반복삼축압축시험과 수치해석을 통한 모래의 액상화 거동특성 분석

Abstract

Liquefaction refers to losing shear strength as pore water stress within the ground rises and effective stress drops. In other words, effective stress becomes zero and soil flows like liquid by losing shear strength. The Pohang Earthquake that occurred in 2017 showed that Korea is no longer a safe zone from earthquakes. Since then, various studies on liquefaction have been conducted in Korea, including the development of the risk of liquefaction suitable for domestic ground conditions. The government, based on these research results, sought to develop a liquefaction evaluation technique and common standard to suit the Korean situation, and in 2018, the seismic design standard was revised to suit the domestic ground conditions, and accordingly, the liquefaction evaluation standard was also revised. Since the laboratory experiment for the evaluation of liquefaction basically uses the repeated triaxial compression test or the repeated shear test, in this study, the Cyclic Resistance Ratio(CRR) values were compared and analyzed using the repeated triaxial compression equipment. The repeated triaxial compression tests were performed to analyze the characteristics of the liquefaction strength according to the particle size distribution of the sand and the content of fine particles by dividing the relative density of each sample into three sections of 40%, 55%, and 70%. In addition, a ground motion analysis was performed using FLAC V.7.0., a ground analysis program, to verify the reliability of the repeated triaxial compression test results and, to compensate for the limitations of the existing liquefaction equipments, additional analysis was performed by varying frequency domain and confining pressure. The existing confining pressure was 100kPa, and the calculated depth based on the conditions of and unit weight in water is 12-13 m. The confining pressures for 50kPa and 150kPa are about 6-7m and 18-19m, respectively. Since the liquefaction evaluation standard is less than 20m in depth, the behavioral characteristics of sandy ground for liquefaction from shallow to deep were analyzed by performing experiments additionally in the range of 50kPa and 150kPa. The results of laboratory tests showed that the measured liquefaction strength of weathered soil is higher than that of Jumunjin Standard Sand, suggesting that the better particle size distribution, due to smaller gap, is associated with being stronger to liquefaction resistance. On the contrary, the higher fine contents tended to decrease the liquefaction strength, which suggested that fine particles filled between the sand particles caused the particles to slip.