Mudhoney, the tunnel boring machine (TBM) used for the Ship Canal Water Quality Project (SCWQP), successfully broke through in June after completing its 14,000-foot-long drive (4,267 m). It was a critical milestone for the project, which will reduce combined sewer overflows into the Lake Washington Ship Canal and Lake Union in Seattle.
Delve Underground is the project’s prime design consultant and engineer of record for the tunnel. The project is being constructed by The Lane Construction Co. (Lane) under a design-bid-build contract. Delve Underground has been providing engineering support and part-time geotechnical inspection during construction.
Tunneling commenced in August 2021 using a pressurized steel bell launch system. The system was proposed by the contractor as an alternative to ground improvement outside of the shaft. The steel bell consisted of a segmented steel cylinder sealed against the shaft headwall with the TBM inside. This allowed the contractor to increase pressure in front of the machine to match the ground pressure outside the shaft prior to break out.
Following its successful launch, the 21.6-foot-diameter (6.6 m) Herrenknecht earth pressure balance TBM encountered several notable challenges. The TBM mined through a 12-foot-diameter (3.7 m) boulder approximately 3,000 feet (914 m) into the 14,000-foot drive, and cut through 15 rows of detensioned wire strand tiebacks approximately 200 feet (61 m) before breaking into the reception shaft. The tiebacks were anticipated, and their removal was required per contract documents. Lane’s solution, coordinated closely with the TBM manufacturer, Herrenknecht, was to mine slowly through the obstructions by implementing tight controls on TBM advance parameters, creating access points in the screw conveyor housing for clearing potential blockages, and accessing the cutterhead frequently to evaluate conditions. No notable damage to the cutterhead or screw was observed and most of the recovered tieback strands were pulled off the muck belt.
The contractor performed 19 interventions during the drive, and 18,000 precast segments were installed. Two segment sizes were designed by Lane: a typical 5-foot-long (1.5 m) segment and a shorter 3.3-foot-long (1 m) segment to help navigate some of the tighter curves. The contractor’s segmental lining design used steel fiber for typical reinforcement and had no bolted connections at the longitudinal joints.
Navigating a Curved Microtunnel Drive
Another important design element constructed in 2023 was a curved microtunnel drive beneath the Ship Canal to connect two drop shafts via an inverted siphon. The 600-foot-long (183 m) drive was completed with a slurry microtunnel boring machine (MTBM) through dense glacially overconsolidated soils. Lane’s microtunneling subcontractor, Northwest Boring, designed the 96-inch inside diameter (2,440 mm) reinforced concrete jacking pipe and hydraulic joint system used to navigate the 650-foot radius horizontal curve. The system allowed the team to continuously monitor joint articulation and thrust forces along the pipe string during excavation. After excavation, conduits and carrier pipes were installed within the casing—42-inch (1,065 mm) and 18-inch (455 mm) fiberglass reinforced pipe (FRP) siphon pipes for combined sewer overflows (CSO), and a 30-inch (760 mm) FRP foul air vent pipe to push air to the carbon scrubber odor control system on the other side of the canal.
Two SEM Adits for Access and Ventilation
Lane is finishing the last few pieces of underground work, which includes two permanent adit connections designed by Delve Underground between the tunnel and off-line drop shafts. Both adits are 8-foot (2.4 m) inside diameter reinforced concrete connections. One adit at the tunnel crown provides necessary ventilation during tunnel filling, and the other connects at the tunnel invert to provide SPU access to the tunnel with an inspection vehicle. The adits are being mined using Sequential Excavation Method (SEM) techniques within glacially overconsolidated soils improved with jet grout.
SCWQP is being delivered under partnership between Seattle Public Utilities (SPU) and King County to improve water quality in the Lake Washington Ship Canal. The project will reduce the volume of CSOs discharging into receiving waters by over 75 million gallons (274 million liters) each year. Substantial completion for the tunnel contract is expected in early 2024. SPU and King County are currently preparing to bid the follow-on tunnel effluent pump station (TEPS) contract, which Delve Underground is also managing as the prime design consultant.