The Study of Liquefaction: Its Definition and History

The term “Liquefied” was first used by Hazen in soil mechanics field, referring to the 1918 failure of the Calaveras Dam in California.

Later, “Liquefaction is a phenomena wherein a mass of soil loses a large percentage of its shear resistance, when subjected to monotonic, cyclic, or shocking loading, and flows in a manner resembling a liquid until the shear stresses acting on the mass are as low as the reduced shear resistance” defined by Sladen et al (1985).

Liquefaction has become a big deal in the last 50 years. Extensive efforts have been devoted to understand the main reason of liquefaction of sands. Seismic liquefaction was first approached by Arthur Cassagrande in 1965, following the earthquake on 29 April 1964 in Alaska, Good Friday, (MW = 9.2) and Japan on June 16, 1964, Niigata (MS = 7.5).Firstly, evaluation of liquefaction started when Seed and Idriss (1971) issued a methodology based on empirical work termed as ‘simplified procedure’.

Liquefaction occurs in saturated soils and saturated soils are the soils in which the space between individual particles is completely filled with water. This water gets pressure on the soil particles. In contrary to, unsaturated soils are not subject to liquefaction because volume compression does not generate excess pore water pressure. However pore water pressure is lower than before the occurrence of earthquake. Resulting from earthquake shaking, the water pressure starts to increase to the point at which the soil particles can move in terms of each other.

Liquefaction and large deformations are more associated with contractive soils while cyclic softening and limited deformations are more likely with expansive soils. In case large deformations are prevented after initial liquefaction due to increased undrained shear strength, then this is called “limited liquefaction” (Finn 1990). When dense saturated sands are subjected to static loading, they tend to progressively soften in undrained cyclic shear, achieving limiting strains that is known as cyclic mobility (Castro 1975; Castro and Poulos 1979). Cyclic mobility should not be confused with liquefaction. It is possible to distinguish both from the fact that a liquefied soil does not display any appreciable increase in shear resistance, regardless of the magnitude of deformation (Seed 1979). Soils undergoing cyclic mobility first soften under cyclic loading, but later, when monotonically loaded without drainage, harden as the tendency to increase in volume reduces the pore pressures. During cyclic mobility, the driving static shear stress is less than the residual shear resistance and deformations accumulate during cyclic loading only. However, in layman’s language, a soil failure arising out of cyclic mobility is referred to as liquefaction.

According to Selig and Chang (1981) and Robertson (1994), a dilative soil can reach a state of zero effective stress and shear resistance. Cyclic loads may generate a reversal in the shear stress direction when the initial static shear stress is low, in other words, the stress path undergoes a condition known as state of zero shear stress. Under such condition, a dilative soil may accumulate enough pore pressures to help attain a condition of zero effective stress, and large deformations may develop. However, deformations stabilize when cyclic loading comes to an end, because the inclination to expand with further shearing increases the effective stresses, and hence shear resistance. Robertson (1994) called this “cyclic liquefaction”. It involves some deformation occurring while static shear stresses exceed the shear resistance of the soil (when the state of zero effective stress is obtained). However, the deformations finish after cyclic loading ends, because the tendency to expand quickly results in strain hardening. This type of failure in saturated, dense cohesionless soils is also called “liquefaction”, but with limited deformations.

Robertson (1994) and Robertson et al (1994) have suggested a complete classification system to define “soil liquefaction”. The latest put forward by Robertson and Fear (1996) are given below:

1. Flow Liquefaction-The undrained flow of saturated, contractive soil when subjected to cyclic or monotonic shear loading as the static shear stress exceeds the residual strength of the soil
2. Cyclic softening-Large deformations occurring during cyclic shear due to increase in pore water pressure that would tend to dilate in undrained, monotonic shear. Cyclic softening, in which deformations do not continue once cyclic loading ceases, can be further classified as

• Cyclic liquefaction-It occurs when the initial, static shear stress is exceeded by the cyclic shear stresses to produce a stress reversal. This may help attaining a condition of zero effective stress during which large deformations may develop.
• Cyclic mobility-Cyclic loads do not result in a reversal of shear stress and condition of zero effective stress does not occur. Deformations accumulate in each cycle of shear stress.