Evaluation of liquefaction resistance of unsaturated clayey sand

Soil mixtures such as clayey sands, silty sands, or clayey silts are among the categories of common natural soils observed in liquefied sites. The substantial amount of liquefactions discussed in previous contributions appeared to occur in sands containing plastic fines. In saturated soils, a notable amount of experimental studies were performed in past to examine the influence of fine content on the liquefaction potential of sands. In spite to the occurrence of liquefaction in unsaturated zones due to ground motions observed in past, there are few amount of experimental data that relate the potential of liquefaction with degree of saturation, Sr, specifically for soils with high degrees of saturations. In this article, the results of a series of careful laboratory test program is represented to determine the liquefaction behaviour of a sand mixed with a range of kaolinite including zero to 30 perect at elevated saturation conditions. This is experimentally achieved using a double-walled suction controlled triaxial cell specifically developed to conduct cyclic triaxial tests at high degree of saturations that were 80, 85, 90, 95 and 100 percent. The stress-strain behaviour of the soil is represented and compared with respect to the amount existing data available in the literature. The variation of excess pore water pressure during the cyclic loading indicate that, in saturated pure sand, the generation of excess pore water pressure was mainly occurred at higher cycle of loads while, in saturated specimens with 30% clay content, it is observed from the early cycle of loading stage. Also a change in suction of specimen during cyclic loading under undrained condition is observed. Due to the presence of air in unsaturated soil volume change occurs during cyclic loading. It can be observed that void ratio decrease while saturation ratio increases. Matric suction is almost constant during cyclic loading until pore air pressure reached at maximum value and by increasing pore water pressure matric suction decrease. During cyclic loading axial strain is small until pore water pressure reached the effective confining pressure. In this case sudden increase in axial strain occurs and liquefaction starts. Accordingly, it is seen that during the cyclic loads all tested specimens reached to the liquefaction state. The liquefaction potential within the soil is represented according to CSR20 and is found to be a function of fine content. It is appeared to be initially decreased within the increment of fine content up to 20%, and consequently, it is slightly increased with increasing the percentage of fines up to 30 percent. The above behaviour aspect was obvious in all the range of degree of saturation considered. Additionally, it is seen that at a given fine content, a slight desaturation of specimens caused a significant increment in the liquefaction resistance ratio (LRR) within the soil and was more evident within the decrement of the fine content. The trend observed for the variation of liquefaction resistance ratio versus the potential volumetric strains in pure sands appeared to be consistent to the logarithmic relationship as suggested in the literature.