【作者】 周云东；

【导师】 刘汉龙； 高玉峰；

【作者基本信息】 河海大学 ， 岩土工程， 2003， 博士

【摘要】 饱水砂土地震液化引起的地面大变形会对液化区的各种结构产生灾难性的破坏，在过去的几次大地震中均发现有因地面大变形而导致的破坏现象，但目前对该问题的研究较为欠缺，有关饱水砂土液化问题现有的研究基本都局限在初始液化之前，在饱水砂土地震液化的影响因素、产生的机理及条件、液化势的判别等方面积累了大量的研究成果，而液化后问题的研究才处于起步阶段。本文结合国家自然科学基金“地震液化引起的地面大位移研究”(No．59809004)对饱水砂土液化后的相关问题进行了研究。主要工作内容如下： 参与研制了振动扭剪全自动多功能三轴仪，并利用其独特试验功能设计了一套饱水砂土液化后特性的试验方法，试验过程中模拟地震作用的动加载过程及模拟大变形发生的静加载过程均采用应力控制的方式进行，试验方法跟现场条件更为接近并用该试验方法对相对密实度、固结压力、液化度等对液化后变形特性的影响进行了研究。结果表明液化后加载时变形将会表现出截然不同的两段，即低强度段和强度恢复段，其中低强度段试样的强度极低，在剪应力增加极小的情况下土体将发生大的变形，随试样的相对密实度不同低强度段的应变可达百分之几到百分之几十，该段试样中的有效应力近似为零；而强度恢复段随应变的增长试样中的有效应力将逐步上升，土体的模量逐步得到恢复直至与液化前的模量相近。砂土液化后模量随应变增加而增大的特性与常规三轴试验中模量随应变增大而减小的特性不同，相对密实度、液化度、初始有效固结压力等都以特定的方式对影响着液化后的变形特性。 采用常规动三轴试验对饱水砂土动加载后的再固结体变特性及其相对密实度、动荷载波形等影响因素进行了试验研究，重点对初始有效固结压力对再固结体变的影响进行了研究，研究发现动荷载波形及频率不会对土体的再固结体变特性构成影响，在动荷载使得土体达到初始液化之前固结压力对再固结体变的影响也较小，而当土体达到初始液化之后，固结压力对再固结体变的影响不可忽略，根据大量有关固结压力影响的试验结果提出了一个可考虑初始有效固结压力、相对密实度、液化度等影响的再固结体变计算公式。 根据动加载液化后静扭剪时试样将在近乎为零的有效应力下发生大的应变，以及液化后再固结时体变的绝大部分都发生于有效应力近似为零的阶段的试验现象，对饱水砂土液化后大变形机理进行了分析，提出了大变形的机理，指出动加载使得饱水砂土颗粒趋于紧密，土体中的水体有被压缩的趋势以及液化后砂土的剪胀特性是造成液化后特殊应力应变关系的根本原因，其中动加载导致的土体体变势反映了水体的压缩程度，可由再固结规律来描述，砂土液化后的剪胀特性对于一定颗粒特征及相对密实度的土体而言可以认为是维持不变的。摘要 基于大变形机理以及文中提出的再固结体变公式提出了一个饱水砂土液化后本构模型，模型不寻求液化后整个加载过程中的应力应变反应，将液化后过程分成低强度段和强度恢复段两段来分别进行表达，因为低强度段应变产生于极小的剪应力作用下因此认为该段应变是必然会发生的，强度恢复段应变的大小取决于土体所受剪应力的大小。该模型可考虑初始有效固结压力、相对密实度、液化度等对液化后变形特性的影响。 基于上述本构模型及分层总和法的概念提出了一个大变形简化预测分析方法，并用该方法给出了一个预测示例。更多还原

【Abstract】 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更多还原