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Infiltration Rates

Infiltration Rates – How Thick Is Thick Enough?

Before any infiltration system is designed, soil investigation must be done. This consists of two steps. First, simple test pits six to eight feet in depth are excavated with a backhoe and the soil conditions are observed. Next, infiltration measurements are performed at the anticipated bed bottom location. Based on these investigations, plus knowledge of the design storm, the optimum depth of the reservoir course can be determined.

The key is to design the reservoir course to hold water for the design storm while making sure that the water will drain within a 24- to 72-hour period for proper water treatment. Most references suggest that the soils underlying a porous pavement should have a minimum infiltration rate of 0.50 inches per hour. Some reports suggest that soils with permeability less than 0.25 inch per hour are probably not suitable for porous pavement applications without substantial additional facilities. However, porous pavements have been successfully used with soil infiltration rates as low as 0.1 inches/hour. Other engineered options such as infiltration trenches and bioswales, in combination with porous permeable pavement, may be considered for areas with slow infiltration rates.

Underlying geology must also be considered in areas such as those underlain by limestone or dolomite formations. In that situation, more detailed site investigation may include borings and ground-penetrating radar. Contrary to popular belief, properly designed infiltration systems do not create sinkholes. A number of systems designed by Cahill Associates, including older systems, are located in carbonate areas. In several situations they have successfully installed porous pavement infiltration systems adjacent to areas where detention basins created sinkholes.

Porous pavements should never be built on landfill, or on areas that may be subject to hazardous material spills such as fueling areas.