Cordova Electric Cooperative, Inc. operates a 1.25 megawatt (MW) hydroelectric facility located approximately 5 miles north of Cordova, Alaska. In December 2005, a powerhouse fire required the Humpback Creek facility to be taken offline. Less than a year later, in October 2006, massive flooding destroyed additional aspects of the facility including the intake/diversion structure, penstock, and tailrace. In order to complete repairs and build a replacement intake/diversion structure, a tunnel to temporarily divert Humpback Creek was constructed. The diversion tunnel consisted of a 10-ft by 10-ft straight-leg horseshoe section constructed using drill and blast methods.
The ground conditions along the tunnel alignment consisted mainly of poor to very poor rock (8<RMR<36); and due to orientation of the existing joints and the direction of the tunnel drive, difficult tunneling conditions were expected to be encountered at the intake portal.
Working as the Engineer of Record, Lachel developed four ground support classes to address the variability in ground conditions. Due to limited site access and resource constraints, the ground support options provided took into account equipment and material availability. During construction, Lachel monitored and evaluated ground conditions in the tunnel and optimized the support design as needed.