Comparison of standard structural mapping results to 3-D photogrammetric model results: Boundary transformer banks rockfall mitigation project, Metaline Falls, Washington

The Boundary Dam hydroelectric power plant on the Pend Oreille River in northeastern Washington has been producing a significant proportion of Seattle's electricity since 1967. Six hydro-generating units are housed in a large machine room within a dolomitic rock massif with individual step-up transformers in rock bays, located at the bottom of a 150 m high cliff above the tailrace. Transmission lines exit the bays on concrete outriggers, run nearly vertically up the rock face to stand-off structures, and then into the power grid. Rockfall has been a constant problem, causing damage to outriggers, power line accessories, and appurtenances. Because of electrical interference and difficult access, traditional mapping of the geologic structure was complemented with close range terrestrial photogrammetric 3-D models on which discontinuities could be mapped remotely. Rock mass discontinuity orientations from historical mapping data, new tunnel mapping, and photogrammetric modeling of the face agreed well and fell into distinct sets. This facilitated evaluation of the rock mass structure and kinematic analyses used in tunnel design and rock face protection. Remote techniques such as photogrammetry are important tools that complement manual rock face mapping. However, they cannot and should not replace traditional hands-on-the-rock geological fieldwork, observation, and interpretation.