- Also called:
- earthquake liquefaction
- Related Topics:
- earthquake
- soil
- secondary wave
soil liquefaction
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What is soil liquefaction?
Soil liquefaction happens when seismic S waves shake water-saturated, unconsolidated soil during earthquakes, causing it to lose strength and act like a viscous liquid. Earthquake shock is the best-known cause of liquefaction, but construction practices, such as blasting, can also cause this phenomenon. Sandy, silty, and gravelly soils are the most susceptible to liquefaction. Buildings on loose soil can pitch or tilt because the soil's resistance to shear stress lowers, and it no longer supports the structure's foundations.
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When and where can soil liquefaction occur?
Soil liquefaction happens when saturated, unconsolidated soils lose strength and act like a viscous liquid soil liquefaction. This occurs mainly in sandy, silty, and gravelly soils because they don't drain well.
Earthquake vibrations are a common cause, but construction practices such as blasting, soil compaction, and vibroflotation can also trigger it.
Liquefaction is more likely to occur in areas with seismic activity and poorly drained soil. Examples include the 1976 Tangshan earthquake in China, where over 2,400 sq km were affected, and the 1906 San Francisco earthquake, where buildings collapsed in the Mission and Market districts due to the ground liquefaction.
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What causes soil to become liquefied?
Soil liquefaction happens when vibrations, often from earthquakes, turn solid ground into a viscous liquid. This occurs in water-saturated, unconsolidated soils when seismic waves cause ground vibrations. Earthquake shock collapses the water-filled pore spaces, increasing water pressure between soil grains, which then move freely. The soil loses its resistance to shear stress and behaves like a liquid, deforming easily and causing heavy objects and structures to sink or tilt due to loss of support. Poorly drained fine-grained soils such as sandy, silty, and gravelly soils are the most susceptible to liquefaction.
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How does an earthquake trigger soil liquefaction?
During an earthquake, soil liquefaction happens when seismic S waves affect water-saturated, unconsolidated soils, causing ground vibrations. The water-filled pore spaces collapse, which decreases the soil's volume. Consequently, the water pressure rises between soil grains, allowing them to move freely within the watery mix. The soil's resistance to shear stress is substantially reduced, and the soil mass takes on liquid-like properties. Heavy objects and structures can be damaged from the sudden loss of support from below because the soil deforms easily in its liquefied state.
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What are some signs or effects of soil liquefaction on buildings or land?
Soil liquefaction happens when vibrations, often from earthquakes, cause saturated soil to lose strength and act like a liquid. Buildings on such soil may sink or tilt. The soil's structure collapses, increasing pressure and expelling water and sand, leading to sand blows, also called sand boils or sand volcanoes.
Liquefaction can also trigger landslides, as seen in the 1964 Alaska earthquake, where it caused a suburb of Anchorage to slide, destroying homes and utilities.
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How can engineers prevent or reduce the dangers of soil liquefaction?
To reduce dangers from soil liquefaction, engineers can implement several strategies. Structures can be anchored to bedrock or stiff soils to minimize vibration transmission. Drainage systems can alleviate steep seepage gradients by moving them to where the soil is constrained. Freely draining fill zones on the upstream face of dams can counter rapid water drawdown. Grouting foundations, cutoff trenches, drainage blankets, and extending seepage paths can also prevent weakening of the soil.
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soil liquefaction, ground failure or loss of strength that causes otherwise solid soil to behave temporarily as a viscous liquid. The phenomenon occurs in water-saturated unconsolidated soils affected by seismic S waves (secondary waves), which cause ground vibrations during earthquakes. Although earthquake shock is the best known cause of liquefaction, certain construction practices, including blasting and soil compaction and vibroflotation (which uses a vibrating probe to change the grain structure of the surrounding soil), produce this phenomenon intentionally. Poorly drained fine-grained soils such as sandy, silty, and gravelly soils are the most susceptible to liquefaction.
Granular soils are made up of a mix of soil and pore spaces. When earthquake shock occurs in waterlogged soils, the water-filled pore spaces collapse, which decreases the overall volume of the soil. This process increases the water pressure between individual soil grains, and the grains can then move freely in the watery matrix. This substantially lowers the soil’s resistance to shear stress and causes the mass of soil to take on the characteristics of a liquid. In its liquefied state, soil deforms easily, and heavy objects such as structures can be damaged from the sudden loss of support from below.
Buildings constructed on loose soil pitch and tilt easily when liquefaction occurs, since the soil no longer supports the structures’ foundations. In contrast, structures anchored to bedrock or stiff soils in earthquake-prone areas suffer less damage, because less vibration is transmitted through the foundation to the structure above. In addition, buildings anchored to bedrock have a reduced risk of pitching and tilting.
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One of the most severe episodes of liquefaction in modern times occurred in China during the Tangshan earthquake of 1976. Some scientists estimate that an area of more than 2,400 sq km (about 925 sq miles) was subjected to severe liquefaction, which contributed to the extensive damage that took place in the southern part of the city. The liquefaction of the soft lake sediment upon which central Mexico City was built amplified the effects of the 1985 earthquake, the epicentre of which was located hundreds of miles away. In addition, the liquefaction of the ground beneath the Mission and Market districts in San Francisco during the 1906 earthquake caused several structures to pitch and collapse. These districts were built on poorly filled reclaimed wetlands and shallow-water areas.
Liquefaction may also contribute to sand blows, which are also known as sand boils or sand volcanoes. Sand blows often accompany the liquefaction of sandy or silty soil. With the collapse of the soil’s granular structure, the density of the soil increases. This increased pressure squeezes the water out of the pore spaces between the soil grains and expels wet sand from the ground. Sand blows have been observed in the aftermath of several earthquakes, including the New Madrid earthquakes of 1811–12, the Tangshan earthquake of 1976, the San Francisco–Oakland earthquake of 1989, and the Christchurch earthquakes of 2010–11.
In addition, liquefaction may also cause landslides. For example, during the Alaska earthquake of 1964, the liquefaction of a sandy layer of soft clay beneath Turnagain Heights, a suburb of Anchorage, caused a landslide in the mass of ground above that destroyed approximately 75 homes and disrupted utilities.