Abstract: Liquefaction is the cause of many earthquake-induced failures in loose to semi-dense saturated deposits. Most recently published works have been focused on the liquefaction potential of clean sands, but the studies on silt and silty sands particularly the effect of cyclic stress ratio (CSR) on the critical silt content in the evaluation of sand liquefaction potential with 40% fine grains and more, have received less attention. Hence, the present work attempts to determine the effects of CSR, backpressure, the percentage of non-plastic fine-grain contents, and the effect of pressure on the saturated soil using cyclic triaxial experiment at a constant confining pressure. Samples were prepared by mixing 161-Firoozkuh Sand with three different amounts of silt including 0, 30, and 60 wt%. The experiment continued by the “wet tamping method” in which samples were made with a diameter of 5 cm and a height of 10 cm. All samples were compacted under constant confining pressure of 100 kPa at a relative density of 32%, following the recommendation of ASTM.D-5311. According to the obtained results, with increasing the silt in the sand by 30%, a decrease in liquefaction resistance occurred, but with a further increase of silt to sandy soil with 60% silt, an increase in liquefaction resistance was observed. This indicates that systematic progress from pure sand to sand with 30% silt, led the fine particles of silt to fill the voids between coarser particles of sand. This resulted in reducing the soil drainage capacity during earthquake vibrations or cyclic loading. Therefore, the liquefaction potential increases in these conditions, but the sand-like behavior still prevails up to 30% fine-grained, and then a further increase beyond 30% changes the soil behavior and the soil adopts fine-grained behavior, which reduces the liquefaction potential. The effects of CSR on liquefaction behavior of all soil samples of this study have been evident that with changes in the CSR, the percentage of fine particles that cause the highest pore water pressure, respectively, change. In this regard, the equation of liquefaction curve is presented in the range of sandy to loamy sands. The results showed that by increasing the silt content up to 30%, a decrease in liquefaction resistance occurred, and then a further increase in the silt content caused an increase in the resistance. It was observed that by increasing fine grains up to 30%, the behavior of sand is predominant, however, when the content of fine grains exceeds 30%, the behavior of fine grains and silt is dominant. Collectively, the results show that changes in the value of CSR causes a regular change in the percentage of fine particles that cause the highest pore water pressure. On the other hand, the effect of backpressure on the soil skeleton in the saturation state (B-value) of the samples on the result of liquefaction potential and the resulting strains was insignificant in sand with 60% silt and somewhat more pronounced in sand with 30% silt.

 
Keywords: Cyclic triaxial testing, Liquefaction potential, Silty sand, Cyclic Stress Ratio, Non-Plastic