Pacoima Dam after the 1994 Northridge earthquake: Design, installation, and maintenance of high strength steel rock anchors, epoxy injection & extensometers

Pacoima Dam is a 365-ft high flood control concrete arch dam located in a highly seismic area in the San Gabriel Mountains of Southern California. The dam is maintained and operated by the Los Angeles County Department of Public Works (LACDPW). On January 17, 1994, the dam was severely shaken by the Northridge Earthquake (6.8 Richter Scale magnitude). Seismic instrumentation installed at the site showed that the dam experienced a peak ground acceleration of up to 0.5 g horizontal and 0.4 g vertical near the base of the dam. Peak ground accelerations on the abutments were measured at 2.0 g horizontal and 1.3 g vertical. Following the earthquake, extensive investigations by a team evaluated the effects of the earthquake on the dam and reported various damages such as tilting of the dam, deflection of the crest, vertical contraction joint opening, cracking, and abutment movement. The team recommended a number of permanent repairs to restore the dam to its full service that was constructed in 1999-2000.

To restore the left abutment's shear friction factor of safety to 1.5, the repairs included installing 8 permanent high strength double corrosion-protected and epoxy coated tieback rock anchors. Each anchor had a permanent vibrating wire load cell, a bond length of 40-ft, an average lock-off load of 740 kips, and a depth-range of 212-ft to 250-ft. To monitor and measure the internal movements of the dam, foundation, and abutments, an additional 7 multi-point (multi anchors) extensometers equipped with load cells were installed at the left and right abutments. To measure the vertical movements of the dam, two inclinometers were installed at the left abutment. Stress cracks at the upstream and downstream faces of the dam were sealed using epoxy injection techniques. LACDPW recently conducted lift-off tests to all 8 rock anchors and performed an extensive evaluation of the newly installed and existing dam safety monitoring system. This article discusses the specifics of the construction and evaluation of the permanent repairs and presents conclusions and recommendations regarding the design and maintenance of dam safety monitoring instrumentation. 11 pp., 4 references.