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3.2. DESIGN APPROACHES 3.2.1. General Design Approach Design work was completed by forming multi-disciplined sub-teams assigned to specific design tasks, such as determining slope stabilization methods, excavation depths, foundation objectives and treatments, underdrain designs, structural concrete designs, RCC designs, and secant pile wall designs. Leads and members of the sub-teams were organized beginning on March 6. Eventually, over 100 professionals were involved in developing over 50 design technical memorandums and contract plans and specifications. The work for the preliminary designs was generally completed in Sacramento, with final designs being completed by moving design team members to work on-site in Oroville. This latter work was facilitated by mobilizing over a dozen portable trailers with communication and computer equipment to a large parking area adjacent to the Emergency Spillway. This allowed the design groups to be co-located with the contractor’s (Kiewit’s) forces and to be on-site as the construction work was being done. This also helped obtain expedited dam safety regulatory approval as the designs were being finalized and the construction work was being completed, as dam safety regulatory staff (FERC, DSOD) were provided temporary office space in the trailers. 3.2.2. Geologic Investigations During the original design, boreholes had been completed along the FCO Spillway chute alignment and along the monoliths/weir sections of the Emergency Spillway. There were also very detailed geologic maps of the FCO Spillway chute and the monolith foundations completed during original construction, but relatively little detailed geologic information was available on the Emergency Spillway area downstream of the monoliths/weir sections. Geologic investigations carried out in 2017 for the OER are described in detail in a companion paper [2]. The 2017 geologic explorations included the use of 10 drill rigs to complete over 104 new borings, install 26 piezometers, and install 15 inclinometers, all by the end of July 2017. In addition, 22 seismic refraction lines were completed, and extensive geologic mapping of exposed rock was carried out at both spillways. The geologic investigations confirmed that the rock foundation at both spillways consisted generally of amphibolite with varying degrees of weathering. The degree of weathering ranged from intensely weathered or decomposed to fresh, with most of the surface exposures consisting of moderately-weathered rock. There were also numerous shears across both spillways, which significantly contributed to both slope instability and erosion problems. Along the FCO Spillway chute alignment, the vast majority of the intensely-weathered rock was found at the two scour holes, with almost all of this original foundation 76