Outline of soil liquefaction due to unstable layer after an earthquake.

Guide To Soil Liquefaction

Soil liquefaction is a geological phenomenon that changes the structure of the Earth’s soil. External stressors loosen the soil particles, changing the ground into a less stable base for buildings and other structures.

Soil liquefaction presents risks to construction companies building in high-soil liquefaction areas, such as California. If builders construct on top of loose soil, it can damage the integrity of the buildings and cause structural damage or collapse. To mitigate these risks, builders should work with a qualified geotechnical engineer to assess the soil before beginning a construction project.

Learn why soil liquefaction occurs and how to reduce the hazards of soil liquefaction with the help of a geotechnical engineer.

What Causes Liquefaction?

Soil liquefaction occurs when external stressors like earthquakes, blasting, or soil compaction cause soil particles to lose their strength and firmness. The shaking loosens dense soil particles when an earthquake occurs, creating high, unstable pressure.

When affected by this phenomenon, the soil particles follow the path of least resistance and become more viscous, resembling quicksand. This type of soil loses its ability to support loads like engineered buildings.

Structures standing on liquefying soil lose their structural integrity, causing them to tilt or collapse. Most objects like cars and trucks sink under the surface, and underground utility lines such as water and natural gas rupture, causing severe damage and presenting a significant threat to life and property.

Origins of the Term

Although soil liquefaction events have occurred numerous times throughout history, hydraulics expert Allen Hazen first coined the term in 1918 to describe the failure of the Calaveras Dam in Fremont, California. Hazen described the phenomenon as “similar to quicksand.”

However, it wasn’t until the 1964 earthquakes in Niigata, Japan, and throughout Alaska that the geo-engineering community took an interest in studying soil liquefaction. Today, the International Building Code and most countries’ national building regulations require new construction to factor soil and earthquake liquefaction into the design of new buildings, bridges, and dams.

What is a Liquefaction Zone?

A ​​liquefaction zone is an area assessed by local geotechnical engineers where the soil is compacted or mainly composed of low-density particles, such as sand, landfill, or silt. These zones are the most susceptible to soil liquefaction damage.

According to the United States Geological Survey (USGS), many areas in California are in a liquefaction zone, including the Los Angeles Basin, Orange County, the San Francisco Bay Area, northwestern Alameda County, and northern Santa Clara Valley.

For instance, the USGS estimates that approximately 25% of the SF Bay Area is in the three highest liquefaction risk levels, which are moderate, high, and very high. A prominent example is the 58-story Millennium Tower in downtown San Francisco. In 2015, property developers discovered that the tower was slowly sinking and tilting due to soil liquefaction.

What is the Process of Liquefaction?

The soil in a liquefaction zone mainly comprises loosely-packed particles held together by the friction created by groundwater molecules. When an earthquake disturbs the soil, the soil particles soften, destabilize, and lose their strength, causing the surface to shift and sink downward in a process called ground surface subsidence until the soil particles naturally settle in a more compact state.

The result causes the soil to become slurry or semi-liquid, damaging or destroying structures, buildings, and other objects. If the event occurs on a slope, the ground may slip down and cause lateral spreading, inducing a tearing effect on structures and underground utilities.

In extreme cases, soil flotation can even occur, causing light structures buried at shallow ground depths like a sewer line to float to the surface.

How to Reduce the Hazards of Soil Liquefaction

The easiest solution to prevent soil liquefaction damage is to avoid building in a liquefaction zone. However, this option is not always viable in California due to the high prevalence of liquefaction zones.

Geo-engineers and construction project planners have access to remedial measures to improve a building’s resilience. These engineers help construction companies use soil improvement techniques and soil liquefaction-resistant building designs to build more safely in these unstable zones.

Soil improvement techniques aim to increase the density and strength of the underlying soil, using tools such as vibroflotation probes, dynamic soil compaction equipment, improving the soil’s drainage capabilities with wick drains, or implanting stone columns into the ground.

Examples of liquefaction-resistant building design elements include:

  • Foundation mats: Builders can use slabs of stone or concrete designed to keep a house or building upright to prevent the structure from sinking during a liquefaction event
  • Pliable utility pipes: Construction crews can use utility pipes with sections of soft, ductile elements to withstand motion and displacement induced by an earthquake.
  • Reinforced pile foundations: Builders use load-bearing, structure-supporting piles featuring sufficient vertical and lateral strength to support the structure’s weight and resist the horizontal forces of a liquefying soil.

Hire a Team of Qualified Los Angeles Geo-Engineers

At Applied Earth Sciences (AES), we aim to provide expert geotechnical engineering solutions for building projects of all sizes. We have over three decades of experience analyzing soil and partnering with companies in Southern California to ensure your structures’ safety, stability, and reliability when building in a soil liquefaction zone.

Call us today at (818) 552-6000 to learn how we can help you build successfully in the Los Angeles area and beyond.

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