Driveway soakaways & infiltration drainage
In short
A soakaway takes collected surface water and lets it infiltrate into the ground rather than discharging it to a sewer or the road. Sized for the area draining to it, sited well clear of foundations, and only worth building where the ground will actually accept water — which is a test, not an assumption.
Also called: Install a soakaway · Soakaway crate · Driveway soak away · Infiltration drainage
Signs you need this
- An impermeable drive that currently discharges to the road
- Needing to satisfy the front-garden planning rule without permeable paving
- A channel drain with nowhere sensible to discharge to
- An existing soakaway that fills and backs up in heavy rain
- Rainwater downpipes discharging onto the drive surface
- Ground that is known to drain reasonably well
A soakaway is a void in the ground — traditionally rubble, now more often modular plastic crates wrapped in geotextile — that receives collected water and lets it soak away into the surrounding soil over the following hours. It is the first option in the Part H drainage hierarchy for a reason: it puts the water back where it fell, keeps it out of an overloaded sewer network, and satisfies the front-garden planning rule in the process.
Whether one will work on your ground is a question with an actual answer, and it is worth getting before spending anything. A percolation test — digging a pit, filling it with water and timing how fast the level drops, repeated — tells you the infiltration rate, and that rate plus the area draining to the soakaway determines how big it has to be. Free-draining sand and gravel takes water quickly and needs a modest structure. Heavy clay may barely take it at all, in which case a soakaway is the wrong solution and something else is needed.
Siting is governed by common sense and by a rule worth knowing. A soakaway should sit at least five metres from a building's foundations, because saturating the ground next to a footing is a good way to cause the settlement you were trying to avoid. It also wants to be clear of boundaries, away from trees whose roots will find it, and downhill of the area it serves so water actually reaches it.
Sizing is a calculation rather than a guess, and the calculation is not complicated: the volume has to store the design rainfall event falling on the connected area faster than the ground can take it away. A large drive on slow-draining ground needs a substantial structure. This is exactly where the cheap version fails — a builder's rubble pit the size of a wheelie bin under a hundred square metres of tarmac fills in ten minutes of heavy rain and does nothing for the rest of the storm.
Modern crate systems are a genuine improvement on the old rubble-filled pit. Modular plastic cells give around 95% void ratio against roughly 30% for rubble, so the same storage takes far less digging; they are wrapped in a geotextile that keeps silt out and soil from migrating in; and they can be built to a shape that fits the space available. They also need a silt trap upstream, because a soakaway that fills with silt is a soakaway that has stopped working and cannot easily be cleaned.
One caution about permission and about neighbours. Concentrating your drive's run-off into one point in the ground affects the ground around it, so a soakaway close to a boundary can genuinely become a neighbour's problem. Keeping it well within your own land, sized properly and clear of structures on both sides of the fence is both good practice and the way to avoid a dispute that outlasts the driveway.
What the job covers
- Percolation testing to establish whether infiltration is viable at all
- Soakaway sized for the actual area draining to it
- Sited at least five metres from foundations and clear of boundaries
- Modular crate or stone-filled construction as appropriate
- Geotextile wrap to keep silt out and soil from migrating in
- Silt trap or catchpit upstream so the soakaway stays serviceable
- Overflow route designed for events beyond the design storm
- An honest answer if the ground will not take it
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your contractor has to honour it. On driveways it would also be close to meaningless: the groundworks are usually the biggest single factor, and how deep the dig has to go varies enormously house to house. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
How do I know if a soakaway will work here?
A percolation test — a pit dug, filled with water and the drop timed, repeated a few times. That gives an infiltration rate, and the rate plus the connected area gives the size needed. It is a modest cost and it is the difference between a soakaway that works and a hole that fills up and stays full. On heavy clay the answer may be that infiltration is not the right approach.
How far from the house does it need to be?
At least five metres from foundations is the standard guidance, and further is better on shrinkable clay. The reason is straightforward: repeatedly saturating the ground beside a footing is a way of causing settlement. It also wants to be clear of the boundary, away from trees and downhill of the area it drains.
Are crates better than a rubble-filled pit?
Generally yes. Modular crates give roughly 95% void against about 30% for rubble, so the same storage takes a much smaller excavation, and they can be shaped to fit the space. Both need a geotextile wrap and a silt trap upstream. A rubble pit still works — it just has to be considerably bigger than most people build it.
My soakaway keeps backing up. Why?
Most often it is undersized for the area draining to it, or it has silted up because there was no silt trap upstream. Occasionally the ground around it has become saturated because it is too close to something else, or the water table is high. Digging a bigger hole is not always the answer — the diagnosis comes first.
Need this doing in Macclesfield?
Tell us the job and it goes straight to our local contractor — one call back, usually the same day. Free quote, no obligation.