Cornforth Consultants
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Sain Creek Water Supply Tunnel

Cherry Grove, Oregon
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.
Type 2 RHRS Slope on Highway 26
Sain Creek North Portal Explorations

​Services Provided:

  • Site geology reconnaissance
  • Review of geomorphic conditions
  • Plan of explorations
  • Geophysical testing seismic refraction
  • Geologic hazards
  • Field drilling and test pit explorations
  • Laboratory testing rock quality
  • Geotechnical instrumentation
  • Seepage evaluation
  • Rock mass rating (RMR) and classification
  • Analyses of rock mechanics and stability
  • Tunneling feasibility and production rate estimates
  • Geotechnical data report
  • Home
  • Services
    • Earth & Rockfill Dams
    • Reservoirs, Canals, and Levees
    • Spillway Safety Inspections
    • Landslides and Rockfall
    • Earthquake Engineering
    • Geologic Hazards
    • Geotechnical Site Characterization
    • Hydrogeology
    • Expert Witness
    • Concrete Dams and Locks
    • Geotechnical Instrumentation
    • Deep and Shallow Foundations
    • Geotechnical Manuals
  • Industries
    • Water Resources
    • Energy
    • Natural Resources
    • Industrial
    • Transportation
    • Ports and Airports
    • Expert Witness
  • Projects
    • Earth and Rockfill Dams >
      • Barney Reservoir
      • Frog Lake Reservoir
      • Iron Gate Dam Raise
      • McGuire Dam Raise
      • Bonneville Cofferdam
      • Bull Run No. 2 Seismic Stability Analysis
    • Levees and Canals >
      • Libby Dike Rehab.
      • St. Johns Dike Stabilization
      • Cowlitz River Levees
      • Swift Canal Failure
    • Earthquake Engineering >
      • Barney Dam Seismic Analysis
      • Fern Hill WTP Liquefacation Study
      • Bull Run No. 2 Seismic Stability Analysis
      • Mt. Tabor Seismic Stability Analysis
      • Port Westward Power Plant Seismic Design
    • Rock Slopes and Tunnels >
      • Libby Rockfall Mitigation
      • Sandy River Pipeline Tunnels
      • Ketchikan Third Avenue Extension
      • Sain Creek Water Supply Tunnel
      • Montana Rockfall Hazard Rating System
    • Foundations >
      • Port Westward Power Plant
      • Cascade Locks Casino
      • Air National Guard Hangar
      • Biglow Canyon Windfarm
      • Oakridge Water System Improvements
    • Concrete Dams and Locks >
      • John Day Lock & Dam
      • Marmot Dam Removal
      • The Dalles Dam
      • Willamette Falls Dam
      • Bonneville Dam
    • Transportation >
      • OTIA Cracked Bridge Geotechnical Baselines
      • Newburg Dundee Bypass Design DEIS
      • I-84 Sandy River Bridge Investigation
      • Tanana River Bridge Foundation, Tok, Alaska
      • Montana RHRS Implementation
      • Ketchikan Third Avenue Extension
    • Ports and Airports >
      • Navy CVN Drydock
      • Depoe Bay Seawall Distress
      • Willamette Falls Flow Control Structure
      • Air National Guard Hangar
    • Water and Wastewater >
      • Fern Hill Water Treatment Plant
      • Tualatin Valley North Transmission Line
      • Evaluation of 28 Water Storage Facilities
      • City of Sandy Water Treatment Plant
      • Mt. Tabor Reservoir No. 5 Cap Study
      • Oakridge Water System Improvements
  • Staff
  • Resources
  • Careers
    • Geotechnical Engineer
  • Contact Us