Porous Paving
Technical Standards for Porous Paving 
Porous pavement may be used to meet water quality and water volume standards performance criteria. Design criteria for the porous pavement stone reservoir, drainage design and the under laying soils are similar as for infiltration devices. Additional design is required for designing the pattern, jointing and strength of the wearing course. Pavement design is based on foundation soil strength, projected traffic intensities and the storage capacity of the reservoir and base.
The following represents a general list of design specifications that should be considered in any porous pavement design:
1. Anticipated traffic intensities, purpose and location of tree planters or landscape beds.
2. Test for foundation soils using California Bearing Ratio (CBR) or equivalent method.
3. Positive drainage is required with the use of under drains and edge drains within and washed, open-graded aggregate reservoir below the wearing course.
4. Geo-synthetic fabric separation between stone reservoir and sub-grade soil may be required in soft soil situations. SF
5. Three (3) types of porous pavement can be used for storm water infiltration including Portland Cement Porous pavement porous bituminous pavement (bituminous) and permeable sand jointed Paver stones.
a. Portland Cement Porous Pavement may be used for parking lots, driveways, walkways, terraces and other acceptable light traffic load structures. Non-porous reinforced concrete pavements shall be used for heavy load travel lanes within parking lots and loading areas.
b. Porous Asphalt Pavement may be used in parking stalls and other acceptable light load structures. Minimum three (3) inch pavement shall be used for light loading. Minimum foundation depth with stone courses shall be six (6) inches on soils of normal bearing capacity.
c. Precast pavers may be used for all parking spaces, articulate cross walks and interior walkways in and around parking lots. They may also be use as joints with travel lanes if laid on a concrete base. Precast pavers would normally be laid on a prepared porous foundation of sand and mixed gravel base with joint sand used on the wearing course. Joints not to exceed ¼” in width, reinforce all edges or precise pavement with reinforced concrete headers.
6. Use CU structural soils or equivalent with a minimum porosity of 26% under all pavements to allow for storm water infiltration and root growth from tree planters. Minimum depth shall be twelve (12) inches for simple pavements and thirty-six (36) inches for below grade storm water infiltration. Structural soils shall consist of 80%, ¾” – 1” angular crushed stone and 20% sandy clay loam soil. Effective infiltration shall be a minimum of 5.6 gallon per sf or 50.5 gallon per s y.
7. Porous Concrete Infiltration Calculation.
To determine the amount of storm water that can be infiltrated into porous paving one must just calculate the area of the vehicular use area and multiply it by the infiltration capacity (IC) in gallons per square foot of surface area.
Therefore the formula is L x W x IC
This can be converted cubic yards (CY) to determine the volume of water pulled from the storm water train and infiltrated on the site.
As and example consider a vehicular use area that will park thirty (30) automobiles. A porous paved area of 72,000 SF will be required in this example. The calculation to determine the amount of water is;
240 x 300 x 5gal = 360,000/202=1782 CY
This makes the assumption that the slope of the parking surface is less than .015 and that the parking surface can absorb five (5) gallons per square foot. On steeper slopes and more intense rainfall events the amount of absorption will decrease. Two hundred and two gallons of water equals one cubic yard.
Another way of looking at the absorption rate of porous paving is to look at from a design storm event of a one (1) inch storm.
Each square foot of a five (5)inch cross section with a minimum twenty percent void space (20% porosity) will absorb one (1) inch of rainfall. Therefore in our example;
SF/27= Load CY
72,000/12= 6000CF/27= 222.22 CY. Our parking lot captures just over one (1) cubic yard of water for a one (1) inch design storm.
In the previous calculation, the total amount 1782 CY would suggest a rain event of almost 1800 inches and suggests that the capacity of porous paving at five gallons per square foot is more than adequate to infiltrate storm water.
Interlocking paver stones with solid edge constraints may be used for light load parking lots, light load driveways, walkways and terraces and as pavement articulation within PCC surface.
Permeable pave stone material should meet ASTM 936. Refer to paving stone manufacturer’s installation specifications for standard design details for foundation, jointing, color, size, texture, use and slope placement. Minimum four (5) inch pavement shall be used for light loading. Minimum foundation depth with stone courses shall be six (6) inches. Minimum paver size shall be six by six (6) x (6) inches. Maximum paver size shall be eighteen by eighteen (18) x (18) inches. Minimum joint width shall be three eighths (3/8) inch and maximum joint shall be one half (1/2) inch.
b. Reservoir for each paving system shall be sized in depth based upon contributing drainage basin.
c. Observation wells shall be provides at the lowest elevation of the paved surface using pre-manufactured containers installed per manufacturer’s specifications
d. Coring may be required for all heavy traffic pavement structures.
e. Operations and Maintenance must be carried out on all porous pavement surfaces with the use of inspections, wet vacuuming, cleaning, sweeping and on a routine basis, inspection of observation wells. If the voids in porous pavement become fully clogged, the only maintenance measure is refreshment or replacement.
