【作者】 王富强；

【导师】 张建民；

【作者基本信息】 清华大学 ， 土木工程， 2010， 博士

【摘要】 饱和砂土地震液化和液化后大变形问题是岩土工程抗震领域热点与难点研究课题之一。本文采用震害资料调研、材料试验和数值分析等手段,以自然排水条件下砂土液化变形规律和本构模型研究为重点,以“液化大变形与剪切吸水效应”的定量描述为核心,在下述方面取得新成果:(1)开发了一套较高精度的饱和砂土剪切吸水试验测控装置,完成了系统的单调与循环剪切吸水试验,揭示了部分吸水剪切、自由吸水剪切、强制吸水剪切三种不同排水剪切作用下饱和砂土可呈现出硬化、理想塑性、软化甚至流滑失稳的三类不同本构响应的特征规律,指出了剪切吸水效应本质上体现在使饱和砂土发生更明显体胀和剪切放大作用两方面,阐明了地震荷载作用下饱和砂土在液化前震动、液化后震动、震后超孔压扩散与消散的3个不同阶段上均可能发生大变形和趋向流滑失稳。(2)基于体应变划分和应变约束条件,揭示了上述的三类本构响应以及液化大变形和流滑失稳破坏的形成机理与产生条件,提出了考虑剪切吸水效应的临界应力状态方程,试验表明相对剪切吸水率是表征应力应变类型的有效指标。(3)基于液化后大变形物理机制以及可逆和不可逆剪胀的研究成果,引入物态相关概念和强度演变规律,采用边界面模型的理论框架,建立了可考虑剪切吸水效应、可描述从小变形到大变形的弹塑性循环本构模型,通过对不同类型试验结果的模拟,初步验证了该模型的有效性。(4)发展了本构模型的数值格式以及合理描述液化大变形和流滑失稳破坏问题的方法。对新泻地震饱和砂土地基液化侧向大变形实例的分析表明:该数值方法较好地实现了对液化大变形发展的“3个过程”(包括自然排水条件下震动循环剪切小变形到大变形的瞬态演变过程、震后伴随超孔压扩散和消散的变形累积发展过程以及伴随着整个变形的物态演化过程)的定量描述。(5)基于经过验证的数值方法,再现了美国下圣菲尔南多土坝震后数十秒发生流滑失稳破坏的发展过程和主要现象,计算分析得到的震害特征规律与Seed等的调查结果吻合较好,从而较为圆满地解释了该震害形成的主要原因。更多还原

【Abstract】 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.更多还原