AEG Monthly Meeting—Thursday, November 7

Chad W. Carlson, Ph.D.

Oroville Dam Emergency Response: Geologic Considerations for Spillway Repairs and the Engineering Geologists’ Contributions during Construction

Abstract: At 770 feet (235 m) high, Oroville Dam is the tallest dam in the U.S. An earthfill-embankment dam on the Feather River just east of the city of Oroville, California, Oroville Dam serves mainly for water supply, hydroelectricity generation, and flood control. The catastrophic failure of the Oroville Dam main spillway during use forced dam operations to stop release of reservoir waters to minimize damage. The subsequent rise in reservoir levels brought into use the emergency (auxiliary) spillway for the first time since dam completion in 1968. The rapid headward erosion across the native landscape toward the crest of the emergency spillway, and its potential for an uncontrolled release of flood waters, was the deciding factor to the downstream evacuation of nearly 200,000 residents on February 12th, 2017. Two and a half years later, with temporary and permanent repairs in place during the runoff seasons of 2018 and 2019, respectively, the majority of work at Oroville Dam is now complete and public access across the dam and spillways to the boat ramp has been restored.

Construction on or into competent rock was a primary consideration during repairs and new construction of Oroville Dam spillways. While the high strength (~50,000 psi) of metamorphic rock (amphibolite) provided sound foundation, localized zones of deeply-weathered bedrock were present across the site. The original construction of main spillway infrastructure on severely weathered bedrock in some places was determined to be the principal contributing factor to its failure. These weathered zones are commonly associated to regional deformation (e.g., foliation and periods of subsequent faulting/shearing). Detailed geologic mapping, exploratory drilling, and geophysical methods were used to assess rock characteristics prior to construction.

During construction, engineering geologists monitored drilling of Main Spillway anchors into bedrock, Secant Pile Wall installation at the downstream end of the Emergency Spillway, slope stability, and numerous other tasks onsite. The in-field engineering geologists’ real-time observations during construction verified spillway design specification were met and aided in the rapid completion of Oroville Dam repairs.

Biography: After 13 years working in Naval and commercial aviation, I found myself with an opportunity to change the direction of my professional life. Being a Fresno native, I returned home in 2005 and registered for courses at Fresno City College. Not long after, I chose to enter studies focused in the Geological Sciences. Having completed much of my lower division requirements, I transferred into the Earth and Environmental Sciences Department at Fresno State the fall of 2007. Working with Dr. John Wakabayashi, I completed a senior thesis interpreting the late Cenozoic uplift history of the central Sierra Nevada and graduated in 2009 with my undergraduate degree in Geology. My earlier goals of going into industry and applied geology were thwarted with my new found passion for geologic research. The fall of the same year I re-entered Fresno State to begin a Master of Science degree working with Dr. Christopher Pluhar. Using paleomagnetics of volcanic rocks I studied the vertical-axis rotations of crust east of the Sierra Nevada in the west-central Walker Lane. As my master’s work was drawing to an end, I considered my options and decided to try applying to a doctorate program. After acceptance to the University of Nevada, Reno, and while still writing my Master’s thesis, I began working with Dr. James Faulds in fall of 2011 to further the understanding of dextral shear accommodation and strain transfer at the transition between the northern and central Walker Lane. While juggling a teaching assistantship and coursework the first year in Reno, I managed to complete my Master’s in Geology the spring of 2012 at Fresno State. Combining detailed geologic mapping and paleomagnetic research, I finished my Ph.D. in Geology the summer of 2017. With little time to relish my accomplishment, I soon found myself full circle back to applied geology working on the spillway repairs of the Oroville Dam as an independent geologic consultant for InfraTerra, Inc. With my experiences from Oroville, and a new position with Lettis Consultants International, Inc., I look forward to becoming a licensed Professional Geologist while continuing my research endeavors to expand my knowledge base in all aspects of geology.

Chad W. Carlson, Lettis Consultants International, Inc. (InfraTerra, Inc.) carlson@nevada.unr .edu




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