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the duration of the field exploration program. The QA/QC function included daily monitoring of drilling activities at each drilling location to confirm compliance with the work plan, performing daily review of borehole logs to ensure completeness and accuracy of the logged conditions, and reviewing the final, completed borehole logs. The borehole logging QA/QC review used a multi-step approach to oversee the process. The initial step consisted of a QA/QC review that was performed in the field by the on-site QA/QC Engineering Geologist to verify that each drill rig engineering geologist was meeting DWR’s borehole logging standards for the project. This review was performed daily at the drill rig, during drilling and sampling. Upon completion of each borehole field log, the QA/QC Engineering Geologist performed an initial field log review to check consistency of logging and completeness of the field log. Then, the borehole log was reviewed and compared to the recovered rock core. If necessary, revisions to the borehole log were made as part of the technical review. The field log was then drafted using Bentley gINT with the QA/QC Engineering Geologist maintaining control over the process until the draft borehole log was accepted as final. 5. CHALLENGES AND LESSONS LEARNED The geological and geotechnical explorations for the design of OER was a complete success. This success was made possible through the commitment of DWR staff and DWR’s key support personnel sourced from the public and private sectors, along with field services organizations. This integrated team truly rose to meet the challenges. One of the major challenges for the geologic and geotechnical design team was the highly variable geologic conditions, which changed significantly both laterally and vertically within short distances. While there was only one geologic rock mass (i.e., amphibolite) truly present across the site, the cross-cutting shears created a patchwork of rock weathering and fracturing conditions. The overall rock mass strength of the amphibolite was controlled by the degree of weathering and the intensity and orientation of discontinuities (e.g., shears and joints) within the rock. The integration of a detailed surface and borehole geophysics program, coupled with extensive rock-core borings, proved invaluable in making proper assessments of foundation rock quality for this project. The surface refraction methods greatly assisted in developing initial interpretations of rock uniformity (horizontal and vertical) and estimates of rock mass strength through correlations to nearby boreholes. It was critical to this success that the OER design team assembled very early on to discuss geology and geotechnical needs that might arise during the design process. This allowed a relatively small group of geologists, geotechnical engineers, and construction specialists to develop a comprehensive, but adjustable, exploration program to address design needs. As a result, the geologic and geotechnical exploration program provided high-quality results, while 118