The Influence of Specimen Diameter and Aspect Ratio on the Shear Mechanical Behavior of S-RM in Triaxial Shear Test Using Coupled PFC3D with FLAC3D

Soil-rock mixture (S-RM), an important geotechnical material, is widely used in subgrade and slope engineering. Triaxial shear test can effectively simulate the complex mechanical properties and test the mechanical parameters. However, there is a specimen size effect during testing, and the choice of different specimen sizes can influence the test results. In order to investigate the specimen size effect in the test, this paper first proposes a three-dimensional discrete element refined modeling method for S-RM, then develops a flexible boundary loading method for triaxial shear test using coupled PFC3D with FLAC3D, and subsequently conducts triaxial shear tests on S-RM of different sizes. The test results show that as the ratio of specimen diameter to maximum particle size of the rock block (lambda) or the aspect ratio (alpha) increases, both the quantity of force chains and cracks increase, while shear band evolves into conjugate "X" shape. With the increase of lambda and alpha, the stress-strain curves transition from strain hardening to strain softening. Both cohesion (c) and internal friction angle (phi) exhibit continuous reduction, characterized by an initial rapid decline followed by a gradual deceleration in rate, eventually stabilizing when lambda and alpha reach sufficiently large values. Therefore, the specimen size effect becomes less significant when the specimen size is relatively large. When the lambda exceeds 5.0 and the alpha surpasses 2.0, the specimen size effect in triaxial shear tests for S-RM diminishes to an acceptable range. Consequently, it is rec-ommended to employ specimens with lambda greater than 5.0 and alpha greater than 2.0 in triaxial shear tests for S-RM.