透水型级配碎石基层填料强度演化特征的离散元模拟

骨架空隙型级配碎石填料因其较好的渗透性能而广泛应用于透水型基床结构层，但其在不同级配下的抗剪强度特征及演化规律仍不明确。首先针对6种不同级配类型的碎石填料试样开展了不同围压下的室内单调加载三轴压缩试验，进而采用考虑颗粒真实不规则形状的三维离散单元方法对室内试验结果进行了数值模拟研究，从颗粒间接触、微观组构和颗粒运动等角度揭示了透水型级配碎石基层(UPAB)填料抗剪强度机理及其随级配变化的演化特征，验证了室内试验得出的级配影响规律及级配优化设计方法。研究结果表明：细颗粒通过改变颗粒体系的内部排列结构(即各向异性程度)而直接影响级配碎石填料宏观抗剪强度，细颗粒含量对试样的剪切破坏形态有直接影响，随着细颗粒含量的减少，试样整体发生较大变形时表现为内部颗粒发生转动的比例下降但发生颗粒间滑动的比例增加；试样失稳的主要原因是由于颗粒拓扑结构的改变，最终发展成不可逆的塑性变形；级配控制参数G/S=1.8时试样表现出最低的各向异性特征，相同应变条件下的颗粒旋转角均值最小，表明此时试样内部的颗粒堆积排列最优且抗剪强度最高，G/S=1.8可取作“填充密实”型与“残余空隙”型结构的分界；不同级配的试样内部颗...

Discrete Element Modeling of Shear Strength Evolution Characteristics of Unbound Permeable Aggregate Base Materials

Open-graded aggregate base (OGAB) materials featuring large air void and load-carrying skeleton have been increasingly used in permeable base layers due to preferable drainability; however, the mechanism of shear strength behavior of such materials evolving with gradation remains unclear. In this study, laboratory monotonic load triaxial compression tests were conducted on OGAB specimens with six different types of gradation at different levels of confining pressure. The testing results were then modeled by using the discrete element method (DEM) with realistic irregular particle shape considered. The mechanism of shear strength of such OGAB materials and its evolution characteristics with gradation were disclosed from the perspectives of inter-particle contact, microstructural fabric, and particle movement, whereas the gradation effect and gradation optimization method derived from laboratory testing results were validated. It was found that the macroscopic shear strength behavior is directly affected by the fine fraction that changes the internal packing structure (or the degree of anisotropy). The content of fine fraction directly affects the shear failure pattern, i.e., as the fines content decreases, the proportion of rotating particles reduces and that of sliding particles increases upon the occurrence of large deformation in the specimens. The instability of the specimens is mainly attributable to changing topological structure of internal particles that eventually develops into irreversible plastic deformation. The specimen with G/S value of 1.8 exhibits the least anisotropy as well as the smallest mean value of particle rotation angles at the same strain level, thus indicating the optimal packing arrangement of internal particles and the highest shear strength. The G/S value of 1.8 could be used as the threshold value separating floating skeleton and underfilled skeleton type packing structures. The significantly reduced variation in Euler angle of internal particles of specimens with varying gradations was observed for the size range from 4.75 mm to 9.50 mm, indicating that such a size range separates particle rolling and sliding. Since particle rolling and sliding are directly related to shear strength behavior, this validates the rationality of the parameter G/S for controlling and optimizing gradations from the perspective of particle movement.