Investigation of the damage characteristics of rock material under the combined effect of cyclic load and impact load is extremely crucial for many mining engineering applications. Based on energy dissipation theory, we considered factors such as circulation times, the cyclic stress of a uniaxial cyclic load test, and the impact pressure (strain rate) of a split Hopkinson pressure bar (SHPB) test, studying the damage mechanism of red sandstone under the combined action of a uniaxial cyclic and impact loads. We found that when the load stress is 60%, 70%, and 80% of the uniaxial compressive strength (UCS) of red sandstone, the stress can still promote the development of microcracks and the generation of new cracks in the rock, increasing the inner damage of the rock so that it reduces the rock strength. As the cyclic time increases, the energy dissipation ratio presented a trend of decreasing dramatically and then maintaining basically no change, and the peak strain and the damage variable of the rock both increased first and then tended to remain relatively constant. The damage variable of the rock specimen under the combined action of the uniaxial cyclic loading and impact load had a significant corresponding relationship with the initial damage. As the rock specimen initial damage increases, the damage variable and the peak stress of the rock specimen both decreases almost linearly with initial damage. Meanwhile, the average particle diameter of the rock specimen after breakage gradually increased, showing a positively correlated tendency. The ratio between the fragment quality of the bigger particle diameter in the total rock specimen quality gradually increased. Under the conditions of the same initial damage, a higher impact pressure resulted in smaller rock fragment average particle diameters.

Acoustic emission (AE) source location is an effective method used to reveal the deformation and damage characteristics of materials. Improving the positioning accuracy of AE events is extremely important for both experimental research and engineering practice. An experiment was performed on a standard rock specimen to investigate the key factors that affect the positioning accuracy of an AE event, such as the computational combinations and spatial layout of the sensors and the picking accuracy of the first arrival signal. The results show that as the number of sensors involved in the positioning calculation increases, both the maximum and average positioning errors clearly decrease, and the positioning accuracy increases. Meanwhile, a better result is achieved when the number of sensors involved in the positioning calculation reaches three-fourths of the total number of monitoring sensors used. Under this condition, both the minimum positioning error and the error dispersion are sufficiently low, and the reliability of the positioning results is greatly improved. Furthermore, when different types of and deviations in the first arrival signal are used for the location calculation of the same AE event, the positioning results are quite different, and large errors are found. Finally, the accurate positioning of an AE event is realized by adjusting the number of sensors and improving the picking accuracy of the first arrival time of the signal. The maximum reduction in the positioning error achieved is 96.62%, and the minimum reduction is 57.61%. Hence, the positioning error decreases significantly. The investigation results can provide an important reference for the precise positioning of AE events involved in experimental research and engineering.

In this paper, uniaxial cyclic compression and shear test was carried out for lignite samples. The effects of inclination angle (θ) and upper limit of cyclic stress (σ_max) on mechanical properties of coal samples were analyzed, and the damage variables of coal samples were studied based on energy dissipation theory. The results show that the uniaxial compressive strength (UCS) of coal samples after uniaxial cyclic compression and shear tests decreases with the increase of the upper limit of cyclic stress and inclination angle. The shear stress component generated by the increase of inclination angle can effectively reduce the UCS and increase the damage degree of coal samples. With the increase of inclination angle, the failure mode of coal samples is changed from tensile failure (θ = 0°), the combined tensile failure and shear failure (θ = 5°) to shear failure (θ = 10°). The peak axial and radial strain of coal samples first increases rapidly and then stagnates. The peak volume strain rapid increases and then stagnates (θ = 0° and θ = 5°). When the inclination angle is 10°, the peak volume strain first decreases rapidly and then stagnates. Even if the upper limit of cyclic stress is lower than its UCS, it will still promote the propagation of micro cracks and the generation of new cracks and increase the internal damage of coal samples. With the increase of the cycle number, damage variables of coal samples after uniaxial cyclic compression and shear tests nonlinearly increase, and the growth rate decreases gradually.

Purpose. The purpose of the scientific article is to conduct a numerical analysis of the metro running tunnel to determine, based on the results, the dependence of the lining's stress-strain state on its shape. To achieve this goal, a modeling of a tunnel with a circular outline and an ovoid shape was performed. Methodology. LIRA-CAD is a multifunctional software package for calculation, research and design of structures for various purposes. It is based on the use of the finite element method (FEM), recognized worldwide as the main tool for numerical analysis of the building structures’ strength and stability. Calculation of forces, modeling of changes in the stress-strain state of the structure at the stages of its construction and research of the operation of building structures using the finite element method are performed after specifying the materials and properties of the elements. Findings. Finite element models of the soil massif and tunnel linings of various shapes (circular outline and ovoid shape) were developed for numerical analysis of the system. An analysis of the results of the stress-strain state of the soil massif and tunnel linings was performed, which allowed to draw a conclusion about the discrepancy of the results for different lining shapes. The analysis was conducted for the stress-strain state of linings of various shapes, taking into account the method of construction and their waterproofing, allowing to conclude that ovoid linings, which are constructed by NATM, have advantages in strength and durability. Originality. The originality of the scientific research lies in the analysis of the stress-strain state of the metro running tunnel lining in various engineering and geological conditions, which provides an opportunity to reasonably use calculation methods, the results of which reflect the most reliable operation of the structure for specific engineering and geological conditions. Practical value. The practical value lies in the fact that the developed methodology allows for considering underground structures of complex spatial forms, various models of soil behavior, taking into account the technology of tunnel construction.