Windsor Elementary School, located in Arlington Heights, Ill., is expanding due to an increase in student enrollment. With the addition of the new facilities, it was necessary to make site infrastructure improvements and provide additional storm water management. An underground storm water management system was designed to meet project goals and satisfy the storm water requirements of both the village of Arlington Heights and the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC). The project is located within the delineated combined sewer limits of the MWRDGC’s jurisdiction.
“Due to the project being located within the limits of the MWRDGC’s combined sewer area, we were responsible for designing infiltration based BMPs that are required to maintain a minimum separation of 3.5 ft from the seasonal high groundwater table to the bottom of the proposed infiltration practice to promote water quality,” said Josh Cap, design engineer at Eriksson Engineering Associates Ltd.
A geotechnical report indicated a seasonal high groundwater elevation that ranged as close as 8 ft (elevation+/-669 mean sea level) from existing grade. Because of the limited available depth and the corresponding MWRDGC requirements, the storm water detention system had to be watertight.
StormTrap worked with Eriksson Engineering Associates Ltd. to design the 1.35-acre-ft system, which was installed under the school soccer field, so it would comply with MWRDGC storm water regulations. The 5-ft-by-8-in. SingleTrap basin—made up of 91 precast pieces—was installed on a reinforced concrete foundation and is capable of storing 58,836 cu ft of storm water.
Because infiltration could not be completed due to the high groundwater level, SiteSaver, a BMP measure, was added to treat the runoff from the site and to meet water quality requirements. The system uses a disposable mesh net, inclined plates and oil baffles to collect trash, sediment, hydrocarbons and other impurities before water is discharged to the municipal storm sewer.
“We proposed the use of the SiteSaver as a substitute to make up and accommodate for the volume control (infiltration practices) that was unable to be provided onsite due to the high seasonal groundwater noted in the geotechnical report,” Cap said.
The StormTrap detention system, together with the SiteSaver treatment device, provided a flexible option to the owner to meet total water storage requirements and treat storm water runoff on site, while providing a usable area above the system which will be utilized by the students for leisure.
