Page - 112 - in Book of Full Papers - Symposium Hydro Engineering

downhole televiewer, surficial geologic mapping, seismic lines, and instrumentation. 3.2. EXPLORATION FOR DESIGN OF OROVILLE EMERGENCY RECOVERY At the onset of the OER design effort, there were many spillway repair alternatives being examined. Not knowing which design alternative would be selected, the exploration plan accommodated each of the repair alternatives and the issues specific to each alternative. The major design components included repairs or replacement of the FCO Spillway chute, fortifying the Emergency Spillway monolith weirs against potential sliding or over-turning, construction of an erosion cutoff wall downstream from the Emergency Spillway, and erosion protection of the ground surface between the erosion cutoff wall and the Emergency Spillway. The replacement of the FCO Spillway chute included modifications to the original design features that would bring the chute to current design standards. These modifications included longer chute slab anchors, new underdrains below the slabs (not within them), enhanced underdrain filter designs, and so on. To ensure adequate chute slab anchor lengths, unconfined compressive strength testing was performed on core samples from the foundation rock to accurately calculate the required bond length for anchorage into moderately-weathered or better rock, and for anchorage into intensely-weathered or decomposed rock. Geotechnical borings were also added to collect representative samples of the intensely-weathered to decomposed rock that might be in contact with the chute underdrain materials. These materials were then tested to help design the filters for the underdrain system. The design criteria for the erosion-resistant cutoff wall within the unlined Emergency Spillway foundation required at least 4.5 m (15 ft) of embedment into the slightly-weathered or better rock. Many rock-core borings were added to the exploration plan to determine the geologic conditions for the proposed erosion- resistant cutoff wall. Several surface P-wave geophysical lines were also performed to examine the variability of rock weathering in the upper 15 m (50 ft). The alignment of the erosion-resistant cutoff wall, which later was designed as a secant pile wall, was adjusted during the design to minimize pile depths based on the geologic conditions encountered. 3.3. FCO SPILLWAY CHUTE EXPLORATION A major component of the geologic and geotechnical explorations took place within the FCO Spillway chute alignment. As the design process progressed, it became apparent that the reconstruction of the chute would likely take at least two years to complete. Between the two construction seasons, during the Winter Flood Season of 2017-2018, the upstream portion of the original chute nearest the 112