考虑真实颗粒形态效应的砂土宏微观剪切特性试验与模拟研究

Particle morphology is an essential characteristic in determining the mechanical properties of natural sands, especially the shear resistance and dilatancy behavior at lower stress level. In this proposal, the standard quartz sand from Pingtan area of Fujian province will be selected as the main research material. In this study, the quantitative relationship between the particle morphological features and the shear resistance as well as the deformation parameters of the sand will be established based on the statistical analysis of particle morphological parameters and triaxial test results. Furthermore, based on the X-ray micro-computed tomography (CT) data, a mathematical approach using spherical harmonics will be introduced to globally characterize and accurately reconstruct the multi-scale particle morphology in three dimensions. By using principal component analysis for the obtained spherical harmonic descriptors of the scanned sand particles, a realistic sand assembly, consisting of a large number of virtual particles with random shapes but major morphological features of the real sand, will be generated and further implemented into the efficient discrete element modelling (DEM). In DEM simulations of the realistic sands, the development and microstructures of the shear band can be identified and characterized by the distribution of the local void and the micro-strain localization of the sample. Meanwhile, a deep insight of micromechanics of the local shear failure and shear induced dilatancy of the sand considering the significant effect of the particle morphology will be gained by conducting a series detailed studies on the microscopic behaviors inside the shear band, such as particle movements, evolution of the contact fabric, structural stability of the force chain and the allocation of the energy dissipation. Finally, a micro-based stress-dilatancy relationship of sands will be originally introduced to consider the effect of particle morphology, which will support significant theoretical foundations for the establishment of the scientific constitutive model of sands.

颗粒形态是影响砂土在低应力状态下力学特性的重要因素。选取我国福建平潭地区的标准石英砂为主要研究对象，根据砂样的粒形参数统计分析和三轴试验建立颗粒形态特征与砂土抗剪强度指标以及变形参数之间的相关性。然后，借助于高精度的X射线微型断层扫描和图像处理技术获取砂粒形态的三维空间信息，采用球谐函数分析实现三维砂粒形态在多尺度观测下的全局表征和准确重建。进而，通过扫描砂粒的球谐表征参数的主成分分析，重构出形态随机、但服从原有形态特征统计规律的逼真砂粒集，并应用到砂土三维离散元模拟中。最后，根据砂土的局部孔隙分布、微观应变局部化来识别剪切带的微观结构，在砂土剪切过程中，从剪切带内颗粒的运动行为、接触组构、力链结构和能量耗散等方面深入揭示颗粒形态效应下砂土局部剪切破坏和剪胀行为的微观力学机理,并提出考虑颗粒形态效应的微观应力剪胀关系。期待对砂土真实力学特性的把握和本构模型的建立提供强有力的科学理论依据。

本项目选取了我国福建平潭地区的标准石英砂和广东地区风化花岗岩残积砂为主要研究对象，借助于高精度的X射线微型断层扫描和先进的三维图像处理技术获取单个砂粒形态的三维空间信息，采用球谐函数分析实现砂粒三维形态在多尺度观测下的全局表征和准确重建。基于球谐函数重建的砂粒三维表面网格，提出了一系列的实用性算法计算单个砂粒的三维形态参数，如体积、表面积、三维尺寸、球度、圆度和分形维度等，这些些三维形态参数都可以直接应用到工程实践中。此外，通过对大量砂粒的球谐表征系数做主成分分析，发现不同种类砂土的特征模式，并提出随机重构算法反演出形态随机、但服从原有砂土形态特征统计规律的大型逼真砂粒集体系，并成功地应用到砂土三维离散元模拟中。将该方法反演生成的逼真砂粒集的三维形态信息输出到离散元分析软件中，建立了考虑砂土真实颗粒形态效应的三维离散元模型，并实现了其三轴压缩过程和直剪过程的模拟，全面分析颗粒形态效应对砂土宏微观力学行为的影响，并揭示了颗粒形态影响砂土破碎行为、剪切带发展和剪胀行为的微观力学机理。该项目研究成果将对砂土真实力学特性的把握和本构模型的建立提供强有力的理论依据。