To investigate the interaction mechanism between the sand-structure interface under cyclic loading, a series of cyclicdirect shear tests were conducted. These tests were designed with various surface roughness values represented by the jointroughness coefficient (JRC) of 0.4, 5.8, 9.5, 12.8, and 16.7, and normal stresses of 50, 100, 150, and 200 kPa. A 3D printerwas employed to accurately control the surface roughness and obtain concrete samples with varyingJRCvalues. The testresults were used to establish discrete element method models, which facilitated the analysis of the mesoscopic shearbehavior at the sand-structure interface during the cyclic direct shear process. The results revealed that the sand-concreteinterface demonstrated softening behavior. There is a critical value for the surface roughness corresponding to themaximum interface shear strength. The thickness of shear band, where the changes in porosity were concentrated within,increases with higher surface roughness and cycle number. The coordination number stabilizes after 80 cycles. Thedistributions of the contact normal direction and tangential contact force exhibited nearly isotropic characteristics aftercyclic loading. It was observed that surface roughness amplifies the deflection angle of the main axis in the normal contactforce distribution, while reducing that in the shear contact force distribution.

In cold-region tunnel engineering, the bonding surface between concrete and surrounding rock is highly susceptible to engineering disasters such as the cracking of support structures under the influence of -freeze-thaw cycles, which severely affects the stability of tunnel engineering. This study investigates the macroscopic and microscopic mechanical properties of the sandstone-concrete interface under freeze-thaw cycles and reveals the microdamage and fracture evolutionary laws during the freeze-thaw loading process via the coupled expansion of water-ice particle phase changes via particle flow numerical simulation methods, aiming to provide theoretical support for the design and construction of rock-soil and tunnel engineering in high-altitude frozen soil areas. The results indicate that the interface strength characteristics of the sandstone-concrete specimens exhibit a stable decreasing trend with an increase in the number of -freeze-thaw cycles and a decrease in roughness. Taking the most unfavorable condition as an example, the exacerbation of freeze-thaw- deterioration resulted in shear strength reductions of 16.53%, 27.01%, and 37.17% for specimens (JRC=3.727), while an increase in roughness led to shear strength increases of 11.00% and 15.14% for specimens (NT=60 cycles). The acoustic emission characteristics during the loading process of the sandstone-concrete interface specimens reflect the microcracking evolutionary activity of the specimens quite well, and setting 1.0 as the recommended value for the precursor determination of the b-value and CV(k) index is most reasonable. Under -freeze-thaw cycles, cracks first initiate partial microcracks at the interface and on the outer side of the specimen, primarily dominated by tensile cracks and evolving with a slow-fast trend toward both sides of the interface. Under shear stress, particles at the interface first undergo slip dislocation due to their lower bonding strength, generating cracks, subsequently inducing significant displacement of particles on both sides of the interface, resulting in crack propagation toward both sides of the interface, ultimately penetrating and forming shear bands leading to macroscopic failure.

Pile foundations frequently endure dynamic loads, necessitating an in-depth examination of the cyclic shear properties at the pile-soil interface. This study involved a series of cyclic direct shear (CDS) tests conducted on sand and concrete with irregular surface, utilizing varying displacement amplitudes (1, 3, 6, and 10 mm) and joint roughness coefficients (0.4, 5.8, 9.5, 12.8, and 16.7). Discrete Element Method (DEM) models, informed by experimental data, facilitated mesoscopic mechanical response analyses. Findings indicate that the sand-concrete interface undergoes softening, with hysteresis loops' morphology dependent largely on displacement amplitude. A maximum ultimate shear stress corresponds to a specific critical surface roughness, while the initial tangent modulus escalates with increased concrete roughness. Volume variations of the specimen inversely correlate with displacement amplitude and directly with surface roughness. As displacement amplitude expands, there is a reduction in the maximum shear stiffness and an elevation in the maximum damping ratio. Empirical formulas for the surface roughness and normalized shear stiffness were proposed. Larger displacement amplitudes result in more substantial shear bands and heightened energy dissipation, yet the incremental energy ratio remains largely unaffected. Predominant energy dissipation mechanisms include both slip and rolling slip, with the former surpassing the latter in energy dissipation capacity. The anisotropy directions of contact normal, normal contact forces, and tangential contact forces consistently fluctuate with shear direction alterations.