Green Peter Dam is a concrete gravity structure located in west-central Oregon on the Middle Santiam River within the Willamette Valley Basin. A risk assessment for the project identified continuous and adversely oriented low-angled shear zones underlying portions of the foundation that could potentially facilitate sliding instability of one or more monoliths during earthquake loading. Conceptually, a potential foundation rock wedge could be formed with a shear zone as its sliding surface and joints as the side planes. This wedge, which would otherwise be stable under static conditions, could feasibly be displaced and/or shifted during seismic ground shaking, resulting in significant structural damage and/or breach of the dam. A qualitative evaluation was performed to characterize the geomechanical conditions and geometry of movement (i.e., kinematics) of the dam-foundation system associated with rock wedges. The study revealed that wedges could indeed be formed by adversely oriented and intersecting rock mass discontinuities. The qualitative evaluation concluded that the displacement geometry and geologic conditions in the foundation collectively suggest that the wedges would likely be stable under even large probabilistic seismic loading. While no concrete dams are known to have failed due to seismic loading, an increased knowledge of higher seismicity in the Pacific Northwest region warranted a careful evaluation to ensure that the risks of foundation rock wedge deformation are well characterized, and that our level of confidence in the available data is acceptable to better constrain the potential risk posed by this failure mode. This paper summarizes the background, findings, and results of the preliminary and qualitative dam-foundation system stability evaluation that was performed for Green Peter Dam.

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