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 loadcarrying 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. © 2023 Xi'an Highway University. All rights reserved.