Soakaway installation in Liverpool

Sandstone, till and the coast

The rock under the city is the same red sandstone its cathedral was built from. On much of Merseyside it is buried under glacial clay, which drains poorly, but on the higher ground and the Wirral ridges the clay is thinner and infiltration rates can be good. North of the city, the blown sand behind the Sefton coast drains freely too.

Good drainage is only half the test. The sandstone is an important aquifer, and in the dunes and near the estuary the groundwater may lie only a short way down. Your engineer has to show enough depth between the base of the soakaway and the groundwater, and decide what treatment the runoff needs before it goes into the ground.

Where soakaways get built

Most of the Liverpool soakaways we see are on residential and light commercial schemes in the suburbs and the Wirral: small housing developments, school buildings and single units with space for the clearances from foundations. On the waterfront and in the docks the ground is too wet or too contaminated, and the drainage goes to a sealed tank.

Woolton and the ridge running through Childwall and Allerton are good examples of where the test results can come back strong. Low-lying ground toward the Alt and Ditton Brook is usually less promising.

Keeping the silt out

A soakaway in sand or sandstone works for as long as its faces stay clean. Fines carried in with the runoff gradually clog the surrounding ground, so the design puts a catchpit or filter upstream. We make those silt management connections as part of the install.

• Formation checked first
• Wrap that passes water, not fines
• Crates to the design volume
• Inlets and silt traps connected
• Wrap closed for backfill

Who does what

Your engineer arranges the percolation testing and designs the system. The hole belongs to your groundworks contractor, at both ends of our visit: they open it before we arrive and fill it once we've gone. In between, we build the wrapped crate structure, inspect it and leave you the records building control will want under Approved Document H.

The Wirral side

Across the river, the Wirral has the same sandstone under much of it, showing at the surface on ridges like Bidston Hill and Thurstaston. Development around Heswall, West Kirby and Greasby is mostly housing on generous plots, which leaves room for the separation a soakaway needs. Closer to the Dee and the north Wirral coast, the ground is lower and the groundwater nearer the surface, and results there are more mixed.

We cover both banks of the Mersey from the same team, so a soakaway in Birkenhead gets the same checks and records as one in Woolton.