Elkridge Library & DIY Education Center
New Permeable Parking Lot Ties in With Existing System to Achieve Stormwater Management Goals
Every Project Starts with a Purpose
Demand for the community resources provided by the Elkridge campus of the Howard County Library System had outgrown the facility. With adequate space available on site, the county moved forward with a $28.9 million project to modernize the library and expand its footprint to more than twice its original size.
At the same time, the existing stormwater management system—made up of traditional storm drains and a pond—was no longer sufficient. “Rainfall events in the past few years have been increasingly intense in the region, and the older stormwater facilities weren’t designed for the volume,” said project engineer Sharon K. Cruz, PE of Pennoni Associates.
Since the original construction of the facility, Howard County had adopted new stormwater management requirements. Any new pavement needed to be compatible with the existing system while also complying with updated guidelines. “The intent of the new requirements was to treat runoff in a way to mimic existing site conditions and provide even distribution,” Cruz said.
The system also needed to manage runoff from upstream properties in addition to the project site, without overwhelming the facility. “And with a roadway immediately downstream of the existing facility, ensuring that the facility wasn’t overwhelmed was extremely important.”
While the county had seen mixed results with permeable interlocking concrete pavements (PICPs) on other projects, Cruz noted those outcomes were often tied to compacted soils that limited infiltration. “For this project, though,” she said, “the biggest challenge for PICP was the slope.”
The Solution
Solving for Style, Function, and Durability
Parking spaces in the main parking lot were designed to be permeable. Joint openings between the pavers ranged from 7 to 8 mm and were filled with a highly permeable filter aggregate (ASTM No. 9 stone). The open-graded aggregate base beneath the pavers was designed to act as a reservoir, holding stormwater like a detention basin until it could be released back into the environment, minimizing runoff from the property.
Entrances and driving aisles were not permeable in order to protect underlying utilities from being impacted by detained rainwater. Cruz specified Aqualine® 9L pavers for their permeable capabilities and traffic load capacity.
Cruz preferred Belgard’s localized manufacturing network. The proximity of a plant located just 25 miles from the job site resulted in significant time and cost savings. “We try to specify a product available locally when we can,” Cruz said. “Not only is that a better cost savings in terms of shipping, it also keeps the economics local, which is better for everyone.”
Adjustments were made to the PICP system design to account for the site’s slope. “We had to level the bottom of the base and step some sections down instead of sloping them, which also helped reduce the price of the installation,” Cruz said.
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The Result
A Lasting Impression:
The Final Transformation
The new facility opened in 2018. The use of Aqualine® pavers helped the project achieve LEED® Gold certification through reduced impervious area and local sourcing, and the project also received a USGBC® Wintergreen Award for environmental stewardship. Because of the PICP system’s infiltration rates, expansion of the existing pond was not required, and the combined stormwater systems are functioning as designed, according to Cruz.
One year after installation, the Belgard team returned to test surface infiltration in accordance with ASTM C1781, Standard Test Method for Surface Infiltration Rate of Permeable Unit Pavement Systems. Four representative locations were tested using an infiltrometer ring, and the average surface infiltration rate measured was 155 inches per hour, confirming the system was functioning as designed without generating surface runoff.
“We are using this project as a case study to monitor over the next couple of years for Howard County,” said Kathy Walsh, Belgard Commercial Sales. “This is an opportunity to show the long-term performance of a large-scale PICP project in helping to meet their new regulations.”
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