This chapter deals with the liquefaction of soil. An introduction to liquefaction was presented
in Sec. 3.4. The concept of liquefaction was first introduced by Casagrande in the
late 1930s (also see Casagrande 1975).
As mentioned in Sec. 3.4, the typical subsurface soil condition that is susceptible to liquefaction
is a loose sand, which has been newly deposited or placed, with a groundwater
table near ground surface. During an earthquake, the application of cyclic shear stresses
induced by the propagation of shear waves causes the loose sand to contract, resulting in an
increase in pore water pressure. Because the seismic shaking occurs so quickly, the cohesionless
soil is subjected to an undrained loading (total stress analysis). The increase in pore
water pressure causes an upward flow of water to the ground surface, where it emerges in
the form of mud spouts or sand boils. The development of high pore water pressures due to
the ground shaking and the upward flow of water may turn the sand into a liquefied condition,
which has been termed liquefaction. For this state of liquefaction, the effective stress
is zero, and the individual soil particles are released from any confinement, as if the soil
particles were floating in water (Ishihara 1985).

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