Ottumwa Water & Hydro River Crossing Project Receives Engineering Excellence Award
On May 17, 2023, Ottumwa Water & Hydro, along with the design team of Brierly Associates and French-Reneker-Associates, was recognized with the Honor Award
in the water and wastewater category at the Iowa Engineering Society Annual Awards Banquet. The project, a 1,395 linear foot water main bore under the Des Moines River, successfully navigated multiple design challenges.
In March of 2019, the City of Ottumwa experienced heavy rainfall. The Des Moines River surged from a stage of 2.62 feet to 10.44 feet—just 1.06 feet shy of flood stage. The river stayed at this near-flood stage for much of March and all of April 2019. With the Des Moines River still ice-bound, this extreme rainfall and flooding caused the ice to “go out” in dramatic fashion, causing significant damage to a 100+-year-old 10-inch water main crossing the river at Vine Street. While water pressure was not lost and the water main was still functional, the water main needed to be replaced to prevent it from being compromised in the future. The team of Brierley Associates, French-Reneker-Associates, Inc., and Kaskaskia Engineering Group, LLC was selected for engineering design and construction-related services.
Horizontal directional drilling (HDD) was the most cost-effective method to install a new water main. The final design consisted of casing the HDPE water main from the entry point to a depth at which the new water main reached bedrock. This consisted of 70 linear feet of steel casing. A special welded end plate connection was designed to allow a ductile iron water main fitting to be connected to the HDPE and prevent any HDPE from being exposed to a potential contamination plume on the site. The annular space between the steel casing and HDPE pipe was filled with flowable mortar to provide a seal at the interface between the bedrock and the steel casing pipe.
The Vine Street replacement is the first instance of a water main of this material and size being constructed via trenchless methods beneath the Des Moines River in Ottumwa, Iowa. Since Ottumwa Water and Hydro (OWH) was unfamiliar with longer HDD installations, the project team worked collaboratively with OWH to educate, mitigate risks and develop a level of comfort with the HDD method.
The trenchless methods employed in this project enabled the existing water main to stay in service and minimized the potential for the new one to impact it. The new pipe was selected for its viability for HDD, but also is a more robust pipe material with electro-fusion connections that effectively make the pipe one continuous piece.
The complexity of this project stems from the challenging site conditions and constraints. The overburdened soils were very soft, and the layers of rock that most of the drill was constructed in varied between fractured shale, very strong limestone, and soft friable sandstone.
The new water main was constructed at a depth of approximately 40 feet below the river bottom, effectively eliminating any potential that a future ice floe could damage the new water main. The project was completed within the expected timeframe to allow the new water main to be online and the construction sites restored prior to the next winter. The project also demonstrated the viability of trenchless construction for Ottumwa Water and Hydro and allows them to consider these methods for future enhancement of critical underground infrastructure.