Research On Liquefaction Behavior And Constitutive Model Of Sand Under Naturally Drained Condition

Study on liquefaction and large post-liquefaction deformation of saturated sand is one of hot and difficult topics in research field of soil dynamics and geotechnical earthquake engineering. Based on literatures investigation, material tests and numerical analysis, objective of this dissertation is to study liquefaction behavior and constitutive model of saturated sand under naturally drained condition, with emphasis on quantitative evaluation of large post-liquefaction deformation considering'water-absorption effect in shearing'. The main achievements and conclusions can be drawn as follows.1. A new system suitable for water-absorption shear testing is developed and series of water-absorption shear tests of both monotonic and cyclic loading conditions are carried out. Three different types of stress-strain response, including strain harding, perfectly plastic and strain soften or instability, are found to be triggered under three different states of partial water-absorption, free water-absorption and compulsory water-absorption in shearing. Such different water-absorption effects in shearing are essentially to significantly increase the expansive volumetric strain and amplify the shear application. Special investigation is paid to a new finding that large deformation and instability can always occur within any one of three different stages including pre-liquefaction shaking, post-liquefaction shaking and post-shaking diffusion and dissipation of excess pore water pressure, provided that free and compulsory water-absorption in shearing is triggered.2. A new index, defined as'relative rate of water-absorption in shearing'is found to be an effective indicator to classify three different types of the stress-strain response mentioned above. Mechanisms and conditions are provided to explain and evaluate the occurrence and development for three different types of the stress-strain response. And a formulation is given for describing the critical stress states dependent on the relative rate of water-absorption in shearing.3. A new cyclic elastic-plastic constitutive model is established based on mechanism of large post-liquefaction deformation, concept of the reversible and irreversible dilatancy and bounding surface constitutive theoretical frame. The state-dependent concept and effect of the water-absorption in shearing on the critical stress state are also considered to reflect the changes in physical state of material and shear strength during the water-absorption in shearing. Effectiveness of present model is preliminarily checked through comparing the tested and calculated results for different experimental paths of nomotonic and cyclic loading in naturally drained conditions.4. A new numerical algorithm of the present constitutive model and related practical numerical method are developed for evaluating the large post-liquefaction deformation and flowsliding. Case study on lateral ground spreading during the 1964 Niigata earthquake is made to confirm the effectiveness of the numerical algorithm and method. It is shown that such a new developed numerical analysis can be used to well reproduce the three processes: (1) transient evolution process of small to large ground deformation during an earthquake in naturally drained conditions, (2) accumulative developing process of post-earthquake ground deformation during diffusion and dissipation of the excess pore water pressures, and (3) gradual change process of material physical states during development of deformation.5. Another case study on delayed flowsliding failure of the Lower San Fernando dam in earthquake was preformed well using the present numerical method. The calculated dynamic behaviors and deformation of the dam are shown to be considerably consistent with those observations and main investigations made by Seed et al in the past. Satisfactory conclusions are consequently obtained to explain the main reasons why heavy damages to the dam were induced.