Brightwater Conveyance System
Topographic conditions along the tunnel alignment range from hills to intervening valleys, resulting in external groundwater pressures that vary greatly, up to a maximum of 7.3 bars. This geologic factor significantly influenced the design of the tunnel lining system—bolted and gasketed precast concrete segments—and type of tunnel boring machines used for excavation—earth pressure balance (EPB) and slurry shield. In fact, the Brightwater Conveyance System marks the first specified use of slurry shield technology in the United States on a competitively-bid contract.
King County packaged the project into multiple construction contracts, advertised and bid separately. This strategy kept the three tunnel contracts below $300 million each, thus encouraging competitive bidding and allowing for concurrent construction necessary to meet the project schedule.
Jacobs Associates, in joint venture with MWH, provided project management for final design of tunnels and shafts and ancillary facilities, construction packaging, permitting, community outreach, and bidding and engineering assistance during construction. During the pre-design phase, Jacobs Associates provided shaft and tunnel design services as part of an HDR-managed team. The team evaluated various alignment alternatives, performed risk assessments, and contributed to the preparation of environmental documentation.
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