Reacting to the Situation: Slurry Microtunneling for the City of Omaha

Nada Pacific’s SL60 MTBM

Nada Pacific’s SL60 MTBM emerges from a 1,292-lf drive for the City of Omaha.

The Miller Park to Pershing Detention Basin Sewer Separation Project will divert storm water flows to the Pershing Detention Basin and is part of the City of Omaha’s Clean Solutions for Omaha initiative. The majority of this conveyance was an extensive 1,292 lf, 60-in. ID RCP microtunneled alignment where the MTBM was launched from a specialized concrete thrust block, designed to withstand the jacking force for this great distance.

Additional work on the Miller Park to Pershing Detention Basin Sewer Separation Project included 1,008 lf of 60-in. ID RCP open-cut pipelines, outlet modifications in Miller Park, a new inlet into Pershing Detention Basin and construction of a storm water swale. Before construction, storm waters flowed into a combined sewer system that is slated for upgrade in 2016. Construction began in July 2013 and is anticipated for completion in July 2014. Project costs are $5,187,000

The Clean Solutions for Omaha (CSO) initiative contains 92 construction projects outlined in the city’s long-term control plan to improve water quality in the Missouri River and Papillion Creek. When complete, these efforts will reduce the number of combined sewage and storm water flows from 52 occurrences per year to four. Construction on the CSO initiative began in 2010, will continue through 2027 with total project costs estimated at $2 billion.

Slurry Microtunneling

The microtunneling contractor launched from a 40-ft long by 16-ft wide by 8-ft high concrete block (200 CY) on top of 30-ft long H-piles driven into bedrock and anchors drilled into the bedrock at a 45 degree angle.

Graham Construction of Omaha was the project’s general contractor. The overall project designer was Carrollo Engineers of Omaha. Nada Pacific Corporation of Caruthers, California, was responsible for microtunneling construction. Nada was established in 1992 and has installed more than 150,000 lf of pipeline via slurry microtunneling methods since its founding. Bennett Trenchless Engineers of Folsom, California, provided microtunnel design services.

A microtunneled solution was required for this tunnel due to the topography, geotechnical reports reflecting blow counts as low as 2, existing utilities and the presence of ground water above the tunnel zone. The full length of the 1,292-lf drive ran along Redick Avenue, along the northeastern quadrant of Miller Park at a 0.65 percent uphill grade in a mostly residential region of northern Omaha.

The project was awarded in April 2013 and required detailed coordination for the sequence of construction phases by Graham Construction. Nada Pacific had a firm window of opportunity to get the tunnel installed for subsequent project progression, while facing inevitable freezing temperatures.

Nada Pacific crews began microtunneling in September 2013.

Nada Pacific crews began microtunneling in September 2013, with a mere 1.5 ft of ground cover above the MTBM at the launch portal.

Getting Started
Originally, the design called for a traditional shaft, but Graham, with the assistance of Engineering Partners of Eagan, Minn., realized that the native soil would not be able to withstand the jacking force. Clayton Wachal, project manager with Graham, stated: “The original design called for the installation of the jacking shaft 30 ft into the bluff where the shaft would have had a 35-ft head wall and 12-ft back wall. Not only did this create unequal pressures for shoring, it also made access impossible.”

Wachal further described the challenge. “The tunnel began at the bottom of a bluff and was to be pushed into a condensed neighborhood. This provided zero resistance for the machine since the invert of the pipe was 3 ft above existing grade,” he said.

Conversations and calculations ensued between Graham, Nada Pacific and the engineers that resulted in the determination that the custom man-made concrete structure would be needed to bear the reactive thrust load to complete a tunnel of this length. Wachal described the solution as “moving the jacking shaft out 30 ft to level ground to build a thrust block which consisted of a 40-ft long by 16-ft wide by 8-ft high concrete block (200 CY) on top of 30-ft long H-piles driven into bedrock and anchors drilled into the bedrock at a 45 degree angle.”

Pushing Through
Nada Pacific brought its Akkerman SL60 MTBM with a soft -ground cutter face, an MK875T keyed jacking frame, control container, bentonite pump, four jacking stations, a jacking can and a U.N.S. guidance system for the exceptional distance.

American Concrete supplied the concrete pipe featuring a double-confined O-ring gasketed joint in 8-ft segments, valued for its watertight seal properties and jacking load capacity. Although challenged by limited resources in this area to produce this specialized pipe, American was able to satisfy the project’s delivery deadline in order for microtunneling work to commence on time.

The MTBM launch shaft was located in a wetland, at the base of hillside at the eastern end of the alignment. The general contractor began site construction by creating a gravel access road and staging area in early August 2013. Nada Pacific mobilized in the second week of September. Crews positioned the separation plant, pipe and slurry line inventory behind the launch site, with the control container and bentontie pump situated on top of the thrust block. Ground cover was a mere 1.5 ft above the MTBM at the launch portal.
Ted Miller, project engineer for Nada Pacific, reported: “Among the buried utilities near the path of the microtunneled line was a 30-in. brick sewer line and another 18-in. sewer line that both ran parallel on opposite sides of it for about half of the distance.” He added that: “Swelling clay was encountered throughout the entire alignment, which increased jacking loads” and that “special attention was given to the bentonite lubrication, to ensure a successful project.”

The blow counts in this region made it difficult to maintain grade yet their skilled operators managed to persevere on course. To help with thrust distribution, they inserted four evenly spaced intermediate jacking stations along the tunnel and a jacking can behind the MTBM. The average amount of cover above the tunnel was about 40 ft.

Nada Pacific crews pulled out in mid-November and reported a total of 36 working days to complete this installation. They had a crew of six; four came from Nada Pacific and two from local sources. They worked 10- to 12-hour shifts and experienced their best production day when installing 50 lf in one shift.

Job Well Done
Mike Abbott, microtunneling operator for Nada Pacific, noted that the project posed no outstanding difficulties and seconded Miller’s comment that “the use of bentonite on the project was key.” Midway through the alignment, one of the pumps in the bentonite circulation system failed, but Abbott reported that they were able to send an operator to Akkerman headquarters to pick up a replacement and continue production the next working day.

Graham Construction has a long-standing relationship with the City of Omaha. Miller stated that he “appreciated the open dialogue and shared respect for each party’s area of expertise among the owner, general contractor and engineers.” This being the first time that Nada Pacific worked with Graham Construction, Miller felt that, “we were treated as a valued contributor to the process and that’s always a positive way to embark on a project.”

Wachal equally admired the microtunneling subcontractor’s competence, stating: “The team was able to hit the exact dates that were planned five months in advance. … This project gave me a tremendous amount of confidence in Nada Pacific’s knowledge and abilities.”

Laura Anderson is director of marketing and communications at Akkerman Inc., Brownsville, Minnesota.

TBM Online - 2017

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