Using Permeable Pavement on Steeper Roads

[Boltricity]

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.

 

[Ron]

Keep in mind that your estimates of permeability and storage are based on the new condition. In fairly short order, the voids fill with fine debris and the pervious pavement is less pervious.

 

[beej67]

Can you just go impervious for the steep part?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Boltricity]

Ron, you are correct. This is why permeable pavement requires ongoing maintenance, mainly periodic vacuuming of the paved surface to keep fines and other small debris to a minimum. In the research I've done it's been noted that many of these installations eventually fail (from getting completely clogged) due to lack of maintenance.

Thanks, Paul

 

[Boltricity]

Beej67,

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).

 

[beej67]

One idea to bridge the problem Ron brought up, is to put a vertical header curb on both sides, two feet off the edge of pavement, and fill the gap between the pavement and the curb with rounded river stone over the continued road subgrade. Then if the pavement clogs, the water still gets into the subgrade through the decorative stone on the edge, and the curb keeps it in place.

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

 

[agua84]

I have heard of issues with water coming out of the pavement, with flatter grades that the 10-20% you suggest (albeit it was at the entrance to a parking lot and I'm not sure that they did much in the way of subgrade check dams). I don't know about the soils where you are, but inducing infiltration on a steep slope could be asking for trouble, whether it is movement of fines, global stability, or whatever.

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.

 

[dhengr]

Boltricity:
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.

 

[Boltricity]

Aqua, that is a good idea. Unfortunately, the steep part of the driveway is at the bottom. Yes, if it was up higher I could build in additional storage depth in the lower, flatter area and simply let the runoff flow from the upper to the lower area. That would work. But I can’t apply that here. Thanks anyway for the suggestion.

 

[Boltricity]

Dhengr… I think you and I are on the same page here. Before you posted I was digging into this (figuratively) some more, and it seemed like the rock trenches were the way to go. The rock base being on a slope limits its capacity, so the rock trenches can make up for that lost capacity. I only have to treat the first inch of runoff, so I probably won’t need the additional dry wells along the road. Any runoff above and beyond one inch can simply run off and flow downstream untreated. If it percolates through that’s a bonus but it doesn’t have to. I’d calculate the capacity I need for the one inch over the road surface, and then size and/or space the trenches accordingly. I also think a geo-fabric liner is a good idea. I’m getting some recommendations from a soils engineer for this.

In the meantime, I’m still open to other ideas, though.

Thanks everybody for your input.

 

[LincolnPE]

Typically we do everything we can to keep water out of the road structural sections and the subgrade beneath because this is the leading cause of road failure.

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.