Permeable paving on clay soils: when it still works

Permeable paving on clay soil works, and it works more often than the objections suggest. What fails on clay is full infiltration, where water passes through the pavement and soaks away into the ground beneath. The pavement itself is a storage and treatment structure, and it keeps doing that job whether or not anything drains out of the bottom of it.

So a percolation test that returns a poor infiltration rate does not rule out permeable paving. It rules out one of the three ways of building it.

Infiltration rate is a design input, not a verdict

Test the ground properly before you design anything. A soakage test carried out to the method in BRE Digest 365, at the depth the structure will sit and at the location it will sit, gives an infiltration coefficient in metres per second. Firm clays typically return values low enough that a soakaway would take days to empty, which is why a scheme relying on full infiltration across a clay site will not stand up.

Two practical points get missed. Test at the right depth, because a granular made-ground layer at 400mm can give a flattering result that vanishes once you dig to formation. And test more than one trial pit, because London Clay boulder clay and weathered head deposits vary enough across a single plot to change the design.

Record the groundwater level as well. A winter water table close to the underside of the sub-base removes storage volume you were counting on, and it is a question the lead local flood authority will ask about your drainage strategy.

The three build-ups

The SuDS Manual describes permeable pavement systems in three types, and the difference is entirely about what happens at the base.

On clay the answer is almost always full tanking, occasionally partial. A tanked pavement is a shallow, wide attenuation tank that happens to have a driveway on top of it, discharging through a hydrobrake or an orifice plate at a controlled rate to a sewer, a watercourse or a downstream feature.

Tanking matters structurally as well as hydraulically. Clay softens when it is wetted, and a tanked base with a geomembrane keeps water off the formation, which protects the subgrade California Bearing Ratio the pavement design assumed.

Sizing the storage

The storage volume lives in the voids of the sub-base. Clean, single-size angular aggregate is the material to specify, and a void ratio of around 30 per cent is the usual design assumption for it. Fill that space with fines and you lose both the storage and the permeability, which is the single most common way these pavements are ruined on site.

Work out the required volume from the rainfall event the authority asks for, usually including the 1 in 100 year event with a climate change allowance, then divide by the void ratio to get the aggregate depth. A driveway of 60 m² needing 4 m³ of storage needs roughly 13 m³ of sub-base at 30 per cent voids, which over that area is about 220mm of storage depth on top of whatever the structural design already requires. Those numbers are illustrative, and the real depth comes out of the hydraulic calculation for your rainfall and your discharge rate.

Two constraints usually bite before the maths does. Falls, because the sub-base needs a level or near-level base to store water across the whole area, which on a sloping site means check dams or terracing. And cover, because deep storage plus a laying course plus blocks can lift the finished level above a threshold or a neighbouring boundary.

Keep infiltration structures away from foundations. The conventional minimum separation from a building is five metres, and on shrinkable clay near trees the interaction between wetting, drying and heave is a structural question rather than a drainage one.

Silt and the factor of safety

Permeable block paving clogs from the top down. Wind-blown fines, tyre-carried mud, leaf litter and construction silt all collect in the joint filling, and the surface infiltration rate falls year on year from the day it is laid.

Designers handle this with a factor of safety applied to the surface infiltration rate, commonly a factor of ten in the SuDS Manual approach, so that the pavement still accepts rainfall long after it has partially clogged. A new pavement accepts water far faster than any storm can deliver it, and the design question is whether it still does so in year 15.

Construction is where most of the damage happens. Do not use the pavement as a site haul route before the surface course goes down, do not store spoil on it, and do not let a wheel wash discharge across it. If the pavement has to serve traffic during the build, lay the sub-base and a temporary surface, then replace the laying course and blocks at the end.

Maintenance that keeps it working

A maintenance schedule is a planning document as well as an operational one, because the drainage strategy has to show the system will still function for the lifetime of the development.

The realistic regime for a domestic driveway is brushing loose material off a few times a year, keeping joints topped up with the specified grit rather than sand or soil, and a mechanical vacuum sweep at intervals set by how dirty the surroundings are. Once a year is a common starting point for a normal setting, more often where there is overhanging planting or an unsurfaced route onto the drive.

Larger sites should record the regime, the responsible party and the funding route. Who holds that obligation over decades is a question in its own right, and the note on SuDS adoption and long-term maintenance covers it before you assume a management company will pick it up.

Write into the schedule what to do if performance falls. Removing and replacing the top 20mm of jointing material restores most of a clogged pavement, and replacing the blocks is a last resort rather than a first response.

Adoption and the highway authority

Highway authorities remain cautious about adopting permeable surfaces on adoptable roads, and the position varies by county. Some will adopt with a commuted sum, some will adopt the carriageway but not the parking bays, and some refuse. Water and sewerage undertakers will adopt qualifying SuDS under the sector design and construction guidance where the system meets their standards.

Ask the question at pre-application rather than at reserved matters. A layout that assumes adoption and then has to switch to private management under a management company will need a different maintenance regime and a different section 106 clause.

Schedule 3 to the Flood and Water Management Act 2010 would change this picture by creating a SuDS approving body with its own approval and adoption duties. The position in England has moved slowly and should be verified before you rely on it, which is covered in the note on where Schedule 3 stands.

Specifying so the condition can be discharged

A drainage condition is discharged on drawings and calculations, not on intent. Give the authority a cross-section with layer thicknesses and materials named, the void ratio assumed, the storage volume, the discharge rate and the device that controls it, the exceedance route for storms beyond the design event, the infiltration test results with pit locations, and the maintenance schedule with the responsible party.

For a householder scheme, remember that laying more than five square metres of hard surface between the principal elevation and the highway is only permitted development where the surface is permeable or drains to a permeable area within the curtilage. Getting that right is often the whole planning question.

This is general guidance, not legal advice. Drainage requirements are set by your lead local flood authority and by the receiving undertaker, so check their standards before finalising a design.

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