Weather Proves Lucky
When Michels Tunneling bid the Indian Memorial Intake Phase 1 microtunneling job, it anticipated a 60-ft water head in South Dakota’s Lake Oahe for the underwater retrieval of its microtunneling boring machine (MTBM), not 82 ft as it turned out. As is often the case with tunneling contractors, circumstances beyond their control dictate that they must adapt and rise to the occasion.
The water intake pipeline from Lake Oahe will provide drinking water to the Standing Rock Sioux Tribe reservation. The Sioux in this area have been dealing with a repressed water situation for many years. Lake Oahe, a man-made reservoir and the fourth largest in the United States, is part of the Oahe Dam along the Missouri River in Mobridge, S.D. The project owner is Standing Rock Rural Water Systems and the general contractor is Graham Construction Services. Michels Tunneling served as the subcontractor to Graham Construction for the specialized microtunneling segment of this phase. The Indian Memorial Intake received $18.9 million in American Recovery and Reinvestment Act (ARRA) funds to facilitate this project.
Because projects of this nature are seasonally critical, Michels’ operators initially expected this 950-ft drive to take place during the summer. But a delayed start posed the potential for a multitude of setbacks. As stated by Michels Tunneling vice president Ray Post, “The tunneling portion of the project was originally going to take place in late summer, however, we did not start up until October, just as winter was starting to set in. During the bid phase, we did not anticipate any winter work. However, luck was on our side, as we did not have any hard freezes, only some snow and so we were able to complete the project.”
Crews from Michels set up their launch shaft from a 110-ft deep, 20-ft diameter caisson located on the shore of the reservoir. They installed shaft seals then lowered their jacking frame and MTBM onto the base of the caisson. The shaft seal acts as a gasket around the pipe to prevent loss of ground and slurry into the launch. This project required a wet retrieval, where the MTBM meets its target destination and is retrieved and brought to shore by deep-sea divers, rather than reclaimed from a reception shaft.
This past fall, Lake Oahe’s water table happened to be much higher than what it had originally expected. Michels Tunneling general manager Jim Kabat remarked, “We had some concerns when construction started. Due to the spring thaw and heavy summer rains, the underwater retrieval ended up being 82 ft deep, nearly 20 ft more than anticipated. But, after a lot of discussion and a little re-engineering, the job was set in motion.”
Michels used an Akkerman Inc. MTBM SL46 to install the 950 ft of 44-in., 16-ft length Permalok pipe. Michels also used an Akkerman MT 866 jacking frame and control container. The pipeline began in the caisson and progressed 950 ft under the bed of the lake where ground conditions started in shale then transitioned about halfway to silty clay with some sand.
Microtunneling, often referred to as remote-controlled pipejacking, is a system consisting of an MTBM and jacking frame, controlled by an operator in an above-ground control container with the support from several types of pumps, hoses, slurry separation plant, bentonite pump and water cooling tank. Operators are able to monitor and adjust the MTBM’s location and rotation, torque, jetting, pipejacking thrust, steering, slurry flow and pressures from a three-monitor console located in the control container.
The MTBM serves to excavate materials at the cutter face and guide the pipe through the ground. Akkerman’s MTBM ranges in size from 24 to 102 in. in OD. The guidance target, housed inside a heavy-duty steel cylinder, is mounted in the back center of the MTBM. The guidance system reports the MTBM’s position to a monitor in the control console for operator assessment and anticipates the MTBM’s location at the cutter face. After the spoil enters the MTBM cutter face, it is cut and ground up, mixed with water for transport and removed. The steerable, rotating, bi-directional cutter head, fitted with excavating tools accomplishes this mining action. The forward advancement on the pipeline, combined with the rotating action of the cutter head, removes spoils in place and forces them rearward to the crusher cone. This crushes the spoils into pieces small enough to be passed through the slurry system. Slurry lines carry the cuttings away from the machine, out of the tunnel, to the surface and return the slurry back to the machine.
The jacking motion was performed by a MT 866 jacking frame, which specifically can jack up to a 66-in. OD pipe with 800 tons of thrust capacity. The MTBM is launched through a shaft seal. After the jacking frame pushes each length of pipe, the MTBM rotation and slurry flow is stopped, another piece of pipe and new tunnel connection hoses are added and mining is resumed. With this process, the first piece of pipe travels to the far end of the tunnel directly behind the MTBM.
Operators are alerted of the MTBM hitting its target destination by monitoring its progress from the control console. At this point, Michels was finished and the professional divers took over to complete the job.
A.D.I. Marine Professionals president Aaron Faken commented on the wet retrieval of the MTBM. Faken reported that this was the sixth underwater retrieval that they have performed for Michels Tunneling and the second time they retrieved this particular MTBM. Divers located the SL46 about 8 ft below the mud line with air pressure probes. The team of five divers cleaned off the lift eye mounting pockets and installed lift eyes on the MTBM to attach the spreader bar lifting apparatus. Chains linked the spreader bar to winches mounted on the floating barge, where dual 800,000-pound tuggers pulled the MTBM to the shoreline. The retrieval process took two days.
The 950-ft drive was completed in about six-weeks, without a hitch. A re-engineered design to compensate for high water levels coupled with ideal soil conditions, operator expertise and competent divers, made for a successful run.
Laura Anderson is the marketing specialist for Akkerman Inc., Brownsdale, Minn. She would like to acknowledge Ray Post and Jim Kabat with Michels Tunneling and Aaron Faken with A.D.I. Marine Professionals for their input in this article.
