Evaluation of Liquefaction Potential of Soils

It is widely recognized that earthquakes are among the most severe natural disasters causing significant damages such as failure of earth structure, settlement or tipping of buildings, lateral spreading of sloping ground and densification causing vertical settlements. The reasons for these failures can be attributed either due to the compaction of loose deposits of soils or by a phenomenon called liquefaction. The phenomenon of liquefaction is associated with a condition of zero effective stress due to progressive increase in pore water pressure resulting from the tendency to densification of the sand structure subjected to cyclic loading. The generation of excess pore pressure under undrained loading condition is a hallmark of all liquefaction phenomena. The relative incompressibility of the pore water makes the rapid compaction of the sand impossible. Instead, an excess pore water pressure develops whose value increases with the duration of cyclic loading and many a time these pressures only start dissipating after the ground shaking has ended. Due to its high potential to cause damages, this phenomenon of liquefaction during earthquakes has become a prime subject of concern in the geotechnical engineering. Catastrophic failures in recent earthquakes have provided a serious reminder that liquefaction of sandy soils and sands with large amount of non-plastic fines as a result of earthquake ground shaking poses a major threat to the safety of civil engineering structures. Primary seismological factors that control liquefaction are amplitude and frequency of the cyclic shear stress besides the duration of shaking. Whereas, the other site-specific factors that control development of liquefaction of soil are grain size distribution of the soil mass, relative density of the soil deposit, depth and thickness of different soil strata, depth of ground water table etc. Major landslides, lateral spreads, settling and tilting of buildings and failure of waterfront retaining structures were some of the observed excellent examples of liquefaction triggered by the recent Bhuj earthquake on 26 January 2001. In many places of earthquake-affected area several sand boils/water fountains (Figure 1) were developed indicating the occurrence of extensive liquefaction.