Soakaway installation in Manchester

Finding ground that drains in a clay city

The boulder clay left by the last ice sheet blankets much of the conurbation, and it passes water very slowly. Infiltration tests on a typical plot in Moston, Gorton or Wythenshawe tend to fail, and the scheme moves to attenuation instead. That is the right outcome, and we would rather see it than build a soakaway that stays full.

The exceptions are worth knowing about. Sand and gravel lie along stretches of the Mersey and Irwell valleys, and in places the clay thins over the red sandstone beneath Salford and Trafford. A site there can pass its tests comfortably. The catch is that the same valley ground can carry a high water table, and a soakaway needs depth between its base and the groundwater, so the engineer checks both before signing the design off.

The jobs that suit a crate soakaway

Where the numbers work, the sites are usually suburban: a short run of new houses in Sale or Didsbury, a school extension, a small commercial unit with a modest roof. There is space for the clearances from foundations and boundaries, and the runoff is clean enough for the ground to take it once the silt has been caught upstream.

Dense city-centre plots rarely qualify, even on good ground, because the separation distances eat the whole site. Those go to a sealed tank with a flow control.

What we build inside the pit

Your groundworker excavates and levels the pit. We take it from there, then hand it back for your contractor to backfill.

• A look at the formation first, so the crates sit flat and true.
• Permeable geotextile laid out to let water through and keep fines out.
• Crates stacked to the engineer's storage figure, in whichever load rating they picked.
• Inlet pipes and the silt protection the design calls for, usually a catchpit or filter upstream.
• The geotextile folded and closed over the top, ready for cover.

Silt is what shortens a soakaway's life

A crate structure in sandy Mersey valley ground can keep draining for decades, provided its faces stay open. The threat is fine material: road grit off a drive, soil washed from new landscaping, leaf litter from the mature trees that line so many south Manchester streets. Catchpits and filters upstream trap it, and the owner empties them. Our crew connects them to the drawing, and we'll mention it if a lid ends up somewhere awkward to lift.

Records for building control

Soakaways sit under Approved Document H, which puts infiltration first in the order of preference when the ground allows it. Building control will want to see that what went in matches the approved design. We inspect the finished structure and hand over the records and documentation, so the sign-off doesn't stall on paperwork.