Infiltration testing and when soakaways are ruled out

Infiltration sits at the top of the drainage hierarchy, so a council will expect you to prove it does not work before it accepts anything else. Proving it means testing to a recognised method, at the right depth, in the right place, with the results written up properly. A line in a drainage statement saying the ground is clay will not do it.

For a self-builder, the sequence matters as much as the method. Test before the layout is fixed, because a failed test moves storage into the garden and changes the drawings.

Why infiltration comes first

Soaking water into the ground where it falls keeps it out of sewers and watercourses, reduces the volume the downstream system has to carry, and avoids any need for a discharge agreement. National guidance and every local drainage standard start there.

The hierarchy runs: to the ground, then to a surface water body, then to a surface water sewer or highway drain, then to a combined sewer. Each step has to be justified by evidence. A water company will often refuse a surface water connection outright where infiltration has not been tested, and the lead local flood authority will say the same thing on a larger scheme.

So the test is not a formality. It is the document the rest of the design depends on.

The test method and how many trials

BRE Digest 365 is the reference for soakaway design in England, and it sets out both the design equations and the field test.

The test uses a trial pit dug to the depth of the proposed soakaway. The pit is filled with water and the fall in level timed, and this is repeated so that three separate fills are recorded. The infiltration rate is calculated from the time taken for the water level to fall between 75 per cent and 25 per cent of the effective storage depth, and the design uses the slowest of the three results.

Three fills matter because the first often gives an optimistic answer. Dry ground absorbs water quickly until it wets up, and a single test on a summer afternoon can suggest a rate the ground will never deliver in February.

Number and location matter too. One pit does not describe a site with made ground in one corner and natural clay in another. Test at each location where a soakaway is proposed, and record the position, depth and ground conditions of each pit with a log.

Depth relative to the proposed feature

Test where the soakaway will sit, not where digging is convenient. Permeability changes with depth, sometimes sharply, and a rate measured at 1 metre tells you nothing about conditions at 3 metres.

Two related rules shape the design. Building Regulations guidance places soakaways serving a building at least 5 metres away from it, and most councils apply the same separation to boundaries and to other structures. And the base of the infiltration feature needs an unsaturated zone beneath it, commonly at least 1 metre above the highest expected groundwater level, though local standards vary and some authorities require more.

That second requirement is why a test carried out in August can be misleading. Groundwater in many areas peaks between January and March.

Groundwater and seasonal variation

If the trial pit shows water entering from the sides or standing at the base, record it and say so. That observation is evidence, and it is worth more than a calculated rate.

Where groundwater is close to the proposed base level, the honest answer is monitoring. Install a standpipe and take readings through the winter, or use published groundwater data alongside a single dip and state the uncertainty. Councils accept a monitored answer and are sceptical of a confident one based on one dry day.

A Geotechnical Assessment will usually pick up the same information, and where ground investigation is being carried out for foundations it is worth adding the infiltration testing to that scope rather than mobilising a second contractor.

Where infiltration is not permitted

Some sites fail for reasons unrelated to the soil.

The contamination case needs care. Where a site has an industrial history, get the position from a Land Contamination assessment before proposing infiltration, because the Environment Agency's objection arrives late and is difficult to argue with.

Interpreting a failed test

A low rate is not a failure of the process. It is a result, and it justifies the next step in the hierarchy.

As a rough guide, rates below roughly a millionth of a metre per second are generally treated as unsuitable for infiltration drainage, and local standards often set their own cut off. Below that the storage volume required becomes so large that the feature is impractical.

Partial infiltration is worth considering before abandoning the idea. Some sites support shallow infiltration through permeable paving or a lined feature with an infiltrating base, reducing the discharge volume even if they cannot take it all. Councils like this, because it shows the hierarchy was applied rather than skipped.

If the answer is a discharge to a sewer or watercourse, the test report becomes the evidence supporting the request. Attach it. The article on what a consultee checks in a drainage model output covers what happens to that submission next.

Timing the tests within the programme

Do the testing at the same time as the topographical survey and before the layout is finalised. On a small site, that is usually one day of plant hire and half a day of engineer time.

Avoid testing in prolonged dry weather if you can, and if you cannot, say in the report when it was done and what the weather had been. A test carried out during a drought with no commentary invites a request to repeat it.

Leave time for a second round. Where the first results are marginal, additional pits or winter groundwater monitoring may be needed, and those add weeks rather than days.

Reporting the results so they are accepted

The report the council wants is short and specific.

1. A location plan showing every trial pit with a reference.
2. A log for each pit giving depth, strata encountered, groundwater observations and the date.
3. The raw timing data for all three fills, alongside the calculated rate.
4. The calculated infiltration rate for each pit, with the method and the equation used stated.
5. The design rate adopted, with a factor of safety applied and justified.
6. Photographs of each pit, dated.
7. A statement of what the results mean for the drainage design.

That package feeds directly into the Drainage Strategy. Councils reject far more drainage submissions for missing evidence than for bad engineering.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.