Determining the type of base material and how deep to make it for pervious concrete depends on a number of factors. First thing is to determine if any is even needed. If the subbase is primarily a well-draining sandy soil, then often pervious concrete can be placed directly on this soil with no additional base material needed.
This little pervious concrete driveway was placed directly on the sandy soil of San Francisco’s Marina District. Note: the uneven color is the pavement drying from the morning’s fog.
More commonly however is that base material will be needed under the pervious concrete. How much and what depends on how well the subsoil drains, how much slope is present, what percentage of rainfall one is trying to capture and if other elements are to be combined into the system, and structural requirements.
Since pervious concrete’s ability to absorb water is typically well in excess of 200″ an hour, the soil below is usually the limiting factor in being able to absorb and retain all rainwater that falls on a site. Primarily for this reason, a reservoir is needed below the pavement to help store the captured water until it has an opportunity to infiltrate into the slower draining subbase soil.
If capturing a certain percentage of water onsite is a requirement to satisfy local stormwater laws, then the infiltration rate of the subsoil should be determined. A double-ring infiltrometer per ASTM D 3385 can be used to measure this. If other elements, like roof runoff, are also being directed to the drain reservoir, then that needs to be added to the equation as well.
One note of caution here, it is generally not a good idea to use a permeable system as a big ‘catch basin’. Pervious concrete drains very well for the life of the pavement, typically well in excess of 20 years, when no runoff is directed on to the surface of it. This is because of the fines and other debris that surface runoff brings with it. On its own pervious concrete can flush out and filter dust and other pollutants that make their way directly on it. So avoid having soil and mulch drain on to the pavement and especially avoid having asphalt do it. Asphalt in particular has flecks of tar that can really get into pervious concrete and begin to clog it up.
As a general rule of thumb we typically design approximately 6″ thick vehicular pervious concrete pavements to have about the same, 6″ rock reservoir below them. For pedestrian 3 1/2″ thick pavements, 3 1/2″ of rock reservoir is typically called out for them. The rock that is used is typically a 3/4″ crushed drain rock (hopefully with as little dust and fines as possible), which has roughly 40% void space. So in general for a 6″ rock reservoir we end up with about 2.4″ of water storage. Much as in a french drain, to maintain the rock’s ability to store water it is protected from the soil below and on the sides with a fabric, typically a 4 ounce nonwoven geotextile.
This is a simple calculation because it does not take into account the slope of the site.
In almost all situations there is some slope involved, which is generally a good thing, because we want the subbase to be sloping away from any structures. Depending on how much of a slope is present, it can often be minimized by excavating deeper in one area than another while still maintaining some positive slope away from any structures.
On moderately steeper slopes, from about 2 to 5%, simple dams can be created
(ideally these are matched with joints in the pavement above, since any changes in the base material can cause uneven settling in the pavement potentially leading to cracks). Baserock, well-graded rock with lots of fines, can make a good material for these dams. It is best to place the baserock directly on the subbase and then cover with the fabric to keep the fines from infiltrating into the clean drain rock reservoir. Pay careful attention to compact these damns well.
In fact all the base material should be compacted.
Drain rock, even though it is minimal, does compact some, and even more so the thicker it gets. One needs to be careful however not to overcompact the subbase so that its infiltration abilities aren’t diminished. Typically around 90% compaction for the subbase is recommended under pervious concrete pavements.
On slopes steeper than about 5%, implementing a french drain type channel across the slope is a good way to capture water and mitigate any affects from erosion.
Structural considerations can also play a role in how thick to make the base for pervious concrete. As a general rule, just as ordinary pavements can benefit from base material between the pavement and the subsoil, so can pervious concrete. This base can add to the overall structural integrity and provide a bit of a buffer between any movement in the subsoil and the pavement itself. This benefit can be particularly helpful when placing pervious concrete over expansive soils.
We have placed a lot of pervious concrete in the South Bay and Peninsula regions of the San Francisco Bay Area where heavy clay (expansive) soil can be prevalent and have had very good results with little to no cracking. Unlike ordinary pavements where water can only get into the subbase from the sides and thru cracks, which can lead to uneven subbase expansive pressures, pervious concrete pavements allow water to infiltrate equally throughout. This allows the subbase to expand evenly and so raises and lowers the whole pavement as one.
The depth of base material for a pervious concrete pavement can also be dictated by freeze/ thaw. In climates with hard winters, this base should at least extend below the frost line, just as it typically does for ordinary pavements in these conditions. Also pervious concrete has been shown to perform quite well in freeze/ thaw conditions (link to another blog). A thicker base that allows for all the water to drain out of the pervious concrete does seem to be beneficial as well.
Other than in heavy freeze thaw locations, drain rock or at least a lot of it is only necessary under pervious concrete for its reservoir/ water holding capabilities and the desire to capture and retain most or a designed for amount of water that falls on a site. If the pervious concrete is for a residence or other application that does not have to meet certain stormwater or run-off requirements, then one may be perfectly content with capturing the majority of rain. For most soil types, even clay soils in more arid environments, roughly 60 to 80% of rain that falls on a pavement in a year could be retained onsite even without a reservoir below it.
If one doesn’t require a reservoir for the pervious concrete pavement, some base material is generally still helpful just as it is for ordinary pavements. The base material would provide stability to the pavement, be well draining and provide some buffer between potential movement in the subbase and the the pavement. We have often installed our PerkPave pervious concrete just 2″ to 3″ thick over 2″ to 3″ of well compacted baserock, well-graded crushed rock with fines. We then also place a thick, 8 ounce nonwoven geotextile fabric on top of the base to further stabilize it and to reinforce the pervious concrete since it bonds well to it. This makes a great, far more durable, clean, no maintenance substitute for fines/ decomposed granite pedestrian pathways.