Elk Rock Tunnel Assessment and Recommendations

To help evaluate the feasibility of extending the Portland streetcar system from downtown Portland to downtown Lake Oswego, Oregon (a distance of about 10 miles), Jacobs Associates performed a tunnel condition assessment and made recommendations concerning Elk Rock Tunnel, an existing historic freight rail tunnel. Located on the west side of the Willamette River, this 1,395-foot-long (398 m), single-track tunnel was originally built in 1921. The portal structures are constructed of reinforced concrete with an unlined interior.

Jacobs Associates worked as a subconsultant to URS, which was contracted by TriMet to provide conceptual design, environmental analysis, and public information services toward the preparation of a Draft Environmental Impact Study for the Lake Oswego to Portland Transit Corridor Project. We inspected the tunnel and developed rehabilitation options. The inspection found that the majority of the tunnel excavation is located in relatively unweathered basalt bedrock. Rehabilitation options include enlarging the tunnel or constructing a second parallel tunnel to allow double-track rail traffic. Jacobs Associates’ primary tunnel support and repair recommendations include scaling, shotcrete, and repair of rebar in the portals to improve stability. Additional evaluations include accommodations for electric vehicles, walkways for pedestrians, lighting, and an alarm system to warn of incoming trains.

One transportation alternative under consideration includes construction of a streetcar along the existing Lake Oswego trolley right-of-way, which could utilize Elk Rock Tunnel. Until recently, the existing track was used only as a tourist excursion trolley line operated by the Oregon Electric Railway Historical Society.