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Similarly, the recovery work at the Emergency Spillway was carried forward, considering what could be completed during the 2017 construction season and what work could be deprioritized until 2018. It was determined that a deep secant pile wall should be the highest priority in 2017 to address the serious headcutting observed during the original 2017 incident. Following completion of the secant pile wall, an RCC buttress to the ogee spillway and a large RCC apron would be constructed in 2018. Many of the RCC design features and lessons learned rebuilding the FCO Spillway chute have direct relevance to the upcoming Emergency Spillway work. The RCC mix design and construction processes are anticipated to be the same for both structures. 4. RCC AS A “FILL THE HOLE” SOLUTION AT THE FCO SPILLWAY CHUTE Immediately following the incident, the Oroville Emergency Recovery (OER) Spillway Task Force was swiftly assembled. The OER Spillway Task Force developed a wide-ranging list of potential recovery approaches for both the FCO Spillway chute and Emergency Spillway. For the FCO Spillway chute, engineers were faced with a spillway chute that was abruptly terminated (by the foundation erosion and chute loss) at about the mid-point of its length, leaving an approximately 30.5 meter (100-foot) deep hole across the full width of the chute and extending beyond with extensive and irregular areas of exposed rock. An assessment of alternatives to repair the FCO Spillway chute was developed by the OER Spillway Task Force. These alternative concepts were grouped in the following categories:  “Use the Hole” – These concepts involved developing a long-term stilling basin out of the scoured hole and ending the spillway chute near the location remaining after the incident. It was determined that extensive design and modelling would be needed to assure that such a solution would be suitable for flows up to the spillway design flows anticipated under probable maximum flood (PMF) conditions. A key unknown here was whether the foundation rock would continue to erode under these higher flows and possibly endanger the main dam.  “Bridge the Hole” – While minimizing the foundation contact and preparation, these concepts involved construction of a reinforced-concrete aqueduct bridge over the chasm to carry flows over the eroded area. Given the lack of precedent for this type of solution on such a large-capacity, high-discharge, high-velocity spillway, it was determined that extensive structural and physical modelling would probably be needed for such a critical structure. These concepts were ultimately deemed to be structurally too intense for the design and construction time available.  “Fill the Hole” – These concepts centered on replacement of the eroded foundation of the FCO Spillway chute and utilizing the original spillway profile. There were two competing concepts for how this could be achieved: o “Fill the Hole with Rock” – This concept involved filling the eroded hole with large boulders and other rock that could be solidified using flowable concrete. This approach was discounted because of the high variability in the available rock on site and unknown imported rock materials. Again, extensive design and modelling seemed to be required for unknown material sources and material properties, including thermal effects. 90