Permeable paving—whether it is porous asphalt, pervious concrete, or permeable interlocking concrete pavers (PICP)—is revolutionizing urban site design. By allowing rain to pass directly through the surface, designers can eliminate the need for costly catch basins, underground pipes, and sprawling retention ponds.
However, the real engineering of permeable paving happens entirely underground. The surface only lets the water in; it is the deep, crushed stone sub-base that actually stores the water until it can soak into the earth. If a contractor installs permeable pavers over a standard, densely compacted base (like MOT Type 1), the system will fail instantly, flooding the site during the first rainstorm.
The Physics of Void Space (Porosity)
To design a functional permeable system, you must specify Open-Graded Aggregates. These are stones that have been sifted so they are all exactly the same size (e.g., ASTM No. 57 stone). Because there are no “fines” (sand or dust) to fill the gaps between the rocks, massive empty voids are created.
- The 40% Void Rule: A properly specified open-graded clean stone base contains approximately 30% to 40% void space. This means that a 10-inch deep gravel base doesn’t just hold up cars; it can hold roughly 3 to 4 inches of pure rainwater within its gaps.
- Structural Depth vs. Hydrological Depth: You must calculate the depth of the base twice. First, how deep does the gravel need to be to structurally support a fire truck? Second, how deep does it need to be to store a 100-year storm event? You must build the base to satisfy whichever number is larger.
Pro Tip: The Uncompacted Subgrade Hazard
The goal of a permeable system is to infiltrate water into the native soil beneath the gravel reservoir. Therefore, you must never mechanically compact the native soil subgrade during excavation. Running a heavy vibratory roller over the clay subgrade seals the soil pores, reducing its infiltration rate to zero and turning your gravel reservoir into an underground swimming pool that will eventually overflow.
Standard Open-Graded Aggregate Profiles (PICP)
To ensure structural stability while maintaining massive stormwater storage capacity, follow these standard aggregate specifications for permeable paver installations:
| System Layer | Aggregate Specification | Primary Function |
|---|---|---|
| Joint Material | ASTM No. 8 or No. 9 Stone | Allows rapid surface infiltration; locks pavers together. |
| Bedding Course | ASTM No. 8 Stone (2 inches thick) | Provides a level setting bed; highly permeable. |
| Base Reservoir | ASTM No. 57 Stone (4-6 inches) | Primary structural support and moderate water storage. |
| Sub-Base Reservoir | ASTM No. 2 Stone (Depth varies) | Massive void space (40%) for heavy stormwater retention. |
Engineer Your Underground Reservoirs
Calculating exactly how many inches of ASTM No. 2 stone you need to store 4,000 cubic feet of stormwater runoff requires translating void space percentages into volumetric holding capacities.
Eliminate the guesswork with our Stormwater & Hydrology Calculators. Instantly determine the required sub-base depth to capture target storm events, calculate the exact tonnage of open-graded stone required for your site, and design permeable hardscapes that perform flawlessly as invisible stormwater reservoirs.