Research on the Mechanism of Slope Failures in Partially Saturated Slopes during Earthquakes

最近发生的大规模地震，如2004年日本新泻中越地震，突出了前期降雨对斜坡地震不稳定性的作用。地震发生前几天，该地区遭受了大量降雨。地面的饱和和地下水位的增加可能导致土壤强度的下降。这些可能导致了该地区地震引起的边坡破坏的规模和数量。不幸的是，大多数关于地震引起的边坡破坏的研究都集中在极端条件下：干燥的边坡或完全饱和的边坡。关于饱和程度对斜坡地基循环剪切响应影响的定量研究还不多。随着性能化设计的出现，人们认为考虑土体饱和程度的影响可以更准确地评价土体的地震反应。在此基础上，对不同饱和度（S_r）和初始静剪应力（σs）条件下的土样进行了3组试验。所使用的样品是在山口县岩国市获得的分解的花岗岩土壤，在日本更普遍地被称为“Masado”。请注意，由于测试设备的限制，仅使用小于2mm的粒度。物料为粉砂，粉粒含量为F_c=10%。由于土壤水分特征曲线在S_1=30% ~ 60%范围内吸力发生突变，试验中分别考虑S_r=30%、60%和100%的样品。首先通过简单剪切试验和在线拟动力地震反应试验，分析不同倾角（即不同初始剪应力比）和初始饱和度下Masado土的地震反应特征。结果表明：饱和程度越高，边坡越陡，循环抗剪强度越低，边坡残余位移越大；然后进行了不同饱和度和初始剪应力条件下的不排水循环三轴试验。结果表明，在非饱和状态下，未出现完全饱和状态下典型的大强度退化现象。此外，初始静剪应力和外加动应力的组合决定了试样的破坏模式（拉伸或压缩）。最后，将这些实验结果与拟静力边坡稳定性分析和修正版Newmark滑块模型相结合，估计了不同地震荷载作用下部分饱和路堤边坡的地震诱发位移大小。虽然在本质上是简化的，但算例清楚地显示了饱和程度对堤防地震位移大小的影响。这说明需要考虑饱和度对路堤和边坡破坏后行为的影响。少

Recent large-scale earthquakes, such as the 2004 Niigata Chuetsu Earthquake in Japan, have highlighted the role of antecedent rainfall on the seismic instability of slopes. Days prior to the occurrence of the earthquake, the region was subjected to large amount of rainfall. The saturation of the ground and the increase in water table could have led to decrease in the strength of the soil. These may have contributed to the scale and number of earthquake-induced failure of slopes in the region. Unfortunately, most studies concerning thquake-induced slope failures have focused on extreme conditions : dry slopes or completely saturated slopes. There has not been much quantitative investigation on the effect of degree of saturation on the cyclic shear response of sloping grounds. With the advent of performance-based design, it was believed that considering the effect of degree of saturation of the soil could lead to a more accurate evaluation of soil response during earthquakes.In this rese … More arch, three sets of tests were conducted on soil samples under varying degrees of saturation (S_r) and initial static shear stresses (σs). The samples employed were decomposed granite soils, more popularly referred to as "Masado" in Japan, obtained in Iwakuni City, Yamaguchi Prefecture. Note that only particle sizes smaller than 2 mm were used because of the limitation of testing equipment. The material was silty sand with fines content, F_c=10%. Since the soil water characteristic curve indicated abrupt change in suction from S_1=30%-60%, samples with S_r=30%, 60% and 100% were considered in the tests.Initially, simple shear tests and online pseudo-dynamic earthquake response tests were conducted to analyze the earthquake response characteristics of Masado soils with different inclinations (i.e., different initial shear stress ratios) and initial degree of saturation. Results indicated that the higher the degree of saturation and/or the steeper the slope, the lower the cyclic shear strength, leading to larger residual displacements of slopes. Next, undrained cyclic triaxial tests under different degrees of saturation and initial shear stresses were conducted. The results showed that for unsaturated specimens, large strength degradation typically observed in fully saturated specimens were not observed. Moreover, the combination of initial static shear stress and applied dynamic stress determined the mode of failure (extension or compression) of the specimen.Finally, these experimental results were used in conjunction with pseudo-static slope stability analysis and a modified version of the Newmark sliding block model to estimate the magnitude of seismic-induced displacements of partially saturated embankment slopes under different earthquake loads. Although simplified in nature, the example calculations clearly showed the effect of degree of saturation on the magnitude of seismic-induced displacement of embankments. These illustrated the need in considering the effect of degree saturation on the post-failure behavior of embankments and slopes. Less