Soakaway installation in Birmingham

The Birmingham Fault and what it means for infiltration

A geological fault runs roughly north-east to south-west across the city, and it divides Birmingham into two kinds of ground. To its west, Triassic sandstone underlies the city-centre ridge, Edgbaston and much of the south-west. To its east, Mercia Mudstone takes over through Yardley, Hodge Hill and out towards the airport. Sandstone can swallow a storm in hours; mudstone often can't take it at all.

So a soakaway on a Harborne infill plot and one on a Stechford housing site are very different propositions, even though they sit a few miles apart. Patches of glacial clay and sand lie over both rock types, which is why the percolation test, arranged by your engineer, is what settles where a particular plot really stands.

Your engineer sizes it, we build it

We don't dig and we don't size. Your engineer turns the infiltration rate into a crate volume, a depth and the clearances from foundations and boundaries, and your groundworker excavates to suit. We check the base is level and clean, line the pit with permeable geotextile, assemble the crates inside it, connect the inlet and the silt management, then close the wrap over the top.

A soakaway has no membrane, and that is deliberate. The geotextile lets water pass out into the sandstone while holding silt and fines back from the crates, so the laps where the wrap closes matter as much as the crate count.

• A flat, firm base with no loose spoil or brick in it
• The clearance to foundations shown on the drawing
• Catchpits or filters upstream, as the design shows
• Room to hand the crates down into the pit safely

Typical soakaway jobs in the city

Most of our Birmingham soakaway work is residential: small housing schemes in the sandstone suburbs, replacement houses on large Edgbaston plots, and extensions where building control wants the roof water dealt with on site. Parts of Sutton Coldfield, on sandy ground around the park, see similar work.

The sandstone beneath the city is also an aquifer. Engineers here pay close attention to the water table and to any made ground on the plot, because a soakaway should put clean roof and yard water into clean natural ground, never down through old fill.