CONNECTING THE SEATTLE SEWERS

This article describes the intricacies of a sewer overflow project in Seattle using microtunneling and large diameter tunnel boring machine technology. The project described involved a 947-meter long tunnel, 4.5 meters in diameter, plus some smaller tunnels. The south portal shaft required special consideration because it was located in the middle of a busy trucking operation. As expected, about halfway through, soil conditions grew more challenging, where stiff to hard clays changed to outwash till. Describes adjustments in TBM and steps taken to ensure safety of existing utilities near the work. Other soils included wet, flowable alluvial sands that also contained manmade obstructions such as piles and rail ballast. Environmental controls and safety measures are also included.