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To reveal the mechanism of shear failure of en-echelon joints under cyclic loading, such as during earthquakes, we conducted a series of cyclic shear tests of en-echelon joints under constant normal stiffness (CNS) conditions. We analyzed the evolution of shear stress, normal stress, stress path, dilatancy characteristics, and friction coefficient and revealed the failure mechanisms of en-echelon joints at different angles. The results show that the cyclic shear behavior of the en-echelon joints is closely related to the joint angle, with the shear strength at a positive angle exceeding that at a negative angle during shear cycles. As the number of cycles increases, the shear strength decreases rapidly, and the difference between the varying angles gradually decreases. Dilation occurs in the early shear cycles (1 and 2), while contraction is the main feature in later cycles (3−10). The friction coefficient decreases with the number of cycles and exhibits a more significant sensitivity to joint angles than shear cycles. The joint angle determines the asperities on the rupture surfaces and the block size, and thus determines the subsequent shear failure mode (block crushing and asperity degradation). At positive angles, block size is more greater and asperities on the rupture surface are smaller than at nonpositive angles. Therefore, the cyclic shear behavior is controlled by block crushing at positive angles and asperity degradation at negative angles.

Cyclic shear tests on rock joints serve as a practical strategy for understanding the shear behavior of jointed rock masses under seismic conditions. We explored the cyclic shear behavior of en-echelon and how joint persistence and test conditions (initial normal stress, normal stiffness, shear velocity, and cyclic distance) influence it through cyclic shear tests under CNS conditions. The results revealed a through-going shear zone induced by cyclic loads, characterized by abrasive rupture surfaces and brecciated material. Key findings included that increased joint persistence enlarged and smoothened the shear zone, while increased initial normal stress and cyclic distance, and decreased normal stiffness and shear velocity, diminished and roughened the brecciated material. Shear strength decreased across shear cycles, with the most significant reduction in the initial shear cycle. After ten cycles, the shear strength damage factor D varied from 0.785 to 0.909. Shear strength degradation was particularly sensitive to normal stiffness and cyclic distance. Low joint persistence, high initial normal stress, high normal stiffness, slow shear velocity, and large cyclic distance were the most destabilizing combinations. Cyclic loads significantly compressed en-echelon joints, with compressibility highly dependent on normal stress and stiffness. The frictional coefficient initially declined and then increased under a rising cycle number. This work provides crucial insights for understanding and predicting the mechanical response of en-echelon joints under seismic conditions.

En-echelon joints occur widely in rock masses and are often subjected to dynamic loads during earthquakes and rock bursts, which considerably influence stability in rock engineering. However, little is known of the cyclic shear behavior of en-echelon joints. This study conducted a series of cyclic shear tests on en-echelon joints under constant normal load (CNL) boundary conditions to investigate the cyclic shear behavior and dilation characteristics. The results show that shearing mechanisms of en-echelon joints were determined by joint angles during cyclic shearing, leading to differences in the evolution of shear resistance and dilation characteristics. A new phenomenon was observed, in that, shear stresses increased and decreased with an increase in the number of cycles during small and large shear displacements, respectively. The shear strengths and average dilation angles under cyclic shear loads decreased with increasing number of cycles. The difference in the peak shear stresses between en-echelon joints with different joint angles decreased with the number of cycles (N). Finally, dilation only occurred in the first several cycles, contraction predominated after several cycles, and the contraction of the specimens at a positive angle was larger than that at a negative angle. These findings hold crucial implications for predicting surface ruptures after earthquakes. • Effect of cyclic shear loading on en-echelon joints under constant normal load (CNL) is investigated for the first time. • Joint angles affected cyclic shear behavior and dilation characteristics. • Cyclic shear load can reduce shear strength and promote compression. [ABSTRACT FROM AUTHOR]