Influence of Excavation Steps on the Contact Interface between Bedrock and Soil–Rock-Mixture Fillers

A step excavation method commonly is employed on bedrock in road expansion projects, and is aimed at enhancing embankment slope stability. However, the specific impacts of the required number of excavation steps on the shear strength and particle characteristics of the soil–rock mixture remain incompletely understood. In this study, particle flow code (PFC) software was utilized for numerical simulation. The calibration of the parameters and direct shear test simulations on soil–rock mixtures were conducted with various numbers of steps. The effects of step count on the contact interface stress, the shear strength of the mixture, and particle behavior were analyzed. The results indicate that as the number of steps increases, the shear strength of the soil–rock mixture and the degree of rock particle breakage initially increase and then decrease, and the contact interface stress gradually increases and the number of microcracks decreases. These phenomena are attributed primarily to the size effect of the steps, the volume of fillers, and the mechanism of force transmission. Additionally, based on the characteristics of force chains, microcrack distribution, and particle displacement, the stepped soil–rock mixture can be divided into three areas. The first area is inside the steps of the lower shear box, in which most particle motion is inactive. Except for particles at the junction of the steps, the steps will hinder their movement along the shear direction. Therefore, they have to climb over the stairs. The second area is the front half of the upper shear box, in which particles are more active and the contact force between particles is greater, resulting in a large number of dense microcracks; The third area is the rear half of the upper shear box, in which the contact force between particles is small, and there are fewer microcracks.