基于两相介质理论之土层弹塑性大变形地震反应分析

Nonlinear ground response based on the theory of wave propagation in two-phase media

基于土–水两相混合体完全耦合场方程及土体静–动统一本构,构建了新型普适性土层弹塑性大变形地震反应分析方法。考虑强震中土体大变形动力特性描述的困难,基于客观性张量推导,给出了严格的土–水两相混合体平衡方程式和连续方程式,分别进行空间和时间离散,编制了显式有限元–有限差分程序。通过超固结、结构性和应力诱导各向异性状态变量的引入,建立了物理意义明确的土体静–动统一本构,能够合理的表征土体的各类力学特性。数值计算中采用转换应力法,实现了试验应力状态向一般应力状态的拓展,满足复杂三维地层的动力计算需求。通过局部透射人工边界的设置,形成了辐射边界条件,构建了完备的土层反应程序。将分析结果与等效线性化方法和单相介质时域非线性分析结果进行对比,指出饱水砂土层的存在对地震波传播和地表地震动反应特性的影响。

The nonlinear ground response considering the influence of pore water pressure is implemented by a new method. The cyclic mobility（CM） model, an elastoplastic model with rotation hardening which can systematically describe the monotonic and cyclic mechanical behaviors of soils combining the subloading, normal and superloading yield surfaces, is employed in the nonlinear numerical analysis. Using the transform stress method, this model can uniquely describe the overall mechanical properties of soils under general stress states, without changing the values of parameters. Based on the CM model and two-phase field theory, an effective stress-based, fully coupled, explicit finite element-finite difference method（FE-FD） is established. The finite element method and explicit integration method are applied in spatial and temporal discretization, respectively. The multi-transmitting boundary is adopted on the artificial boundary. By introducing the Green-Naghdi rate tensor, the finite deformation analysis is presented. This method is strictly verified, and the calculated results of a real site are quite different among the authors＇, the equivalent linearization method and the normal nonlinear analysis method in one-phase media. The long-period value of ground response spectrum will be higher owing to sand liquefaction. Moreover, the liquefaction process of the saturated sand layer and its influence factors are also studied.