Washington's King County is constructing a third regional wastewater treatment facility, called Brightwater, in response to growth in the greater Seattle region. The project includes a new treatment plant and an extensive conveyance system. Brightwater will provide secondary treatment capacity in 2010 for 36 mgd, with anticipated expansion in 2040 to 54 mgd. The associated design peak hourly flows are 130 mgd and 170 mgd, respectively.

The Brightwater Conveyance System consists of more than 13 miles of tunneled influent and effluent conveyance lines, five tunnel portals/shafts, microtunneled influent and outfall connections, a large pump station, and a 1-mile-long marine outfall extending to a depth of 605 ft in Puget Sound. The conveyance alignment traverses topographically complex terrain and all conveyance pipes are being constructed using tunnel boring machines (TBMs) and microtunnel boring machines (MTBMs) to limit impacts to roads, residences and commercial buildings. The complex geology, depth of the tunnels and shafts, and high variation of overburden and groundwater head along the tunnel alignment all contribute to one of the most challenging soft ground tunnel projects in United States tunneling practice.

Recognizing the importance of consistency in addressing geotechnical issues associated with the deep tunnels and shafts, WTD selected a single geotechnical engineering team led by CDM to perform all the conveyance geotechnical work from predesign through construction. The exploration program included more than 200 exploration borings to an average depth of 265 ft, cone penetration testing, pressuremeter testing, and slug testing and pumping tests. The geotechnical explorations provided a unique regional understanding of the complex glacial and interglacial deposits. They also allowed correlation between various geologic units and physical soil properties. All investigation data was included in the project Geotechnical Data Report. A separate Geotechnical Baseline Report was prepared for each contract to address varying design and construction issues. Data used to develop the baseline parameter values was biased based on the location of the data for each contract. Construction is now under way on most of the Brightwater Conveyance System and the actual conditions encountered are being tracked for comparison to the baseline values.

The presentation will present an overview of the geotechnical investigation program and approach used to characterize complex geologic and groundwater conditions. Geotechnical issues addressed in the design will be summarized including selection of the tunnel vertical alignment; establishment of baseline soil properties; addressing high groundwater head; limiting dewatering impacts to aquifers and streams; dealing with boulders, abrasion, sticky clays; and required construction methods such as inspection and maintenance stops and ground freezing. A summary of the construction completed to date will also be presented.

John Newby is serving as project manager for the CDM Team providing geotechnical engineering services for the Brightwater Conveyance System. He is a professional civil engineer with over 30 years experience in geotechnical and environmental engineering.

Mr. Newby began his professional career in 1976 with HLA in the S.F. Bay Area, transferred to the Seattle area in 1980, purchased HLA's local office in 1983 to create AGI Technologies, and then managed its growth from a 6-person local office to a 100-person diverse regional geotechnical engineering practice. Since AGI's acquisition by CDM in 1999, he has assisted with CDM's geotechnical discipline's strategic technical and personnel development. He is currently responsible for the geotechnical practice in the western U.S.