Under the Clean Water Act, U.S. federal law has mandated that states control non-point source water pollution. Since the late 1970’s pavement systems have been developed to meet this requirement, including porous asphalt pavements, pervious concrete pavements and permeable paving blocks. All of these move surface water to subsurface storm water infiltration and retention systems.
One thing these systems have in common is the need for a sub-base material. SUDAS (Statewide Urban Designs and Specifications) recommends using either:
1. Iowa DOT gradation #3 (1 1/2″ clean stone)
2. Iowa DOT #5 (1″ clean stone)
3. SUDAS Class 1 Bedding Stone(1″ bedding stone similar to Iowa DOT gradation #3). SUDAS also recommends the courser gradations to provide better stability and drainability.
With porous paving surfaces the flow rate through the pavements allows rainfall to be captured and to percolate into the ground. This reduces stormwater runoff, recharges groundwater, supports sustainable construction, and provides a solution for construction that is sensitive to environmental concerns and helps owners comply with EPA stormwater regulations.
This can be of a particular interest in urban areas, or where land is very expensive. Depending on local regulations and environment, these pavements and their sub-base may provide enough water storage capacity to offset the need for conventional retention ponds, and other precipitation runoff containment strategies.
The first step is to determine the size of the chamber. The formula used by many is the cubic feet of storage required divided by 0.40. (40% void space).
Prior to digging, a soils investigation is recommended to determine the specific site conditions. The depth to bedrock and water table are part of the main design information needed.
A non-woven geotextile is typically used to line the basin and prevent fines in the soil from migrating into the stone chamber.
Planners should consider installing a smaller diameter outlet pipe near the bottom of the stone chamber to allow complete drain down. Another pipe near the top of the system can aid in preventing overflow in extreme cases.
As noted earlier, aggregate for the stone chamber needs to be clean, crushed stone. Top sizes from 1 to 3 inches are commonly used. An optional course of smaller clean stone is often used to provide a more uniform surface for paving. This material is typically in the 1/2 inch range.
Aggregate based rock chambers are the answer to stormwater retention systems. This is true even using intakes and standard pavement. Aggregate systems are less expensive, provide superior support and stability, and allow the flexibility needed for detention basin applications. Readers are encouraged to consult a professional engineer for design specifics.
The Porous Pavement Section Rendering graphic is used with permission of Iowa Natural Resource Conservation Services. Illustration by Doug Adamson, RDG Planning.