Soakaway installation in Wolverhampton

Two sides of the city

West of the ring road, much of the ground is Triassic sandstone, the same rock that shows in the cuttings along the Smestow valley. Percolation tests on it frequently pass, and on housing plots and estates out towards the South Staffordshire villages a soakaway is often the first system the engineer reaches for.

East of the centre the picture changes. Bilston, Ettingshall and Wednesfield sit on the Coal Measures, with clays, old workings and a lot of made ground on top. Infiltration there is rarely allowed, and the design usually becomes an attenuation tank instead.

Good ground still needs care

The sandstone is a principal aquifer, so the engineer has to think about what the runoff carries before letting it in. Roof water is one thing; water off a yard or a car park may need treatment upstream. That is why silt management and any interceptor or filter on the drawing matter as much as the crates.

Sandstone can also be fissured, and Approved Document H keeps soakaways a safe distance from foundations. On the tight plots around Tettenhall and Penn, where the garden is small and the neighbours are close, the position on the drawing has been chosen carefully. We set out to it exactly.

Building the soakaway

Sandy ground brings fines with it, and fines are what kill a soakaway. We keep the geotextile clean while we work, close it with full overlaps and check that every inlet passes through the silt management first.

• Pit dug by your groundworker; base looked over by us
• Permeable geotextile spread to enclose the whole structure
• Crate units built up inside to the volume on the drawing
• Catchpit, filter and inlet pipework connected
• Geotextile lapped over the top for others to backfill
• Inspection record and handover documentation

Estates as well as single plots

Larger housing schemes on the western edge sometimes use a series of soakaways under open space or private drives rather than one big structure. Each one gets its own inspection record, so the paperwork lines up with the drainage layout.

Single plots are just as common: a new bungalow in Perton, a garage conversion in Finchfield that adds roof area, a replacement house on a large Tettenhall garden. Building control on those jobs wants to see that the soakaway went in as drawn, and our handover note covers the volume, the wrap and the connections. Where access is through a side gate, the crates come in by hand.