The result is a velocity, usually expressed in metres per second, and it is the number every soakaway calculation depends on.

Two details are worth getting right on site.

Measure the pit accurately after excavation rather than assuming the dimensions you asked for. Trial pit sides collapse and bases fill with spoil, and a pit measured from the drawing rather than from the hole gives a wrong area and a wrong rate.

Record the water levels against a fixed datum with a dipmeter, and note the times. Guessing the 75 and 25 per cent points from the pit edge is the commonest source of unreliable data.

Why three repeats matter

The test is run three times, each begun as soon as practicable after the previous one has drained.

The reason is saturation. The first fill runs into soil that is relatively dry, so the rate looks good. By the third fill the surrounding ground is much closer to the condition it will be in during a real winter storm, when the soakaway is being asked to work.

Design on the slowest of the three results. Taking an average, or worse taking the first, flatters the ground and produces an undersized structure.

Where the three results are very different from each other, that is information rather than noise. Widely varying rates usually mean a layered soil profile, a preferential pathway, or a pit that has silted. Say so in the report rather than presenting a tidy average that hides it.

Trial pit against borehole

Trial pits are the usual method for shallow soakaways serving houses and small schemes. They are quick, they let you log the soil profile directly, and they test the horizon the soakaway will sit in.

Borehole soakaways are used where the near-surface soils are impermeable but a permeable stratum sits beneath, which is common where clay overlies chalk or sand. The testing method is different, the design is different and the risks are different, because a deep bore can create a direct pathway to groundwater.

That last point matters more than the hydraulics. Where contamination is present, or the site sits over a sensitive aquifer, the Environment Agency will look hard at any deep infiltration proposal and may refuse to accept one at all.

Log the ground conditions as you go. A Geotechnical Assessment covering soil description, groundwater and the position of any made ground turns the infiltration result from a number into evidence.

Groundwater and the metre of unsaturated ground

A soakaway needs unsaturated ground beneath it to work. The standard requirement is a separation of at least one metre between the base of the soakaway and the highest expected groundwater level.

Highest expected is doing the work in that sentence. Groundwater in most of England peaks in late winter and early spring, and a level measured in August tells you very little about February.

The way to establish it properly is to install a standpipe in the trial pit and monitor it over a winter period. That is not always possible on a self-build programme, and where it is not, the fallback is to use published geological and hydrogeological information, note the limitation explicitly, and design conservatively.

Two other separation rules apply. Approved Document H sets a minimum distance between a soakaway and any building or road, to avoid saturating foundations and sub-base. And on a sloping site, water entering a soakaway can travel laterally along a permeable layer and reappear downslope, so consider where it goes as well as how fast it leaves.

Testing at the wrong time of year

A test carried out in a dry August on a site with shallow groundwater will pass. The same site in February may not have a metre of unsaturated ground beneath the soakaway at all.

Lead local flood authorities know this, and testing dates are one of the first things a reviewer checks. Where the test was carried out in a dry period, expect to be asked to justify the design or to repeat the work.

The practical answer is to test as early in the project as possible, in a period that is at least representative, and to record the antecedent weather conditions in the report. Where testing has to happen in summer, install monitoring and commit to a winter check as a condition rather than pretending the issue does not exist.

Reading a result that fails

Not every site can infiltrate, and finding that out is a useful result rather than a failed test.

Rates below roughly one millionth of a metre per second are generally regarded as impractical for soakaways, because the structure needed to store water for long enough becomes unreasonably large and the half drain time cannot be met. Clay sites frequently come in below that.

When the result is poor, the options in order of preference are these.

1. Test elsewhere on the site. Infiltration varies a great deal over short distances, particularly where made ground or a former yard covers part of the plot.
2. Test at a different depth. A permeable horizon beneath a clay cap can support a deeper structure.
3. Use partial infiltration, with an overflow to a positive outfall. This is often the realistic answer and is widely accepted.
4. Abandon infiltration and discharge to a sewer or watercourse, at a restricted rate, with attenuation storage.

Source protection zones are the one case where a good result does not help. Infiltration within the inner protection zone around a public water abstraction is generally unacceptable regardless of how well the ground drains, and the same applies where contamination could be mobilised. Check the constraint before you book the testing.

Where the answer moves to a restricted discharge, the storage volume needed rises quickly, which is why the test should happen before the layout is fixed rather than after. The consequences of that arithmetic are set out in the Drainage Strategy.

Reporting so the strategy stands up

A reviewer should be able to reproduce your number from the report. Include:

• A plan showing the trial pit positions against the proposed soakaway locations, with levels.
• Measured pit dimensions and the effective storage depth used.
• The full water level and time record for each of the three tests, alongside the calculated rates.
• The calculated rate for each test and the design value adopted, with the reason it was chosen.
• Groundwater observations, including strike and rest levels, and the dates.
• The date of testing and a note of the preceding weather.
• Photographs of each pit and a description of the soil profile.

Attach the test report as an appendix rather than quoting a single number in the text. Where the site also lies in Flood Zone 2 or 3, or exceeds one hectare, the infiltration evidence forms part of the wider Flood Risk Assessment, and the two documents must use the same figure.

Enter your postcode on PlanScope to see which drainage documents your plot is likely to need before you book a testing contractor.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Infiltration testing and soakaway design are normally expected from a suitably qualified author. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.