Soakaway installation in the Black Country

Where the ground can take it

The western and southern fringes sit on different rock from the industrial core. Around Stourbridge, Tettenhall and the western edge of Wolverhampton, Triassic sandstone lies near the surface, and on undisturbed ground it can drain well. That is where most of the area's soakaway work is found.

Across the middle, from Tipton to Wednesbury and Darlaston, it is another story. The Coal Measures there are clays and mudstones with old shafts and seams beneath, and the top layer is often made ground. Infiltration on that is usually ruled out, and for good reason.

Why your engineer may say no

Water fed into made ground can pick up contamination and carry it down. Water directed at shallow workings can wash out fill and open voids. Under parts of Dudley the limestone was mined out in great caverns, and nobody wants a storm sent into those. The site investigation and percolation testing separate the plots that can infiltrate from the ones that can't.

Where the answer is no, the usual alternative is a lined attenuation tank, which we also install.

• Made ground or fill of unknown make-up
• Shallow coal or limestone workings
• Contamination found by the site investigation
• Poor percolation through Coal Measures clay

Building it on a site that passes

When the results support a soakaway, the job is simple. Your groundworker digs to the engineer's depth and clearances. We check the base, line the pit with permeable geotextile, stack the crates inside to the designed volume, make the inlet and silt-management connections and fold the wrap shut. It goes back to your contractor ready to backfill, with inspection records for building control.

Fine silt washing off yards and drives will clog a Black Country soakaway long before the crates wear out, so the catchpits and filters upstream matter. We connect them as drawn and point out anything that would let fines through to the crates.

Sandstone plots on the western side

Most of what we build here is for housing on the Wolverhampton and Stourbridge side: small estates, single plots and extensions where the sandstone is close under the topsoil. Those sites are often tight, with neighbours close by, so we keep the crates on pallets inside the boundary until the dig is open and build out from one end.

Aldridge and Streetly, on the Walsall side, can also turn up sandy, free-draining ground. As everywhere here, it's the test pit and the percolation figures that decide, not the postcode.