Case Study: Regional Wastewater Conveyance Tunnel

Client: King County / CDM Location: King & Snohomish Counties, WA Contact for geotechnical projects: John Peterson

Project Background & Challenges

To help meet growing demands in the Puget Sound region, King and Snohomish Counties have planned a new regional wastewater treatment plant and conveyance system. Known as Brightwater, this facility will feature six underground structures, including deep shafts and pump stations, about 12 miles of deep effluent tunnel, 7 miles of influent lines and microtunnels, and a deep-water, offshore outfall to and from a treatment plant. Several factors will drive the construction and design of this multi-year project: the technical challenges of tunneling in difficult ground, the feasibility of dewatering, and the potential impacts to groundwater and surface water. Because of the site’s complex glacial geology, detailed geologic analyses are required to minimize uncertainty and to reduce construction risks and costs. A particular concern is the proximity of water supply aquifers — resources that King County is committed to protecting throughout construction and operation.

Solution

Aspect Consulting, a member of the geotechnical team, was responsible for characterizing geologic and hydrogeologic conditions along the proposed tunnel alignments. Characterization has entailed gathering and interpreting data from over 200 boreholes, as well as from multi-depth piezometers and aquifer tests. We also used age-dating and other analytical methods to reconstruct subsurface geologic conditions along the alignment in unprecedented detail. The characterization work is designed to support the geotechnical design and the construction dewatering facets of this project. It also allowed us to analyze impacts to surface water and groundwater, as well as to select techniques and equipment for constructing tunnels and other underground structures.

GIS played an important role in our understanding of conditions beneath the alignments — specifically, their relationships to topographic, surface water, geologic, and groundwater features. As part of this analysis, we integrated GIS subroutines developed in conjunction with GeoMap Northwest (the Seattle Geologic Mapping Project). Our work identified areas where additional subsurface exploration was needed to provide critical information.