How does permeable paving work?

If you’re reading this article, it’s likely that you’ve had a quick look around the website and have worked out that the aim of permeable paving is to get water to drain through the ground, and not into the stormwater system.

But how is that actually achieved? And how do we do it? We’ll have a look at that in this blog post.

What is a permeable pavement made of?

Any pavement (permeable or not!) is made up of various layers. In a permeable pavement, there are typically two key layers – the surface layer, and the base (which supports the pavement). In a typical “permeable paver scenario this is the surface layer of paving blocks on a bedding course of 7mm no fines aggregate and then a sub base of a free draining aggregate (such as a 14-20 mm no fines crushed rock).

In our pavement design, we utilise a surface course of resin bound paving over a fully reinforced custom permeable concrete base. The surface layer, 20 mm of Pebble Pave is a hard wearing surface made of interlocking rocks bound with a UV stable, long lasting polyurethane resin. It looks very similar to exposed aggregate concrete, but the gaps between the rocks allow water to drain through. The base course is a custom permeable concrete mix (the mix design? a secret, don’t ask…) featuring hot dip galvanised reinforcing steel. The base course, like any pavement, does the heavy lifting, with the thickness of the concrete and grade of steel selected according to the intended use and desired weight tolerance of the pavement.

What drainage requirements do you need in a permeable pavement?

… In theory, none! But, even with the most permeable of pavements, it’s worth considering a scenario where the pavement doesn’t drain water through it. The way we tell it to our clients is that we can’t control the end user. In a typical installation, a permeable pavement remains permeable through it’s lifespan. But, what if someone doing some adjacent landscaping (who hasn’t read our aftercare documentation) dumps a load of soil on the pavement, blocking the surface pores?

It’s for this reason that, in regards to drainage, we always follow paving and concreting best practices, and relevant standards such as the NCC and AS 3727-1993 (Guide to residential pavements). Basically, we always grade the pavements away from buildings, to drainage pits or garden beds, and install trench grates at garage entrances if the site slopes towards the dwelling.

Surface falls on a typical permeable pavement installation

What also needs to be considered is the drainage below the pavement. Most of Melbourne is on clay soils, which do not absorb water quickly, and are highly reactive (meaning they swell and shrink with moisture changes). Without giving away too many of our trade secrets, proper soil preparation prior to pavement installation is key to avoiding excessive pavement movement and possible failure. Also, designing a subsurface drainage strategy is important to prevent moisture buildup under adjacent structures (something we’ve written about here).

Do you need anything else to control the soil underneath a permeable pavement?

Sometimes a significant amount of water can be be expected to be present under a permeable pavement. As mentioned above, subsoil drainage is important. Also, if water is expected to flow through or under a permeable pavement, then soil stabilisation might need to be considered. If the soil type is very loose and sandy, then geotextile fabric can help keep the water from rutting out the sand under the pavement.

Another scenario that requires geotextile fabric is if the site gradients result in a puddle forming in the middle of the pavement. Whilst this is not a structural concern for the pavement, it can bring fines (i.e. mud) from the muddy ground up with the water and through the top of the pavement. In this scenario, if geotextile fabrics are used under the pavement then this can prevent the upward migration of fines to the surface. A great example of where this is important is in car park tree surrounds, where large amounts of surface water is drained to permeable paved tree pits.

Our recommendations for a perfect permeable tree pit.

What’s the best way to achieve a perfect permeable pavement?

Honestly, the best way to achieve a long lasting, stable permeable pavement is to engage a permeable pavement professional. There are a lot of subtle nuances that make a huge difference in permeable paving longevity, and frankly you’re putting your whole project at risk if you use a regular landscaper or concreter for permeable paving. If you’d like to know more or  chat about some upcoming projects then get in touch with us today! Call (03) 9543 3013 or email [email protected]