Soakaway installation in Preston

Clay over sandstone: why results vary so much

There is sandstone deep under Preston, but across most of the city and its suburbs it is buried beneath a thick blanket of glacial till. That clay takes water slowly, and a trial pit dug into it will often fill and simply sit there. Two plots a mile apart can give very different percolation figures, depending on whether the dig reaches a sand lens, a gravel bed or nothing but clay.

The better ground tends to follow the water. The Ribble has left terraces of sand and gravel along its valley, and behind the dunes at Lytham St Annes there is wind-blown sand. Sites on those deposits can infiltrate. Sites on the till around Fulwood, Cottam or Leyland frequently can't.

The percolation test decides

Sizing a soakaway without testing is guesswork, and in central Lancashire guesswork tends to end with a waterlogged garden. Your engineer arranges the tests, reads the results and designs the structure, including its depth and how far it stays from buildings and boundaries. We build what that drawing shows.

When results come back marginal, we would rather raise it before anything is ordered. A crate block that cannot empty between storms just becomes a buried pond. On heavy ground the engineer will normally switch to storage with a throttled outfall to a brook or sewer. We build those as well; the 101m³ tank at Whittingham, north-east of the city, is one of ours.

Building it into the groundworker's pit

Your groundworker excavates and trims the hole. We check the base is level and firm enough for the crates to sit true, because a block that rocks puts strain on the wrap and on every connection.

• Permeable geotextile spread over the floor of the pit and taken up each wall, letting water out and keeping soil fines from washing in.
• Crates stacked inside it until the block matches the volume and the loading the drawing asks for.
• Inlet pipework and the catchpit or filter connections made to the drainage layout.
• The wrap closed over the top and lapped, ready for your contractor to backfill.
• A set of inspection records and a handover file for building control and the owner.

Keeping silt out in a wet county

A soakaway carries no membrane at all. The water is meant to leave through the sides and base, so the geotextile acts as a filter rather than a seal, and anything that blinds it shortens the soakaway's life.

Preston gets plenty of rain off the Irish Sea, and runoff from drives, yards and roads carries grit with it. Once fines reach the crates they clog the fabric from the inside and the infiltration rate drops year after year. The catchpits and filters in the design are what protect the structure, so we connect them with care and make sure the owner knows they need emptying.