A soakaway is a hole in the ground, filled with something that holds water in the gaps — crates, or clean stone — that takes rainwater from a roof or a hard surface and lets it soak away into the surrounding soil instead of running into a sewer.
That is the whole idea. The complications are all in the sizing.
Why it has to be sized, not guessed
A soakaway has to hold the water that arrives in a storm for long enough for the ground to absorb it. Two numbers decide how big it needs to be: how much water will arrive, and how quickly the ground takes it away.
The first comes from the area draining into it and the rainfall figures for the location. The second can only be measured on site, which is what a percolation test to BRE Digest 365 does. Build one too small and it backs up and floods; build one enormous to be safe and you have spent money on excavation and crates you did not need.
Why the ground varies so much
Two plots a few hundred metres apart can behave completely differently. Sand and gravel drain freely. Clay barely drains at all. Most sites are somewhere in between, and many are layered — free draining sand over an impermeable clay, or made ground of unknown composition sitting over both.
Made ground is the one that catches people out. On former industrial land, and across much of the Black Country, generations of quarrying, brickmaking and shallow coal working have left fill of very variable depth. Infiltration through it can be quick in one trial pit and almost nil in the next, a few metres away.
Where a soakaway can go
There are rules, and they rule out more sites than people expect:
- It must sit a minimum distance from buildings so that water soaking away does not affect foundations. Approved Document H gives 5 metres as the usual figure.
- The base must stay clear of the water table. If groundwater rises to meet it, it cannot work, no matter how well the soil drains.
- It should not sit where the water will re-emerge downslope or run onto a neighbour’s land.
- It needs to be reachable for maintenance, and not underneath something you cannot dig up later.
What happens if the ground will not take it
Then infiltration is ruled out, and that is a legitimate outcome. Planning authorities work down the SuDS hierarchy: infiltrate if you can; if you cannot, discharge to a watercourse; and only then to a sewer. You are expected to show evidence for ruling each option out, and a properly conducted percolation test is that evidence.
It is also what a water authority wants to see under a Section 106 application before they will accept a surface water connection.
So a failed test is not wasted money. It is the document that lets your engineer design something that will actually work, and get it approved.