长期循环荷载作用下软土结构性损伤机理与累积变形特性研究

Under the long-term cyclic loading of traffic and waves, the foundation constructed on structural soft clay emerges a considerable additional post-construction settlement due to the stiffness soften of the structural soft clay. However, the current prediction method of the settlement is still based on empirical models, where the deformation mechanism of the settlement has not been well addressed yet. This study will investigate the fundamental characteristics of the structural damage within the soft clay. Both the undisturbed soil samples and reconstituted soil samples will be tested with using the cyclic triaxial compressive tests. The degradation of the microstructure of the soft clay under the cyclic loading will be intensively analyzed so that a reasonable variable can be proposed to characterize the damage of the soil structure. Based on damage mechanics and the second law of thermodynamics, the inherent relationship will be revealed between the damage variables under the cyclic loading, dissipated energy of the cementitious structure, stiffness soften index and the accumulated plastic strain. This project will establish a damage evolution model for the cementitious soil structure and reveal the soil structural dynamic damage mechanism. In addition, by proposing a dynamic elastic-plastic constitutive model with the consideration of the soft clay structure damage, a finite element numerical model will be formulated to simulate and analyze the soil structural degradation under cycling loading. The finite element model will also be employed to study the influence of the original soil structure, the properties of the cyclic loading and the drainage conditions on the accumulated deformation. Finally, based on the testing results and mechanistic models proposed in this study, a calculating methodology will be provided to estimate the accumulated deformation of the structural soft soil foundation under a long-term cyclic loading. The productions of this study will be capable of predicting the post-construction settlement for those foundations built on soft ground. Consequently effective measures will be suggested based on the results of this study to control and reduce the long-term settlement for the foundations under construction as well as the structures on schedule.

交通、波浪等长期循环荷载作用下结构性软土地基由于刚度软化产生可观的工后附加沉降，而目前对其计算预测处于经验模型阶段，变形机理研究存在较大缺陷。本项目针对结构破损导致地基循环软化的本质特征，通过原状样与重塑样在循环加载三轴剪切后细观结构变化分析，确定合理表征土结构损伤的变量；基于损伤力学理论与热力学第二定律，探明循环荷载作用过程中损伤变量、胶结结构破损耗散能、刚度软化指数、累积塑性应变之间的内在关系，建立胶结结构的损伤演化方程，揭示土结构性动力损伤机理与演化规律；进而考虑土结构损伤效应，在临界状态理论框架内建立弹塑性动力本构模型，并实现高效稳定的数值算法，与有限元相结合分析初始结构性强弱、动静荷载参数和排水条件对累积变形的影响规律，提出长期循环荷载下结构性软土地基的累积变形计算方法。研究成果可为软土地基上已建工程的工后沉降合理预测、在建和拟建工程采取有效措施控制和减小长期沉降提供科学依据。

以软土地区交通荷载引起的长期沉降给行车安全带来的隐患和危害为背景，从影响力学特性的土结构性本质特征入手，通过宏观土力学试验、微观结构试验和理论分析，提出了天然沉积软黏土的结构性评价参数及其取值范围，确定了合理表征土结构损伤的变量，揭示了不同应力路径下结构性土的损伤机理；探讨了结构性参数与压缩指标之间的关系，建立了考虑结构性影响的压缩模型与天然地基的沉降计算方法；分析了原状样和重塑样在压缩变形过程中渗透系数随孔隙比减小的变化模式，给出了双对数渗透模型，综合压缩模型建立了任意荷载下天然结构性软土地基一维非线性固结分析方法。进一步制备了不同胶结强度和初始孔隙比的人工结构性土，开展了人工结构性土与相应重塑土的动三轴试验，分析了土体胶结强度、初始孔隙比、围压和动应力幅值对累积变形和动强度的影响规律，提出了考虑结构性影响的天然沉积软土累积变形预测模型；研究了不同幅值和频率的动荷载作用下软黏土强度弱化规律。再结合结构性参数与损伤变量，构造了屈服后阶段的结构性演化方程，建立了考虑结构性和初始各向异性影响的天然沉积粘土弹塑性损伤本构模型，并实现了模型的程序化，分析了初始结构性大小、结构破损速率和施工扰动度对软土地基变形和孔压发展规律的影响。研究成果发表学术论文9篇，其中SCI收录2篇、EI收录7篇，获铁道学会科学技术一等奖（第十完成人）。研究成果为进一步明确结构性对土体静动力特性的影响提供数据支撑，并为采取有效措施控制和减小软土路基长期运营沉降提供理论依据。