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detailed geologic and geotechnical information that was relied upon after the incident and during the subsequent development of the exploration program for the OER design efforts. 2.1. GEOLOGIC INVESTIGATIONS PRE-1960S CONSTRUCTION The Oroville Dam site was well-studied prior to construction in the 1960s. Specifically related to the FCO Spillway chute and Emergency Spillway, 45 rock- core borings were drilled and logged, 11 P-wave surface geophysical profiles were imaged, and geologic mapping of natural outcrops was performed. In addition, 51 bulldozer trenches were excavated to estimate the thickness of overburden that could be removed without blasting and to help determine the rock’s excavation characteristics. Most of these explorations were performed along the alignments of the FCO Spillway chute and the Emergency Spillway monolith/weir section. The initial phase of rock-core borings was laid out along a preliminary FCO Spillway chute alignment that was later abandoned (Fig. 2). This preliminary alignment was also where most of the 51 bulldozer trenches were excavated. The rock-core borings ranged up to 29 m (95 ft) deep and were vertical to inclined. The associated boring logs documented the rock lithology, degree of weathering and fracturing, discontinuities and infilling, and descriptions of other relevant geologic features, and included packer test results and water levels where measured. High- quality photographs of the rock cores were also documented for most of the rock- core borings. The results of the pre-1960s-construction site geologic studies indicated that un-rippable rock was present at relatively shallow depths – generally about 0.9 to 1.2 m (3 to 4 ft) below the surface [5]. Subsurface rock-core borings and P-wave geophysical lines were used to develop contour maps and profiles showing depths to sound rock. Sound rock included fresh and slightly weathered rock, and might be slightly to moderately fractured. These maps and profiles were used by the engineering geologists and design engineers to help establish the anticipated foundation rock quality at the invert excavations for both the FCO Spillway chute and the Emergency Spillway monolith foundations. 105