ROCK ABUTMENTS, CONSERVATIVE ANCHORING SCHEMES VS. SUBSEQUENT ANALYSES

摘要

Big Tujunga Dam is a variable radius thin arch dam. Built in 1930-31 for flood control and water conservation, the dam's crest is 244 feet high, crest length 400 feet, and thickness varies from 8 feet at the crest to 73 feet at the base of the maximum section. The reservoir operating level has been restricted by order of the California Department of Water Resources, Division of Safety of Dams (DSOD) due to seismic stability concerns, reducing storage capacity by 75%. Based on studies that confirmed the existing dam was insufficient to handle the seismic loading from the Maximum Credible Earthquake (MCE) or the discharge flow from the Probable Maximum Flood (PMF), MWH Americas, Inc. designed rehabilitation of the dam by addition of concrete against the downstream side of the dam to create a strengthened "thick arch" in combination with construction of a new ogee crest spillway with flip-bucket. Geotechnical components of the rehabilitation design included addressing the thickened dam foundation and abutment slope stability. During the rehabilitation design phase, abutment rock slope stability evaluation and stabilization design was conducted using global assumptions from limited geologic mapping of the abutments and rock foundation, which were concealed because of previously installed shotcrete. Thus, conservative assumptions were made during design that postulated formation of large scale wedges and planar instabilities that could affect dam stability. The design team and stakeholders agreed to delay detailed mapping until existing shotcrete was removed during construction to confirm design assumptions. Detailed mapping during construction revealed less critical conditions than initially inferred, allowing initial conclusions on rock abutment stability and corresponding anchor design to be revised. The paper will focus on initial design assumptions for rock abutment stability, field findings, subsequent design revisions, and lessons learned, highlighting costs and benefits of sequenced design phases and reevaluations. The paper will also include a detailed discussion of the methodology used to analyze the dam rock abutments following detailed geologic mapping, and provide a comparison between the initial and final kinematic and limit equilibrium slope stability analyses.