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Big Walnut Outfall Augmentation Sewer

• Location: Columbus, OH
• Owner: Department of Public Utilities, City of Columbus
• General Contractor: McNally/Kiewit JV
• Technique(s) Utilized: Diaphragm Wall, Jet Grouting
• Subsurface Conditions: Glacial Tills, Sands, Gravels, Cobbles, and Boulders
• Approximate Key Quantities: Diaphragm Wall - 28,000 square feet, Jet Grout Columns - 294, Jet Grout Column Depth - 90 feet, Jet Grout Column Diameter - 5 feet, Shaft Depth - 90 feet, Shaft Diameter - 40 feet

 

Columbus, Ohio, like many cities in the United States, was built on a centuries-old sewer system that overflowed directly into its waterways during periods of heavy rain. Combined Sewer Overflows (CSOs) carry sewage and rainwater in a single pipe and were designed to overflow into rivers when they reached their maximum capacities. Sanitary Sewer Overflows (SSOs) were designed to carry waste water only. SSOs would often overflow or back up into businesses and residential buildings due to the seepage of excess water into tunnel cracks and from outdated downspout connections.

At the time, CSOs and SSOs were the best methods for managing sewage and rainwater.

Decades later, with new environmental regulations in place and new engineering and waste-water treatment options available, reducing the amount of sewage overflow into commonly-used waterways became a priority for many cities, including Columbus.

PROJECT BACKGROUND

To address the pollution of its waterways, used for both recreation and for drinking water, the city of Columbus and the state of Ohio developed a multi-phased plan to address sewer overflows into the Scioto River.

The Big Walnut Augmentation/Rickenbacker Interceptor (BWARI) sewer project was designed to increase the capacity of the existing waste water collection system and to allow the sewer to better support the future needs of the Big Walnut, Blacklick and Alum Creek basins in eastern Columbus and Franklin County.

The two-phase tunnel project would significantly reduce the amount of untreated waste water that overflowed into the Southerly Waste Water Plant and the Scioto River by providing in-pipe storage.

The first phase of the project included sewerage plant and conveyance improvements, followed by the second phase, which involved the construction of 15,850 linear feet of 144-inch-diameter concrete sewer tunnel and the 5,393 linear feet of 42-inch-diameter sewer.

THE WORK

The initial work on the second phase of the project included the construction of the launch shaft, drilling of the four intermediate shafts, and setup of the main shaft site. The 38-foot-diameter main shaft was constructed using slurry panels. The ground outside of the shaft in the zone of the incoming and outgoing sewers was stabilized using jet grouting. The intermediate shafts were constructed using large diameter drilling and also included the stabilization of the adjacent soils with jet grouting.

Nicholson was also contracted to install jet grout columns to reduce loss and water infiltration at various locations where the tunnel alignment intersected the vertical access shafts and other miscellaneous structures. In total, Nicholson constructed 294, five-foot-diameter jet grout columns that were installed at depths of up to 90 feet.

To reach the column depth during the grouting execution, Nicholson had to drill through gravel layers and cobbles. Special procedures were developed to maintain an open hole while drilling to guarantee proper return of jetting spoils to the surface.

In addition, the standard sampling methods had to be modified to enable the retrieval of the as-built jet grout soil-cement matrix for laboratory strength testing. Modified coring methods were used to retrieve the required samples while providing access to the jet grouted zone for the purpose of water tightness testing.

THE RESULT

The Big Walnut Augmentation/Rickenbacker Interceptor was the initial project of Columbus's overall sanitary sewer improvement project. The $300 million project was the largest public works initiative the city has ever undertaken.

Research has shown that approximately 90 percent of the bacteria that exists in the Scioto River were caused by sewerage overflows. The updates to Columbus's sewer systems are estimated to reduce the polluting effects of water overflows by an estimated 1.4 gallons annually.

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