2.1 General literature review
A more exact definition given by Sladen et al (1985)6 states that “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”
Soils tend to diminish in volume when subjected to shearing stresses. The soil grains are inclined to configure themselves into denser packing with less space inside the voids, as water is forced to move out of the pore spaces. If this pore water is prevented from being drained, then the pore water pressure increases with the shearing load. Therefore, there is a transfer of stress, in other words, effective stress, and hence shearing resistance of the soil decrease. In case the static, driving shear stress is greater than the shear resistance of the soil, then it undergoes deformations, which we call liquefaction. It is possible to observe liquefaction of loose, cohesionless soils under monotonic as well as cyclic shear loads.
When dense sands are sheared monotonically, the soil gets first compressed and then dilated, as sand particles move up and over one another. When dense saturated sands are sheared, impeding the pore water drainage, their tendency to increase in volume results in the pore water pressure to decrease and the effective stress and shear strength to increase. When dense sand is subjected to cyclic small shear strains under undrained pore water conditions, excess pore water pressure may be generated in each load cycle, resulting in softened and the accumulated deformations. However, at lager shear strains, increase in volume relieves the excess pore water pressure, leading to an increased shear resistance of the soil.
In case large deformations are prevented after initial liquefaction due to increased undrained shear strength, then this is called “limited liquefaction” (Finn 1990)7. 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)8. 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)9. 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)10 and Robertson (1994)11, a dilative soil can attain a state of zero effective stress and shear resistance. Cyclic loads may produce 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)11 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 approached). However, the deformations cease 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.
Compiling all these ground failure mechanisms, Robertson (1994) and Robertson et al(1994)11 have suggested a complete classification system to define “soil liquefaction”. The latest put forward by Robertson and Fear (1996)12 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.
None of the definition or classification systems appears to be entirely satisfactorily. Hence, a broad definition of soil liquefaction will be adopted for our future study. As defined by the National Research Council’s Committee on Earthquake Engineering (1985)13, soil liquefaction is defined as the phenomena in which there is a loss of shearing resistance or the development of excessive strains as a result of transient or repeated disturbance of saturated cohesionless soils.
2.2 Susceptibility of Soils to Liquefaction during Earthquakes
Liquefaction is most commonly observed in shallow, loose, saturated cohesionless soils subjected to strong ground motions during earthquakes. Unsaturated soils are not subject to liquefaction as volume compression does not generate excess pore water pressure. Liquefaction and large deformations are more associated with contractive soils, while cyclic softening and limited deformations are more likely with expansive soils. In practice, the liquefaction potential in a given soil deposit during an earthquake is often evaluated using in-situ penetration tests and empirical procedures.
Since liquefaction phenomena arises due to the tendency of soil grains to rearrange when sheared, any factor that prevents the movement of soil grains will increase the liquefaction resistance of a soil deposit. Particle cementation, soil fabric, and aging are some of the major factors that can prevent soil particle movement.
Stress history is also crucial in determining the liquefaction resistance of a soil. For example, soil deposits with an initial static shear stress, in other words, anisotropic consolidation conditions are generally
more resistant to pore water pressure generation (Seed 1979)9, although static shear stresses may lead to greater deformations as liquefaction gets initiated.
Over consolidated soils (i.e. the soils subjected to greater static pressures in the past) are more resistant to particle rearrangement and hence liquefaction, because the soil grains tends to be in a more stable arrangement.
Liquefaction resistance of a soil deposit increases with depth as overburden pressure increases. That is why soil deposits deeper than about 15 m are rarely found to be liquefied (Krinitzky et al.1993)14
Characteristics of the soil grains, like distribution of shapes, sizes, shape, composition etc., influence the susceptibility of a soil to liquefy (Seed 1979)9. While sands or silts are most commonly observed to liquefy, gravelly soils have also been known to have liquefied.
Rounded soil particles of uniform size are mostly susceptible to liquefaction (Poulus et al. 1985)15. Well graded soils are less likely to liquefaction due to their stable inter-locking configuration. Natural silty sands tend to be deposited in a looser state, and hence are more likely to display contractive shear behavior as compared to clear sands.
Clays with appreciable plasticity are resistant to relative movement of particles during shear cyclic shear loading and hence are usually not prone to pore water pressure generation and liquefaction. Soils with an appreciable plastic content are rarely observed to liquefy in earthquakes. Ishihara’s (1993)16 theory is that, non-plastic soil fines with dry surface texture do not yield adhesion and hence do not provide appreciable resistance to particle rearrangement and liquefaction. Koester (1994)17 stated that sandy soils with appreciable fines content may be inherently collapsible, perhaps because of greater compressibility of the fines between the sand grains.
Permeability also plays an important role in liquefaction. When movement of pore water within the soil is slowed down by low permeability, pore water pressures are likely to form during the cyclic loading. Soils with large non-plastic fines content are more likely to get liquefied as the fines inhibit drainage of excess pore pressures. The permeability of surrounding soils also has an impact the vulnerability of the soil deposit. Less pervious soils such as clay can prevent the rapid dissipation of excess pore water pressures that may have generated in the adjacent saturated sand deposit. Sufficient drainage above or below a saturated deposit may inhibit the accumulation of
excess pore water pressure and hence liquefaction. Gravelly soils are less prone to liquefaction due to a relatively high permeability unless pore water drainage is impeded by less pervious, adjoining deposits.
2.3 Ground Failure Resulting from Soil Liquefaction
The National Research Council (Liquefaction…1985)13 lists eight types of ground failure commonly associated with the soil liquefaction during earthquakes:
· Sand boils resulting in land subsidence accompanied by relatively minor change.
· Failure of retaining walls due to increased lateral loads from liquefied backfill or loss of support from the liquefied foundation soils.
· Ground settlement, generally linked with some other failure mechanism.
· Flow failures of slopes resulting in large down slope movements of a soil mass.
· Buoyant rise of buried structures such as tanks.
· Lateral spreads resulting from the lateral movements of gently sloping ground.
· Loss of bearing capacity resulting in foundation failures.
· Ground oscillation involving back and forth displacements of intact blocks of surface soil.
It can be said that the nature and severity of soil liquefaction damage is a function of both reduced shear strength and the magnitude of the static shear loads acting on the soil deposit. When the reduced strength of a liquefied soil deposit becomes less than the driving shear loads, there is a loss of stability leading to extensive ground failures or flow slides. And if the shear strength is greater than the driving shear stresses only limited shear deformations are likely to occur, perhaps due to the expansion at larger strains. On level ground with no shear stresses acting on it, excess pore water pressures may come out to the surface, leading to the formation of sand boils, while the venting of liquefied soil deposits may result in settlements, damages are generally not extensive in the absence of static shear loads.
Ground failures associated with the phenomena of liquefaction under cyclic loading can be classified in a broader sense as (Liquefaction… 1985: Robertson et al.1992)18:
(1) Flow failures-It is observed when the liquefaction of loose, contractive soils (i.e. the soils with no increase in strength at larger shear strains) results in very large deformations.
(2) Deformation failures-It is observed when there is a gain in shear resistance of the liquefied soil at larger strain, resulting in limited deformations but no loss of stability.
However, putting an end to the confusion in terminology, all types of ground failure resulting from built-up pore water pressure and consequent loss in the shear strength of the soils during cyclic loading is commonly termed as liquefaction.