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sf_dam09St. Francis Dam was built by the City of Los AngelesBureau of Water Works and Supply in 1925-26 as a curved concrete gravity dam, approximately 200 feet high in San Francisquito Canyon, about 5 miles northeast of what is now Magic Mountain, California. The stated purpose of the dam was to provide an additional 38,000 acre-feet of storage for Los Angeles – Owens River Aqueduct water in close proximity to Los Angeles. The dam failed catastrophically upon its first full filling, near midnight on March 12, 1928, killing at least 450 people in the San Francisquito and Santa Clara River valleys. It was the greatest American civil engineering failure in the twentieth century.
No less than a dozen separate investigations of the failure followed, the most cited being the state commission appointed by Governor C.C. Young, which convened on March 19th, made one site visit, and issued their report (known at the time as the “blue book” report) five days later. That board concluded that the dam’s failure was most likely ascirbable to hydraulic piping of the dam’s right abutment, which had been built upon a fault contact between the Sespe conglomerate and the Pelona Schist. This somewhat simplistic explanation was offered after observing that blocks from the dam’s west abutment were supposedly found further downstream than those of the opposing, east side.

sf_dam02The failure of St. Francis Dam represents but one of a number of important dam failures that occurred in the 1920s and 30s when American civil engineers began to push the limits of a technology then it its infancy. The dam’s high-profile failure led to the immediate and irrevocable demise of William Mullholand, architect of the Los Angeles water supply system. Like most notorious engineering failures, looking back we can take some measure of satisfaction in knowing that considerable long-term societal benefit resulted from public outcry following the disaster. Some of the most important consequences included:

a. the formulation of the world’s first Dam Safety agency;
b. normalization of uniform engineering criteria for testing of compacted earthen materials still in use world-wide

sf_dam03tIn recent years detailed geologic assessments have shown that the eastern, or left abutment of St. Francis Dam was unknowingly founded upon massive paleo mega-slides, developed within the Pelona Schist. The balance of this article explores what is currently understood about the St. Francis disaster by reassessing its failure with modern forensics analytical techniques, most of which were unavailable to civil engineers and geologists in 1928.

c. a reassessment of all Los Angeles Department of Water and Power dams and reservoirs which led to an extensive retrofit of Mulholland Dam; and
d. the formulation of a state-mandated process for arbitration of wrongful death suits that forms the basis of similar legislatiion following the 1989 Loma Prieta Earthquake.

sf_dam04_tResearch-to-date suggests that St. Francis was in all likelihood not designed with a proper appreciation of uplift theory; the dam’s base width was not as thick as previously assumed; and the designers were not aware that the left abutment was a paleo mega-landslide or that the Sespe red beds would slake upon submersion; and that it was actually pieces of the left (eastern) abutment, against the Pelona schist, that were actually found furthest downstream following the dam’s collapse. A review of the available evidence suggests the dam failure sequence was likely brought about by a combinatioin of factors, including excessive tilting when fully loaded, and absence of seepage relief in the dam’s sloping abutmnts, and the partial reactivating of underlying paleo mega-slides within the Pelona Schist. Upon forces acting to destablize the sloping abutments would appear to have been similar to those which fostered the disastrous failure of  Malpasset arch dam in France in 1959, which took more than five years to sort out and understand (Londe, 1968, 1979, 1970).

sf_dam05In recent years detailed geologic assessments have shown that the eastern, or left abutment of St. Francis Dam was unknowingly founded upon massive paleo mega-slides, developed within te Pelona Schist. The balance of this article explores what is currently understood about the St. Francis disaster by reassessing its failure with modern forensics analytical techniques, most of which were unavailable to civil engineers and geologists in 1928.

Research-to-date suggests that St. Francis was in all likelihood not designed with a proper appreciation of uplift theory; the dam’s base width was not as thick as previously assumed; and the designers were not aware that the left aubtment was a paleo mega-landslide or that the Sespe red beds would slake upon submersion; and that it was actually pieces of the left (eastern) abutment, against the Pelona schist, that were actually found furthest downstream following the dam’s collapse.

sf_dam06_tA review of the available evidence suggests the dam failure sequence was likely brought about by a combinatioin of factors, including excessive tilting when fully loaded, and absence of seepage relief in the dam’s sloping abutmnts, and the partial reactivating of underlying paleo mega-slides within the Pelona Schist. Upon forces acting to destablize the sloping abutments would appear to have been similar to those which fostered the disastrous failure of, which took more than five years to sort out and understand (Londe, 1968, 1979, 1970).

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Resources:

St. Francis Dam Disaster – Water and Power Associates

Engenniring Dissaster by Cesar Sosa on Prezi

Owens Valley History

St. Francis Dam

geo earthquakes, RIS & Dam Failures

Outrage in Owens Valley a century after LA began taking its

Regulatory Frameworks for Dam SafetyWorld Bank eLibrary

The L.A. Aqueduct at 100 – Los Angeles Times – Graphics

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