黏滞性软土的固结模型及软基长期变形计算

The deformation of soft soil under effective stress consists of two parts. The first part is compression, produced by consolidation phenomenon, and the second part is creep, in response to the viscosity. They are interrelated, and both control the long-term deformation and stability of soft foundation. Our previous studies show that the coupling effect of consolidation and creep is closely related to permeability of soil and drainage boundary conditions, and the creep properties can be weakened by the consolidation effect.By means of laboratory experiments, theoretical derivation and numerical simulation and so on, the consolidation behavior of soils exhibiting nonlinear viscosity are investigated in this project. The key scientific problems are that: 1) the influence mechanism of instantaneous deformation condition and drainage distance on the initial elastic-like stage of consolidation before yielding, and the unique relationship of effective stress-viscous strain-viscous strain rate after yielding under one-dimensional consolidation state; 2) the creep strain-rate variation that behaves as bulk creep or both bulk creep and shear creep characteristics considering the drainage conditions under triaxial shear state; 3)the coupled model of consolidation and creep based on classic consolidation theory, and a simplified calculation method of long-term deformation and stability of soft foundation. According to the research, this project will further reveal the coupling mechanism of creep and consolidation, and propose the consolidation model of soils exhibiting viscosity and the methods for determining creep parameters. This project will provide the fundamental theory and well as technical support for the practical engineering calculation.

软土受力变形过程既包括基于固结原理的压密过程也包括基于黏滞特性的蠕变过程，两者相互影响，共同控制着软基的长期变形和稳定。申请者的前期研究表明：蠕变固结的耦合效应与土的渗透性和排水边界条件密切相关；蠕变作用可以被固结作用弱化。本项目拟综合利用试验、理论推导和数值模拟等手段更深入研究非线性黏滞性土体的固结特性。关键科学问题：1）一维压密状态下瞬时变形条件和排水距离对初始似弹性固结阶段的影响机制及正常固结阶段的一致性应力-应变-应变率关系；2）三轴剪切状态下考虑了不同排水条件的体积蠕变或兼具体积蠕变和剪切蠕变特性的蠕应变率变化规律； 3）基于经典固结理论的蠕变固结模型和软土地基长期变形及稳定性的简化计算方法。根据研究进一步揭示软土变形过程中蠕变与固结的耦合机制，提出黏滞性软土的固结模型和蠕变参数的确定方法，为固结理论的完善和实际工程的分析计算提供理论依据和技术支持。

软黏土的变形过程与时间密切相关，既包括超静孔隙水压力消散、土体压密的固结过程，也包括由于黏滞特性导致的蠕变变形，两者互相影响，共同决定着土体的长期变形。基于土体变形的时间效应，建立起可以反映软黏土蠕变特性的本构关系，并实现与经典固结理论的耦合，对于土力学中固结理论的完善和实际工程的分析计算都是十分有益的。本项研究综合利用室内试验、理论分析及数值模拟等方法对软黏土的固结和蠕变特性进行了综合研究。主要研究内容及研究成果如下：.(1)通过一维K0侧限固结试验、柔性等K侧限固结试验、渗压试验等，揭示了一维压密状态下黏滞性软土的弹-黏塑性固结特征，分析了蠕应变的主要影响因素，验证了瞬时变形对压缩性指标的影响的存在并提出了量化修正方法。在Bjerrum等时间线理论的基础上证明了将蠕变变形当作与时间相关的塑性变形这一假设的合理性，分别从蠕变效应及应变率效应角度推导了一维弹-黏塑性本构关系，明确了模型参数的定义及确定方法。.(2)通过三轴蠕变试验研究，揭示了排水条件对蠕应变的影响及黏滞变形特征，根据Perzyna过应力理论，引入参考面的概念建立了一般应力条件下的三维弹-黏塑性模型，并考虑了初始各向异性的影响，给出了模型参数的确定方法。.(3)将所建立的一维、三维弹-黏塑性本构方程分别与经典固结理论结合，推导了基于Terzaghi固结理论的一维蠕变固结控制方程、及基于Biot固结理论的三维蠕变固结控制方程，并通过建立Crank-Nicolson有限差分格式对一维控制方程进行求解、通过Abaqus有限元软件中的 UMAT子程序对三维控制方程进行求解，使基于经典固结理论的黏滞性固结模型能够用于软基长期变形计算，模拟结果验证了模型的适用性。.(4)基于试验成果和理论分析，提出了可供实际工程选择采用的简化计算方法，以用于设计阶段软基长期变形的初步预测，包括基于元件模型的分数阶导数经验模型、基于现行规范的参数修正方法。