Liquefaction: When Solid Ground Turns To Mush

Many areas of the San Francisco Bay area are subject to liquefaction. (Map courtesy of the Association of Bay Area Governments)

Liquefaction is a rare geological condition caused when certain soils with a high water table are violently shaken by an earthquake. The shaking earth mixes the soil particles with the water table and creates soil with the consistency of a milkshake. Buildings sink into the liquified soil because it does not have the same bearing strength it had before the quake.

Recent earthquakes in Japan and New Zealand caused certain soils to liquefy. All of the soils were fine-grained and had a shallow water table. Check out this video from New Zealand’s Christchurch quake to see what liquefaction really looks like.

Watch Liquefaction In Action



The Christchurch, New Zealand earthquake and resulting liquefaction damaged or destroyed much of this beautiful city. The same thing could happen in populated areas of California, too.

Inventory High-Risk Locations

Some soils in California are at risk for liquefaction. Fortunately, there are some resources to help landscape architects (and other design professionals) identify areas that are prone to liquefaction.

The United States Geological Survey (USGS) produces maps identifying seismic risks for many areas in California.

California also publishes official Seismic Hazard Zone Maps of selected areas of the state more prone to destructive seismic events.

Northern California Seismic Hazard Areas

Northern California Seismic Hazard Areas

Southern California Seismic Hazard Areas

Southern California Seismic Hazard Areas

If you site is located in one of the areas seismic risk areas delineated on the map, additional site investigations are required by the state and local agencies during the site inventory process. Landscape architects need to bring on certified geologists and registered civil engineers to complete the intensive investigation and mitigation requirements.

Why is Liquefaction Important For Landscape Architects?

There are a few reasons why landscape architects should identify liquefaction risks during a site inventory.

First, you can identify of parts of the site are more prone to liquefaction than others on the site. If your site’s geology has some different soils with less risk, you may choose to locate structures on solid ground. Second, you can make better large-scale planning decisions if you know where at-risk soils are located.

Second, the California Seismic Hazards Mapping Act and other regulations require project developers to identify potential seismic risks and create plans to mitigate potentially damaging ground movement, landslide, and liquefaction risks.

The California Supplemental Exam for landscape architects may include liquefaction as one of the site inventory questions on the test.

For More Information

For more information on the identifying and inventorying a site’s siesmic risk factors and potential, read the following articles:

About

John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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