Understanding the asperity damage behaviors of joints during shearing is critical for evaluating the stability of deep underground engineering structures. In this paper, we prepared plaster joints and used them for direct shear tests under different normal stiffness (0–7 MPa/mm) and various shear rate (0.5–20 mm/min) conditions. The effects of normal stiffness and shear rate on mechanical behavior and AE characteristics were studied. With the increase of normal stiffness, the damaged area of the surface of the joint and the weight of the damaged, rough body basically show a linear increase. With the increase of the shear rate, the peak shear stress and the final shear stress of the joint are non-linearly decreased (the decrease rate at the shear rate of 0.5–5 mm/min is much larger than that at the shear rate of 5–20 mm/min), more local cracks appear on the surface of the joint, and the dilatancy of the joint slightly decreases. More than 60% of the acoustic emission signals in the shearing process of the joint are concentrated in the post-peak phase. With the increase of normal stiffness, the cumulative number of acoustic emission impacts and cumulative energy both increase. With the increase in shear rate, the accumulated acoustic emission impact number decreases, and the accumulated AE energy tends to increase when the shear rate is 0.5–5 mm/min and decreases when the shear rate increases to 5–20 mm/min.

Anisotropic discontinuity exists widely in rock masses of mines, tunnels, slopes, water conservancy and hydropower projects. The shear characteristics of bolted anisotropic rock joints are extremely important for the stability design of engineering rock mass. However, few scholars have studied the bolted anisotropic rock joint. The different rock properties on both sides of the rock joint, especially the different rock strengths, will greatly affect the deformation characteristics and failure mode of the rock mass. Based on this, a series of shear tests were carried out on the bolted anisotropic rock joint under different normal stresses, and the characteristics of shear stress–shear displacement curve, shear strength, failure characteristics of the rock joint and deformation characteristics of the bolt are discussed. λ is defined as the strength ratio of upper and lower rock on the structural surface. The results show that the effect of λ on the shear stress–shear displacement curve is not obvious at the pre-fracture stage. The shear stress–shear displacement curve at the pre-breaking stage of the bolt presents a softening stage when the normal stress is equal to 0.5 MPa, tends to be horizontal when the normal stress is equal to 1 MPa and presents a hardening stage when the normal stress is greater than 1 MPa. After the bolt is broken, the shear stress–shear displacement curve presents a stepped-down descent. With the increase in λ, the breaking shear stress of the bolt increases. Elliptic failure occurs on the surface of the bolted anisotropic rock joint, and the length of the major axis of the ellipse decreases with the increase in λ value and normal stress. The bolts with different λ values of anisotropic rock joint show “Z-shaped” tensile bending deformation characteristics after shear fracture, and the horizontal and vertical components of the bolt deformation decrease with the increase in λ value and normal stress. The fracture shear displacement of the bolt increases with the increase in normal stress and decreases with the increase in λ value. The research results are helpful to further understand the shear mechanical characteristics and differences of bolted rock joints and provide a reference for solving the engineering problems of the composite layered rock mass.

The physical and mechanical properties, microstructure degradation characteristics, and damage mechanism of natural gypsum rock under 15, 30, and 45 freeze–thaw cycles were studied by using a self-developed program-controlled freeze–thaw experimental device. The results show that compared with natural gypsum, with the increase in freeze–thaw cycles, the mass of the sample increases first and then decreases, Poisson's ratio increases, the density decreases, and the longitudinal wave velocity, the uniaxial compressive strength, and the elastic modulus decrease. Indexes decrease greatly in the early stage of freeze–thaw and slowdown in the later stage. With the increase in freeze–thaw cycles, the slope of the pre-peak stage of the stress–strain curve decreases, the compaction stage is significantly prolonged, and the brittleness of the post-peak failure process decreases. Failure cracks of the specimens after freeze–thaw cycles are mainly inclined shear failure and axial tensile cracks. The analysis shows that the freeze–thaw damage of gypsum rock is caused by the superposition and mutual promotion of water-rock softening and pore frost heave. The specific performance is the damage of crystal particles and the loose pore structure at the mesoscale. On this basis, the cracks gradually develop and expand, which is consistent with the macroscopic test results of the sample. The research results have reference significance for the construction scheme design and frost damage prevention of gypsum surrounding rock tunnels in cold regions.

To analyze the shear characteristics and mesoscopic failure mechanism of irregular serrated rock discontinuities, a great deal of interview samples of irregular serrated structures were made by 3D printing technology, and laboratory shear tests were carried out on them under different normal stresses. At the same time, PFC numerical simulation software is used to establish relevant models to study the evolution of microcracks and the distribution characteristics of the force chain on the rock discontinuity during the shear process. The results show that the shear mechanical properties of irregular serrated rock discontinuities are affected by normal stress, undulating angle, and undulating height. The shear strength increases with the increase of normal stress and undulating height, and decreases with the increase of undulating angle. The numerical simulation results show that the irregular structural surface cracks under different undulation angles, which first start at the near force end serration root on both sides and further evolve to the adjacent serrations, while the irregular structural surface cracks under different undulation heights, which first start at the serration root with the lowest height and expand to the adjacent serrations. At the same time, the number of cracks increases with the increase of normal stress and the force chain is mainly distributed near the sawtooth surface. The force chain is more concentrated near the near force end sawtooth and at the tip and root of the rest of the sawtooth. At the same time, the direction of the force chain is approximately perpendicular to the force surface of the sawtooth. The research results are helpful in further understanding the shear mechanical properties and differences of irregular serrated rock discontinuities.

To explore the mechanical failure characteristics of bolted joints under different bolt installation angles and the effect of bolting on the shear strength of joints, a numerical model of structural plane anchoring with different bolt installation angles was established based on the improved Pile element, and a series of uniaxial compression numerical tests were carried out to systematically study the effects of bolt installation angle on bolts. The results show that as the bolt installation angle increases, the peak stress of the specimen is first constant and then decreases, and the elastic modulus of the specimen decreases nonlinearly. When the bolt installation angle is lower than 45°, the bearing capacity of the joints is higher. The interaction between the bolt and the specimen’s force is mainly concentrated at the intersection of the structural plane and the area where the nut gaskets are installed at both ends of the bolt. The horizontal stress is higher in the area where the nut gaskets are installed at both ends of the bolt. With an increase in bolt installation angle, the plastic zone volume of the anchored joint specimen increases linearly with an exponential function. When the bolt installation angle is lower than 45°, the plastic zone volume increases slowly, and when the bolt installation angle is higher than 45°, the plastic zone volume increases rapidly. When the bolt installation angle is small, the contribution of the bolt axial force is greater than that of the bolt shear force. In contrast, when the bolt installation angle is large, the contribution of the bolt axial force is lower than the contribution of the bolt shear force. With an increase in bolt installation angle, the contribution of the bolt axial force decreases nonlinearly, the contribution of the bolt shear force increases linearly, and the shear resistance decreases nonlinearly. The optimal bolt installation angle is about 45°, but the optimal bolt installation angle also changes constantly under the influence of factors such as bolt type, rock strength, and external load.