Understanding the shear mechanical behaviors and instability mechanisms of rock joints under dynamic loading remains a complex challenge. This research conducts a series of direct shear tests on real rock joints subjected to cyclic normal loads to assess the influence of dynamic normal loading amplitude (Fd), dynamic normal loading frequency (fv), initial normal loading (Fs), and the joint roughness coefficient (JRC) on the mechanical properties and instability responses of these joints. The results show that unstable sliding is often accompanied by friction weakening due to dynamic normal loads. A significant negative correlation exists between cyclic normal loads and the normal displacement during the shearing process. Dynamic normal load paths vary the contact states of asperities on the rough joint surfaces, impacting the stick-slip instability mechanism of the joints, which in turn affects both the magnitude and location of the stress drop during the stick-slip events, particularly during the unloading phases. An increasing Fd results in a more stable shearing behavior and a reduction in the amplitude of stick-slip stress drops. The variation in fv influences the amplitude of stress drop for the joints during shear, characterized by an initial decrease (fv = 0.25-2 Hz) before exhibiting an increment (fv = 2-4 Hz). As Fs increases, sudden failures of the interlocked rough surfaces are more prone to occur, thus producing enhanced instability and a more substantial stress drop. Additionally, a larger JRC intensifies the instability of the joints, which would induce a more pronounced decline in the stick-slip stress. The Rate and state friction (RSF) law can provide an effective explanation for the unstable sliding phenomena of joints during the oscillations of normal loads. The findings may provide certain useful references for a deeper comprehension of the sliding behaviors exhibited by rock joints when subjected to cyclic dynamic disturbances. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

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.