In the waterway construction projects of the upper reaches of the Yangtze River, crushed mudstone particles are widely used to backfill the foundations of rock-socketed concrete-filled steel tube (RSCFST) piles, a structure widely adopted in port constructions. In these projects, the steel-mudstone interfaces experience complex loading conditions, and the surface profile tends to vary within certain ranges during construction and operation. The changes in boundary conditions and material profile significantly impact the bearing performance of these piles when subjected to cyclic loads, such as ship impacts, water level fluctuations, and wave-induced loads. Therefore, it is necessary to investigate the shear characteristics of the RSCFST pile-soil interface under cyclic vertical loading, particularly in relation to varying deformation levels in the steel casing's outer profile. In this study, a series of cyclic direct shear tests are carried out to investigate the influential mechanisms of roughness on the cyclic behavior of RSCFST pile-soil interfaces. The impacts of roughness on shear stress, shear stiffness, damping ratio, normal stress, and particle breakage ratio are discussed separately and can be summarized as follows: (1) During the initial phase of cyclic shearing, increased roughness correlates with higher interfacial shear strength and anisotropy, but also exacerbates interfacial particle breakage. Consequently, the sample undergoes more significant shear contraction, leading to reduced interfacial shear strength and anisotropy in the later stages. (2) The damping ratio of the rough interface exhibits an initial increase followed by a decrease, while the smooth interface demonstrates the exact opposite trend. The variation in damping ratio characteristics corresponds to the transition from soil-structure to soil-soil interfacial shearing. (3) Shear contraction is more pronounced in rough interface samples compared to the smooth interface, indicating that particle breakage has a greater impact on soil shear contraction compared to densification.

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.

The bearing resistance provided by the geogrid's transverse ribs is a non-negligible aspect of the strength mechanism in mobilizing the geogrid-soil interface. Therefore, studying its influence on the response mechanism of geosynthetic-reinforced soil structures under cyclic loading is crucial. The stereoscopic geogrids were manufactured using 3D printing technology by quantitatively thickening the transverse ribs of planar geogrids. To investigate the cyclic hysteresis relationship and stress-dilatancy phase-transformation characteristics of the stereoscopic geogrid-coarse particle interface, cyclic direct shear tests were conducted. Additionally, a discrete element method (DEM) was employed to study the evolution of shear bands and fabric anisotropy at the interface under cyclic loading. The results of the study indicate that the stress-displacement phase angle of the stereoscopic geogrid in the horizontal direction of cyclic shear is smaller compared to the planar geogrid. Furthermore, thickening the transverse ribs decreases the stress-dilatancy phase-transformation angle of the interface. The thickness of the interface shear band in the stereoscopic geogrid is greater than that of the planar geogrid. Moreover, as the transverse-rib thickness increases, the principal direction of the average normal contact force and average tangential contact force under cyclic loading also increases.