Rubber-sand mixtures (RSM) have the potential to be used as eco-friendly geotechnical materials for the reinforcement of roadbeds and other projects. By a series of monotonic direct shear tests under normal cyclic loading (NCMDS), the impact of rubber contents, initial stresses, stress amplitudes, and loading frequencies on the shear properties of the geogrid and RSM interface was studied. Shear models for pure sand and RSM were formulated using PFC3D, and the mesoscopic behaviors during the shearing were investigated. The findings indicated that the interface exhibited prominent softening characteristics. It was observed that a lower rubber content corresponded to a more pronounced softening phenomenon. For a given rubber content, with a rise in frequency, there was a decline in both the peak stress and stress fluctuation amplitude of the interface, and the overall dilatancy decreased. The RSM had slightly more contact points than pure sand, and the count of contact points during the peak state surpassed that during the valley state. Throughout the shearing, the coordination showcased cyclic fluctuations. The coordination near the interface initially diminished, then gradually leveled out, mirroring the macroscopic dilation effect. Under cyclic loading, the kinetic energy of particles exhibited more pronounced fluctuations compared to the damping energy, and the damping energy in RSM exceeded that in pure sand.

The application of waste rubber for soil improvement is feasible, and the static and dynamic properties of rubber-reinforced soils have been extensively studied. However, the mechanical properties of frozen rubber-reinforced expansive soils have not been effectively studied due to the complexity of multiphase media under the action of multiple fields, and no applicable constitutive models describe them. In this paper, the stress-strain relationship model for frozen rubber-reinforced expansive soils is investigated over a range of strain rates from 0.18% to 0.3% and the following conclusions were obtained: (1) The structural model of the frozen rubber-reinforced expansive soil can be considered a ternary medium model that consists of elasto-brittle bonding elements, elasto-plastic friction elements and elastic friction elements. (2) The stress-strain relationship can be divided into three stages: linear elastic stage, elasto-plastic stage and strain softening (R-C <= 15%) or hardening (R-C = 20%) stage. The ternary medium model can better describe the three stages deformation process. (3) The rubber content has a greater influence on the stress-strain relationship. When the rubber content reaches 20%, the expression of the stress-strain curve changes from strain softening to strain hardening, at which time the rubber dominates. (4) The maximum shear strength of frozen rubber-reinforced expansive soil is obtained at 10% rubber content.