Sain Creek North Portal Explorations Cornforth Consultants (CCI) was retained to complete a geotechnical feasibility study for a proposed water supply tunnel for the City of Hillsboro. The proposed 10-foot diameter, 13,000-foot long tunnel would improve the reliability of the Tualatin River Water Supply Project by transferring permitted water from the Tualatin River Basin to Sain Creek just upstream from Hagg Lake. The tunnel would have an intake portal approximately 1,500 feet upstream of Lee Falls near elevation 400 feet and an outlet portal on Sain Creek approximately 3,000 feet upstream from Hagg Lake near elevation 320 feet. The project would also include 10,000 feet of cut-and-cover pipeline along Lee Falls Road between Hillsboro Reservoir and the tunnel inlet portal. Primary objectives of CCI's work were to evaluate geologic, geotechnical, and hydrogeologic conditions along the proposed pipeline and tunnel alignments to assess design and construction costs.
Subsurface explorations included four deep borings (vertical and angled), four shallow borings, 27 test pits, and 1,265 feet of seismic refraction geophysics. Borehole imagery and packer testing were completed on portions of deep borings near the proposed tunnel elevation. Vibrating wire piezometers were installed in two deep borings near the tunnel elevation. Laboratory testing on rock core samples included: 1) point load index; 2) unconfined compression; 3) slake durability; 4) tensile strength; 5) abrasivity; 6) mechanical boreability; and 7) thin-section petrographic analyses. Results from subsurface explorations and laboratory testing were summarized in a geotechnical data report.
Geotechnical data analysis included evaluating the Rock Mass Rating (RMR) for geologic units in the tunnel alignments. Projected groundwater inflows were estimated for the proposed tunnel geometry and alignment. RMR and other laboratory test results were used to summarize the likely response and necessary effort required to advance a tunnel in each rock unit. Production rates were estimated for both drill-and-blast and tunnel boring machine tunneling techniques. Geotechnical analyses were completed to estimate tunnel pressures from volume change (slaking), face standup time, the need for rock bolting/rock support, and anticipated bulking rates for diabase and siltstone bedrock. Similar estimates were prepared for the proposed tunnel portal locations.