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Missoula, Montana
Missoula Wastewater Treatment Plant Headworks
New Solution for Odor Control
The Missoula Wastewater Treatment Plant Headworks project resulted from an increasing number of resident complaints. The City of Missoula received an Order to Take Corrective Action from the Missoula City-County Health Department regarding odors from EKO Compost and the City’s Wastewater Treatment Plant (WWTP). The order required the city to complete an odor-characterization study and submit a plan for corrective action, which recommended improvements to the WWTP Headworks and aerated grit chamber, which were the largest single source of odors at the plant.
Replacement of the Headworks required that the existing headworks facility remain in service until the new facility was put on line. This necessitated construction of the new facility in a very small footprint, surrounded by existing plant process units and directly adjacent to the Clark Fork River.
Client
City of Missoula
Location
Missoula, Montana
Award
Montana ACEC Engineering Excellence Award
Services and Highlights
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Civil engineering design
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Bidding
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Construction-phase engineering services
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Design of new grit chambers
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Effluent flow measurement structure design
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Wastewater process design
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Structural design
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Mechanical and plumbing design
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Electrical design
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Site civil design
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Structural inspections of reinforced concrete, masonry shear walls, and structural steel
Due to the varying depths of the building foundation and the large concrete seal slabs located beneath, there was a potential for differential settlement. The building foundation sections are designed as an integral unit with the deepest portion of the foundation acting as an anchor and other portions attaching to this anchor.
This photo shows the depth of the lower-level of the Headworks foundation and the elevated concrete floor slab.
Steel cofferdam sheet piles were designed to be driven into place and the excavation for the building was then executed “in the wet” with no dewatering in place.
One of the primary social drivers for the City of Missoula’s project was to contain and treat odors associated with the Headworks facility in order to meet the compliance order.
While the building footprint was fairly small, the building’s lower level needed to be approximately 32 feet deep. Groundwater levels at the site fluctuated between 22 feet to eight feet below the surface. Conventional dewatering of the excavation was not possible due to the proximity to the river. In addition, the lower level had to be completed and backfilled prior to spring runoff and coinciding high groundwater levels to prevent buoyancy uplift forces from moving the building.
The small working space, close proximity to the Clark Fork River and the weather made this an unusually difficult project. The deep excavation construction necessitated the use of cofferdams and a tremie concrete seal at the base of the excavation to minimize the groundwater flow into the excavation.
This work was initiated in October and the cofferdam installation proceeded during the winter months adding adverse weather conditions to the host of challenges. The critical path scheduling required completion and backfill of the lower level foundation and 32-foot high walls before the annual spring runoff and concurrent increase in groundwater levels.
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