Overview and History
While most pavements are impervious, porous asphalt pavements (also know as open-graded or pervious pavements) are designed specifically to allow water to drain through the pavement. These can be constructed as full-depth porous pavements, where water drains through the pavement to the soil; or they can be constructed as an open-graded friction course (OGFC), which helps move water to the side of a pavement, improving friction while reducing both road spray and noise. Full-depth porous pavements are an EPA best practice for stormwater management.
Porous pavements are highly effective in reducing pollution in stormwater runoff from pavements. Cahill reports that, although sampling on porous pavement systems has been limited, the available data indicate a high removal rate for total suspended solids (TSS), metals, and oil and grease.
From the bottom up, the standard porous asphalt pavement structure consists of:
An uncompacted subgrade to maximize the infiltration rate of the soil.
A geotextile fabric that allows water to pass through, but prevents migration of fine material from the subgrade into the stone recharge bed.
A stone recharge bed consisting of clean single-size crushed large stone with about 40 percent voids. This serves as a structural layer and also temporarily stores stormwater as it infiltrates into the soil below.
- A stabilizing course or "choker course” consisting of a clean single-size crushed stone smaller than the stone in the recharge bed to stabilize the surface for paving equipment.
- An open-graded asphalt surface with interconnected voids that allow stormwater to flow through the pavement into the stone recharge bed.
The general guidelines for the porous asphalt pavement design are:
Consider the location for porous pavements early in the site design process.
- Soil infiltration rates of 0.1 to 10 inches/hour work best.
- Minimum depth to bedrock or seasonal high water should be greater than two feet.
- The bottom of the infiltration bed should be flat to maximize the infiltration area.
- Limit the maximum slope of porous pavement surface to 5 percent. For parking areas on steeper slopes, terrace the parking areas with berms between parking areas.
- Look for opportunities to route runoff from nearby impervious areas to the infiltration bed to minimize stormwater structures. Pretreatment may be required.
- Spread out the infiltration. The maximum ratio of impervious to pervious area should be 5:1. For carbonate soils where there is a risk of sinkholes, the maximum ratio should be 3:1. Do not place porous pavements over known sinkhole areas.
- The design should provide for an alternate path for stormwater to enter the stone recharge bed in the event that the pavement surface becomes plugged or experiences extreme storm events.
- An overflow system should be included to prevent water in the stone bed from rising into the pavement surface during extreme storm events.
- The stone recharge bed should be able to drain within 12 and 72 hours.