Soil liquefaction describes the behavior of loose saturated cohesionless soils, i.e. loose sands, which go from a solid state to have the consistency of a heavy liquid, or reach a liquefied state as a consequence of increasing porewater pressures, and thus decreasing effective stress, induced by their tendency to decrease in volume when subjected to cyclic undrained loading (e.g. earthquake loading). Liquefaction is more likely to occur in loose to moderate granular soils with poor drainage, such as silty sands or sands and gravels capped or containing seams of impermeable sediments . Deposits most susceptible to liquefaction are young (Holocene-age, deposited within the last 10,000 years) sands and silts of similar grain size (well-sorted), in beds at least several feet thick, and saturated with water. Such deposits are often found along riverbeds, beaches, dunes, and areas where windblown silt (loess) and sand have accumulated. Some examples of liquefaction include quicksand, quick clay, turbidity currents, and earthquake liquefaction.

Depending on the initial void ratio, the soil material can respond to loading either strain-softening or strain-hardening. Strain-softened soils, e.g. loose sands, can be triggered to collapse, either monotonically or cyclically, if the static shear stress is greater than the ultimate or steady-state shear strength of the soil. In this case flow liquefaction occurs, where the soil deforms at a low constant residual shear stress. If the soil strain-hardens, e.g. moderately dense to dense sand, flow liquefaction will generally not occur. However, cyclic softening can occur due to cyclic undrained loading, e.g. earthquake loading. Deformation during cyclic loading will depend on the density of the soil, the magnitude and duration of the cyclic loading, and amount of shear stress reversal. If stress reversal occurs, the effective shear stress could reach zero, then cyclic liquefaction can take place. If stress reversal does not occur, zero effective stress is not possible to occur, then cyclic mobility takes place .

The resistance of the cohesionless soil to liquefaction will depend on the density of the soil, confining stresses, soil structure (fabric, age and cementation), the magnitude and duration of the cyclic loading, and the extent to which shear stress reversal occurs .

Although the effects of liquefaction have been long understood, it was more thoroughly brought to the attention of engineers and seismologists in the 1964 Niigata, Japan and Alaska earthquakes. It was also a major factor in the destruction in San Francisco's Marina District during the 1989 Loma Prieta earthquake.