基于统计力学原理的可液化土体流动性理论和分析方法

Based on the statistic mechanics principle, it is predicted that the macroscopic physical state of liquefiable soil can be viewed as the disorder of soil particle movement and array. Based on the laboratory tests in both element and filed scales to study the evaluation method of soil flowability and analyze the development of flowability. Micro force chain classification and characteristic force chain are determined using the discrete element simulation method. It is expected to prove that the disorder of characteristic force chain is approximately in accord with the Boltzmann distribution. The Boltzmann model is established to describe the flowability curve of the liquefiable soils. The generation mechanism of flowability is revealed from the viewpoint of the statistic mechanics. The liquefaction criterion is constructed based on the flowability. The relationship between the flowability level and shear wave velocity of soil is studied. The liquefaction discriminant (or graph) based on the flowability criterion and shear wave velocity is established and verified through case studies. The risk assessment method is proposed considering the flowability level and site characteristics. The pore pressure thixotropic fluid constitutive model is modified in order to apply in the whole process of the existence of the flowability. The yield stress of the liquefiable soil is determined according to the flowability threshold. The semi-theoretical and semi-empirical expression regarding the model parameters is given. On the basis of the existing fluid velocity distribution model, the numerical simulation method regarding the soil liquefaction effect is developed based on the pore pressure thixotropic fluid constitutive model and lattice Boltzmann method. The reliability of this method is verified through a large-scale shaking table test based on the liquefaction failure case of the buried pipelines. The influences of the flowability on the liquefaction failure are revealed. It is expected to provide a new theoretical basis and technical approach for the analysis on the soil liquefaction.

由统计力学原理推测可液化土物态为细观颗粒排列、运动无序度的宏观映射。以单元体和场地尺度室内试验研究土体流动性评价方法，分析流动性演变规律；以离散元模拟研究细观颗粒力链分级和特征力链确定方法，证明特征力链无序度近似符合玻尔兹曼分布，建立流动性曲线玻尔兹曼模型，揭示流动性产生和演变的统计力学机制；构建以流动性表征的液化准则，研究流动性水平与土体剪切波速关系，建立基于流动性准则和剪切波速的液化判别式(图)并实证，提出考虑流动性水平和场地特征的液化危险性评价方法；改进孔压触变流体本构以适用流动性显现的整个过程，依据流动性阈值确定屈服应力，给出模型参数确定方法。借鉴已有流体离散速度分布模型，发展基于孔压触变流体本构和格子-玻尔兹曼法的土体液化效应数值模拟技术，以地下管线液化破坏实例为背景开展大型振动台试验验证方法的可靠性，揭示土体流动性对液化效应影响规律。为土体液化分析提供新的理论基础和技术手段。

土体液化是岩土地震工程领域的热点问题。然而，现有的土体液化判别、分析及评价方法仍然存在以经验为主、缺少理论支撑的弊端，尤其是液化引起的大变形及液化土-结构相互作用模拟方面尚不能满足实际工程需求。为此，本项目提出可液化土体的流动性的概念，从统计力学原理出发，以单元体室内试验研究土体流动性评价方法，提出并定义了平均流动系数、流动性曲线、平均流动系数-孔压比关系曲线以及流动性水平等概念，深入分析了可液化土体的流动性演变规律及渐进液化特征；以离散元模拟研究细观颗粒力链分级和特征力链确定方法，证明了力链强度分布与土体振动孔压的相关性。继而，发现可液化土体的流动性演变过程符合玻尔兹曼分布律特征，实证了可液化土体流动性产生的机制。针对土体液化势分析，构建了描述土体液化前后动力学行为的统一触变模型，并对模型参数的敏感性进行了深入讨论，提出了参数确定方法。在此基础上，提出了基于流动性理论的可液化土体振动孔压增长模型以及基于统一触变模型的土体液化判别方法，并以300多个国内外液化实例进行了验证。设计并开展了可液化地基中综合管廊振动台模型试验以及土体液化侧向扩展下的桩基地震反应振动台模型试验。将可液化土体视为孔压触变流体，发展了基于流动性理论的土-结构相互作用数值模拟方法。通过试验及数值模拟的对比分析，验证了数值方法的可靠性，揭示了土体流动性演变过程中的土体大变形及土-结构率相关相互作用机制。