多维度循环应力路径下超固结软粘土动力特性和长期循环应变累积模型研究

The stress path induced by traffic load has the characteristics of coupling effect of vertical cyclic stress and horizontal cyclic stress and principal stress rotation induced by cyclic shear stress. This project intends to simulate the multi-dimensional cyclic stress path through multidirection hollow cylinder apparatus manufactured by GDS Corporation. Systematic static and cyclic experimental tests are conducted on normal consolidated and overconsolidated soft clay to research the influence of cyclic deviator stress and spherical coupling and principal stress rotation on strain accumulation, stress-strain hysteretic and noncoaxial behavior considering the effects of consolidation method, drained condition, stress level, load frequency and OCR. Based on the test results, the limit cyclic stress level of soft clay under multi-dimensional cyclic stress path is confirmed and the yield characteristics, nonlinear kinematic hardening rules and non-associated flow rules are revealed. Then a cyclic elastoviscoplastic constitutive model for clay considering nonlinear kinematic hardening rules and structural degradation is established by modified that proposed by Kimoto. By combining the tensor expression of accumulated strain rate, an explicit-implicit associated high cycle strain accumulation model is established. Finally, the results of model and in situ tests are used for model verification and improvement. This project is helps to develop the basic characteristic and constitutive model of overconsolidated soft clay under complex stress paths and provide theoretical direction for the traffic engineering in soft clay area.

交通荷载引起的应力路径既有竖向动应力与水平动应力的同步作用，还存在循环剪应力所引起的主应力轴旋转现象。本项目拟利用GDS多向空心圆柱扭剪仪实现交通荷载多维度耦合循环应力路径的模拟，考虑固结方式、排水条件、应力水平、加载频率、超固结比等因素，对正常和超固结软粘土试样进行系统性静动力试验，研究循环偏应力与球应力耦合、主应力轴旋转等对软粘土应变累积、应力应变滞回及非共轴特性的影响。在此基础上，确定多维度耦合循环应力路径下软粘土的界限动应力水平，揭示超固结软粘土的屈服特性、运动硬化规律和非相关联流动法则，修正Kimoto等提出的考虑非线性运动硬化和结构退化的循环弹粘塑性本构方程。结合累积应变速率张量表达式建立显隐式联合的长期循环累积应变模型，并通过模型试验和现场测试进行验证和完善。本项目有助于发展复杂应力路径下超固结软粘土基本特性及本构模型研究，为软粘土地区交通工程建设提供理论指导。

本项目针对交通动荷载长期循环往复作用引起地基土应变累积、强度降低，导致重大工程过大变形和失稳等灾变的问题，利用三维有限元程序获得交通荷载作用下路基土体单元所受的动应力，采用双向动三轴和多向空心圆柱扭剪系统，对正常固结和超固结软粘土试样开展不同大主应力方向下的定向循环剪切试验、偏应力与球应力耦合、主应力轴旋转等复杂条件耦合的多维度循环应力路径下的循环加载试验，研究了应变累积、孔压演化、模量软化和应力应变曲线发展等动力特性。揭示软粘土在三维应力加载下的静力屈服特性及强度包络面，主应力轴旋转引起的非共轴特性的基本规律及偏应力与球应力耦合及主应力轴旋转下的应变累积特性。提出能同时考虑竖向动应力、水平动应力和扭剪动应力的循环应变比表达式，在分析长期累积应变随循环应力比发展模式的基础上，建立了交通荷载引起的多维度耦合应力路径下软土地基沉降计算方法，并通过现场沉降测试数据进行了验证，确定多维度循环应力路径下正常固结和超固结软粘土的界限动应力比，为沉降控制准则的提出提供了理论依据。本项目有助于发展复杂应力路径下超固结软粘土基本特性及本构模型研究，为软粘土地区交通工程建设提供理论指导。