Deep Portal Shafts for the Brightwater Tunnels: Geotechnical Design Considerations and Construction Experience

The gravity-driven effluent conveyance from Seattle’s third wastewater treatment plant to a Puget Sound marine outfall required a deep-lying, 21-kilometerlong tunnel alignment. Access for tunnel boring machine launch and reception operations, as well as conveyance system functions to pump influent to the plant, necessitated the design and construction of deep shaft structures. Geologic settings at the shaft locations ranged from liquefiable alluvial valley deposits to glacially overconsolidated sequences of glacial and non-glacial deposits. This paper discusses site-specific geotechnical design considerations, including safety against uplift, seismic hazard analysis and seismic design, baseline values for boulders, the selection of suitable construction methods, and prevention of third party impacts. The final design for most deep shafts included large-diameter diaphragm wall structures constructed by slurry wall trenching. The 62-meter-deep Ballinger Way portal shaft was constructed using ground freezing technology for ground support. Construction monitoring included tracking geotechnical conditions, groundwater response, shaft deformation and subsoil temperature development at the freeze shaft location. The monitoring results provided the basis for a comparison with and the verification of design considerations, allowing a better understanding of the geological conditions and the geotechnical system behavior.