Written by William G. Handlos, P.E.
For two decades, I held the position of City Engineer and frequently had to explain to disbelieving homeowners, developers and elected officials – that gravel driveways and parking lots were not porous. I would explain that for purposes of stormwater runoff, such gravel pavements must be treated exactly as we would concrete and asphaltic pavements. They often bristled at the idea that gravel did not percolate water and were upset to find that their gravel driveway, lots and roadways were assumed to shed 100% of rainwater.
So it is with more than a dose of irony that I now have found myself in the position of regularly explaining to local and state officials that aggregate pavements are not necessarily impervious. Stakeholders are so accustomed to repeating the impervious gravel mantra…that they forget to notice that porous aggregate is not the same thing as gravel. I think it is best to start with a primer on the vernacular.
A “good” gravel (Image 1) should have 40 to 70% stone, well-graded from 1/4” to 2-1/2” diameter; 20 to 50% sand; and 10% +/- fines. It should resist abrasion, shed water and be capable of being compacted.
A “good” porous aggregate (Image 2) should be poorly-graded from 3/8” to ¾” diameter; with 25 to 40% porosity. It should pass the heaviest of rainfall, is easily rutted/shoved and nearly impossible to suitably compact. Such good porous aggregate is often called open-graded base course (OBGC) when used under pavements.
Some municipal and state regulations continue to deem aggregate surfaces as impervious. This is due to inertia and old school thinking and it is past time to change these outdated standards. With the help of engineered high density polyethylene containment products, such as the GEOPAVE® porous paving system, OBGC porous aggregate can be stabilized to offer a highly porous, strong, abrasion resistant, rut proof, aesthetically pleasing and inexpensive alternative to porous asphalt or pervious concrete.
Injection molded and designed specifically for use with open aggregate designs, this system confines the otherwise unruly OGBC to individual cells with an attractive herringbone wall pattern that is designed to be seen and is reminiscent of a paver system. Resistant to frost heave when placed over an OGBC base for storage or sandy soils for fast infiltration, the system gives a very low cost alternative that is highly sought after by those municipalities seeking to meet demanding stormwater goals.
Check to see if your local or state regulations allow for Porous Aggregate Systems and, if not; send a note to email@example.com with your location, the regulation, and a contact at the agency. I will follow up on all requests personally.