Challenge
Extended high river levels in 2019 caused the levee adjacent to the intake to breach, causing a scour hole that completely enveloped one 60-foot-tall grit tower and was undermining the foundation of the second tower. The first challenge was to shore up the remaining tower foundation and piping to ensure water service to nearby residents and major industries. The next challenge was to find and recover the tower buried in the scour hole. The site was constrained by a USACE levee and the Mississippi River, so the team wanted to build within the footprint of the existing tower. Once enough of the tower was recovered to get an adequate footprint, Goodwin Brothers then moved to filling the scour hole with 30,000 tons of dredged sand. All work was adjacent to the Mississippi Riverbank and sitting on coarse sands. Dewatering was not an option, so work could only be performed when river levels allowed. The final challenge was to design and construct a new grit tower that was located on an island in the Mississippi River as quickly as possible while ensuring that new construction would withstand future flood damage. This included requirements that deep foundations alone would support the new tower and piping if another scour hole completely undermined the structures. The team also designed the new tower to take the impacts of barges and river debris during high river stages.
Approach
The original grit tanks were constructed in 1927. Design was underway to replace both tanks with similar units using design-bid-build prior to the 2019 flooding. The flood event and damage obviously accelerated the replacement schedule, but also exposed potential shortcomings of the new design. With a designer already selected, Illinois American Water chose to utilize CMAR delivery to address design, constructability, and schedule concerns. With Goodwin Brothers on board, they were able to evaluate with actual costs, schedule, and constructability input, which proved invaluable when doing construction on an island where most materials and equipment were delivered by barge.
Results
After bringing on Goodwin Brothers it was quickly determined that the original concept to construct two similar replacement tanks was not the best path. Different steel tank configurations, composite steel/concrete and concrete tanks in both one- and two-tank configurations were quickly evaluated. Material availability, site constraints, constructability, and longevity concerns led the team to the decision to construct a single cast-in-place tank with an internal separation wall. This resulted in shorter lead times for materials and construction materials better suited to withstand future flooding. The single tower design worked well within the site constraints, accelerated the schedule, and saved costs. Utilizing cast-in-place concrete used local, readily available materials, while providing greater longevity and protection against impacts of high river stages. Illinois American was able to make informed decisions to determine the best option that provided the most schedule certainty and greatest protection against future failures.
“This was an unexpected project with a quick turnaround that posed many challenges. Goodwin Brothers’ adaptability and creative problem solving played a substantial role in the success of this project.”
– Illinois American Water, Design Engineer
