Curved Microtunnelling on the Ship Canal Water Quality Project

Abstract

This paper presents the design of a curved microtunnel for the Ship Canal Water Quality Project, including challenges encountered during design. This is a joint project being constructed by Seattle Public Utilities and the King County Department of Natural Resources Wastewater Treatment Division to meet federal Environmental Protection Agency Consent Decrees to limit combined sewer overflow discharges by 2025. The project will control combined wastewater and stormwater overflows from seven combined sewer basins in Seattle, Washington by storing excess flows from the affected basins. The major project elements include a 110,000 m3 storage tunnel, a 0.5 m3/s effluent pump station, five drop structures along the alignment, ancillary facilities at the five project sites to support operation of the tunnel, and conveyance pipelines and facilities to convey combined sewage to the tunnel. As the storage tunnel is located along the north side of the Lake Washington Ship Canal, the project also includes a microtunnel under the Ship Canal to convey flows from one basin on the south side. The preliminary design for the microtunnel includes two straight drives between three shafts to route the microtunnel around an existing deep utility tunnel. To avoid the need for the intermediate shaft, a curved alignment was adopted during detailed design. The 200-metre radius curve will be built in a single, 220-metre-long drive. Detailed design of the project’s tunnel, microtunnel, and pump station elements commenced in 2015. The project was issued for bid in early 2019, with construction commencement anticipated in late 2019.

Techniques