LIQUEFACTION

         Liquefaction is the rapid loss of shear strength in cohesionless soils subjected to dynamic loading such as from an earthquake. Or we can say that it is a state of saturated cohesionless soil when its entire shear strength is reduced to zero due to pore water pressure caused by vibration during an earthquake. Liquefaction is often accompanied by sand boils, which are made of liquefied sand ejected from the ground as shown. If the sand boils are obtained we are sure liquefaction has occurred.

         Liquefaction depends on the nature, magnitude and type of dynamic loading. An entire stratum may be liquefied at the same time under shock loading, or liquefaction may start at the top and proceed downward with steady-state vibrations. Soils that have liquefied in the past may be subject to re-liquefaction depending on the extent of densification of both underlying and adjacent soils, the level of saturation following an earthquake, and the magnitude of vibration from previous earthquake motions.

TYPES OF LIQUEFACTION

Following are the types of liquefaction :

1. COMPLETE LIQUEFACTION: When the shear strength falls to nearly zero, i.e., when complete transfer of intergranular stresses to pore water pressure takes place. The sand water mixture behaves like a viscous liquid after complete liquefaction.

2. PARTIAL LIQUEFACTION: When shear strength of soil drops to a lower than normal value. i.e., when transfer of stress is incomplete, there is a partial loss of strength and partial liquefaction occurs.

3. FLOW LIQUEFACTION: It occurs when the static shear stresses in the soil exceed the shear strength of liquefied soil. This usually leads to large and sudden shear movements in the soil.

4. CYCLIC MOBILITY: It occurs when the static shear stresses are slightly less than the liquefied shear strength, but the static plus dynamic stresses are greater than the liquefied shear  strength. This produces incremental shear movements that are generally not as dramatic as flow liquefaction, but still can be a source of great damage.

                                       Flow Liquefaction

            Flow liquefaction is a phenomenon in which the static equilibrium is destroyed by static or dynamic loads in a soil deposit with low residual strength. Residual strength is the strength of a liquefied soil. Static loading, for example, can be applied by new buildings on a slope that exert additional forces on the soil beneath the foundations. Earthquakes, blasting, and pile driving are all example of dynamic loads that could trigger flow liquefaction. Once triggered, the strength of a soil susceptible to flow liquefaction is no longer sufficient to withstand the static stresses that were acting on the soil before the disturbance.

              An analogy can be seen in the situation, where the static stability of a ski jumper in the starting gate is disturbed when the jumper pushes himself from the start seat. After this relatively small disturbance, the static driving force caused by gravity, being greater than the frictional resisting force between the ski and snow, causes the skier to accelerate down the ramp. The path that brings the ski jumper to an unstable state is analogous to the static or dynamic disturbance that triggers flow liquefaction - in both cases, a relatively small disturbance proceeds an instability that allows gravity to take over and produce large, rapid movements.

             Failures caused by flow liquefaction are often characterized by large and rapid movements which can produce the type of disastrous effects experienced by the Kawagishi-cho apartment buildings, which suffered a remarkable bearing capacity failure during the Niigata Earthquake 1964. The Turnagain Heights landslide, Alaska Earthquake 1964 which is thought to be triggered by liquefaction of sand lenses in the 130-acre slide area provides another example of flow liquefaction. Sheffield Dam suffered a flow failure triggered by the Santa Barbara Earthquake in 1925. A 300 ft section (of the 720 feet long dam) moved as much as 100 ft downstream. The dam consisted mainly of silty sands and sandy silts excavated from the reservoir and compacted by routing construction equipment over the fill (Seed, 1968).
            As these case histories illustrate, flow failures, can involve the flow of considerable volumes of material, which undergoes very large movements that are actually driven by static stresses. As described in the state criteria section, the disturbance needed to trigger flow liquefaction can, in some instances, be very small. Read more about the initiation of flow liquefaction.