Wastewater treatment facility overcomes aggressive soils & fluctuating groundwater
More than 6,800 cu yd of structural concrete are being used in the construction of a new trickling filter and trickling filter pump station in Bismarck, N.D. These structures are being built well below the area’s water table, and the concrete is one of many features protecting them against the effects of Mother Nature.
The $21.9 million Bismarck Wastewater Treatment Facility Trickling Filter Improvements project includes the construction of a trickling filter pump station and a 150-ft trickling filter. The expansion not only will replace an aging pump station, but also will provide the city with 50% more treatment capacity, allowing it to meet anticipated growth over the next 20 years and improve operational flexibility. The project began in October 2015 and is scheduled for completion in July 2017.
Pipe vs. Mother Nature
Battling the elements proved to be an ever-present challenge in the project.
One component of the project already is protected: V-Bio-wrapped and zinc-coated AMERICAN ductile iron pipe. The zinc coating system is a life-extending product for iron pipe that recently became available in domestic markets. Zinc has been used overseas to extend the life of iron pipe for many years. V-Bio enhanced polyethylene encasement provides additional protection in aggressive soils.
“Ductile iron pipe was specified for the project due to aggressive soils in the area and the depth of bury,” said Ken Weber, P.E., senior project manager for AE2S. “Testing was done to confirm the corrosiveness of the soil, and V-Bio wrap was included in the specifications. Once we learned of the zinc coating, we also felt it was the right product for this application.”
The project requires a substantial amount of subsurface dewatering in order to excavate at depths exceeding 40 ft. The groundwater level in the area fluctuates with the elevation of the nearby Missouri River, and under normal conditions, the water level will range from 8 to 14 ft below the existing grade at the jobsite.
“Excavating at these depths presents a unique challenge for any installation,” said Darin Pfingsten, president of PKG Contracting Inc. “With one end of the pump station bearing at a depth of about 44 ft below the existing surface, we elected to sub-cut the entire area to an elevation 15 ft below the surface. It is also at this elevation that a nearly 100-ft-diameter cofferdam retention structure was installed, which consists of steel sheet pilings driven to a depth of 65 ft below the existing ground surface. A total of just under 21,000 cu yd was excavated to complete the cofferdam structure.”
Wastewater Treatment Teamwork
Bismarck’s wastewater plant currently treats an average of more than 6 million gal of wastewater per day. Wastewater is collected from nearly 18,000 customers and delivered to the treatment facility through a 290-mile pipeline network.
PKG Contracting Inc. is the general contractor on the improvement project. Once complete, it will have self-performed more than 70% of the work. Major work includes earthwork, site piping, cast-in-place concrete, helical piers, precast work, process equipment and process piping.
AE2S is providing engineering services for the project, including designing, bidding and providing construction management services.
AMERICAN is providing approximately 2,000 ln ft of Class 250 Fastite and Flex-Ring ductile iron pipe in sizes ranging from 24 to 48 in.
“Because of existing soil conditions, the owner decided to add a measure of safety and longevity to the installation by using zinc-coated AMERICAN ductile iron pipe and V-Bio enhanced polyethylene encasement,” Pfingsten said.
With strategic choices in construction materials, the facility will be protected from the effects of Mother Nature.