Liquefaction is a phenomenon in which a loose sandy soil layer behaves like a liquid when the effective stress is reduced by the generation of excessive interstitial water pressure due to vibrations such as earthquakes and instantaneous shocks, and the shear strength within the sandy soil layer is lost.
It can cause ground settlement or flow, resulting in the collapse of buildings, structures, etc.
The term liquefaction was first coined by Mogami and Kubo in 1953, and was studied in earnest after liquefaction was observed in the 1964 magnitude 7.5 Niigata earthquake and the 1964 magnitude 8.3 Alaska earthquake.
Liquefaction generally occurs when a saturated sandy soil is subjected to earthquake-like vibrations, the effective stress of the saturated sandy soil decreases and the interstitial water pressure increases, causing it to lose shear stress and behave like a liquid.
At this time, the behavior of liquefaction is mainly divided into flow liquefaction and cyclic mobility.
Flow liquefaction occurs when the static equilibrium shear stress in the saturated sandy soil is greater than the shear strength in the soil body, and large-scale deformation is caused by static shear stress.
This type of liquefaction occurs mainly in sloping soils during and after the earthquake, causing flow failure, which is the most damaging type of liquefaction damage.
Cyclic mobility is caused by both static and oscillatory shear stresses and occurs when the static equilibrium stress or oscillatory shear stress is less than the shear strength in the soil.
It occurs in flat or rolling terrain and causes lateral spreading that can damage structures.
However, the terms liquefaction and cyclic mobility are commonly used interchangeably.
When liquefaction occurs, the direct destruction of the ground and the resulting ground settlement and unequal ground settlement can cause great damage to structures.
Therefore, liquefaction evaluation is essential for seismic design of all structures in medium and high seismic areas.
Research on liquefaction is being conducted in various fields such as vibration shear stress of liquefaction-prone sandy soil, vibration resistance stress evaluation, liquefaction potential evaluation, liquefaction prevention ground improvement methods, and liquefaction damage reduction measures.
Mechanism of Liquefaction
Before vibration, the particles of loose sandy soil are in contact with each other, and external forces such as shear strength are transmitted through the interaction between the particles.
When an earthquake occurs, the volume of saturated sandy soil decreases instantaneously, and when the volume decreases without drainage, excess interstitial water pressure is generated, and the effective vertical stress becomes zero, causing liquefaction.
After the earthquake stops, the interstitial water is discharged as the sand particles in the sandy soil layer where the liquefaction phenomenon occurred are relocated, and at this time, the ground subsidence occurs, unequal subsidence occurs, and ground flow occurs.
Conditions for liquefaction
The factors that influence liquefaction are earthquake and ground conditions, but generally include the intensity and duration of the earthquake, the interstitial ratio of the sandy soil, and the groundwater level.
In general, earthquakes of magnitude 5 and above are considered dangerous, and the longer the duration of the shaking, the greater the damage.
However, liquefaction can also occur in magnitude 5 and below due to ground amplification and shaking duration. The groundwater level is within 10 meters of the surface, and the shallower the depth, the more likely liquefaction is to occur.
Examples of liquefaction damage include Valdivia, Chile in 1960, Niigata, Japan in 1965, Tangshan, China in 1976, and Christchurch, New Zealand in 2011.
In particular, in the Niigata earthquake, liquefaction caused the collapse of apartments and bridges, and in the Tangshan earthquake, liquefaction caused material and human damage, killing about 240,000 people.