Soakaway installation in Derbyshire

Rock that can take water, and rock that can't

The county has a few ground types that tend to drain. The Magnesian Limestone ridge around Bolsover and Whitwell is permeable, and so are the sand and gravel terraces along the Trent, the Dove and the lower Derwent. The Carboniferous limestone of the White Peak passes water readily too, though its open fissures mean the engineer has to think about where the water goes once it leaves the crates.

Elsewhere the picture is poorer. The Mercia Mudstone around Derby, the Coal Measures of the north-east and the shales of the Dark Peak usually give slow results. On those, a soakaway that can't empty between storms is worse than none, and attenuation is the honest answer.

Before the pit is dug

Your engineer arranges the percolation testing and sizes the soakaway from it, along with its depth and distance from buildings. In the river valleys the winter water table matters as much as the test rate, because a crate structure sitting in groundwater has nowhere to drain.

We work from that design. If the numbers look marginal to us, we raise it before your groundworker opens the ground, not after the crates are in.

Building it

Our crew takes over once the pit is formed and works through a fixed sequence:

• Formation inspected so the crates bed level
• Permeable geotextile spread across the pit to stop fines reaching the structure
• Crates assembled inside the wrap to the design volume
• Catchpit or filter pipework coupled up as the drawing shows
• Geotextile lapped over the top, then the job inspected and written up

Typical Derbyshire jobs

Many are small: a barn conversion in a Peak village, a new house on an infill plot in Ashbourne, an extension that adds roof area. Others are housing phases on gravel near Hilton or Melbourne, where several soakaways go in as the roads advance. Each one gets its own inspection record.

Approved Document H puts infiltration ahead of a sewer connection where the ground allows it, which is why building control will often ask for a test before anything else on a small Derbyshire plot.

On Peak village plots the access can be the hardest part. Crates come flat-packed or stacked on pallets, so they can be barrowed through a gateway that a lorry could never reach, and the structure is built up by hand inside the pit.