To save the aesthetics of Dellrose Street, an aging, 900 ft. long, brick road, the city of Pittsburgh wanted to limit traditional stormwater infrastructure (see part 1). Jason Borne, a stormwater engineer for ms consultants and his team decided permeable pavers was a viable option, and used two different types of infiltrometers to determine soil infiltration potential. Here’s how they compared.
Shortened Test Times Allow Design Changes on the Fly
Though most of the subsoil was a clay urban fill, there was a distinct transition between that clay material to a broken shale/clay mixture. Borne says, “After excavation, it rained, and we saw that the water was disappearing through the broken shale/clay material. When we did the infiltration tests, the broken shale/clay showed a higher infiltration potential than the clay fill material. That led us to modify the design of the subsurface flow barriers based on specific observed infiltration rates of the subsoils. Where the tests showed higher hydraulic conductivity values, we were able to rely on infiltration entirely to remove the water from behind the check dams.” Borne adds that in the areas where infiltration was poor, they augmented infiltration with a slow release concept. “We put some weep holes in the flow barrier and let the water trickle out down to the next barrier and so on. Basically, the automated SATURO infiltrometer allowed us to do many tests in a short amount of time to establish a threshold of where good infiltrating soils and poor infiltrating soils were located. This enabled us to change the design on the fly. The double ring infiltrometer takes significantly more time to do a test, and time is of the essence when the contractor wants to backfill the area and get things moving. It was nice to have a tool that got us the information we needed more rapidly.”
How did the Double Ring and SATURO Compare?
Borne says the SATURO Infiltrometer was faster and reduced the possibility of human error. He adds, “We liked the idea of it being very standardized. The automated plot of flux over time was also of great interest to us, because we could see a trend, or anomalies that might invalidate the results we were getting. The double ring infiltrometer takes a long time to achieve a state of equilibrium, and it’s hard to know when that occurs. You’re following the Pennsylvania Department of Environmental Protection suggested guidelines, but they’re very generalized. To me it doesn’t suit all situations. What we found with the SATURO infiltrometer is it records information at very discreet intervals, plots a curve of the flux over time, and when it levels out, you basically achieve equilibrium. You get to that state of equilibrium faster. There’s a water savings, but there’s also a time savings. And there’s the satisfaction of getting standardized results rather than the possibility of each technician applying the principles in a slightly different way, as they might with the double ring infiltrometer.”
Borne and his team were ultimately able to prepare a permeable paver street design which allowed for the exclusion of traditional storm sewer infrastructure, reducing both capital costs and long-term maintenance life cycle costs. The permeable paver concept is intended to provide a template for the city of Pittsburgh to apply to the future reconstruction of other city streets.
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