土体稳定与承载特性的分析方法

Analysis methods for stability and bearing capacity of soils

结合研究团队多年的研究积累,分别从砂土与饱和软黏土两个方面阐述了在土体失稳与承载特性分析方法方面的理论研究成果。排水条件下密砂在到达峰值强度前可能会出现应变局部化失稳,而饱和松砂则可能出现分散性失稳;建立了基于三维非共轴塑性理论的变形分叉分析方法,分析了砂土平面应变试验和真三轴试验的应变局部化现象,进一步分析了各向异性砂土的强度特性和应变局部化现象,以及砂土的状态相关特性对应变局部化的影响;建立了基于状态相关本构模型的砂土应变局部化失稳弹塑性有限元分析方法,探讨了复合体理论和非局部塑性理论在消除有限元网格依赖性问题方面的可行性;建立了基于二阶功准则和状态相关本构模型的饱和砂土分散性失稳数值分析方法。提出了基于临界状态理论的饱和软黏土各向异性不排水强度理论公式,以及基于循环累积塑性应变的强度弱化公式,形成了适合低渗透性饱和软黏土的弹塑性静动力有限元分析方法;建立了非均质与各向异性软黏土地基稳定性问题的极限分析上限方法,进一步提出了不排水地基承载特性的虚拟加载上限分析方法,形成了分析地基承载特性循环弱化的弹塑性有限元法和虚拟加载上限法。所建立的分析方法将为砂土与饱和软黏土地基稳定性和承载特性的理论预测提供重要的技术手段。

By summarizing years of theoretical research achievements gained by the author and his research team members, the analysis methods to study the instability and behaviors capacity of soils were concluded from two aspects, sands and saturated soft clays, respectively. Under drained loading conditions, the deformation of dense sand may accompany with strain localization before reaching the plastic limit, but that of saturated loose sand may accompany with diffusive instability. The phenomenon of strain localization in plane strain tests and true triaxial tests of sands was studied by the bifurcation analysis based on the three-dimensional non-coaxial plasticity theory. Further, the strength properties and strain localization of anisotropic sands were analyzed, including the strain localization analysis using a state-dependent constitutive model for sands. In addition, the feasibility to overcome the mesh-size dependence problems with the homogenization procedure and the non-local regularization was explored in the finite element analysis. By using the state-dependent constitutive model, the elasto-plastic finite element procedure accounting for the strain localization of drained sands and diffusive instability of saturated sands was proposed. For the saturated soft clays, a simplified formula for calculating their anisotropic undrained shear strength was put forward based on the critical state theory, and the undrained shear strength degradation was also predicted according to the plastic strain accumulated during cyclic loading. Additionally, a rational static and dynamic finite element analysis method for analyzing the saturated soft clays with low permeability was proposed, which effectively solves the problem caused by the incompressible condition. The upper bound limit analysis theory was applied in analyzing foundation stability problems considering the strength nonhomogeneity and anisotropy of saturated soft clays. Furthermore, the fictitious loading upper bound limit analysis approach for undrained soft clay, through which the ultimate bearing capacity and load-displacement relationship can be obtained simultaneously, was also proposed. Besides, the elasto-plastic finite element method and fictitious loading upper bound limit analysis approach, which are capable of analyzing the degradation of the ultimate bearing capacity, were formed. The established analysis methods will serve as important means in estimating the ground stability and bearing behaviors of both the sands and saturated soft clays.