What Is Soil Liquefaction?
Soil liquefaction, also called earthquake 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.
There are two types of soil liquefaction
1- Flow liquefaction
2- Cyclic Mobility
Liquefaction during Seismic events
Seismic events affect ground conditions. Liquefaction of soil causes structural instability in buildings. This occurs due to various instances of structural failure. The liquefied ground cannot sustain the stresses of its load from the foundations. Foundations will sink into the sand deposit and cause the building to lean and eventually collapse. Soil liquefaction occurs only in areas which have saturated soils. Most of these areas are located near a water body such as lakes, ponds, rivers etc.
Buildings constructed in this zone must adhere to strict codes and bylaws. The soil can sustain the ground forces in general conditions. But an earthquake or strong motion/vibrations in the ground can cause waterlogging which increases the liquid consistency in the soil. The soil loses its rigidity and the ground cannot support the loads causing them to sink or collapse.
Collapsed buildings are not the only problem that liquefaction causes. Since minor earthquakes, vibrations from construction and other incidents happen frequently; soil liquefaction happens often. Some of the other problems are:
- Flotation of buried objects, such as pipelines, fuel tanks and more
- Lateral spreads, or landslides on gentle slopes
- Sand boils, or sand volcano
Determining the liquefaction potential
In order to estimate the occurrence of liquefaction at a location during a strong earthquake, it is necessary to investigate the location and make a geotechnical report that should contain the following data:
1- Position of the location
2- Granulometric composition of the soil on location (presence of soil particles susceptible to liquefaction)
3- Soil compaction on location
4- The degree of saturation with water
5- Seismic activity zone of the location
6- Existing data on previous occurrences of liquefaction on location
How can we prevent soil liquefaction?
Since the cost of the damage by liquefaction is so high, especially in the loss of human life, researchers are testing new ways to prevent it. One of the oldest methods is replacing the loose soil with denser soil and material. This is an inefficient method, in cost or effectiveness.
Often, builders set the footings of the foundation deeper than the layer of unstable soil. Wherever possible, builders try to set the footings onto bedrock. This is especially important for bridges, dams and other building sites near water. However, this is not always practical or possible.
A newer method is Vibroflotation, which is proving to be effective prevention. Technicians insert vibrating probes into the soil at deep levels, and the trembling shakes the loose soil. The compression of the loose soil particles lessens the number of air pockets where water can settle.
Another new method includes injecting the soil with stabilizing materials. Expert geologists and geotechnical engineers are studying the effectiveness of this process.