15.2 Permeable Surfaces and Construction Protection

The Golden Construction-Phase Protection Rule for Infiltration & Filtration BMPs

Low Impact Development (LID) infiltration and filtration practices rely on the high hydraulic conductivity of engineered filter media and the natural macropore structure of uncompacted native subsoils. Because these pores are easily destroyed during site grading, CESSWI inspectors must enforce the Golden Construction-Phase Protection Rule:

1. Never Allow Muddy Construction Runoff to Enter an Infiltration or Filtration BMP: Fine clay and silt washed off active grading areas penetrate deep into open-graded stone reservoirs, sand beds, and native subsoil pores, causing irreversible blinding (clogging) before construction is even finished. Never use a permanent infiltration basin, infiltration trench, or bioretention footprint as a temporary construction sediment basin unless the subgrade is left $1\text{ to }2\text{ feet}$ above final design invert and later excavated to uncompacted virgin soil.
2. Never Drive Heavy Earthmoving Equipment Across the Infiltration Subgrade: Rubber-tired scrapers, loaded dump trucks, and heavy bulldozers compact permeable Hydrologic Soil Group (HSG) A or B native soils down to near-impervious HSG D conditions.
3. Mandatory Sequencing & Excavation Protocol: Surround all proposed infiltration footprints with high-visibility fencing and diversion berms prior to mass grading. Excavate the final subgrade using a long-reach excavator or backhoe stationed outside the footprint working backward, scarify (ripping $6\text{ to }12\text{ inches}$ deep) any accidentally compacted soil before placing stone or media, and keep the facility offline until $100\%$ of the contributing drainage area is paved and permanently stabilized.

1. Permeable Surfaces (12.7.3)

Permeable Surfaces (Permeable Pavements) transform parking stalls, low-speed access lanes, alleys, plazas, and sidewalks from runoff generators into structural infiltration systems.

Four Major Permeable Surface Types & Engineered Cross-Section

• Porous Asphalt: Standard bituminous asphalt pavement formulated by omitting fine sand from the aggregate gradation, leaving $15\%\text{ to }20\%$ interconnected air voids.
• Pervious Concrete: Specialized Portland cement concrete mixed with little or no fine aggregate (sand) and a controlled water-to-cement ratio, producing $15\%\text{ to }25\%$ interconnected void space so rainwater drains straight through the rigid slab.
• Permeable Interlocking Concrete Pavers (PICP): High-strength, solid precast concrete pavers cast with integral spacer lugs that create $1/4\text{- to }1/2\text{-inch}$ wide open joints filled with clean, washed, angular ASTM #8 or #9 stone chips.
• Plastic or Concrete Turf / Gravel Grid Pavers: Cellular high-density polyethylene (HDPE) geocells or open-cell precast concrete grids filled with sandy topsoil and turfgrass or clean pea gravel (widely used for emergency fire lanes and overflow event parking).

Beneath the surface course lies a $1\text{- to }2\text{-inch}$ open-graded bedding/choker course (clean ASTM #8 stone) over a $6\text{- to }24\text{-inch}$ deep structural reservoir base constructed of clean, double-washed, open-graded angular crushed stone (typically ASTM #57 stone over coarse $2\text{- to }3\text{-inch}$ ASTM #2 stone, providing $30\%\text{ to }40\%$ void space for structural load support and stormwater storage). The bottom of the stone reservoir rests on uncompacted native subsoil wrapped on the vertical sides with non-woven geotextile (and includes a perforated PVC underdrain pipe when installed over slow-draining HSG C or D soils).

Critical O&M Prohibitions & Inspection Checkpoints

• Permanent Signage Prohibition — "Do Not Sand or Sealcoat!": Winter traction sand fills the surface pores and joint stone, while asphalt or coal-tar sealcoat forms an impermeable membrane across the pavement. Either mistake permanently ruins a permeable parking lot! Winter deicing must rely on liquid brines or sparing deicing salt (or clean #8/#9 angular chip stone on PICP).
• Regenerative-Air / Vacuum Street Sweeping: Inspect during rain events for surface ponding or runoff sheet-flowing off the lot. Restore surface permeability by sweeping 2 to 4 times per year with a regenerative-air or pure vacuum street sweeper (never use high-pressure water nozzles that wash fine silt deeper into the stone base), and replenish any vacuumed joint stone on PICP surfaces.