In Bend, Ore., local contractor Stadeli Boring & Tunneling had a unique set of circumstances for a new gravity sewer interceptor.
The contractor had a contract with general contractor Taylor NW to furnish and install 323 ft of 36-in. steel casing under live railroad tracks. Line and grade were very crucial on the project, and the rocky conditions limited its trenchless boring options. The contractor’s ultimate decision to use a new piece of trenchless construction equipment was based on many parameters, from crossing length to casing requirements and location.
A Unique Set of Requirements
The Southeast Interceptor Project is a new gravity sewer pipeline that will provide service to rapidly developing areas east and southeast of the City of Bend’s core area. The pipeline ranges in diameter from 24 to 30 in. and extends from the existing plant interceptor in the northeast quadrant of the city to about six miles south along the eastern edge of the city. The alignment includes a 323-ft long, 36-in. diameter trenchless crossing of the Burlington Northern Santa Fe (BNSF) railroad in southeast Bend.
Two borings of the crossing were conducted from the surface, on either side of the BNSF tracks. Rock was encountered in both borings at 2.5 to 4 ft below the ground surface, and both borings were completed using rock coring techniques to depths approximately 10 ft below the tunnel alignment. No groundwater was encountered in these project borings. Based on observations from nearby past projects, groundwater in the region is approximately 200 to 400 ft deep, well below the tunnel alignment.
During the geotechnical investigation, the rock encountered was hard, gray vesicular basalt with significant void space. The unconfined compressive strength (UCS) of the basalt within the tunnel alignment typically ranged from 3,000 to 5,000 psi, however, it was baselined up to 15,000 psi because of the variability of this rock and the results of UCS tests on nearby rock. The RQD of the rock encountered within the tunnel horizon was baselined between 70 and 100 percent, and the rock was considered blocky. The abrasivity index of the rock was baselined at 4.0, indicating hard rock abrasiveness. The condition of the rock, particularly its high strength, its blocky, void-filled nature, and its high abrasivity, were significant features to consider for the design and construction of this tunnel.
Choosing the Machine
Contractor Stadeli Boring and Tunneling felt auger boring would be the best method, as the crossing lacked groundwater. The company currently owns two standard Small Boring Units (SBU-As), a type of hard rock cutting head using disc cutters, but it felt the equipment lacked the precision guidance needed for the hard rock crossing. “We met with Robbins in Ohio and told them what our needs were. They felt like their 36-in. prototype machine, which they had tested at one other job in Oman, would be a good fit. They listened to what we were wanting and needing to have done,” said Larry Stadeli, president and owner of Stadeli Boring & Tunneling.
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The machine, known as the SBU-RC, for Remote Controlled Small Boring Unit, was equipped with a smart guidance system by TACS. The guidance system could show an operator projections of the future bore path so steering corrections could be made before the machine was ever out of line and grade. The feature was critical for the crossing below the railroad tracks, which could not be shut down if problems occurred.
During tunneling, an operator in a cabin on the surface would be able to minutely adjust the steering within two degrees in any direction using an articulating front shield. While not for curved tunnels, the system would be able to make the necessary adjustments required for a straight, line-and-grade-sensitive tunnel like the Bend crossing.
The SBU-RC would operate much like other SBUs, with the closest similarity to the Motorized SBU (SBU-M). A circular cutterhead and cutting tools would excavate hard rock up to 20,000 psi or mixed ground conditions, while an in-shield drive motor would provide torque to the cutterhead of up to 16,000 lb-ft at 7 to 10 rpm. A pipe jacking system, or in this case a standard Auger Boring Machine (ABM), could provide thrust.
The team explained the unique differences of the SBU-RC. The machine, with its remote guidance, would be able to stay on line and grade and lessen the possibility of an event below the railroad tracks that would require intervention. Muck removal would be accomplished via a vacuum system connected to a vacuum truck. The entire setup, with SBU-RC, ABM, and vacuum system, was capable of excavating crossings up to 500 ft long. The system was also simpler than a slurry microtunneling operation, as vacuum suction would be used rather than slurry. No separation plant or cleanup would be required, and the cost-effectiveness of the SBU-RC operation would be far better.
Based on those parameters, Stadeli accepted the SBU-RC and it was delivered to the site on April 14, 2015, then lowered into a launch pit 26 ft deep. In the beginning stages of tunnel construction, some modifications were made to the system. In the first few feet of tunneling, the crew quickly identified that rock particles larger than 4 in. could not pass through the vacuum line, and the machine was retracted for modifications to the cutterhead, including grill bars, to restrict the size of the spoils allowed to enter the vacuum system.
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In addition, the capability of the 4,000-cfm vacuum truck to clear spoils from the line slowly dwindled, until finally, at about 100 ft of advancement, it was no longer effective and the spoils clogged the vacuum line. This truck was replaced with a 6,000-cfm vacuum truck, increasing the suction by 50 percent. After replacing the clogged lines, this truck was sufficient for the remainder of the 323-ft tunnel. Additionally, the truck had a larger tank capacity, allowing the installation of an entire 20-ft casing before the tank had to be emptied, eliminating the need to stop production between the placement of casing pipes.
The machine bored through volcanic basalt rock that was full of fissures, fractures and rubble pockets between 5,000 and 7,000 psi UCS. While the start-up was rough going, crews quickly began getting rates of 20 ft per day after the modifications to the cutterhead and vacuum system. “As we got used to the machine, we went up to 40 ft, and one day we even got 50 ft. We were able to cut off a couple weeks of our schedule time. Taylor NW was very pleased about it. When you look down the pipe now after it’s finished, it looks like a rifle barrel. There is no sag, it’s all in one straight line,” said Stadeli.
With the clear success in Oregon, Robbins is looking to lease the machine on more projects and expand their offerings. The modifications required in Bend will now be incorporated on all future models, including cutterhead grill bars, and the requirement for larger vacuum trucks will be communicated to contractors. “I think the SBU-RC is an exciting piece of equipment that has been compressed into a 36-in. size. To make it all work it is very compact. It’s impressive that the components have been sized down and it still works so efficiently,” said Stadeli.