Lower San Fernando Dam — the Earthquake That Liquefied a Dam Above 80,000 People

At about six o’clock on the morning of 9 February 1971, the magnitude-6.6 San Fernando earthquake shook the Lower San Fernando Dam — a 142-foot (43-metre) earth embankment built by the hydraulic-fill method between 1912 and 1916 at the head of the San Fernando Valley in Los Angeles — and the upstream face of the dam slid bodily into the reservoir. The loose sand at the core of the embankment liquefied, lost almost all of its strength, and flowed. The crest dropped, the upstream shell spread some 250 feet beyond its original toe, and when the movement stopped only about five feet of soil — roughly a metre and a half — separated a full reservoir from the breach. No one died at the dam. Below it lay a residential district of some 80,000 people, and the cause was the seismic liquefaction of the dam’s own fill.

The dam did not fail because the earthquake pushed it over. It failed because the shaking destroyed the strength of the soil holding it up. For about twelve seconds of strong motion, cyclic stress drove pore-water pressure in the saturated hydraulic-fill sand of the upstream shell until the effective stress between grains approached zero and the material behaved as a heavy liquid. Then — and this is the detail that made the case famous — the dam did not move during the shaking. The major slide occurred an estimated twenty to thirty seconds after the ground stopped moving, when the now-liquefied mass could no longer carry the dead weight of the embankment above it and the whole upstream slope ran out under static gravity alone.

Authorities did not know how close they had come until daylight. The Los Angeles Department of Water and Power began an emergency drawdown of the reservoir within hours and the police evacuated roughly 80,000 residents from the valley below while the lake was lowered over three days. The Lower dam held by a margin measured in feet. Had the reservoir stood a little higher, or the slide run a little farther, the case would read like Malpasset or Vaiont. Instead it became the most studied near-miss in geotechnical history.

The investigation, led by H. Bolton Seed and his colleagues at the University of California, Berkeley, and later re-examined by the US Army Corps of Engineers Waterways Experiment Station, established the modern understanding of how earthquakes destroy dams: not by inertia, but by liquefaction, and not necessarily during the shaking, but in the seconds after it. The finding ended the use of hydraulic fill for embankment dams in seismic regions and rebuilt the way every earth dam in earthquake country is analysed. The Lower San Fernando Dam is the canonical case of seismic soil liquefaction.

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