The Fremont Siphon Replacement Project included twin 2.15-meter (84-inch) diameter, 141-meter (464-foot) long microtunnel bores beneath the Lake Washington Ship Canal in Seattle, Washington. In addition to the microtunnel bores, the project included two secant pile shafts up to 9.8 meters (32 feet) in diameter and up to 24.4 meters (80 feet) deep, two near-surface connections to the existing trunk sewer system, and a new odor control facility. To mitigate microtunnel risk on the project, King County (the project owner) selected a unique baselining approach where baseline geotechnical conditions for the shafts were described in a traditional geotechnical baseline report, while baseline conditions for the microtunnel drives were presented in the project specifications, with values limited to a cobble and boulder strength range, and a maximum boulder size. The baseline also described the ground as highly abrasive, with the number of cobbles and boulders encountered being incidental to tunneling. To facilitate collaboration with the contractor and mitigate the county's financial risks associated with differing site conditions (DSC) claims, an incentive provision was included in the contract which allowed the contractor to claim a bonus at the completion of each drive as long as a DSC-related claim was not submitted for that drive. In addition, tunnel mining was paid by the hour, with a specified number of hours and minimum unit cost required on the bid form. This innovative approach proved successful, as the contractor completed both microtunnel drives within the allotted time, collecting both incentive payments, despite challenges. This paper details the unique contracting approach used on this project and discusses some of the lessons learned that are being applied to other microtunnel projects in the Seattle area.
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