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University of Michigan Infiltration System – Central Campus

Ann Arbor, Michigan


University of Michigan


CDM  Smith 


Site Development of Madison Heights, Mi


August 2018

Technical Description:

  • CON/SPAN® O-Series® Stormwater Detention System
  • EXPRESS® Foundations 
  • CDS® Hydrodynamic Separator
  • Armorflex® Hard Armour 

The University of Michigan’s Central Campus located along South State Street includes historic halls, the Museum of Art, the Graduate Library, the President’s Residence, and the famous Diag – an open area known as the busiest site on campus, hosting concerts, ceremonies, fundraisers, and demonstrations. Where the Central Campus is the gathering spot for students and visitors, it is also a gathering location for stormwater in the downtown Ann Arbor area, particularly during large storm events.

The University, along with the project engineer, CDM Smith-Detroit, devised a long-term stormwater management plan to correct flooding and damage occurring within the buildings located at the Central Campus.

The new Central Campus stormwater system follows other stormwater-management initiatives that the University of Michigan has implemented to benefit the entire campus and the community, such as rain gardens, green roofs, porous surfaces, and infiltration or holding basins.

The new underground stormwater infiltration system was designed to protect the campus from a 100-year, 24-hour storm event while providing additional capacity in the university's and City of Ann Arbor's stormwater systems. The new system also provides water quality improvements by removing sediment and nutrients that would ultimately flow into the Huron River Watershed.

The project was unique and challenging in several ways. The first challenge was designing a system to manage the 750,000 gallons or storage needed during the largest stormwater events while providing a large infiltration bed to allow natural infiltration. The second challenge was developing this large underground basin in the limited space at Central Campus without damaging the existing building and infrastructure.

To solve these challenges, CDM Smith and the University coordinated with Contech Engineered Solutions to develop a plan to address all onsite stormwater management issues.The plan used the following components

  • A CDS Hydrodynamic Separator – a pre-treatment structure to remove trash, debris, sediment, and hydrocarbons from stormwater runoff.
  • A CON/SPAN O-Series Precast Arch Infiltration System – 175 ft of a 35 ft span x 11 ft rise precast arch with integral end walls and maintenance access vault.
  • A CON/SPAN Express Foundation System - a prefabricated spread foundation system combining the speed of precast with the economy of cast-in-place construction
  • Armorflex Articulating Concrete Block – an engineered erosion control concrete mat system to manage 10 ft per second high velocity flows into the infiltration system.

Stormwater runoff is first treated using the CDS, a hydrodynamic separator that uses swirl concentration and continuous deflective separation to screen, separate and trap trash, debris, sediment, and hydrocarbons from stormwater runoff. Pretreating the runoff reduces loading and maintenance of downstream structures.

Treated water then flows to the CON/SPAN underground infiltration system. CON/SPAN is a modular, precast, three-sided concrete arch system designed to provide economical, large volume stormwater detention or infiltration below grade. CON/SPAN’s distinctive arch top and vertical sidewalls maximize storage volume while providing enhanced structural integrity. The open bottom of the CON/SPAN system allows the collected runoff to infiltrate into the ground. The Armorflex materials were placed within the CON/SPAN to minimize scour from stormwater entering the structure during high intensity rain events.

The CON/SPAN units were placed on EXPRESS Foundations, a unique foundation system that combines the speed of precast with the economy of cast-in-place construction. Contech was able to reduce the size of the EXPRESS Foundations to allow for on-site excavators to lift and place the foundation pieces without requiring a crane. This reduced the on-site use of a crane by one week and resulted in significant cost savings.

The CON/SPAN system offered many benefits to the engineers and contractors. The access from State Street severely limited size of the crane that could be used during installation. The use of the lightweight CON/SPAN system was able to meet the size and weight limitations of the crane.

The entire project was constructed in a tight schedule, as the project was required to be completed within a 10 week time period while not interfering with the nationally-renowned annual Ann Arbor Art Fair and begin completed prior to the 2018 school year. Contech engineers worked closely with CDM Smith and the University throughout the design and construction process, providing design, review, onsite technical assistance and specifications support.

“We are pleased with the effort that Contech assisted with the installation of the CON/SPAN system,” said Brain Callaway, estimator at Site Development of Madison Heights. “The Field Engineer assisted in every aspect of installation – from locating and placing the Express Foundations system to showing our field crew special techniques for installing the large CON/SPAN structures and making the units fit within one-half inch from end to end. The project was successful from a schedule and overall timing as we appreciate the efforts that Contech had with this project.”

Technical Description:

  • CON/SPAN® O-Series® Stormwater Detention System
  • EXPRESS® Foundations 
  • CDS® Hydrodynamic Separator
  • Armorflex® Hard Armour 

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