Using Permeable Pavement on Steeper Roads
Using Permeable Pavement on Steeper Roads
(OP)
I’m in Southern California and working on a single family residential project with a fairly long driveway (600 feet) from the main road to the house. The project is required to provide water quality treatment. Instead of putting in typical impervious pavement for the driveway, I’m using a pervious (porous) pavement design. The owner likes the idea and I think it’s got a lot of advantages such as reducing runoff, providing water quality treatment, and providing groundwater recharge. All good.
The pervious pavement will incorporate a porous (no fines) Portland cement concrete surface, an open graded gravel base (4 inches or more) and possibly a geo-filter below that. The gravel base has a void ratio of 40% so it will provide short term storage of runoff that percolates through the concrete above. As an example, 4 inches of gravel can store 1.6 inches of water. Over time (which could be fairly quickly if subsurface soils have a good percolation rate) the stored runoff will infiltrate into the soils below. By local ordinance, I’m required to store one inch of runoff over the pavement area.
The issue is this: Most of the driveway is fairly flat (1% to 2% grade), however, due to grading constraints a portion of the driveway will need to be much steeper (10% to 20% grade). Pervious pavement lends itself best to flat or nearly flat paved applications. On a steeper grade, the water will tend to collect in the base material at the lower end of the road and actually start to come out of the pavement surface if there’s enough water (simple fluid hydraulics at play here). So the effectiveness of the permeable pavement is greatly reduced in this case.
Some potential solutions to this that I have heard of include a) putting in impervious “check dams” across the road to create a series of smaller, more level base areas, b) installing additional rock trenches across the road (under the gravel base course) to provide additional storage, or c) possibly using some variation of a manufactured web or grid system, whereby the smaller “cells” (possibly filled with gravel) would provide the needed runoff storage. I’ve done some research online but haven’t seen much addressing this specific situation. So at this point I’m really not sure exactly how to deal with the steeper portion of the driveway.
Does anyone out there have any experience in designing and/or installing permeable pavement on a steeper road such as this? I would greatly appreciate any ideas, suggestions or links to useful sites that would address this specific issue.
Thanks in advance for any help.
The pervious pavement will incorporate a porous (no fines) Portland cement concrete surface, an open graded gravel base (4 inches or more) and possibly a geo-filter below that. The gravel base has a void ratio of 40% so it will provide short term storage of runoff that percolates through the concrete above. As an example, 4 inches of gravel can store 1.6 inches of water. Over time (which could be fairly quickly if subsurface soils have a good percolation rate) the stored runoff will infiltrate into the soils below. By local ordinance, I’m required to store one inch of runoff over the pavement area.
The issue is this: Most of the driveway is fairly flat (1% to 2% grade), however, due to grading constraints a portion of the driveway will need to be much steeper (10% to 20% grade). Pervious pavement lends itself best to flat or nearly flat paved applications. On a steeper grade, the water will tend to collect in the base material at the lower end of the road and actually start to come out of the pavement surface if there’s enough water (simple fluid hydraulics at play here). So the effectiveness of the permeable pavement is greatly reduced in this case.
Some potential solutions to this that I have heard of include a) putting in impervious “check dams” across the road to create a series of smaller, more level base areas, b) installing additional rock trenches across the road (under the gravel base course) to provide additional storage, or c) possibly using some variation of a manufactured web or grid system, whereby the smaller “cells” (possibly filled with gravel) would provide the needed runoff storage. I’ve done some research online but haven’t seen much addressing this specific situation. So at this point I’m really not sure exactly how to deal with the steeper portion of the driveway.
Does anyone out there have any experience in designing and/or installing permeable pavement on a steeper road such as this? I would greatly appreciate any ideas, suggestions or links to useful sites that would address this specific issue.
Thanks in advance for any help.





RE: Using Permeable Pavement on Steeper Roads
RE: Using Permeable Pavement on Steeper Roads
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Using Permeable Pavement on Steeper Roads
Thanks, Paul
RE: Using Permeable Pavement on Steeper Roads
Yes, I could go with impervious pavement for the steep part of the driveway. But then I'd have to treat the runoff from that area somehow. That means installing some other type of system. In that case we'd now have two kinds of systems to maintain. So that's why I'm trying to go with the permeable pavement in the steep area also, if there is a way to make it work there (without incurring ridiculous costs).
RE: Using Permeable Pavement on Steeper Roads
I saw an engineering outfit in Maryland brew some details up to that effect, and it seemed like a good idea.
Still don't think I'd put the stuff on a steep grade though, because water is going to run along the subgrade and displace the underlying soil. I'd do something else in that area.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Using Permeable Pavement on Steeper Roads
How about impervious pavement on the steep part, some sort of conveyance for the runoff, and a deeper crushed stone course on the flatter section to store and infiltrate more water. Sort of like an underground detention pond. Of course it would only work if the flat part is downhill of the steep part.
RE: Using Permeable Pavement on Steeper Roads
I think you’ve already suggested the best idea for the steep slope, if you only develop it a bit more. You suggested “ b) installing additional rock trenches across the roadway (under the gravel base course) to provide additional storage.” Your ‘rock trenches’ are my ‘french drains,’ and their top is already down 8" when you consider the paving and gravel base and geo filter fabric, don’t forget the fabric. You line them with geo filter fabric, fill them with clean gravel base mat’l. The trench depth is a couple feet, and spacing on the slope is ever 5 or 10', both to be determined with some engineering thought. Extend these trenches out beyond the edge of the paving a couple feet to some 24-36" dia., gravel filled dry wells, again depth to fit storage and percolation needs. These dry wells are lined and topped with filter fabric, and the top 8" can be/have soil and plantings. As the water flows down the sloped paving, you want to capture it before it ever gets to the bottom of the slope and comes percolating back up. I’ve used these french drains to capture surface water flowing toward a structure, when I had no other grading options. They can have a sloped bottom and perforated drain pipe which runs to daylight, or a dry well, or some such.
RE: Using Permeable Pavement on Steeper Roads
RE: Using Permeable Pavement on Steeper Roads
In the meantime, I’m still open to other ideas, though.
Thanks everybody for your input.
RE: Using Permeable Pavement on Steeper Roads
Take the water rapidly off the road section into a side swale that is grass lined for WQ treatment. Use channel armour and drops where needed in the steep section.