A cooperative project with the California Geological Survey

About Liquefaction

What Is Liquefaction?
Loose sand and silt that is saturated with water can behave like a liquid when shaken by an earthquake.

How does it work:
Earthquake waves cause water pressures to increase in the sediment and the sand grains to lose contact with each other, leading the sediment to lose strength and behave like a liquid. The soil can loose its ability to support structures, flow down even very gentle slopes, and erupt to the ground surface to form sand boils. Many of these phenomena are accompanied by settlement of the ground surface — usually in uneven patterns that damage buildings, roads and pipelines. Three factors are required for liquefaction to occur.

Where can it happen:
Much of the urban development in the San Francisco Bay region is in the flatlands around the Bay margin, where liquefaction is most likely. Particularly vulnerable areas lie around the margins of San Francisco Bay and along the larger streams and rivers. The most vulnerable are areas of bay or marshland that were filled with pumped or dredged material many decades ago to create "made" land. These and other potentially hazardous settings can be mapped in advance for planning and design purposes. All parts of the San Francisco Bay region have the potential to be shaken hard enough for susceptible sediment to liquefy.

The expected severity of ground shaking and damage in the San Francisco Bay region from anticipated future earthquakes.
(click to enlarge)

Typical effects of liquefaction include:

loss of bearing strength –the ground can liquefy and lose its ability to support structures.

Tilted apartment buildings, 1964, Niigata, Japan earthquake (click to enlarge)

Tilted home in Mission District, 1906 San Francisco earthquake

lateral spreading - the ground can slide down very gentle slopes or toward stream banks riding on a buried liquefied layer.

Burning gas main ruptured by lateral movement, Balboa Blvd in Granada Hills, 1994 Northridge earthquake (click to enlarge)

Road damaged by lateral spread, near Pajaro River, 1989 Loma Prieta earthquake. (click to enlarge)

sand boils - sand-laden water can be ejected from a buried liquefied layer and erupt at the surface to form sand volcanoes; the surrounding ground often fractures and settles.

Sand boils in Milpitas mud flat, 1906 San Francisco earthquake (click to enlarge)

Sand boils along a fissure near the Pajaro River, 1989 Loma Prieta earthquake (click to enlarge)

flow failures — earth moves down steep slope with large displacement and much internal disruption of material.

ground oscillation — the surface layer, riding on a buried liquefied layer, is thrown back and forth by the shaking and can be severely deformed.

flotation — light structures that are buried in the ground (like pipelines, sewers and nearly empty fuel tanks) can float to the surface when they are surrounded by liquefied soil.

settlement — when liquefied ground re-consolidates following an earthquake, the ground surface may settle or subside as shaking decreases and the underlying liquefied soil becomes more dense.