Pilot Tube Microtunneling in the City of Omaha, Neb.
During the past two years, the City of Omaha, Neb.’s sewer network received 13,000 ft of new pipelines — all installed by pilot tube microtunneling (PTMT). Two of the four projects are the separation of combined sewers where all linear footage was completed with this method. With two projects completed and one currently under way, PTMT is exceeding expectations for the City of Omaha.
In order to comply with the federal Clean Water Act, the City is investing millions of dollars over the next 15 years in the separation of combined sewers, named the Clean Solutions for Omaha Program. There are a total of 510 miles of combined sewer pipelines and 345 miles of sanitary sewers that discharge into the CSOs in Douglas and Sarpay counties. Omaha’s two wastewater treatment facilities filter wastewater and discharge it into the Papillion Creek and Missouri River. The federal mandate states that Omaha must reduce the number of combined sewer overflow occurrences from 58 to four per year. Improvements are funded through incremental user rate increases. Aside from reduced backups and water quality improvement, additional criterion outlined in the City of Omaha’s long-term plan includes minimized community disruption, infrastructure improvements, reduction of street flooding, simplified solutions, compatibility with the community and green opportunities.
In summer 2008, a short amount of pilot tube microtunneling work was completed where 100 ft of 15-in., vitrified clay jacking pipe was tunneled under a high-traffic road and a World War II memorial to ultimately tie into a manhole. This short drive, performed by NorthCore of Fargo, N.D., had to be completed without disturbance.
In July 2008, the OPW 50762 26th & Grant Area Sewer Separation project commenced to create 7,087 ft of 8- to 15-in., vitrified No-Dig clay pipe in residential and business streets. PTMT accounted for $4.1 million or almost half of the $8.3 million total. Drive accuracy was critical due to weak soil conditions, strict grade tolerances and dense buried utility lines. The city emphasized the importance of unencumbered traffic flow during the project. Pipe installation within the confines of these particulars must be completed via microtunneling or pilot tube microtunneling. With PTMT being the faster and less costly method of installation, the City of Omaha found that this method best fit the bill.
Construction for the 26th & Grant Area Sewer Separation project was completed by Roloff Construction Co. Inc. of Omaha with pilot tube work performed by NorthCore. Ehrhart, Griffin & Associates of Omaha were the civil engineers on this project.
Complex in design, this job featured 32 drives with four drives connecting two manholes. Shaft depths ranged from 20 to 32 ft. In an effort to reduce manhole construction costs, Roloff Construction and NorthCore altered the design and completed four drives where they bored through a manhole mid-drive before emerging in the reception shaft. With an estimated launch shaft costing around $30,000, this unique application offered many levels of cost-savings.
With the need for speed and accuracy and an ambitious agenda of 32 drives, NorthCore used a McLaughlin auger boring rig with a 308A Akkerman Guided Boring Machine (GBM) mounted on the front to provide accuracy. All casings and pipe adapters were assembled by NorthCore. The power and speed of the auger boring rig, combined with the precision offered by the GBM made for a good union.
Once the launch and reception shafts were constructed and shored, the auger boring rig was set up with the GBM mounted in front, secured to the track with adapter brackets. A soil appropriate steering head was mounted on the first length of pilot tube. Line and grade are established by aligning the LED target, housed in the first pilot tube section. The camera and theodolite was mounted in a keyhole notch on the back of the GBM frame for protection. Operators accessed and controlled the progress of the target on a monitor mounted to the top of the GBM frame, above the controls area. Pilot tubes were installed on line and grade until the full drive length was achieved. The GBM frame was then removed and the auger boring rig assumed regular operation to install the final product pipe. As a section of final product pipe was installed, a section of pilot tube was removed in the reception shaft. PTMT line and grade can typically be maintained within a quarter of an inch at distances of approximately 500 ft in soil conditions under the 50 blow count.
A total 2,167 ft of 8-in., 2,654 ft of 12-in., 840 ft of 18-in. and 1,427 ft of 21-in., vitrified clay pipe was installed. All drives were completed in 305 days, with the shortest drive recorded at 124 ft and the longest at 350 ft, with nothing more than a half-inch variance in bore accuracy. Most of the drives were completed in three to five days, with the longest drives averaging about seven to nine days with a six-person crew during single shift work days. The flattest grade was 0.3 percent and the steepest was 1.9 percent. All drives were completed by July 2009 in 305 working days, one month ahead of schedule.
Mike Nesler of NorthCore was chief foreman and operator for this job. After spending a year encamped on the project, he became familiar with all its intricacies. He commented that the drives connecting the two manholes involved a new technique for him. He stated, “Although there were 32 drives, each drive came with a unique set of circumstances.” The area that most of the work took place, referred to as the Old Town section of Omaha featured heavily trafficked, narrow streets. He expressed satisfaction with the GBM’s accuracy so he could “bore with confidence.”
The City of Omaha sought bids for a 1,864 ft of PTMT work for the OPW 51450 96 & I to 88 & L St. Sanitary Relief Sewer. Construction began in the fall 2009 by Horizontal Boring & Tunneling of Exeter, Neb. Approximately $608,000 of the $1.4 million total project costs were for installation of 18-in., vitrified clay jacking pipe in a high truck trafficked industrial area featuring Kellogg’s, Precision Industries and Omaha Steaks businesses. With minimized traffic disruption being key in the industrial park, PTMT was again an excellent choice for sewer pipeline installation. Horizontal Boring & Tunneling constructed seven shafts where depth ranged from 9 to 29 ft. The 1,864 ft was accomplished in six drives.
The most recent project currently in progress for the City of Omaha and Horizontal Boring & Tunneling is the OPW 51861 South Omaha Industrial Area Sewer Separation. This is another CSO project where all 2,980 lf of 8-to 15-in., VCP pipe will be installed with the pilot tube technique. Approximately $800,000 of the $2 million cost constitutes PTMT work. Pipelines will be installed in an industrial area where several meat packing industries and the Metro Community College south campus are located. Construction began in June 2010 and is estimated to continue through December 2010.
Laura Anderson is a marketing specialist at Akkerman.