The site. The Musketeer, Avon Road, Kidderminster, where a proposed development needed soakaway testing for its planning application.
What we did. On 18 September 2018, in dry, bright weather, we dug two trial pits:
- Pit A: 350 mm wide, 1,000 mm long, 1.6 m deep
- Pit B: 450 mm wide, 1,100 mm long, 1.5 m deep
Each was filled to a water depth of 1 m and tested in line with BRE 365 and BS 6297. For BRE 365, each pit had three fills, each timed as the water fell from 750 mm to 250 mm.
What we found. No groundwater came into either pit. The water fell steadily in both:
| Pit | Test 1 | Test 2 | Test 3 |
|---|---|---|---|
| A | 160 minutes | 170 minutes | 195 minutes |
| B | 264 minutes | 280 minutes | 292 minutes |
The slowest run (Pit B, test 3) gives an infiltration rate of about 6.9 × 10⁻⁶ m/s.
What it meant. The ground was suitable for soakaways, so the engineer could design them using the slowest rate recorded, subject to the design depth and size.
The lesson. In both pits each fill took longer than the last, as the ground around the pit wetted up. That’s why BRE 365 asks for three fills and the design uses the slowest, not the first or the best.
Need a percolation test for your own site? See how we test and report.