Modelling of liquenfaction by numerical methods

Loss of bearing capacity is one of the major causes of earthquake damage on structures.Liquefaction phenomenon is an important factor leading to loss of bearing capacity when thesandy soil deposits comprise the foundation layers.In this study, a series of numerical analyses are carried out to shed light on factors affectingliquefaction and the mechanisms involved. The numerical analyses are performed with thecomputer code LASS III (Dikmen, 1979). The values of constants needed for the materialmodel employed are determined by performing a parametric study through a series ofanalysis.An extensive number of site response analyses are performed on a typical soil profile in whichthe values of parameters known to affect liquefaction are varied within expected ranges. Thesoil profile analysed comprised of an uniform sand deposit 20 m thick underlaid by bedrock.The 1999 Kocaeli Earthquake Arçelik strong motion record is used as the bedrock motion toanalyse the field behavior.The ground water level is considered to be shallow, and results of numerical analysis havedemonstrated the importance of ground water level on the development of liquefaction and itsdepth. The effects of the relative density of the sand deposit and its permeability are studiedextensively, being subjected to ground motions of different shaking intensities.The results of analyses have shown that for highly permeable sands (i.e. k = 10−2 m / s ) therisk of liquefaction is negligible regardless of density and shaking intensity if only onehorizontal component of the earthquake motion is considered in the analysis. When the sanddeposit is subjected to both horizontal components of the earthquake motion, a large adverseaffect on the pore pressure accumulation and development of liquefaction are observed. Thedepth of liquefaction is shown to be less than 10 m under strong ground motions (limited to 6-7 m in medium dense sands) when the soil deposit is subjected to only one horizontalcomponent, whereas it might exceed 15 m?s when both horizontal components are effective.The results of numerical analyses have also demonstrated that surcharge loading at the groundsurface can lead to a considerable degree of increase in liquefaction resistance and itseffective depth. On the other hand, presence of a less permeable layer in the sand deposit isshown to have an adverse effect, leading to development of large pore pressures under the lesspermeable layer and increase in the depth of liquefaction. A limited number of analyses haveindicated that the characteristics of the earthquake motion can also have a large influence onsoil behavior.Key Words: Liquefaction, numerical modelling, effective stress analysis