| Summary: | 碩士 === 國立臺灣大學 === 土木工程學研究所 === 89 === The Chi-Chi earthquake, with a magnitude of 7.3 on Richter scale, happened on September 21, 1999 in Taiwan. It led to a large scale of soil liquefaction in the Yuan-Lin area, Chunghwa county, which caused sand boiling, road subsidence or rupture, and foundation subsidence and tilt. The National Science Council appointed MAA Group Consulting Engineers to investigate the liquefaction in Yuan-Lin and its neighboring towns, Sheh-Tou and Da-Chun, and to conduct in-situ tests and measurements for soil properties in these areas. During the subsurface exploration, ”undisturbed” soil samples were acquired with thin-walled tubes. In this study, the cyclic triaxial tests were performed to investigate the stiffness change of the soil stiffness under cyclic loading due to the excess pore pressure excitation. Meanwhile, the liquefaction potential of the soil in the Yuan-Lin area is also assessed.
According to the experiment results, the weak sandy soils in the shallow layer would be liquefied under the cyclic loading equivalent to the shaking induced by the Chi-Chi earthquake. The higher the liquefaction resistance of a sand, the less the stiffness reduction under seismic shaking. But when the cyclic shear stress ratio increases or dry density decreases, the degree of stiffness reduction tends to be getting larger.
For clayey soils in the Yuan-Lin area, the shear modulus also reduces under the earthquake loading, similar to the response of the sandy soil. However, in the early stage of excess pore pressure excitation, the degree of stiffness reduction is much higher than that of the sandy soil. For the sandy soil with a large amount of fines content, or the clayey soil with sandy silt seams, the way of stiffness reduction, tends to follow of sandy soil or clayey soil. The test results depending on the distribution of the coarse and fine grains show that, the effective stress is the most important factor on the soil stiffness reduction. But, under large shear strain that the stiffness is significantly affected by the shear strain amplitude.
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