Permeable Concrete vs Soakwells
Roof water still needs somewhere to go. The ground under a permeable concrete driveway stops being catchment, though, and the soakwells on your plan change with it.
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Permeable concrete vs soakwells: the short answer
A sealed driveway is catchment. Every litre that lands on it has to be sent somewhere, and on a WA block that somewhere is usually a soakwell fed by a grate at the garage door. A permeable driveway keeps its own rain, which takes the paved area off the drainage plan. That can shrink the soakwells that served the driveway, and on the right site the paved area needs none at all. How far that goes is for your drainage designer or council to decide, from your ground and your plans.
What a soakwell does
Perth sits on sand that takes water quickly, which is why a new house plan there usually shows several soakwells. Each is a buried chamber, concrete or plastic. It's open at the bottom and perforated around the sides, so as roof and hardstand runoff arrives through downpipes and grated drains, the chamber fills and water seeps out into the sand around it. An engineer sizes each one from the roof or paved area it serves.
Everything has to travel to it, though. That's the weak point. A pipe runs from every downpipe, and a grate sits across the foot of the driveway. On a wet July night, the chamber then has to hold everything they send at once.
How a permeable pavement holds the rain
Under the slab sits a layer of clean screenings, and that layer is the storage. Rain soaks into the pavement exactly where it hits and falls through the voids, which take up 20 to 25 per cent of the concrete, into the stone. You end up with a shallow reservoir as big as the driveway instead of a single chamber tucked in the corner.
For the paved area, then, the storage is part of the pavement, and depth decides how much. At 85 per cent design capacity, each square metre of screenings 100 mm deep stores 28 litres; a typical driveway base stores from 37 litres to 42 litres a square metre. The sand then takes it, the same as it would from a soakwell. No pipe carries it there, and it drains through far more ground.
When you still need soakwells
The roof is why most houses keep some soakwells. Its water comes down a downpipe as a concentrated flow, and that needs a soakwell or another outlet designed to take it; a downpipe turned onto the driveway isn't drainage design. Any hardstand left sealed, such as the alfresco slab, keeps shedding water as well.
What changes is the size of the job. The engineer sizes soakwells for the catchment that's left, and the permeable paving has dropped out of it. On a typical block that can mean fewer soakwells, or smaller ones. Where the old soakwells already overflow in a big winter storm, pulling the driveway off them can give them the room to cope.
Slow ground changes the base design. On some blocks it's clay; on others it's a water table that climbs through winter, or a cemented layer under the sand. Each one means the ground can't take water as quickly, so the screenings go deeper to hold more and an under-drain releases water at a controlled rate. Whether soakwells still suit the rest of the site is worked out from the same soil.
The answer for your block
Your drainage designer and your council settle it, and what we give them is a pavement design they can check against their own calculations. We will work with anyone that wants the Permcon product. We just need to make a design that suits their needs. On your block that starts with an estimator in WA looking at how quickly your ground takes water and which roof and hardstand areas still drain to soakwells, then putting the base depth and storage for your paving in writing.
Engineers who want the storage figures for each profile early will find them on our technical page. Permcon in Perth covers slab and finish options and how a job runs, and our Western Australia page covers the rest of the state. When you're ready, send the address and the plan that calls for soakwells and we'll tell you what a permeable surface could change.
Soakwell questions
With a permeable driveway, can the soakwells go altogether?+
Sometimes, for the driveway's share. A soakwell that only ever took the paved area's runoff can come off the plan. The roof still needs its own outlet, so most houses keep some soakwells (the section on when they're still needed explains why). Your drainage designer makes that call on site.
Will my council accept a permeable driveway over soakwells?+
Victoria's Clause 55 guidance, for one, counts pervious driveways toward the minimum 20 per cent of a site that should let water reach deep soil. Your council applies its own rules, on your drawings, provided the design shows the driveway holding and soaking away its own rain. We supply the storage figures your designer needs.
Where does the roof water go, then?+
Into soakwells, usually, sized by your engineer for the roof area alone. The permeable driveway only handles rain that lands on it. Take the driveway out of the sum and those soakwells carry the roof and nothing more, which is where any saving in chamber size comes from.
What about a clay block?+
Clay doesn't rule it out. The base does more of the holding, and water leaves through the clay slowly rather than soaking straight down, with the under-drain described above taking the rest. On the surface nothing changes: it still drains the moment rain hits. A soakwell sitting in clay slows down for exactly the same reason, so its size changes as well, and that's why we look at the soil before anyone sizes anything.
Before the soakwells go in, ask us
Send the address and your drainage plan. Catching it at plan stage means the soakwells are sized once, for the catchment that's actually left.
Check if permeable concrete suits your site
A few quick questions about your site. You get a straight answer about whether permeable concrete is the right call, including when it isn't, before anyone asks for your details.
What are you surfacing?
Tell us about your project
Tell us where the site is and what you're planning. An estimator from your state team will call you back, look at the site with you and give you a real figure, from a single driveway to a council car park.