The site. A site off Dilmore Lane in Fernhill Heath, on the northern edge of Worcester. The planning permission needed evidence that surface water could go to ground before a soakaway design would be accepted.
What we did. We dug trial pits at the proposed soakaway positions and tested them to BRE 365: each pit filled and timed three times, from 75% to 25% of its effective depth.
What we found. Infiltration was very low to negligible. The water level barely moved, and the ground could not take surface water at a rate a soakaway could use.
What it meant. A soakaway was not viable. That is still a result the Council needs: with infiltration ruled out by a proper test, the drainage strategy could move on to another way of discharging surface water without further questions.
The lesson. Much of the ground around Worcester is mudstone and clay. Testing before the drainage is designed saves redesigning it later.