SR-99 Tunnel: Construction of the TBM Launching Pit
The SR-99 Tunnel Project for the Washington Department of Transportation parallels the waterfront through the south Seattle area and was constructed utilizing the world’s largest tunnel boring machine (TBM). Malcolm teamed with the Seattle Tunnel Partners JV to construct and dewater the TBM launching pit, which exceeded 85 feet in depth. Additional ancillary work included the support of excavation for several cut-and-cover roadways, along with jet grouting for ground improvement.
The site conditions consisted of very soft, highly saturated, and unstable material, along with abandoned wood and timber foundations. To meet these various geotechnical challenges and to accommodate clearance issues at the site, a retention system of heavily reinforced secant pile walls supplemented with high-capacity strand anchors was installed. Pile verticality was critical to handle the horizontal loads and to minimize water inflow. With pile lengths up to 140 feet on this project, the task of advancing sectional casing through several layers of wood and debris, as well as soft, silty sand, required the most current state-of-practice equipment and techniques. Malcolm had the necessary high-torque rotary drills and oscillators to advance and retract casing within critical tolerances and to great depths to successfully complete this project.
