What is soil liquefaction?

Soil liquefaction is a phenomenon where a block of soil behaves as a liquid instead of a solid for a short period of time. It occurs most often when an applied load, such as earthquake shaking, causes an increase in pore pressure, reducing the soil’s effective stress by the equation:

Effective vertical stress = (Vertical stress) – (Pore pressure)

When a soil’s effective vertical stress becomes negative, meaning the pore pressure is greater than the vertical stress, the soil loses its internal cohesion and begins to behave as a liquid instead of a solid, which can result in heavy damage to structures on top of the soil. As liquefaction most often occurs in response to an earthquake’s shaking, it is known as a “secondary hazard”.

What soils can liquefy?

Liquefaction is most likely to occur in wet, sandy, soils. Soils with large grains, such as sands, don’t fit together very well and have large void spaces (“high porosity”). In wet regions of the world, this allows more water to infiltrate the soil. If a load, such as shaking, is applied to a block of soil with high porosity and high water content, its pore pressure will increase much easier than if the same load was applied to a soil with low porosity or low water content.

Where can liquefaction happen?

As strong loading, high porosity, and high water contents are all generally needed for liquefaction, soils typically liquefy in sandy, coastal regions in response to large earthquakes. In the United States, liquefaction occurs most often along the West Coast which has areas of high seismic vulnerability, porosity, and water content. Damages due to liquefaction impact primarily metropolitan areas such as San Francisco, Los Angeles, and Seattle because of their high liquefaction vulnerability and high infrastructure density.

How much damage does liquefaction cause?

This is one of our research group’s ongoing efforts. Starting in 2013, the Tufts Geohazards Research Lab led by Professor Baise has created and implemented models estimating spatial probability of liquefaction given shaking parameters. The most recently published version of this model is in use by the PAGER alert system of the USGS.

One lab member, Alex, is working on a model to estimate economic impacts of liquefaction given the same shaking parameters, which the USGS also plans to implement in their PAGER system.

Another lab member, Lekan, uses satellite imagery to identify damage levels of different infrastructure by several types of natural disasters, including liquefaction.