STABILIZATION OF A POTENTIAL ROCKSLIDE AT BOUNDARY DAM, WA

The Boundary Hydroelectric Dam Project facility is located in Pend Oreille County, Washington. The dam was constructed in a narrow canyon on the Pend Oreille River near the town of Metaline Falls, WA, and is founded on the Cambrian age Metaline Limestone. In 2016, Schnabel Engineering performed a geologic assessment of the right abutment, and identified two potentially unstable rock blocks, both of which formed the foundation for a bridge that spans from the right abutment to the downstream face of the dam. A subsequent planar sliding stability analysis of the two rock masses resulted in unacceptably high probabilities of failure. Failure of these rock blocks would result in failure of the end-span of the bridge.

Schnabel Engineering developed a series of alternatives to address the unstable rock blocks, totaling over 700 cubic meters, which ranged from full rock block removal to in-place stabilization. Viable alternatives had to accommodate several key constraints, including no vehicular access on the end-span of the bridge during construction. Drilling through the unstable rock blocks in their current condition is also disallowed, due to concerns about vibrations and drilling pressures further destabilizing the blocks. Due to these constraints, access to the project area is extremely difficult. Available access points include the crest of the dam and the downstream pool, which are 51 meters above and 33 meters below the unstable rock blocks, respectively. The selected alternative relied on a two-phase approach to incrementally increase the factor of safety. Phase I includes construction of temporary stabilization consisting of cable lashing and mesh to improve the stability of the rock blocks sufficient to allow safe access to the bridge end-span for drilling equipment. Phase II includes drilling through the bridge deck and into the foundation rock blocks to install post-tensioned anchors, tying the blocks into the rock abutment. This presentation will include an overview of the site geology, 3D geologic model, stability analyses, rock slope stabilization technologies, drilling access on steep slopes, and risk reduction measures. Phase I construction is anticipated to begin in September of 2017.