Cornforth Consultants
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Mount Tabor Seismic Stability Analysis

Portland, Oregon
The Mount Tabor reservoir complex was built between 1894 and 1911 and was an integral part of the City of Portland’s water supply system. The Mount Tabor complex includes dam No.'s 1, 5, and 6. The dam at Reservoir No. 1 is a 30-foot high concrete structure with a downstream earthfill buttress. The dam at Reservoir No.5 is similar but 55 feet high. Both are built on the upper slopes of Mount Tabor, a dormant volcanic vent. The dam enclosing Reservoir No. 6 is an earth embankment 2,100 feet long and up to 28 feet high. The complex includes the ability to generate power from the 110 feet of head between Reservoirs No. 5 and No. 6.

The foundation below the embankment dam at Reservoir No. 6 contains clean, alluvial sand deposits, 14 to 47 feet thick. A FERC dam safety study raised concern about potential liquefaction of the foundation sands. Cornforth Consultants was retained by the Portland Water Bureau to assess the liquefaction hazard and risk posed to the reservoirs. The study included 19 exploratory borings to evaluate soil type and density; gradation testing to determine silt content; a probabilistic seismic hazard evaluation to develop the design earthquake; a finite element analysis to determine in-situ stresses below the embankment; and an assessment of cyclic stresses within the foundation.

​The results of the analyses determined that localized pockets of foundation sand could liquefy; however, the vulnerable pockets were isolated and not laterally continuous. Therefore; excess pore-water pressures generated in the localized pockets would dissipate into denser soil without threatening the stability of the embankment.
Deep Joint above Highway 2
Mount Tabor Reservoirs

​Services Provided:

  • Soil Response
  • Liquefaction Susceptibility
  • Finite Element Seismic Stability Evaluation
  • 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