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Garney Constructs First of Its Kind Tunnel Rearmoring

July 2019

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In a highly rural area, at the base of one of the tallest mountains in the lower 48 states, the Mud Mountain Dam towers over the White River in Enumclaw, Washington. Constructed at the base of Mount Rainer in the 1940s by the U.S. Army Corp of Engineers (USACE), it was the tallest rock/earthen dam of its time at a height of 430 feet.

Two tunnels channel the river around and under the dam. A 9-foot-wide tunnel passes normal flows and relieves the pressure of debris buildup on the backside of the dam. The debris flows consist of 2-foot diameter boulders flowing at velocities as high as 75 feet per second. The flat bottom, high carbon steel liner began to develop multiple scour holes after a decade of use and required maintenance repairs one to two times annually.

Awarded the first project of its kind in North America, Garney and our engineering partner, ILF, proposed an innovative approach to reline the tunnel using granite blocks rather than steel. This extended the design life to 40 years and reduced overall construction and capital lifecycle costs.

Garney adopted the concept of using granite for Mud Mountain Dam from the Pfaffensprung tunnel in Switzerland instead of using steel. This tunnel served as a basis for information and inspiration for the alternative granite proposal to USACE.

Garney performed all construction work on the site to line 1,685 linear feet of the 1,800-foot tunnel. A 4-inch layer of cellular concrete was installed over the floor, drains, and alarm system. Precision-cut granite blocks were installed tightly together in a specific sequence on top of the cellular concrete. The scope required dewatering, 1,600 linear feet of 3″ bypass pumping, and temporary power. The main challenge included the logistics of getting the required materials in place.

Check out this video to learn more about this forward-thinking project.

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