Challenging installations of gravity sewer systems are benefitting from advancements in auger boring machine technology and specialized cutterheads. The technology has grown in power and diameter range, enabling longer, more accurate boring in rock and mixed-ground conditions. The trend has been exemplified by a recent record-breaking gravity sewer crossing in Tigard, Oregon. This paper will focus on the Locust Street Sanitary Improvements Project, consisting of three deep gravity sewer crossings up to 600 ft in length through rock and mixed ground for Clean Water Services of Hillsboro, Oregon. The environmental considerations of working in highly prized and regulated stream corridors in the Pacific Northwest prompted the owner to embrace more environmentally sound routing solutions. The project involved 1.1 miles of system upgrade, with almost 1200 ft of system bored through bedrock and mixed ground in an effort to avoid environmental impacts. Gonzales Boring selected a 72-inch ABM and a Robbins SBU-A with a specialized disc cutterhead to tackle 7,000 to 17,000 psi basalt at depths of 20 to 30 feet. The crossings spanned open ground, residential properties, and a large Army reserve maintenance facility. In addition to the project design, setup, performance data, and unique machine adaptations, the paper will discuss trenchless boring methods as a way to reduce environmental impact compared to customary open-trench projects.
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