Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world. In recent times, frequent earthquakes have occurred in the last few years and these events have made it even more important to study about soil liquefaction because the best way to minimize earthquake aftermaths is to study and understand every aspect associated with them. Effects of soil liquefaction in the design of new buildings and infrastructure such as embankment dams, bridges, and retaining structures have to be studied in detail to ensure erection of strong and durable structure.
What Is Soil Liquefaction
Soil liquefaction and sensitive clay (leda clay) liquefaction illustrate the behavior of soil under load, and the change from a solid condition to a liquid state. Alternately, the soil may be in the state of a heavy liquid. The occurrence of liquefaction is enhanced in loose or fairly saturated coarse soils with reduced drainage characteristics. Examples of such soils are silt sands, clay sands, and sands containing impermeable sediments. During the loading of the soil during earthquakes or other conditions, the volume of loose sands is likely to be decreased. Consequently, the pore water pressure is increased, and the shear strength is decreased. The real soil strength is reduced considerably depending upon the loading conditions. The soil condition will influence the soil liquefaction that can be determined by soil element analysis, conducted by soil test kits. Deposits that are most vulnerable to liquefaction are relatively young sands and sediments that are saturated with water. In such sands, making quicksand is simple. Such deposits are normally created along river beds, dunes, beaches, and regions where windblown silt and sand have been accumulated. The examples of liquefaction include earthquake liquefaction, quicksand, and quick clay.
During a strong earthquake when the ground shaking starts, the soil grains at shallow depth are sheared into a solid composition.
However, due to the presence of water, the soil grains are saturated. The sand grains may turn into sand volcanoes. The sediments at the depth have a strong bond, with greater confining pressures, and are thus less probable to liquefy. When the sand under a clay or silt layer liquefies, the top layer may slide due to gravity towards a slope, producing ground cracking. Foundations of bridges, highways, buildings, and the gas and sewer lines may be severely damaged by these movements. Earthquake liquefaction contributes significantly towards seismic damage.
Quicksand is developed when water saturates loose sand, and the normal sand is disturbed. When the water held in the sand is not able to escape, it produces liquefied soil. Quicksand is produced by the standing, or underground flowing water, like a spring. In underground flowing water, the water flow force resists the gravitational force, causing buoyancy in the sand granules. The strength of the liquefied surface is reduced, causing the sinking, or falling of structures. The saturated sediments look fairly solid, until a shock starts the liquefaction, producing suspended sand, with the grains being surrounded by a fine water film. This quicksand has an elastic texture. Objects sink in the liquefied sand till their weight is same as that of the displaced sand and water mixture.
Reduce Consequences of Liquefaction
Liquefaction causes intensive human and property loss due to which scientists are continuously studying methods to mitigate the harmful effects. Slope slumping, lateral spreading, floating of light structures, ejection from surface fissures, and ground settlement are some of the most commonly occurring problems because of soil liquefaction. Therefore, the buildings and structures are designed accordingly. Initially, the soil is tested for vulnerability to liquefaction. Geological studies identify the landfill sediments, and water table in a seismic region. Soil composed of a mix of small and big grains, can safely endure liquefaction, since the smaller grains fill the pores between the larger grains. Thus, the soil strength is maintained. Furthermore, modern buildings are designed to be liquefaction resistant. In a shallow foundation, the foundation constituents are joined to ensure uniform settlement of the foundation. Thus, the induced shear forces are decreased. The soil characteristics may also be enhanced by improving the soil density, strength, and drainage characteristics. Popular mitigation methods to reduce effects of soil liquefaction are:
- Mitigation by deep soil mixing method
- Compaction, Permeation, and Jet Grouting
- Drain Pile technique
- Dynamic compaction and construction of stone columns
Pinopolis Dam in South Carolina, Riverside Avenue Bridge in Santa Cruz, Highway Viaduct in San Diego, and Jackson Lake dam in Wyoming are some of the structures that have been saved by using some of the above-mentioned techniques of soil liquefaction prevention.
- Image: Liquefaction at Niigata, Wikipedia
- Digital Library and Archives, Soil Liquefaction in Earthquakes (pdf)
- Ronald D. Andrus et al 1995, NIST.gov, Ground Improvement Techniques (pdf)