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年日本新泻地震，突出了前期降雨对斜坡地震失稳的作用。在地震发生的前几天，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会导致土壤强度的降低。这些可能是导致该地区因地震而导致的斜坡坍塌的规模和数量的原因之一。不幸的是，大多数关于地震引起的斜坡破坏的研究都集中在极端条件下：干燥的斜坡或完全饱和的斜坡。关于饱和度对斜坡地基周向剪切响应影响的定量研究还很少。随着基于性能的设计的出现，人们认为考虑土的饱和度的影响可以更准确地评价土体在地震中的反应。在本研究中…在此基础上，对不同饱和度(S R)和初始静剪应力(σS)的土样进行了三组试验。所使用的样品是在山口县岩谷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，只使用了小于2 mm的颗粒。材料为粉细砂，Fc=10%。由于土壤水分特征曲线显示吸力在S_1=30%~60%之间发生突变，因此试验中考虑了S_r=30%、60%和100%的试件。首先，通过单剪试验和在线拟动力地震反应试验，分析了不同倾角(即不同初始剪切应力比)和初始饱和度的Masado土的地震反应特性。结果表明，饱和程度越高和(或)坡度越陡，土体的循环抗剪强度越低，导致边坡的残余位移越大。其次，进行了不同饱和度和初始剪应力下的不排水循环三轴试验。结果表明，对于非饱和试件，没有观察到通常在完全饱和试件中观察到的大的强度退化。此外，初始静剪应力和外加动应力的组合决定了试件的破坏模式(拉伸或压缩)，最后，将这些试验结果与拟静力边坡稳定性分析和修正的Newmark滑块模型结合起来，估计了不同地震荷载下部分饱和路堤边坡的地震诱发位移的大小。虽然简化了计算，但实例计算清楚地显示了饱和度对路堤地震位移大小的影响。这说明了考虑度饱和度对路堤和边坡破坏后行为的影响的必要性。较少

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