The mechanical properties of mortar materials in construction are influenced both by their own proportions and external loads. The trend of the stress–strain curve in cracks compaction stage has great influence on the relationship between the strength and deformation of cement mortar. Uniaxial compression tests of mortar specimens with different cement–sand ratios and loading rates were carried out, and the stored and dissipated energies were calculated. Results indicated that the elastic modulus and strength of mortar specimens increase with the cement–sand ratio and loading rate. The energy dissipation shows good consistency with the damage evolution. When the loading rate is less than 1.0 mm/min, most of the constitutive energy at the peak point is stored in the specimen and it increase with cement–sand ratio. A simple representation method of axial stress in cracks compaction stage was proposed and an energy-based damage constitutive model—which can describe well the whole process of cement mortar under uniaxial compression—was developed and verified.

There are a lot of fissures, holes, and other defects in the formation of natural rocks. Under the influence of the external loads, these defects may cause engineering problems. Therefore, it is of great significance to analyze the characteristics of damage evolution of the defective rocks. In the study, the double‐hole defective rocks with different angles of the center connection line are considered and the numerical models are established firstly. Then the mechanical behavior and acoustic emission (AE) characteristics are analyzed systematically. Finally the laws of damage evolution of the defective rock materials are investigated based on the AE characteristics. The research results show that the stress‐strain behavior of the defective rocks can be divided into elastic stage, plastic stage and failure stages. The characteristics of acoustic emission evolution and laws of damage evolution are closely related to the stress‐strain relationship. The elastic modulus of the double‐hole defective rocks is similar with different angles of the center connection line, but the peak strength is different. The shape of the peak strength of these defective rocks is a W type owing to the different failure modes. The influences of different angles of the center connection line on the characteristics of AE evolution include the maximum events number, the strain value of the initial AE events and the maximum AE events, and the strain range of the serious AE events. Different angles of the center connection line have different influences on the laws of damage evolution of the double‐hole defective rocks.

Basic rock mechanical parameters, that is, the uniaxial compressive strength σ_c and elastic modulus E, have close relationships with the fractal dimension and inhomogeneity. Scanning electronic microscopy and fractal dimension calculations are applied to four different rock types (mudstone, sandstone, limestone, and basalt) in order to investigate the relationships between the rock mechanical properties, fractal dimensions, and homogeneity. The results show that the fractal dimension of each rock type fluctuates as the scanning electronic microscopy magnification increases. Rocks with different uniaxial compressive strength and elastic modulus values possess different self-similarity properties, and when the uniaxial compressive strength or elastic modulus increases, the fractal dimension of the rock microstructure decreases. The rock homogeneity is consistent with the fractal dimension, that is, the higher the homogeneity is, the larger the fractal dimension. Generally, homogeneity refers to the macroscale, and fractal dimension refers to the microscale. Overall, this research provides an innovative and effective approach for researching the mechanical behavior of rocks through a combination of uniaxial compression tests, homogeneity, and fractal dimensions.

In order to evaluate the stability of deep surrounding rock, all of the affecting factors should be theoretically identified. However, some factors have slight impacts on the stability of deep surrounding rock compared with others. To conduct an effective risk assessment, key factors should be first extracted. The analytic hierarchy process (AHP) and grey relation analysis (GRA) methods are integrated to determine the key factors. First, the AHP method is applied to sort the factors by calculating the weights of them. Seven out of fifteen factors are extracted as the key factors, which account for 80% of the weights. Further, the GCA method is used to validate the effects of these key factors by analyzing the correlation between the performance of each factor and that of the reference. Considering the influence of these key factors and experts’ judgements, the multilevel fuzzy comprehensive evaluation method is adopted to obtain the risk level of the deep surrounding rock stability. Finally, the risk assessment of the deep surrounding rock in the E-Zhuang coal mine of Chinese Xinwen Mining Area illustrates the operability of the proposed method.

Natural rock masses have many internal joints, fractures and faults, which greatly influence their mechanical properties during geological processes. In this paper, considering different joint angles and rock-bridge angles, uniaxial compression numerical tests on double-fractured red sandstone based on particle flow code (PFC) were carried out. The influence laws of the mechanics, acoustic emission (AE) and damage evolution characteristics of double-fractured sandstone under different crack geometry conditions were analysed. The results indicated that the peak stress and elastic modulus increase with increasing fracture angle. The peak stress and elastic modulus of rock samples first increase and then decrease with the increase in the rock-bridge angle, exhibiting a nonlinear distribution; when the angle β (rock-bridge angle) is less than or equal to 45°, the peak strain differs only slightly and decreases gradually with the increase in the rock-bridge angle. There are three stages in the AE evolution of fractured red sandstone: The initial emission of AE, the increase in AE and the decline in AE. The influences of different fracture angles and rock-bridge angles on the AE characteristic rule of rock samples vary. The damage evolution process of sandstone specimens with different joints can be divided into four stages: Initial damage, stable increase, accelerated development and stable damage. The fracture angle mainly affects the damage stage. The smaller the angle of the crack is, the greater the strain value in this stage. The rock-bridge angle mainly influences the damage variable evolution during the stable increase, accelerated damage development and stable damage stages.

Rock burst hazards during deep mining are a dynamic phenomenon of the sudden release of elastic energy aggregated in the roof rock-coal combined body. In this paper, the occurrence mechanism of rock burst for roof rock-coal combined body was studied through energy dissipation theory. And the elastic energy attenuation degree of combined body was defined. Additionally, the effects of height ratio of roof rock to coal and elastic modulus of roof rock or coal on the rock burst were analyzed in the potentiality of the combined body. Results show that the elastic energy attenuation degree is positively correlated with the equivalent elastic modulus of combined body. And the equivalent elastic modulus is shown to a positive correlation with the height ratio of roof rock to coal and elastic modulus of roof rock or coal. Generally, the larger the height ratio and elastic modulus of roof rock or coal are, the great the equivalent elastic modulus is and the stronger the elastic energy attenuation degree is. And the roof rock-coal combined body easily occurs in rock burst. The above achievements are of great significance to understand the elastic energy attenuation characteristics and to prevent rock burst hazards of the roof rock-coal combined body.