Effects of the Northridge Earthquake On Pacoima Dam

Pacoima Dam is a 365-foot high concrete arch flood control dam built by the Los Angeles County Flood Control District between 1925 and 1929 and operated by the Department of Public Works. It is located on the south side of the San Gabriel Mountains, approximately 23 miles northwest of downtown Los Angeles, and 4.5 miles northeast of San Fernando, California. It has a crest length of 640 feet and contains approximately 220,000 cubic yards of concrete. The dam is founded on massive, predominately metamorphic-gneissic formations in a very steep-walled canyon. Because of unfavorable foundation conditions at the left abutment, the dam was built 1 O feet lower than originally planned and a mass concrete gravity thrust block was constructed at this abutment. Vertical contraction joints with keys were provided in the dam at 50-foot intervals including a joint between the thrust block and the left side of the arch.
As a result of a reanalysis of the structural stability of the dam and the distress caused by the 1971 San Fernando Earthquake, 35 post-tensioned rock anchors or tendons were installed in the upper rock mass of the left abutment upon which the concrete thrust block rests, securing the mass to more competent rock at depth. The anchors ranged in length from 130 to 195 feet and consist of 28 one-half inch, 7-wire steel strands designed to develop a working force of 694 kips.