In King County, north of Seattle, WA, the modernization of the sewage disposal system is is being planned. Sewage tunnels have to be bored on a 20-kilometer-long section in order to collect the sewage and pump it to the treatment plant. The project's challenges include high groundwater pressures and the danger of methane gas.
Two Mixshield machines opened up just under 10 kilometers of sewage tunnels. Both started from the same shaft – that of the North Kenmore Portal – the dimensions of which imposed considerable challenges for assembly logistics. This meant, for instance, that the parts of the gantries stayed at the upper edge of the launch shaft. Only when the leading segments of the machines had made some appreciable advances in their tunnels were these parts linked up. S-379, the first machine, started tunnelling through 3,530 meters of rock in October 2007 to be followed by S-380 – its sister machine – which was faced with a 6,200-meter stretch. Both machines completed their tunnelling in 2010.