Porous Pavement Siting Criteria - First Step

Porous Pavement Siting Criteria Decision Tree Introduction

There are many different kinds of porous pavements. To learn the basics of a few of them, please see our fact sheet, “Porous Pavements”. Information on this page is designed to help you understand the questions that follow and answer them to discover whether your site or proposed location is suitable for porous pavement.

While porous pavements for pedestrian walkways may rely on standard section designs, soil conditions can vary widely even across a small site. Porous pavements that must support traffic loads should always be designed by a licensed professional such as geotechnical engineer and/or civil/transportation engineer.

A few terms you’ll see in the questions that follow:

Base rock: This is the rock that is usually located beneath the porous pavement surface. In pervious concrete installations on faster draining soils (i.e. high infiltration rate), the rock is optional and the pervious concrete may be installed without it.

Bottom of the pavement: This is the lowest elevation of excavation. For example, flexible pavements like porous asphalt and permeable pavers will have a base course of aggregate

Crushed Aggregate: Rock quarried with a crusher and is angular, not rounded. Angular rock is required in vehicular applications to ensure structural stability.

Drain rock: Drain rock may be open- or well-graded rock, so take care to make sure you’re ordering open-graded rock.

Open-graded (aka poorly graded): Rock diameters are all similar in size (i.e. 1.5- 1 inch rock, AASHTO No. 57, AASHTO No. 8), which creates voids between the rock where water may be stored. This is the only kind of rock gradation that should be used in porous pavements.

Well-graded rock: Rock diameters may range in size from large to very small, (ex. “3/4-inch minus”). Avoid this kind of rock since the “minus” portion of the rock mix will fill the voids between the large rocks, leaving no room for storing rainfall.


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