Laboratory Study On Large Ground Deformation Induced By Earthquake Liquefaction

Damage to structures may be caused by large ground displacement due to seismic liquefaction. This kind of damage has been observed many times in several strong earthquakes in the past. However, the research concerned is limited. The state-of-art of sand liquefaction is mainly about the behavior of saturated sand under cyclic loading before initial liquefaction. There are many researches about the mechanism, influence factors, judging methods of sand liquefaction. And the research of these is relatively mature. When it comes to the research about the post-liquefaction behavior of saturated sand, the situation is not so famous. The current research is supported by the National Science Foundation project of Large Ground Displacement due to Seismic Liquefaction (No. 59809004), and in this thesis the basic aspects about the post-liquefaction behavior of saturated sand has been thoroughly investigated. The main contents of the current research are as follows:In order to investigate the behavior of saturated sand, a multi-functional tri-axial test equipment has been designed. Using it can do many kinds of dynamic/static tests. And a post-liquefaction test method has been put forward, in which the cyclic loading and the static loading courses are both controlled by stress mode. This method reflects the in-situ conditions well and truly, and a lot of tests have been done using this method. In these tests the effects of confining pressure, relative density, liquefaction severity etc. have been thoroughly investigated. It is found that the post-liquefaction behavior of saturated sand is distinctly different from those without dynamic loading. Large deformation will yield when the sand is liquefied, and the deformation is composed of two parts approximately. The two parts can be called as low-stiffness part and stiffness-recover part separately. The former is mainly developed when the effective confining pressure goes though zero in undrained loading. The stiffness is very low in this part, and the shear strain ranges from several percent to tens of percent. The latter is depending on the magnitude of shear stress. In this part the stiffness recovers step by step till it comes near to the stiffness of sand without dynamic loading. No matter what the relative density, confining pressure, liquefaction severities are, the post-liquefaction deformation will show the same characteristic. The factors mentioned above have their own affecting ways.A lot of tests have been done to investigate the reconsolidation characteristics of sand after dynamic loading. The research focused mainly on the effects of confining pressure. It is found that the effect is not distinctive before initial liquefaction, but when initial liquefaction occurs the effect of confining pressure can't be ignored. Based on test results, a new formula has been put forward. This formula can be used to calculate volumetric strain of reconsolidation after dynamic loading. And it can take the effects of confining pressure,relative density, liquefaction severity etc. into account.Based on the phenomena of post-liquefaction tests, the reason of post-liquefaction large deformation has been analyzed. It is pointed out that the compression of water in saturated sand caused by the densification of sand during dynamic loading and the dilation trend of liquefied sand are related to this. A new mechanism of large post-liquefaction deformation of saturated sand is established.According to the mechanism of large post-liquefaction deformation and the reconsolidation volumetric strain formula, a post-liquefaction constitutive model had been put forward. This model describes the post-liquefaction behavior of sand by two parts, corresponding to low-stiffness part and stiffness-recover part. The prediction results by the model show good consistency with the test results.A simplified large post-liquefaction deformation method has been put forward, which is designed to calculate the post-liquefaction deformation of horizontally-layered or near horizontally-layered...