Trenchless Technology

JAN 2019

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W W W.T R E N C H L E S S T EC H N O LO GY.C O M 37 a big bowl centrifuge in its custom- built separation plant. Line and grade are critical to the success of the siphon project as chal - lenging as this one. After the team was assembled and many design meetings were held, some unique challenges emerged. Among them were: • The slope of the floor at approximate- ly 30 degrees and crew functionality working at such a steep slope. • The front wall, floor and thrust block design, which also had an equivalent 30-degree slope. • The thrust force of the reaction block that could potentially push the non- stable soils and react above ground. • Crossing a U.S Army Corps of Engi- neers waterway (the Tualatin River). When river crossings are encountered, issues that could be generated from tun- neling include inadvertent flows of the drilling fluids, settlement of the river and heave. During the investigation of the soils, it was determined that the upper region of the tunnel would be in the Mis- soula Flood Deposit formation (a sandy clay) with blow counts ranging from as low as 4 to 12, and then entering into the Hillsboro formation (clayey sand) with blow counts ranging from 9 to 44. Since steering concerns were very critical, sev- eral different soil treatment designs were evaluated during the process to steer the machine on line and grade. The origi- nal concept design included a 750- radius vertical curve and soil cemented columns. Aer several iterations, it was determined to tighten the curve radius to 650 and shorten the drive to 470 lf to put more of the tunnel alignment into the Hillsboro formation, which had a better potential for steering reac- tion. However, by tightening the curve radius, additional risk was associated with joint deflection and jacking forces - both proved to be non-issues with the appropriate design. The tighter curve al- leviated the need for the intrusive soil- cemented columns through wetlands, and only needed for the front entry wall. The columns added at the front entry al- lowed for the MTBM to provide a stable steering reaction right from the launch of the tunnel and increased the chances for success. Once all the risks were reviewed, it was time for Michels' tunneling crews to shine. The project created multiple logistical challenges, which also in- cluded the complication of shipping the international RCP pipe and MTBM. International shipping can create mul - tiple complications. However, with knowledgeable management and field staff, these potential problems proved to be non-issues and all shipping was handled on- time. The commencement of the new separation plant and MTBM also can raise multiple potential issues. With the help of local suppliers and other contractors in the area, Michels was able to work through the minor is- sues and prepare for launch. While preparing to launch the MTBM, the Michels team developed a strategy to rig the MTBM and set the head as well as each subsequent section of RCP pipe down at a 30-degree slope. After the MTBM was launched, the Michels crew shifted emphasis to steering the tunnel. Placing the bottom curvature of the tunnel into the Hillsboro forma- tion proved to be successful, and the line and grade were perfectly executed. The MTBM was launched and finished in less than a month, and celebrated as a success to the entire team. Design-build projects always pro- vide unique challenges, however, the team achieved astounding success. This unique project provides the utility in- dustry with an alternate to conventional microtunneling methods and provides new potential for what the industry can achieve. The shallow shas allow for a smaller dewatering footprint and the pre- cise steering demonstrates that microtun- neling can be a viable, successful option for construction on line and grade under waterways, highways, railroads and any other potential obstruction. While the equipment used on this project is not new technology, it was used in new ways. Current potential cost savings could also prove to be use- ful as the shafts no longer need to be constructed at such great depths. In addition, using the U- shaped vertical curve to construct siphons can help push the limits of the industry higher. Brenden Tippets is microtunneling manager at Michels Corp.

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