Soakaway or Attenuation? Ground Suitability Triage

The same StormGrid module becomes a soakaway when it is wrapped in geotextile and an attenuation tank when it is wrapped in welded impermeable membrane. Your ground decides which, and getting it backwards buries a bathtub under your pitch.

Subgrade soil type Percolation testing to BRE 365 settles this, not a borehole log alone.

Maximum likely groundwater level

Known ground contamination

Surface above the tank

Recommended build

Attenuation

Lined attenuation tank with a controlled outfall

Clay will not percolate, so an unlined void becomes a bathtub. Store the water and meter it out at your consented discharge rate.

This triage does not replace a percolation test. National standards require the base of any infiltration system to sit clear of the maximum likely groundwater level, and permitted discharge is the greenfield runoff rate or 3 l/s/ha, whichever is the greater. Where that drives an orifice under 50 mm, the flow control must be protected against blockage.

Send your ground investigation for a build recommendation