Drainage strategy where infiltration testing fails on clay

An infiltration test that fails does not release you from the surface water hierarchy. It moves you one step down it, and the drainage authority will want to see the evidence that justified the move before it looks at anything else in your Drainage Strategy. On clay sites that evidence is usually a set of trial pit records showing a rate too slow to drain a soakaway within a day, backed by groundwater levels and a borehole log, not a sentence saying infiltration was discounted.

The practical consequence is a different drainage design, a different conversation with the sewerage undertaker and a longer programme. Plan for all three.

What a failed infiltration test actually proves

The soakaway test most commonly used in England follows the BRE Digest 365 method: a trial pit of known dimensions at the proposed base level of the soakaway, filled with water, with the time recorded for the level to fall from 75 per cent to 25 per cent of the effective storage depth. The test is repeated three times in the same pit, and the design rate is taken from the slowest of the three because the first fill wets up the soil and flatters the result.

A result that drainage designers treat as unworkable usually shows up as a half-drain time well beyond 24 hours, which on a firm glacial clay or a London Clay site is common. The rate quoted in drainage practice as the practical floor for infiltration systems is around one times ten to the minus six metres per second. Below that, a soakaway sized for a storm has not emptied before the next one arrives.

What the test does not prove is that nothing on the site infiltrates. It proves that the material at that location and that depth does not. Clay sites often carry a granular made ground layer, a gravel terrace beneath the clay or a weathered horizon near the surface, any of which can take a shallow system. Three pits in the footprint of a proposed basement tell you about that basement, not about the garden.

Repeating tests, and the groundwater question

Before accepting a failure, check three things about how the test was done. Were the pits at the depth the soakaway would occupy, or at a convenient depth for the machine. Were they wetted up and tested three times. And were groundwater levels recorded at the time of the test and, ideally, monitored afterwards.

Season matters on this. A test in a dry August on a clay site can report a rate that cracking has temporarily improved, and the same pit in February can be standing in water. Designers normally want at least a metre of unsaturated soil between the base of an infiltration system and the highest expected groundwater level, so a high winter water table rules out infiltration even where the permeability looks acceptable. Monitoring over a winter period adds months to a programme, which is why ground investigation belongs before the design freeze rather than after it.

Where the pits tell different stories, do not average them. Report them separately and design to the worst case for anything you rely on.

Moving down the discharge hierarchy

Approved Document H3 of the Building Regulations sets the order: an adequate soakaway or infiltration system first, a watercourse second where that is not reasonably practicable, and a sewer last. National policy and the Defra non-statutory technical standards for sustainable drainage systems repeat the preference. A lead local flood authority consulted on a major application will test each step rather than take the conclusion on trust.

The second step is the one applicants skip. Many clay sites in England have a ditch, a culverted watercourse or a highway drain within reach, and the fact that discharging to it needs a third party consent is not the same as it being impracticable. Riparian ownership, an ordinary watercourse consent from the lead local flood authority for any works in the channel, and the capacity of the receiving ditch all have to be checked and reported.

Only when no watercourse is available does a sewer connection become the design answer. The right to connect for surface water under section 106 of the Water Industry Act 1991 is not unqualified, and the undertaker will apply its own conditions.

Sizing attenuation once infiltration is out

Attenuation storage is driven by the discharge rate you are allowed, not by the size of the roof. Fix the rate first, then size the tank.

Illustrative example only. A 0.4 hectare brownfield site is limited by the lead local flood authority to a 5 litre per second discharge. The 1 in 100 year event with the current climate change allowance produces around 180 m³ of runoff at the critical six hour duration. Over those six hours the outfall removes roughly 108 m³, which leaves about 72 m³ to hold on site. Halve the permitted rate to 2.5 l/s and the stored volume rises above 125 m³, which is the difference between a crate tank under a car park and a redesign of the layout.

Three points follow from that arithmetic. The critical duration has to be tested across a range rather than assumed at six hours. Climate change allowances change by management catchment and epoch, so take the current Environment Agency figures rather than a number from an earlier scheme. And a low permitted rate makes flow control the hardest part of the design, because a hydrobrake at 2 l/s on a shallow head blocks.

Interception and water quality still apply. An attenuation tank on its own treats nothing, so the first stage of the train normally has to be a permeable surface, a filter trench or a swale even where infiltration is discounted for volume purposes.

Negotiating a discharge to sewer

Sewerage undertakers handle these requests constantly and ask the same questions each time. Expect to be asked for the infiltration test results in full, a plan showing where the pits were, evidence that a watercourse was investigated, the proposed rate with calculations, and a commitment on the existing rate where the site is already developed.

On previously developed land most undertakers and lead local flood authorities look for a reduction against the existing discharge rather than a like for like connection, often expressed as a percentage betterment. The figure varies by authority and by catchment, so take it from the published policy rather than from precedent on another site. Where the receiving sewer is combined, the undertaker may also require a foul and surface water separation on site, which affects the layout and may need a Foul Sewage Assessment of its own.

Start this before you submit. A connection agreement is not a planning requirement, but an undertaker's holding objection on capacity will stall a determination, and capacity evidence takes weeks to obtain. The same lead time applies to utilities capacity evidence generally.

Evidence the drainage authority wants to see

A drainage strategy that survives consultation contains the following, cross-referenced so a reviewer can follow the logic from test result to tank size.

1. Trial pit logs with depths, soil descriptions and the three test runs for each pit, plus a plan showing pit locations against the proposed drainage layout.
2. Groundwater observations at the time of testing and any monitoring record, with the assumed highest groundwater level stated.
3. A written step through the hierarchy, naming the watercourses considered and the reason each was ruled out.
4. The permitted discharge rate, its source, and the greenfield or existing rate it was derived from.
5. Hydraulic calculations for the design events with the climate change allowance stated, the critical duration tested, and the exceedance route for events beyond the design standard.
6. A maintenance plan identifying who owns and maintains each component, since adoption arrangements decide whether a scheme is deliverable.

Where a site also sits in Flood Zone 2 or 3, the drainage strategy sits inside or alongside a Flood Risk Assessment and the two documents must use the same levels and the same assumptions. Inconsistent finished floor levels between the two is one of the more common validation queries.

Writing the conclusion without overclaiming

The temptation in a conclusion is to say the development will not increase flood risk. Say instead what the design achieves and what remains. A defensible closing section states the permitted rate, the storage provided, the events tested, the residual risk in an exceedance event, and the assumptions that would need revisiting if the ground investigation is extended.

Schedule 3 of the Flood and Water Management Act 2010, which would bring a SuDS approving body regime into force in England, remains uncommenced, so approval of your drainage design runs through the planning condition and the undertaker's agreement rather than through a separate consent. Say which route you expect.

And note the uncertainty honestly. On a clay site with made ground, the single largest risk to a drainage strategy is that the intrusive investigation after planning permission finds something different from the three pits that informed the design. A strategy that says so, and identifies which parameters it is sensitive to, reads better at appeal than one that does not.

This is general guidance, not legal advice. Drainage design for a planning application should be prepared or reviewed by a suitably qualified drainage engineer, and the lead local flood authority's published requirements take precedence over any general rule described here.

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