Reassessment of the St. Francis Dam Failure

The St. Francis Dam was a curved concrete gravity dam built by the City of Los Angeles Bureau of Water Works and Supply in 1925-26. The dam failed upon its first full filling near midnight on March 12-13, 1928, killing at least 450 people. Investigations of the failure followed, most of which attributed the failure to a fault that lay beneath the right abutment. Others attributed the failure to softening and swelling of argillaceous sandstones and conglomerates of the Sespe formation, which formed the upper two-thirds of the right abutment (adjacent to the fault). The right abutment appears to have been the focus of most of the investigations because blocks from that side of the dam were found furthest downstream following the failure. A modern review of the evidence suggests that the base of the dam was not as thick as previously assumed, and that portions of the dam were prone to overturning failure. Large arching stresses would have prevented overturning, but may have contributed to block failures along schistosity planes in the east (left) abutment. Piping in the area of the block failures may also have served to undermine the dam's east abutment as the failure sequence initiated. Geomorphic assessments reveal that the entirety of the east abutment was comprised of an ancient megalandslide within the Pelona Schist. Portions of the landslide were buttressed by the dam, however the dam's buttressing effect would have been removed when the structural integrity of the dam became compromised. During the failure sequence, more than 500,000 cubic yards of the left abutment appear to have translated as a large landslide, and this material was then washed away, leaving a sizable void where the left abutment had been. Modem analyses of buoyancy effects (due to the high solids content of the flood wave picking up the 500,000 yards of slide debris) and rapid unsteady channel flow within the canyon can account for blocks from the right abutment ending up furthest downstream. 7 pp.