Permeable paving on clay: designing for zero infiltration

Permeable paving on a clay subgrade still works. It just works as a tank rather than as a soakaway. Water passes through the surface into an open-graded sub-base, sits in the voids between the stones, and leaves through a flow control at a rate the receiving sewer or watercourse can take. Nothing soaks away, and nothing needs to. The real design questions on a permeable paving clay subgrade are how deep the sub-base has to be and where the outfall goes.

Why permeable paving still works without infiltration

Three functions are bundled into one construction. The surface intercepts rainfall where it falls, so no separate gully network is needed. The sub-base stores water in the voids between aggregate particles. And the system treats the water, because silt, hydrocarbons and metals are captured in the upper layers and broken down by biofilms on the aggregate.

Only the second function depends on the subgrade, and even then only for the outlet. On clay you keep the interception and the treatment, and you replace infiltration with a controlled discharge.

That treatment benefit is worth stating in the Drainage Strategy. Where a scheme has a car park and the receiving water is sensitive, permeable paving often removes the need for a separate treatment stage that would otherwise take up land.

Tanked, partially tanked and full infiltration systems

Three arrangements are described in UK drainage guidance and they need to be named correctly, because consultees use the same shorthand.

Tanked systems are the default across the London clay, the Weald clay, the Mercia mudstones and most made ground. They are also required, regardless of soil, where infiltration would mobilise contamination or where a source protection zone rules it out.

Demonstrate why you have chosen the tanked option. An infiltration test carried out to a recognised soakaway method, with the trial pit locations and results reported, is the evidence a lead local flood authority expects before it accepts that infiltration was ruled out. Asserting that the site is clay is not enough.

Sizing the sub-base storage void

The arithmetic is straightforward. Storage volume equals the plan area of the sub-base multiplied by its depth multiplied by the void ratio of the aggregate.

An illustrative example. A car park of 800 square metres with a 400 millimetre sub-base of clean open-graded stone at an assumed void ratio of 30 per cent gives 800 multiplied by 0.4 multiplied by 0.3, which is 96 cubic metres of storage. If the required attenuation volume for the 1 in 100 year event plus climate change is 130 cubic metres, the sub-base needs to be deeper, the paved area larger, or the shortfall taken up elsewhere. That calculation takes two minutes and should be done before the levels are drawn, not after.

Take the void ratio from the aggregate specification rather than from habit. Clean single-sized stone gives the assumed figure only if it is actually clean and single-sized, and a supplier's grading certificate is the way to confirm it.

Depth is limited by the subgrade level, the frost depth, the outfall invert and the water table. On a flat clay site the outfall invert usually controls, and a 600 millimetre sub-base that cannot drain is not storage, it is a permanently saturated layer under a car park.

Falls in the subgrade matter too. Where the site slopes, the sub-base has to be terraced with check dams or baffles, otherwise water runs to the low end and the upper storage never fills.

Flow control and outfall arrangements

Water leaves a tanked system through a perforated pipe in the base connected to a control chamber. Put the flow control in a chamber that can be accessed, rodded and cleared, and never bury a restrictor in the sub-base.

Set the discharge rate from the drainage strategy, which on a greenfield site means greenfield run-off rates and on a brownfield site means a betterment on the existing rate agreed with the authority and the sewerage undertaker. Where the connection is to a public sewer, the undertaker's approval process runs separately from planning and takes its own time.

Provide an overflow. Every tanked system needs a route for water once the storage is full, and it should be a designed exceedance path across the surface rather than a surprise.

Loading, construction and contamination of the sub-base

The sub-base is both a structural layer and a storage layer, and the two requirements pull in different directions. Open-graded stone has less bearing capacity than a dense graded sub-base, so the pavement design has to account for it, with a geotextile or geogrid over the subgrade to prevent clay pumping up into the voids.

Construction sequencing is where these systems are ruined. Silt is the enemy, and the two most common failures are using the completed pavement as a site haul road, and allowing surrounding landscaping to wash soil onto the surface before it is stabilised.

Protect the installation as follows:

1. Complete the sub-base and lay the blocks only when surrounding earthworks are finished.
2. Divert construction traffic away from the finished pavement, or protect it with a sacrificial layer.
3. Stabilise and vegetate adjacent soft landscaping before handover.
4. Sweep the surface at practical completion and record that it was done.
5. Hand over a maintenance manual with the as-built drawings.

Where the surrounding ground is contaminated, the impermeable membrane also serves a containment purpose, and its specification should be agreed with the contamination consultant rather than picked off a supplier's list.

Maintenance and the reality of clogging

Permeable surfaces clog. That is not a design flaw, it is the treatment function working, and the systems are specified with enormous spare capacity precisely because of it. A new surface passes water far faster than any storm will deliver it, so partial clogging is tolerable for years.

The maintenance regime is surface sweeping with a machine that lifts material out of the joints rather than pushing it around, typically twice a year, with joint material replenished where it has been removed. Inspect the control chamber and the silt trap at the same visit.

What kills these systems is neglect combined with a silt source. A car park beside an unsurfaced overflow area will clog in two winters. Deal with the silt source in the design and the maintenance burden drops. The question of who holds SuDS adoption and maintenance matters more here than for below-ground tanks, because a neglected tank still stores water while a neglected surface stops accepting it.

Specifying it so a consultee accepts it

Lead local flood authorities see a lot of permeable paving proposed and rather less of it properly designed. What they check is consistent, and it pays to test your submission against what the lead local flood authority checks in a drainage strategy before you send it.

Include the infiltration test results and the reasoning for the tanked option, a section through the construction with layer thicknesses and the membrane shown, the storage calculation with the void ratio stated and justified, the discharge rate with the flow control device named, the exceedance route, the maintenance schedule with responsibilities, and confirmation that the paved area will not be used for construction access.

One honest note. Some authorities still resist permeable paving in areas where it has been installed badly and failed. Where you meet that, the answer is the construction protection sequence in writing and an offer to condition it. The scheme fails on site, not on paper, and a consultee who has seen three failures is reacting to the site.

Where the wider statutory position matters, the current state of Schedule 3 SuDS approval should be checked, because it changes who signs off the design.

PlanScope drafts a Drainage Strategy and Flood Risk Assessment structured to UK planning policy, with the site-specific test results and calculations flagged for your engineer. Enter your postcode on PlanScope to see which reports your site is likely to need.

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.