March 12, 1928: The geological induced collapse of the St. Francis Dam

Sometimes ignoring the geological structures and resulting problems of construction sites can have disastrous consequences. Following the post of the Stava dam collapse, a similar event from the New World, where ignoring the geological circumstances caused death and destruction:

At the end of the 19th century, the city of Los Angeles was lacking an important factor limiting the economic development - the water supply was insufficient and couldn't satisfy the demand. William Mulholland, an Irish immigrant, and a very ambitious man had managed to become a self-educated engineer and the superintendent of the Los Angeles Bureau of Water Works and Supply. He seemed the right man for the job to bring the needed water to the city.
So he planned and constructed a system of aqueducts that collected and redirected water from the Sierra Nevada to Los Angeles. Almost 72 kilometers north from Los Angeles the main aqueduct ended in the San Francisquito Canyon, where two hydroelectric power stations were built. Mulholland decided to amply the original project and constructed a supplementary dam to regulate the water flow.

Fig.1. The completed St. Francis Dam (figure from CHAMP 1983).

The construction site in the San Francisquito Canyon follows a tectonic lineament that separates conglomerates and sandstones with gypsum from schists with layers of talcum. In addition to these very unstable rocks, surrounding the contact between the two lithologies were lenses of serpentinites, weak rocks inadequate to support a heavy building like a dam.
In 1926 the St. Francis dam was inaugurated, 61 meters high, with a span of 210 meters and at the base 50 meters thick. Soon after the completion of the dam water leaks were observed, but it seemed normal for such a large reservoir. On the morning of May 12, 1928, the technician Tony Harnischfeger observed a leak on the eastern side of the dam. Mulholland was informed and arrived soon thereafter to inspect the damage. The water was clear, sign that no sediment was eroded. At 23.57 p.m. a power blackout in Los Angeles marked the collapse of the dam.
 
Fig.2. The remains of the St. Francis Dam (figure from CHAMP 1983).

The resulting waves inundated highway 126 in the Santa Clara Valley, destroying at least 50 cars and killing an estimated 300 to 600 people, the exact number of victims remains unclear to this day. The waves finally reached the Pacific, more than 70 kilometres distant from the San Francisquito Canyon.

References:

CHAMP, C. (1983): Planet Earth - Flood. Time-Life-Books. Amsterdam: 176