Liquefaction of Soils

What

• Soil liquefaction is a natural phenomenon in which the water pressure between the soil particles increases. This increase in water pressure allows the soil particles to move freely about one another. Liquefaction occurs in saturated soils, which are soils completely filled with water. During an earthquake, the stiffness of the soil is reduced by the shaking of the ground, which packs everything tighter together.

When

• Soil liquefaction has occurred during earthquakes for many years and ancient records describe events during earthquakes that we now know are caused by liquefaction. According to the University of Washington, there are a few earthquakes in recent history that have become greatly associated with liquefaction due to the severity of its effects. In 1989 in Loma Prieta, California, many sandboils, which are large sinkholes that appear in sandy soils, appeared around the Bay Area. In Kobe, Japan, in 1995, the 6.9 earthquake caused major damage to streets and bridges due to the loss of stability in the soil.

Where

• Soil liquefaction only occurs in saturated soils, so it usually occurs in low-lying areas near water, such as lakes, bays and rivers. Liquefaction commonly causes damage to bridges that span bays and rivers by either shifting the support legs or causing them to sink entirely. Also, liquefaction can cause retaining walls near bodies of water to shift towards the water, disrupting whatever it is they support and possibly allowing water to flow over them.

Why

• Soil is made up of tiny particles in contact with neighboring particles on all sides. This contact and the weight of the particles above hold the particles in place and give the soil its strength. The increased force generated from the earthquake causes the soil particles to try to compact themselves more densely, but there is not enough time for the water molecules between the soil particles to escape, so the water is trapped, preventing the soil particles from packing tighter in. The earthquake also causes an increase in water pressure, which reduces the contact between the soil particles, effectively turning the soil in to a liquid.

Prevention

• There are three basic ways to reduce the hazards of liquefaction when constructing new buildings and bridges: avoiding liquefaction-susceptible soils, building liquefaction-resistant structures, and improving the existing soil. Avoiding susceptible soils seems easy, as long as you know what you are looking for: sandy soils. Liquefaction-resistant aspects of a project can be added to the designs during construction if construction must occur in liquefaction-susceptible soils. Finally, the soil itself can be improved through soil additions that will make it a less sandy soil.