| Budget Amount *help |
¥2,730,000 (Direct Cost: ¥2,400,000、Indirect Cost: ¥330,000)
Fiscal Year 2007: ¥1,430,000 (Direct Cost: ¥1,100,000、Indirect Cost: ¥330,000)
Fiscal Year 2006: ¥1,300,000 (Direct Cost: ¥1,300,000)
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| Research Abstract |
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
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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
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