Strength from Within

by Bijan Khaleght, Concrete Specialist; Washington State Dept. of Transportation's Bridge and Structures Ofc., Olympia, WA,
JoAnn Schueler, P.E., Proj. Engr.; Washington State's Transp. Improvement Board, Olympia, WA,

Serial Information: Civil Engineering—ASCE, 2002, Vol. 72, Issue 4, Pg. 54-59

Document Type: Feature article


After decades of being continuously struck by rockfalls and wayward vehicles, Knapps Hill Tunnel�a highway structure built in 1936 and located on state Route 97A in central Washington State�was in need of rehabilitation. The tunnel, which featured timber portals and bent supports, measured 840 ft (260 m) long, 26 ft (8 m) wide, and 21 ft (6.4 m) high. Time constraints and a tight budget prompted highway planners to complete the project in two phases. During the first phase, rocky slopes outside of the tunnel were stabilized and a wing wall was added to the south portal to extend the length of the tunnel. The second phase dealt with fortifying the tunnel's interior timber support system. First, the tunnel was lined with several layers of shotcrete, which was then covered over with grouting. Once the grouting bonded with the rock behind the original timber bents, the now-stabilized bents could be trimmed, thereby expanding the degree of available space in the tunnel. Designers feel confident that the repairs will decrease the number of tunnel closures caused by rockfalls.

Subject Headings: Tunnels | Wood and wood products | Bents (structural) | Landslides | Highway and road structures | Grouting | Walls

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