What subsurface drainage do you need under a permeable pavement?

A good permeable pavement should drain significant quantities of water immediately through both the paving layer and paving base. This means that in all but the heaviest of rain events, very little water remains on the surface of the paving.

But what implications does this have for drainage? Do you still need surface drains for permeable paving? And do you need subsurface drainage? We’ll have a look at these common questions in this blog post.

Do you need surface drainage in a permeable pavement?

Short answer, yes. But not as many as in an impermeable pavement.

Whilst a good permeable pavement will usually drain all water through it, we can’t predict the weather! Surface drainage provides an additional safety net in preventing flooding off of the pavement in very heavy rain events.

We also can’t predict the future usage of the pavements we install. Whilst a good permeable pavement shouldn’t block under normal use, if something like sand is spilt on the pavement causing surface blockage, surface drains then become extremely important.

Surface drains also provide access points for cleaning the stormwater system should it become blocked in the future.

It’s for these reasons that most councils in Melbourne (who love permeable paving for the environmental benefits) still require surface drainage and surface falls on permeable pavements.

Do you need subsurface drainage?

The answer to this question is a little bit more complicated, and depends on a variety of site specific factors. So we’ll have a look at a few different scenarios. Remember, the primary aim of permeable paving is to get as much water to drain into the ground and water table as possible.

The best “rule of thumb” is visualise the pavement not as paving, but as a normal “soft” landscape element such as grass or a garden bed – and if you wouldn’t need subsurface drainage, then you probably don’t need it.

Sandy soils (with high drainage rates)

On sandy soils, soils that are free draining and relatively stable, then generally no subsurface drainage is required. Even in heavy rain events, the high drainage rates of the subsoil will be able to dissipate the water under the paving.

Clay soils (with low drainage rates)

On clay soils, the low drainage rates and more reactive soils mean that the decision whether to include subsurface drainage is a lot more complicated and is generally dependent on the gradient of the site. A variety of common scenarios are presented below.

On a flat site, or a site where ground falls away from the building

On a flat site, requirements for subsoil drainage are generally limited. Because the majority of water build up will occur on the slow-draining subsoil, it is essential that the subsoil is graded away from any building footings. It’s absolutely fine for water to build up in the permeable paving whilst waiting to slowly dissipate into the soil below, just as long as precautions are taken to protect adjacent structures and footings.

Normally on new developments in Melbourne, concrete pits are installed in all driveway areas as part of the drainage design. These pits typically need to be raised to finish surface level during driveway installations. During our permeable driveway pours, these pits are raised with permeable concrete which makes the walls permeable. If the subsoil can be graded towards the pits, then in heavy rain events when subsoil drainage rates cannot cope with the volume of water present, excess water can drain into the drainage pits through the permeable walls. See the schematic below for more detail.

ground falls away from building

In areas where there are no concrete pits (such as in tree protection zones where trenching is forbidden), then the grading of the subsoil needs to take into the account the expected amount of water that might puddle there. If building footings are of a concern, then additional subsoil drainage around the building footings may need to be installed, as per the detail below.

subsoil drainage around the building footings

On a site that falls towards the buildings

On a site where the building is lower than the street and the driveway must slope down towards the building, then subsurface drainage is required. As a broad “rule of thumb”, anywhere you would need a surface strip drain, you will need subsurface drainage too.

But the location of the subsurface drainage isn’t the same as the location of the grate drain. When considering the location of the subsurface drain, it should be remembered that a build up of water could occur in the paving. Therefore, the surface drain should be located slightly further back from the garage slab (e.g. a metre or two) to prevent build up of water directly adjacent to the garage footings. We also grade the subsoil back away from the garage towards the subsurface drain. Then finally, in order to achieve satisfactory driveway gradients, we thicken the permeable concrete base over the subsurface drain as per the detail below.

 permeable concrete base over the subsurface drain

What sort of subsurface drains should you use under a permeable pavement?

The design of the subsurface drainage should normally be part of scope of works for the project engineer. However, designers and engineers often omit permeable paving details due to a lack of standards for permeable paving.

Therefore we recommend that all subsurface drainage should utilise as a minimum 100 mm “AGI pipes” that are socked with a geotextile fabric to prevent blockage with fines over time, and to ensure good flow rates. All drainage trenches should be backfilled with a non-compressible, free draining material such as 14 mm bluestone screenings, or permeable concrete.

The subsurface drainage should also only be installed after the driveway excavations are completed, to ensure satisfactory subsoil grading.

We are able to complete subsoil drainage works as part of our excavation service, or they can be installed between the excavations and driveway pour by the project plumbers.

Subsurface drainage installed