Soakaway installation in Cambridgeshire

Where infiltration has a chance

The chalk rises gently south and east of Cambridge, towards Sawston, Linton and the Gog Magog Hills, and it can drain freely. Sand and gravel left by the rivers underlie parts of St Ives, Huntingdon, St Neots and the villages along the Cam, and they can drain well too, provided the water table is low enough.

Across the fenland of the north and on the gault clay that runs through the middle of the county, a soakaway rarely passes. Peat and silt sit over high groundwater, and the clay barely lets water through. On those sites your engineer will normally specify attenuation instead.

Chalk and drinking water

The chalk under south Cambridgeshire is an important aquifer for public supply, and the Environment Agency defines source protection zones around the boreholes that draw from it. Inside those zones, discharging runoff into the ground can be restricted or need extra treatment upstream.

That check sits with your engineer at design stage, together with the percolation test at the depth of the soakaway and the highest recorded groundwater level.

Building it into the dig

We go through the design and the test results before your groundworker excavates, and question anything that doesn't fit, such as a base that would sit in seasonal groundwater on a gravel site. Your groundworker digs the pit and forms the base; we check it is level and clean before starting.

A permeable geotextile goes round the pit first; water passes through it freely, but silt and chalk fines stay outside the crates. Crates are assembled inside to the designed volume and loading class, the inlet is connected along with any catchpits or filters upstream, and the wrap is closed over the top. The finished soakaway is handed back closed and ready for your backfill, and building control gets our inspection sheets and handover pack.

• Driveway and roof soakaways on chalk village plots
• Larger soakaways under car parks on gravel sites
• Install-only, using crates and geotextile you have bought

Keeping it working

The crates need no attention once buried. What fails a soakaway is silt, so the catchpit or filter upstream should be lifted and emptied on a routine, especially where a drive or yard drains chalky or sandy grit into it. Silt that gets past a neglected catchpit into the crates can only be removed by digging them up.