往复交通动载下饱和软黏土地基安定性机理研究

Over several past decades, an empirical approach based on experimental observations is used to predict traffic-induced pavement settlement, regardless of theoretical analysis of dynamic strength. This research program aims to explore the shakedown behavior of saturated soft clay, within the framework of low limit analysis (Melan’s) shakedown theorem. Meanwhile, experimental studies and dynamic numerical method are also employed in the research program. By incorporating a plastic hardening yield criterion, the classical Melan’s shakedown theorem will be developed to unify elastic shakedown limit, plastic shakedown limit and intermediate shakedown state into newly-developed kinematic-hardening yield shakedown theorem. The IEB(Infinite Element Boundary)- FEM technique is developed to simulate the dynamic stress responses of saturated soft ground to moving loads. A series of cyclic torsion shear tests along the pavement field stress are performed to achieve the traffic-load- induced permanent plastic strain. The residual stress (resulting in residual elastic strain) distribution at the shakedown limit state are available by searching for the maximum yield boundaries based on the shakedown yield criterion. An equivalent viscoplastic FEM method, in which the residual strain is treated as initial strain in the iteration of FEM, is developed to compute the long-term traffic-induced irrecoverable deformation. The framework of shake-down theorem based on an allowable amount of settlement is built for the design of pavement in the soft clay ground. This research program aims to propose a rational technical guideline for a positive evolution from an empirical approach to a theoretical framework used for the design of pavement structure.

长期以来，交通动载下地基累计沉降计算是基于试验观察的经验方法，并独立于动力强度分析。本课题拟将Melan下限分析安定理论引入到饱和软黏土路基分析中，结合试验研究及动力数值分析探讨长期往复交通动载下饱和软黏土的安定性机理。将饱和黏土塑性硬化机理引入到Melan安定理论框架中，将经典的弹性安定、塑性安定及其中间安定状态统一在运动硬化屈服的安定理论框架中。采用无限元人工边界的动力有限元技术模拟高速移动下饱和地基的动应力效应，基于场地动应力路径开展循环扭剪试验以获取残余塑性应变，基于安定极限准则获得长期往复交通动载下地基中极限残余应力（以获得残余弹性应变），将总残余应变视作弹塑性数值迭代的初应变建立路基运营沉降的等效黏塑性有限元预测方法，研究交通动载下软黏土弹-塑性安定状态对应的累计变形，形成基于变形控制的软黏土地基安定性理论框架，为软黏土路基结构设计从经验方法向理论框架转变提供理论支撑。

往复交通动载下，软土路基产生过大沉降或沉降差，进而诱发路基沉陷、车辙、裂缝等工程灾害，目前路基灾害控制的设计方法是以工后沉降允许值为控制目标。然而，由于交通荷载属于大数目往复移动荷载，循环次数可达几十万乃至几百万次，荷载移动诱发主应力轴偏转诱发地基土刚度弱化、车辆制动及路面不平顺性还将在路基中产生不可忽略的动力响应。对于这一复杂的交通岩土课题，既有的试验技术无法再现交通动载下地基土真实应力路径，现有沉降计算理论预精度差，无法为路基工后沉降的定量控制提供技术支撑，同时又缺乏可靠的动力强度理论对交通动载下路基极限承载性能予以合理的预估。.本项目综合应用了解析理论与数值方法，研发了循环扭剪单元试验、常重力及超重力软黏土路基模型试验技术，并提出了动力安定下限理论，针对交通动载下软黏土地基的动应力响应机制、累计变形（孔压）演变规律、运营沉降及差异沉降的计算方法，软黏土安定行为的判别准则及路基安定极限荷载的求解方法等关键性问题开展了系统的研究。首先，开展高速移动荷载下软土路基动应力响应分析，分别应用解析与数值方法建立了车辆-路面（基）-地基动力耦合体系的分析模型，获得了交通动载下柔性与刚性层状路基体系的三维动应力响应规律；其次，开展了土单元动态扭剪试验、常重力与超重力模型试验，从不同尺度揭示交通动载下饱和软黏土的变形弱化机理及累计变形演化规律，获得了大数目交通循环动载下软黏土安定状态的判别方法；此外，基于循环剪切单元体试验，建立了交通动载下饱和土累计变形（孔压）的显示模型，并采用初应变有限元技术，建立了交通动载下饱和软黏土路基沉降（沉降差）的定量分析方法；最后，以经典的Melan静力下限安定理论为基础，构建了动力下限安定理论框架，发展了一套无限元人工边界的动力有限元技术，实现了高速交通动载下软土路基动力安定极限荷载的定量评估。