粗粒土三轴试验端部约束宏细观效应机理与力学参数修正

As the main filling material in earth-rockfill dam, the accuracy of strength and deformation characteristics of coarse-grained soil is very important for the safety stability analysis of the dam. Due to the effects of end restraint, test data of triaxial test often can not really reflect the mechanical properties of coarse-grained soil. Currently, studies of end restraint effects are more limited to the macro scale of phenomenon description and simulation. The related research on coarse-grained soil with the discrete characteristics of multiscale granular mixtures is seldom reported. The project is aimed to study the effects of end restraint on the strength and deformation characteristics under complex stress path of coarse-grained soil by large-scale triaxial tests with the normal constrained end and free end（little friction）, and study the evolution of mesoscopic fabric features by real-time CT scan test. Based on discrete element theory, mesoscopic numerical model considering the characteristics of particle breakage with high efficiency is established so as to realize the mesoscopic particle simulation of end constraint effect of coarse grained soil. By the combination of macro and mesoscopic method, starting from the force chain network evolution mechanism between particles, mechanical mechanism of the strength and deformation evolution of coarse-grained soil under different end constraint conditions is revealed. By introducing the end restraint effect quantitative parameters, the modification method of mechanical parameters on end restraint effect of coarse-grained soil is established and validated in engineerings. Findings could not only improve the cognitive level of end constraint effect, and further improve the triaxial test technology of coarse-grained soil, but also provide a theoretical basic for stability analysis and safety operation of the high earth-rockfill dam engineering.

粗粒土作为主要的筑坝材料，其强度与变形特性的准确性对大坝的安全稳定分析尤为重要。受端部约束效应的影响，三轴试验结果并不能真实反映粗粒土的力学特性。目前有关土体端部约束效应的研究，多局限于宏观尺度的现象描述与模拟重现，而对具有多尺度散粒体离散特征的粗粒土研究鲜有报道。本课题拟通过端部正常约束与端部自由(微摩擦)的大型三轴试验，研究端部约束对复杂应力路径下粗粒土强度与变形特性的影响规律，借助实时CT扫描试验分析其细观组构演化特征；基于离散元理论，建立可考虑颗粒破碎特征的高效细观数值模型，实现粗粒土端部约束效应的细观颗粒流模拟，通过宏细观相结合的方法，从颗粒间组构力链演化机制出发，揭示不同端部约束条件下粗粒土强度与变形演化的力学机制；引入端部约束效应量化参数，建立粗粒土端部约束效应的力学参数修正方法并进行工程验证。研究成果有助于完善粗粒土的三轴试验技术，亦可为高土石坝的安全稳定分析提供理论依据。

粗粒土作为主要的筑坝材料，其强度与变形特性的准确性对大坝安全稳定分析尤为重要。受端部约束效应的影响，三轴试验结果并不能真实反映粗粒土的力学特性。项目以我国西南地区高土石坝典型粗粒土为研究对象，采用宏细观结合的方法，借助大型三轴、CT实时扫描试验和宏细观数值模拟分析，基于其“颗粒-力链-体系”的多尺度结构，研究端部约束对粗粒土强度与变形特性的影响规律和作用机理和参数修正方法。研究成果完善了粗粒土的三轴试验技术，亦可为高土石坝的安全稳定分析提供理论依据。主要工作内容如下：.（1）针对土工三轴仪中加载板与试样接触面存在较大摩擦力的关键技术难题，采用整体接触变为分散式接触、滑动摩擦变为滚动摩擦的减摩新方法，研发了一种粗粒土三轴试验端部减摩装置，破解了土工三轴试验微摩擦加载技术难题。.（2）基于研发减摩装置联合大型高压三轴仪进行了中砂、堆石料和掺砾心墙料一系列端部正常约束与端部自由（微摩擦）的大型三轴试验，研究了端部约束对粗粒土强度与变形特性的影响规律，提出一种端部约束效应强度参数修正公式。.（3）开展了端部约束条件下粗粒土细观实时CT三轴扫描试验，通过对试样CT数和CT图像的定量分析，获得了粗粒土颗粒运动规律，揭示了端部约束条件下粗粒土三轴剪切过程中的细观组构演化特征。.（4）采用ABAQUS有限元分析软件，模拟分析了三轴试验的端部约束效应，得出了不同端部约束条件下三轴试样水平应力和位移的分布，初步分析端部约束的影响机制。.（5）将三维组合墙法模拟柔性薄膜融入细观数值建模，实现粗粒土端部约束效应的数值模拟，获得不同端部约束条件下粗粒土的宏观变形特征和颗粒间接触力、接触法向等细观组构参量演化规律，从本质上揭示不同端部约束条件下粗粒土的强度与变形演变细观力学机制。.（6）开展了粗粒土强度和变形参数对比分析和应用研究，对典型的高土石坝工程进行数值模拟分析，研究了端部约束和端部自由条件下堆石坝应力变形差异性和适用性。