In the current work a new equation for initial damage assessment of limestone based on plane strain theory is proposed. Detailed investigations of the static and dynamic characteristics of limestone with different initial damage degree, using longitudinal wave speed, and static-dynamic compression tests are performed. This study investigated the static and dynamic characteristics of limestone with different initial damage degree, using longitudinal wave speed, and static-dynamic compression tests. Experimental results show that the degree of initial damage decreases with increasing longitudinal wave speed, which reaches the minimum when the longitudinal wave speed is approximately 6000 m/s, and the smaller the longitudinal wave velocity, the greater the degree of initial damage. The static and dynamic compressive strengths of limestone increase with the longitudinal wave velocity and strain rate, but the elastic modulus and Poisson’s ratio do not change significantly. Finally, based on the experimental results, the definitions of damage threshold value and strain softening are proposed, which further verify the influence of strain rate and initial damage on rock compression characteristics. The present study sheds light on the importance of initial damage for the mechanical state of rock in underground engineering.

Sinking and horizontal movements are necessary parameters for assessing the potential impacts of surface subsidence in mining activities. Based on similarity criteria, the surface subsidence mechanism was studied using a physical model composed of similar materials such as sand, cement, and gypsum. With constant field geological parameters maintained in two angles of a coal seam, models of roof subsidence of composite rock were compared for different mining configurations. In accordance with observations from the physical model, it was concluded that subsidence and horizontal movement of strata near to and far from the coal seams were different and divided into five zones. The zone above a mined-out area underwent greater total subsidence compared to unexploited regions on both sides. Correlations between a subsidence curve and the height of a caving zone and the mining dip angle were obtained and verified from numerical model results. According to the roof’s position relative to the goaf, the area above the goaf of the composite rock layer was divided into three regions: a curving zone, a water-conducting fracture zone, and a falling zone, to which the subsidence and movement characteristics of each area could be proposed. Compared with the subsidence and movement characteristics observed from the physical and numerical model, the acquisition of subsidence characteristics and parameters in different areas can provide an idea for improvement, innovation or proposal of a theoretical formula for subsidence prediction of composite rock formations.

The raising and flowing of groundwater caused by coal mining threaten the stability of mining faces, which cause casualties and machine damage accidents. Among the above accidents, the water inrush disaster caused by the water‐rich water‐conducting fault zone is the largest. Considering the complexity of geological structure and the suddenness of water inrush, reserving a reasonable thickness of waterproof coal pillars in front of the fault tectonic belt can effectively predict and control the occurrence of water inrush. The excellent adaptability of the numerical model to the geological conditions makes it an effective research method for simulating waterproof coal pillars. Based on the analysis of the background of on‐site mining, this paper proposes a three‐zone waterproof coal pillar calculation theory and establishes a numerical model for comparative analysis. The comparison results show that (1) the elastic‐plastic theory and fracture theory can be used to calculate the thickness of the disturbed zone and the water‐resisting zone, and the thickness of the fractured zone is positively correlated with the accuracy of the existing detection technology and equipment. (2) For the numerical model results, the increase of tangential stress is positively correlated with the distance of coal seam mining and the thickness of fault; the large plastic zone of the fault causes a higher increase in pore pressure, which ultimately increases the risk of water inrush. (3) The two results are in good agreement. The theoretical results have a safety margin, indicating that the three‐zone theory is reasonable, which are used to guide the actual mining of the project to ensure the smooth passage of the project through the fault area.

Previous research results preliminarily indicated that the Coal Failure Events (CFEs) that occurred during the process of Underground Pressure Relief Drilling (UPRD) represented the phenomenon of coal fracture and energy release. The research results had excellent value for the monitoring and response of pressure relief drilling while drilling, but there were still some special situations that needed to be analyzed and studied in actual on-site testing. So, through on-site testing and data statistical analysis, the study investigated the applicability of the innovative external Monitoring-While-Drilling (MWD) method for UPRD with more coal failure events and made a quantitative statistic of the CFEs and their relationship with abutment pressure to reveal the applicability of the external MWD method and characteristic of CFEs. The results showed that hundreds of CFEs were produced in the UPRD process, which must be removed to ensure the accuracy of the MWD method. Although CFEs bring recognition difficulties, they also provide conditions for studying their own distribution and characteristics. Results showed that more CFEs were produced in the depth of difficult drilling, which indicated that there was a positive correlation between the degree of difficulty in drilling and the number of CFEs. In addition, spectrum analysis showed that the depths with more CFE occurrence were more likely to produce high-frequency events. When the surrounding stress of drilling rocks is high, the occurrence of small fractures with a higher main frequency may become more frequent and consistent; more fractures with similar failure forms would occur, which may have a lower fractal dimension and promote the generation of more failure. The research results were of great significance for the MWD method for UPRD, a quantitative study of CFEs and their generation characteristics during UPRD construction.