Overcoming Challenges Due to Mississippi River Flooding
Grit Tank Replacement at Intake
The two existing welded steel grit removal tanks in Granite City, IL, had been in use since 1927. In early 2019, a project was underway to replace the existing tanks with new bolted steel tanks in the same location and possibly using the existing tank foundations. However, when the levee broke on Chouteau Island during the 2019 flood, the current completely destroyed one of the existing tanks and its foundation and partially undermined the foundation of the second tank, causing it to lean. The Granite City, IL, intake has a very large service area and population, so time was of the essence to get new, reliable grit removal tanks installed. Due to the urgency, high sensitivity, and several challenges associated with the project, Illinois American Water implemented a modified design-build approach, bringing the contractor on board at approximately the 30% design stage.
Because there was no assurance that the damaged levee would be repaired, the site would be swept by the current whenever the river flooded. Illinois American Water was not comfortable with new steel tanks being exposed to this current, given the damage that had been sustained by the existing tanks. After evaluating the options, the project team’s analysis pointed to cast-in-place concrete to achieve both a robust tank and the shortest construction timeframe. The ultimate design, a single tank with an interior divider wall to function as two tanks, provided operational flexibility and allowed the existing tank to remain in service while the new one was built, despite spatial limitations at the site.
Aside from these constraints, the project site had several challenges. The only access to the island was an old bridge with a weight limit of 19 tons, so several options were considered to bring equipment and concrete to the site. Heavy equipment had to be barged, but there were several options for the concrete: bucketing across the river by helicopter, barging full trucks and pumping from the barge, installing a mobile batch plant and barging the raw materials, or bringing concrete across the bridge 3 cubic yards at a time. After evaluating the options, the team decided to cross the bridge using trucks with partial loads of concrete. The site was under water and inaccessible for the first six months and remained in flood stage for several additional months, requiring design changes to accommodate the river. The most notable was the large site piping had to be redesigned during construction to reduce excavation depths. In addition, poor soil conditions and the potential for scour during river flooding required the new tank, valve vaults, and most of the piping onsite to be supported on helical piles.
Flexibility, collaboration, and the ability to change the design rapidly – in large part attributable to the design-build approach – allowed the successful completion of the project despite challenges posed by the river. Substantial completion was in April 2021.