Infiltration testing to BRE Digest 365 and common errors

Infiltration testing to BRE 365 means three successive fillings of a trial pit dug to the depth of the proposed soakaway, timing the fall of the water level from 75 per cent to 25 per cent of the effective storage depth, and designing on the slowest of the three. Anything less than that is not an infiltration rate, and a lead local flood authority reviewer will say so. The rejection usually arrives late, after the drainage strategy has been drawn around a soakaway that may not work.

Why three fillings? Because the first one wets up the soil and gives an optimistic answer, and because a soil that keeps getting slower across three runs is telling you something about siltation that a single test hides.

What the method requires

The trial pit is excavated at the location of the proposed infiltration device and to the level of its base. That last point is the one most often ignored. A soakaway with its base at 2.5 metres tested in a pit 1 metre deep has been tested in a different soil.

Typical pit dimensions are in the order of 0.3 to 1.0 metre wide, 1 to 3 metres long, and the pit is filled with water to at least 75 per cent of its effective storage depth. The effective depth runs from the base to the invert of the proposed inlet, and it must be recorded, because the rate calculation depends on it. Three successive fillings follow, each starting as soon as practicable after the previous one has drained.

For each filling, record the time for the water level to fall from 75 per cent to 25 per cent of the effective depth. The infiltration rate is then the volume of water lost between those two levels, divided by the internal surface area of the pit measured up to 50 per cent of the effective depth including the base, divided by the time taken.

Use the slowest of the three results for design. Averaging the three is the most common arithmetic error in this field, and it produces a soakaway roughly a third too small.

Where the test sits in the design

The rate is an input, not an answer. Design then applies a factor of safety reflecting the consequence of failure and the size of the contributing area, and checks that the device half-empties within 24 hours so it is available for the next storm.

Two geometric constraints usually decide feasibility before the rate does. The base of an infiltration device should sit at least 1 metre above the highest expected groundwater level, which on many sites removes infiltration entirely. And Approved Document H expects soakaways serving paved areas to be at least 5 metres from a building or a road, which on an infill plot can leave nowhere to put one.

Groundwater levels and seasonal variation

An infiltration test in August on a site with a winter water table at 1.2 metres tells you about August. Groundwater in most aquifers peaks in late winter and early spring, which is when the soakaway has to work and when the site is least likely to be tested.

Deal with it in one of three ways: test in the winter recharge period, install a standpipe and monitor the water level over at least one winter, or use published groundwater data and regional records to estimate the highest expected level and apply the 1 metre clearance to that. The third is acceptable on small schemes if the reasoning is stated. It is not acceptable on a scheme where the whole drainage strategy depends on infiltration.

Made ground is the other seasonal trap. A pit through granular fill can drain freely to a perched water table that is empty in summer and full in February, so log the strata rather than recording a rate alone. The wider evidence base for high water tables is covered in the note on groundwater flooding evidence and how to assess it.

Errors that invalidate a result

Most rejected tests fail for one of these reasons, and all of them are avoidable.

• Testing in topsoil or in the upper weathered horizon rather than at the formation level of the proposed device.
• A single filling instead of three, or three fillings reported as an average.
• No record of the effective storage depth or the pit dimensions, so the rate cannot be recalculated.
• Smeared pit sides from a toothless bucket, sealing the infiltrating surface in cohesive soils.
• Testing during or immediately after prolonged wet weather, or after prolonged drought when desiccation cracking gives a rate the soil will not sustain.
• One pit used to represent a site with more than one soil type or a marked fall across it.
• No strata log, so nobody can tell whether the water went into sand or into a land drain.
• The result quoted without the factor of safety applied in the design calculation.

Two further errors are regulatory rather than technical. Infiltration into ground affected by contamination can mobilise contaminants towards a receptor, and the Environment Agency position on discharges to ground in source protection zones restricts what is acceptable. Screen for both before the pit is dug, because a good infiltration rate on a contaminated site is a problem rather than a result.

Reporting the test so a reviewer accepts it

A reviewer wants to be able to reproduce the number. Give them the pit location on a plan, the strata log, the effective depth, the pit dimensions, the three timings with dates, the calculation, the rate adopted and the factor of safety applied.

1. Site plan with pit positions related to the proposed devices.
2. Trial pit logs with depths, strata descriptions and any groundwater strike.
3. A table of the three fillings with start and finish times and water levels.
4. The rate calculation shown as arithmetic, not as a spreadsheet output.
5. The adopted rate, the factor of safety, and the resulting storage volume.
6. A statement on groundwater, contamination and source protection zone status.

Illustrative example. A pit at 2.4 metres on a sandy clay records fall times of 2 hours 10 minutes, 2 hours 45 minutes and 3 hours 20 minutes across the three fillings. The slowest run is adopted, a factor of safety is applied for the size of the contributing area, and the resulting soakaway is around 40 per cent larger than the one the first filling would have justified. The figures are illustrative.

Where infiltration turns out to be unviable, the strategy moves to attenuation and a controlled discharge, and the hierarchy and the evidence for each step belong in the Drainage Strategy. On a single plot the proportionate approach is different again, and the note on drainage evidence for a single new dwelling sets out what a small scheme actually needs. Where the site also sits in a flood zone, the infiltration work feeds the Flood Risk Assessment rather than standing apart from it.

Repeat testing is warranted when the layout moves the device to a different part of the site, when the strata log shows variation between pits, when the first tests were carried out in a dry summer and the scheme depends on infiltration, or when the reviewer asks and the alternative is a refusal.

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. This is general guidance, not legal advice.