Page - 104 - in Book of Full Papers - Symposium Hydro Engineering

This paper is part of a series developed by the State of California, Department of Water Resources (DWR) to describe the Oroville Dam Spillway Incident - Emergency Response and Recovery. This paper focuses on the geologic conditions known prior to the incident and the subsequent development and execution of a geologic and geotechnical investigation program to support the Oroville Emergency Recovery (OER). The geologic materials at the Oroville Dam site are structurally complex, with numerous cross-cutting shear zones that make predicting ground conditions difficult. It was clear, however, that the geology, in particular the cross-cutting shears, controlled the performance of the FCO Spillway chute and Emergency Spillway during the incident, and controlled the designs developed during the OER phase. The complex geology, coupled with the urgent time-frame in which to develop and implement a field and laboratory testing program, and then provide useful high- quality subsurface geologic and geotechnical information to the OER design team, was very challenging. Specific challenges ranged from resource management to assessing a wide range of conceptual and then preferred design components, field quality assurance and quality control (QA/QC), expedited data analysis, and interpretations needed to support development of time-critical project designs. These challenges are discussed in the following sections of the paper. 2. GEOLOGY Oroville Dam is located in the northeastern portion of the Sacramento Valley in northern California in the western Sierra Nevada foothills within the Middle to Late Jurassic arc sequence [4]. The western-most portion of the foothills consist of the Foothills Metamorphic belt, which includes Jurassic-age ophiolitic rocks of the Smartville Complex. The predominant geologic formation where the Oroville Dam and spillways were constructed is an unnamed member of the Smartville Complex, which consists of strongly-foliated green-schist-facies mafic meta-volcanic rocks. The primary mineral assemblage of the on-site rocks consists of actinolite (amphibole) and plagioclase, and is generically referred to as amphibolite. Site geologic studies completed prior to the dam’s construction included regional and site-specific geologic mapping, rock core drilling, packer testing, laboratory shear strength testing, surface geophysics, petrographic studies, exploratory tunnels, and other measures [5, 6]. These studies led to dam and spillway sites that were generally well-characterized, using the standards at the time for design and construction purposes. The following subsections generally describe the geologic studies completed for design of the original FCO Spillway chute and Emergency Spillway, geologic observations and documentation created during the original construction of Oroville Dam, and geologic investigations performed between the original construction of the FCO Spillway chute and Emergency Spillway and the Oroville Dam Spillway Incident. This background information is presented to illustrate the 104