Soakaway or Attenuation? Ground-Suitability Triage

Before a crate is sized, the ground decides how it can work at all. Infiltration, attenuation to a controlled outfall, and a fully tanked system are three different products — and getting this wrong is the mistake that only shows up in the first big storm.

1. Has a percolation test to BRE 365 been carried out?

Yes — it infiltrates readily Yes — infiltration is slow or negligible Not yet

2. Ground conditions at the proposed crate depth

Sand, gravel or other granular soil Mixed / silty, or made ground Clay

3. Groundwater and constraints

Water table well below the crate base; no protection zone High or seasonally high water table Source protection zone or contaminated ground

Answer all three questions first.

Request a scheme-specific management-train review

This triage reflects the decision logic an engineer applies before sizing; it does not replace a percolation test, a ground investigation, or the drainage strategy submitted with the planning application. Where infiltration is ruled out, the crate field still works — as attenuation storage discharging to a controlled outfall.