Soakaways and Sustainable Drainage for Home Projects

A soakaway is a void in the ground that temporarily stores water and lets it seep out into the surrounding soil. Traditionally it was a pit filled with rubble or shingle. Modern practice uses modular plastic crates wrapped in a geotextile, which give far more storage volume per cubic metre of excavation — typically over ninety per cent void, compared with around thirty per cent for loose stone.

Water reaches the soakaway through a pipe from a gully, channel drain or downpipe, usually via a silt trap or catchpit. The silt trap matters: fine material carried in runoff will progressively clog the geotextile and the surrounding soil, and a soakaway that silts up cannot be cleaned out easily.

The system works only as fast as the surrounding soil can accept water. Storage buys time during a storm; infiltration is what empties it before the next one.

A soakaway is sized from two numbers: the area of hard surface draining into it, and the infiltration rate of the soil. The second is measured by a percolation test — digging a trial pit, saturating it, then timing how long the water level takes to fall between set points, repeated several times.

Guessing this is the most common mistake. A soakaway sized for free-draining sandy soil and built into clay will be overwhelmed the first winter, and the overflow will appear as a wet patch or a flooded lawn.

For a driveway, the design should cope with a substantial storm event without surcharging. In practice on clay this often means a larger structure than homeowners expect, and it is one reason permeable surfacing is frequently the cheaper overall solution.

Standard guidance keeps soakaways at least five metres from building foundations, and well clear of boundaries, retaining walls and trees. Water infiltrating close to a foundation, particularly in shrinkable clay, risks affecting the ground the building sits on.

They should not be sited uphill of a basement or below a slope where the water will simply re-emerge lower down. Existing services need locating before excavation, and any adopted sewer running through a garden brings its own build-over restrictions.

On small urban plots the five-metre rule alone often rules out a compliant soakaway, which pushes the design towards permeable surfacing, rain gardens or attenuation with a controlled outfall.

Rather than collecting water and infiltrating it at one point, a permeable surface lets it soak through across the whole area. Resin bound gravel over a permeable base, permeable block paving with open joints, and gravel all work this way.

The build-up matters as much as the surface. Permeable systems use an open-graded, clean stone sub-base — not standard MOT Type 1 with its fines — so the whole structure acts as a reservoir while remaining strong enough to carry vehicles.

This approach also removes the planning question entirely. A permeable front driveway does not require the planning permission that an impermeable one draining to the highway does, which for many Essex homeowners is the deciding factor.

The caveat is that permeable systems still need somewhere for water to go beneath them. On very heavy clay they act mainly as storage, and may still need an outfall or an overflow, though usually a much smaller one.

Sustainable drainage principles favour managing water where it lands, slowly, in the open rather than piping it away. A rain garden is simply a shallow planted depression that collects runoff, holds it briefly and lets it soak in while plants use some of it.

A swale does the same in linear form, moving water gently along a route while it infiltrates. Both handle heavy rainfall gracefully, need no maintenance beyond ordinary gardening, and look like planting rather than plumbing.

Water butts and rainwater storage take roof water out of the equation before it ever reaches the ground, which reduces the size of anything downstream. On a project where drainage capacity is tight, intercepting roof water is often the cheapest single improvement available.

Soakaways fail slowly and invisibly. The usual causes are silt entering the system, tree roots finding a permanent water source, and geotextile blinding as fines migrate from the surrounding soil.

Preventing this is a matter of keeping silt out: a catchpit before the soakaway, cleaned annually, does more for its lifespan than anything else. Gully pots and channel drain gratings should be lifted and cleared each autumn.

If a soakaway overflows every winter, it is either undersized, silted or sited in ground that cannot infiltrate. Adding a second one nearby rarely helps if the soil is the limiting factor; the honest fix is usually a permeable surface, a larger attenuation volume, or a genuine outfall.