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Mitchell Point Tunnel Feasibility Study

Mitchell Point Tunnel Feasibility Study
Hood River, Oregon

Mitchell Point tunnel is located on the Historic Columbia River Highway in Oregon, about 4 miles west of Hood River. The tunnel was closed in 1953 due to increased vehicle size and rockfall hazard. GRI provided geotechnical services for the preliminary design of a 1,200-ft-long tunnel through Mitchell Point to connect the two existing segments of the Historic Columbia River Highway. Several side adits provide windows for viewing the Columbia River. Geotechnical considerations included tunneling conditions, preliminary tunnel design, and stability of massive rock cliffs above the west tunnel portal and the side adits. The tunnel alignment passes through five layered basalt flows with a dipping orientation. GRI’s services included interpretation of geologic structure and geologic mapping of rock outcrops on the site, rock mass characterization, laboratory rock strength testing, evaluation of the suitability of different tunneling methods, and evaluation of rockfall hazard. The majority of the excavation will be made using a road header. Drill-and-blast excavation techniques will be used in areas of massive basalt, which is about a third of the tunnel. Additional consideration will be given to blasting near cliff faces above I-84. GRI assisted with the selection of a preliminary tunnel location and developed a preliminary tunnel design and “sense of magnitude” cost estimate for design. A system of patterned rock bolts in the roof and steel-fiber-reinforced shotcrete lining will be used for tunnel support. Steel sets will be used for additional support where the tunnel passes through a severely weathered basalt flow top.

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