Combined Sewer Overflow Project - Northwest Geotech

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Combined Sewer Overflow Project

Materials Testing/Special Inspection/Laboratory Testing
In 1994 the City of Portland and the Oregon Department of Environmental Quality agreed to a combined sewer abatement plan.  Combined Sewer Overflow Project was the largest project included in the twenty year abatement plan to improve watershed health, restore habitat for endangered salmon and steelhead, and reduce raw sewage overflow from contaminating the Willamette River by 94% by the year 2011. This reduction in combined sewer overflows equates to more than three billion gallons of raw sewage annually.

The East Side Combined Sewer Overflow Project (ESCSO) is the final and one of the largest aspects of the abatement plan, and at $382.5 million, the “Big Pipe Project” is the largest public works contract ever awarded by the City of Portland. The 22-foot diameter ESCSO tunnel spans a 6-mile distance at depths of up to 165 feet below the City of Portland. Seven shafts, ranging from 49 to 67 feet in diameter were also constructed to connect existing outfall pipes to the tunnel. NTI provided materials testing and special inspection in support of the project. NTl’s duties included material quality control for the fiber reinforced and non-fiber reinforced pre-cast concrete tunnel panels, water quality sampling and testing, welding inspection of the TBM cutter heads, and structural steel inspections of welding and high-strength bolting for the waste material conveyor system. In addition, NTI assisted with design, and managed the installation and analysis of geophysical site monitoring systems, including bore hole inclinometers/piezometers and strain gauges. NTl’s material quality control testing included providing the City of Portland an experienced field inspection representative for full-time construction monitoring and quality assurance of the precast plant construction activities. NTI also provided laboratory testing that included split-tensile testing of fiber reinforced concrete, compressive strength testing of the fiber reinforced and non-fiber reinforced concrete, density, absorption and voids of hardened concrete, and flexural performance of fiber reinforced concrete (using beam with third-point loading) for each of the 48,000 tunnel segments that were utilized for the ESCSO.

At 14 feet in diameter, 120 feet deep, and four miles long, the $293 million West Side Combined Sewer Overflow Project (WSCSO) is a close second to its sister tunnel, the ESCSO, as the second largest public works contract ever awarded by the City of Portland. The tunnel runs from SW Clay Street in Downtown Portland, takes a near one-mile journey under the Willamette River, and ends at a 220 million gallon per day pump station on Swan Island. The WSCSO also includes five shafts that connect the tunnel to existing outfalls and allow for maintenance access. For over two years, NTI provided materials testing, laboratory testing, and special inspection in support of this project. Duties included rheological testing of the bentonite slurry utilized throughout the project. Due to geological composition, the material the tunnel boring machines were required to pass through was not self-supporting. This situation required a slurry mixture to be pumped into the ground ahead of the cutters on the tunneling machines to keep the ground from collapsing. While this system of pumping bentonite slurry has been used in Europe, the WSCSO tunnel is the first time it has been utilized in the United States. The rheological properties of the slurry and its ability to “plug” highly permeable gravels were critically important to the effectiveness and productivity of the tunnel boring machines. This dependency on a reliable and consistent slurry mix directly influenced the cost of the tunnel boring operations and consequently the overall cost of the project. The bentonite slurry testing included density, viscosity, yield point, filtration, and soil permeability. In addition, NTI provided special inspection of structural steel construction, including welding and high-strength bolting in support of the construction of the tunnel head houses and shafts.
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