General Design Standards for Porous Paving
Porous paving, paving that allows the infiltration of storm water can be an effective storm water BMP that reduces site run off and infiltrates water back into the ground. Structural soils designed to not only gear the weight of heavy construction such as parking lots, terraces, and courtyard but provide void space for tree roots and storm water infiltration. Paving and under ground water storage space is one of several green parking lot design strategies that can reduce site run off by twenty (20) percent or more if effectively designed.
One of the primary elements of a green parking lot is to use porous paving rather than impervious surface. Porous paving can be constructed with Portland cement concrete, oil based asphalt or precast concrete or masonry pavers. These products can be recycled however asphalt due to its oil base can not be set free in the environment. Therefore porous concrete or colored precast concrete paving units would be a better choice for green parking, low grade streets such as alleys and for small paved outdoor areas such as swimming pool decks, patios, guest parking, and surrounding trees planted in urban areas. Working with porous concrete takes a trained crew that will know how to properly form it, place it, reinforce it, joint it and finish it and cure it. Most landscape contractors can quickly learn the proper way to construct porous precast paving units.
Structural soil makes an ideal root growth media below parking pavements. This same structural soil can also rapidly infiltrate water to satisfy vegetation needs. Uniformly graded 3/4”-1 1/2” angular crushed stone specified for parking lot, courtyard or terrace bases is designed to ensure the greatest porosity. Crushed or angular stone provides more compaction and a load-bearing capacity to meet state department of transportation base course standards while storing a large volume of water.
Since among soil textures, clay has the most water and nutrient-holding capacity, a heavy clay loam or loam with a minimum of 20% clay, with an organic matter content ranging from 2%-5% ensures nutrient uptake and water holding capacity while encouraging beneficial microbial activity. This design soil should be used within the first twelve inches of depth within the projected CRZ (critical root zone) of all parking lot, terrace, courtyard and patio trees.
A paved area built with CU soil is an effective storm water absorber. Each square foot of terrace or parking surface on a twelve (12) inch CU soil base will absorb a little over three (3) inches (26% porosity) of storm water. Four square feet will store one (1) gallon of water. To store added water thicken the CU soil depth to three (3) feet. This same parking lot surface will collect nine (9) inches of water per square foot of surface. That would make a tree planted terrace an exceptional storage area for rainfall during a hurricane. A parking lot or terrace with a thirty six (36) inch tree well space of CU soil will collect 50.5 gallons of water per square yard or 5.6 gallons per square foot. Porous paving with CU structural soils makes an excellent urban storm water BMP as well as an effective place to plant trees with diminutive root balls.
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