Infiltration testing and when a soakaway will not work

A soakaway infiltration test planning submission needs three things the authority can check: a test carried out at the depth the device will actually sit, enough trial pits to show the ground is consistent across the site, and a groundwater level that leaves at least a metre of unsaturated soil beneath the base. Get any of those wrong and the drainage strategy fails at consultation, whatever the headline infiltration rate says.

Infiltration sits at the top of the drainage hierarchy because it is the closest thing to how the site behaved before you built on it. That is also why the lead local flood authority will ask you to justify not using it, rather than the other way round.

Why infiltration sits at the top of the hierarchy

The non-statutory technical standards for sustainable drainage put discharge to the ground first, then to a surface water body, then to a surface water sewer, then to a combined sewer. Each step down the list increases the burden on somebody else's infrastructure.

For a self-builder the practical consequence is that you cannot start with a connection to the sewer. The water company will ask whether infiltration has been tested, and the planning authority will want to see the test results before it accepts a discharge to a pipe. Skipping the test to save a few hundred pounds reliably costs more in delay.

The standards also require you to deal with water quality and with exceedance, so the infiltration question is part of a wider strategy rather than a single calculation. On a plot with no mains sewer the foul side raises separate questions, covered in foul drainage with no mains sewer and what to show.

The test method and how many tests

The standard method is the soakaway test in the Building Research Establishment's digest on soakaway design. The procedure is straightforward and the detail is where people go wrong.

1. Dig a trial pit to the depth and in the position of the proposed soakaway, with the base at the proposed invert level.
2. Record the pit dimensions, the soil profile by horizon, and any groundwater encountered.
3. Fill the pit with water to near the top and record the water level against time as it falls.
4. Calculate the infiltration rate from the time taken for the level to fall from 75 per cent to 25 per cent of the effective storage depth, using the surface area of the pit up to the 50 per cent level.
5. Repeat the filling and timing three times in the same pit, allowing the previous fill to drain fully.
6. Adopt the least favourable of the three results for design.

On the number of pits, the digest expects a minimum of three trial pits, and more where the site is large or the geology varies. Three results from one pit are three results from one point, which tells you nothing about the rest of the plot. On a site with made ground over natural strata, or a geological boundary running through it, test on both sides of the boundary.

Record the water volume used, because on free-draining gravel a pit can take more water than a bowser holds, and running out halfway through is the commonest reason a test has to be repeated.

Depth, water table and the separation requirement

Test at the right depth. An infiltration rate measured in the topsoil at 1 metre has no relationship to the behaviour of a soakaway founded at 3 metres in clay, and a reviewer will say so.

The separation requirement is the rule that kills more schemes than the infiltration rate does. Design guidance expects at least 1 metre of unsaturated ground between the base of the infiltration device and the highest likely groundwater level. That is the highest level, not the level on the day you dug the pit, which on a site tested in August can be two metres lower than the February maximum.

Establishing the seasonal maximum means monitoring. A standpipe or dipwell installed in the trial pit and read through the winter gives you the evidence. Shorter alternatives include local borehole records from the national geological survey, groundwater data from the Environment Agency, and the mottling and gleying in the soil profile itself, which records where water has stood historically. Where groundwater is the governing constraint, the assessment overlaps with the work described in evidencing groundwater flooding in a flood risk assessment.

Soakaways also need to sit clear of buildings and highways. The Building Regulations approved document on drainage expects a separation from structures, and on a small plot that distance plus the separation from the boundary often leaves nowhere to put the device.

Results that rule infiltration out

Four findings end the conversation.

An infiltration rate below roughly 1 x 10 to the power minus 6 metres per second is generally treated as the practical lower limit for infiltration drainage. Below that, the device needed to store and release the design volume becomes so large that it is undeliverable on a normal plot. Heavy clay sites routinely come back below that figure.

A high water table, where the 1 metre separation cannot be achieved at any workable depth. A perched water table over a clay layer produces the same result even where the regional groundwater is deep.

Ground stability. Infiltrating water into shrinkable clay close to foundations, into a slope, into made ground of unknown composition, or above a mineshaft or dissolution feature is a geotechnical risk, and the geotechnical report will say no before the drainage engineer does.

Contamination, dealt with below.

If any of those apply, say so clearly and move down the hierarchy with the evidence attached. An honest negative test is a strong document. A strategy that quietly omits the test invites a holding objection.

Partial infiltration as an answer

The binary framing is the mistake. Where the rate is marginal rather than hopeless, partial infiltration often works.

Permeable paving with an unlined or partially lined sub-base will shed a proportion of the volume to ground and attenuate the rest, with a restricted outfall taking what remains. Lined attenuation with an infiltration base, swales with check dams and underdrains, and tree pits with open bases all do a version of the same thing.

The design then has two outputs: the volume lost to ground, which reduces the attenuation requirement, and a discharge rate for the remainder. Both numbers go into the Drainage Strategy with the test results behind them. Where the residual discharge goes to a ditch or stream, the works at the outfall will need land drainage consent for works to an ordinary watercourse from the lead local flood authority.

Be conservative with the infiltration component. Sub-base voids silt up, and a design that depends on the measured rate holding for thirty years without maintenance is a design that floods in year fifteen.

Contaminated ground and infiltration

Where soil or groundwater is contaminated, infiltration is a pollution pathway. Water moving down through contaminated made ground carries the contaminants into the aquifer, which is why the Environment Agency objects to infiltration on affected sites and why it objects more firmly in a groundwater source protection zone.

The sequence is: find out what is in the ground, then decide. A desk study and intrusive investigation, with a conceptual site model showing sources, pathways and receptors, tells you whether infiltration creates a pathway that did not exist before. Where remediation removes the source, infiltration may become acceptable with the regulator's agreement in writing.

Do not infiltrate through made ground of unknown origin on the assumption that it is probably fine. That assumption is the one that produces an enforcement problem rather than a planning one.

Reporting the test for the authority

The report should let a reviewer repeat your calculation without asking you anything. Include:

• a location plan with each trial pit numbered and positioned
• pit dimensions, base level relative to ordnance datum, and the soil log for each
• photographs of each pit and the soil profile
• the date and recent weather, since antecedent conditions affect the result
• the raw water level against time readings for all three fills in each pit
• the calculated infiltration rate for each test and the value adopted, with the reason
• groundwater observed during the test and the basis for the seasonal maximum used
• the soakaway sizing calculation with the design rainfall, the return period and the climate change allowance

Then state the conclusion in one sentence. Either infiltration is viable at the stated rate and depth, or it is not, and here is what the strategy does instead.

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This is general guidance, not legal advice; check the position with your local planning authority, the lead local flood 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.