On top of the subgrade soil is the open-graded reservoir layer ( also referred to as the subbase ) consisting of opengraded coarse gravel , such as ASTM No . 2 or No . 3 stone ( 1 to 2.5 in ) ( ASTM 2017 ). This layer functions as a reservoir for storing runoff collected by the pervious pavement system . The thickness of the reservoir base course layer is determined by the structural or hydrologic design of the system ( UF IFAS ; Bean 2019 ). For pavement subbases constructed on grade , geotextile layers should be incorporated at equal elevations for every 6 to 12 inches of grade change . These act as internal check dams which prevent sediment from reducing the holding capacity of the reservoir layer . A geotextile layer between the reservoir course and subgrade soil can be added to prevent the migration of aggregate into surrounding soil and erosion on slope , therefore improving structural stability of the profile . In the situation of adding a geotextile layer , it should extend up the sides of the system to the pavement surface .
On top of the reservoir subbase is a layer of choker course ( also referred to as filter course ), consisting of washed ASTM No . 57 size stone ( 3 / 16in-1in ). The choker course prevents smaller aggregate from moving into the voids of the below reservoir base by blocking the void openings at the top of the reservoir course . The porous concrete and porous asphalt applications are installed directly on top of the choker course ( ASTM 2017 ; UF IFAS ; Bean 2019 ).
Depending on the pavement chosen and permitting standards with the County , bedding course material can be added on top of the choker course . This additional layer applies to pervious interlocking concrete pavers , concrete grid pavers , and plastic reinforcing grids with turf options ( Bean et al ., 2019 ). The purpose of this layer is to provide a stable layer to prevent the sinking and shifting of surface materials , as well as fill voids in the final surface application . Bedding course should be ‘ washed ’ stone , or pea gravel , coarse enough to prevent clogging the beneath reservoir aggregate or the surface of the subgrade soil . The bedding course stone should be similar in size to pea gravel ( ASTM No . 8 or 89 stone course ( 3 / 64 to ⅜ in )) ( ASTM 2017 ). The recommended depth of this layer is approximately 1 inch ( WEF-Stormwater Report ; UF-IFAS ; Bean 2019 ).
At the surface and above the prepared subbase aggregate is the pervious pavement surface material . There are a variety of pervious pavement surfaces available in the commercial marketplace , including pervious interlocking concrete pavers ( PICP ), porous asphalt ( PA ), porous concrete ( PC ), and plastic reinforcing grids with turf ( PRG ) ( Bean et al ., 2019 ). PICP and PRG systems have larger voids within the pavers , which are typically filled with a stone aggregate or coarse sand to allow water to infiltrate through the paver surface porous pavements ( PA and PC ) have voids by omitting finer particles that are generally included in their conventional , impervious counterparts . PA and PC are poured and cured in place , while other materials are precast and can be installed without needing to be cured .
The recommended surface material thickness for each of the materials varies ; pervious interlocking concrete ( PICP ) 3 to 3.5 inches in depth ; porous asphalt ( PA ) between 3 to 6 inches in depth ( County minimum 2 inches ); porous concrete ( PC ) between 4 to 8 inches in depth ; and plastic reinforcing grids ( PRG ) with turf between 2 to 4 inches in depth ( WEF-Stormwater Report ; UF-IFAS ; Bean 2019 ). This thickness will vary however , depending on the available material ’ s properties and the amount of expected traffic on-site .
Utility lines should be identified prior to the design and preparation of the site . Property owners or contractors should contact # 811 to make a request for utility lines to be identified .
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NATURE-BASED SOLUTIONS MANUAL FOR KIAWAH ISLAND