Effect on Cyclic Response and Liquefaction Resistance Due to De-Saturation of Sand

It is well known that the presence of air pockets in saturated sand increases its shear strength as well as the resistance to liquefaction. Regardless of the above condition, until now little work was done in this area possibly because the air pockets can be unstable when the granular soils are submerged over prolonged period below ground water table level. In this paper, intrusion of air and its dissolution over prolonged period is reviewed using Centrifuge test. Cyclic triaxial tests were conducted on samples of sand pre-introduced with air pockets to investigate the cyclic response and resistance to liquefaction. The cyclic triaxial test setup and method used to introduce air in sand samples are then described. The tests were conducted on large 100mm diameter and 200mm cylindrical specimens. Results indicate that when the degree of saturation is reduced only slightly, there is considerable increase in number of cycles to induce liquefaction. Also proposed in this paper is the modulus increment ratio, G_r defined as the ratio of shear modulus of de-saturated sample to that of saturated sample. G_r was shown to approach unity at shear strains less than 0.001 and more than 0.012. In between the two limits, the shear modulus of de-saturated samples reached a transient peak for the entire range of degree of saturation investigated in this study. Identical trend was observed in the plots of damping ratio versus shear strain.