The Pacoima Dam Incident During the 1994 Northridge Earthquake

On January 17, 1994, the magnitude 6.7 Northridge Earthquake struck the San Fernando Valley region of the City of Los Angeles. Pacoima Dam, a 365-foot-high concrete arch dam completed in February 1929 and located 11 miles southwest of the earthquake’s epicenter, experienced significant ground accelerations (over 1.7g near the top of the left abutment). The left abutment at the arch dam displaced 19 inches horizontally and 14 inches vertically during the event. The thrust block at the left abutment of the dam experienced a horizontal movement of 2.2 inches and a vertical displacement of 1.1 inches. Some lift joints within the arch dam reportedly opened during the earthquake, and some of the 35 existing post-tensioned anchors at the left abutment (installed after the 1971 San Fernando Earthquake) were damaged and required repairs.
Following the Northridge Earthquake, the dam was one of over 100 dams inspected by the Division of Safety of Dams (DSOD) under the California’s Department of Water Resources. After the earthquake and several aftershocks, Los Angeles County Public Works (owner) immediately conducted in-depth exploratory drilling and engineering analysis of the left abutment. This led to the installation of eight new anchors to strengthen the left abutment and additional repairs to the existing damaged anchors. In addition, DSOD required an operating plan for the interim reservoir level restriction and daily dam surveillance, including the measurement of seepage, piezometers, reservoir water levels, and joint movement data.
This case study will describe the incident at Pacoima Dam during and following the Northridge Earthquake, including the emergency response, the dam’s performance, and subsequent repairs. This study will highlight how the dam owner successfully completed post-earthquake long-term instrumentation monitoring and evaluations while effectively communicating with regulators. Unlike other incidents, this case study at Pacoima dam will emphasize the overall resiliency of the structure and its ability to withstand significant seismic ground accelerations that exceeded the original design parameters established nearly 100 years ago.