Pervious Pavement
More Detailed Information
Design Considerations
Contributing Drainage Area
Offsite runoff onto pervious pavement should be limited to impervious surfaces. The ratio of contributing impervious area to the pervious paver surface should not exceed 3:1. If runoff is coming from adjacent pervious areas, it is important that the area be fully stabilized.
Siting of Facility
Each site should be considered unique. Pervious pavement systems are typically used in low-traffic areas such as: parking pads in parking lots, overflow parking areas, residential driveways, residential street parking lanes, recreational trails and emergency vehicle and fire access lanes. These systems are not recommended on sites with a slope greater than 2%.
Load Bearing Surface
An appropriate modular porous paver should be selected for the intended application. If it is a load bearing surface, then the pavers should be able to support the maximum load.
Insitu Soil
The surface of the insitu soils should be lined with filter fabric or an 8” layer of sand. It should be graded so it is completely flat to promote infiltration across the entire surface. Insitu soils should have field-verified, minimum permeability rates of greater than 0.5 inches per hour to a depth of 3 feet below the bottom of the stone reservoir. During construction and preparation of the subgrade, special care must be taken to avoid compaction of the soils.
Pervious Paver Infill
The pervious paver infill selection is based upon the intended application and required infiltration rate. Masonry sand (or equal) has high infiltration rates and should be used where no vegetation is desired. A sandy loam soil has a substantially lower infiltration rate, but will provide a growth medium for vegetative cover.
Filter Layer
The layer of filter fabric is located below the pervious paver infill and the above the storage layer: 1) Pervious pavers 2) Pervious paver infill 3) Filter fabric 4) Base course/ storage layer.
Base Course/Storage Layer
Regulatory requirements will determine the design storage volume. The stone aggregate used should be washed, bank-run gravel, 1.5 to 2.5 inches in diameter with a void space of approximately 40%. The gravel base course must have a minimum depth of 9 inches. The system should be constructed to infiltrate the design storm within a maximum of 48 hours (24 recommended).
Underdrains and Overflows
Underdrains are used to provide overflow drainage for low permeable subgrades and/or for storms exceeding the design storm. If the in- situ soils exhibit low permeability, the underdrain may be located in the gravel bed and the pervious pavement system will operate as a biofiltration practice. Designs need some method to convey larger storms to the storm drain system. One option is to set storm drain inlets slightly above the surface elevation of the pavement. This allows for temporary ponding above the surface if the surface clogs, but bypasses larger flows that are too large to be treated by the system.
Depth to Water Table and/or Bedrock
Pervious pavement systems should have a minimum of 3 feet between the bottom of the practice and the seasonally high water table or bedrock. This separation is required to maintain groundwater quality and the hydraulic capacity of the practice.