Contractors building two segments of a 13-mile-long sewage conveyance tunnel near Seattle have devised plans to fix in place two stalled tunnel-boring machines that had been working in poor soils and high groundwater pressure.
The tunnel is a key portion of King County, Wash.’s $1.8-billion Brightwater wastewater treatment project. The rims of the cutter heads on the 17.5-ft-dia Herrenknecht slurry machines were damaged, allowing rock and boulders to get stuck, says Gunars Sreibers, King County project manager. The general contractor, the joint venture Vinci/Parsons RCI/Frontier-Kemper, first stopped tunneling in May and laid off about 160 employees. The second machine was idled in June.
Vinci’s $221-million contract includes two tunnel segments. Both the contractor and the manufacturer have recommended adding additional tools to the cutter heads to avoid future problems. There is about 150 feet of water pressure on the machines, says Sreibers. The contractor is sinking dewatering wells 330 ft to the front of the cutter heads to relieve some of the pressure and allow workers to perform repairs under normal atmospheric conditions. The treatment plant will still open on schedule in 2011, but the conveyance tunnel will not be used, initially. Instead effluent will be discharged from two existing plants.