Influence of pore-fracture structure characteristics of coal rock on its damage evolution law

The development and expansion of fractures in coal rocks are the primary causes of mining disasters, such as gas outbursts, rock bursts, and instability of coal pillars. Investigating the fracture evolution process and damage evolution law in coal rocks is crucial for predicting dynamic hazards in coal mines. Fracture inclination angle and porosity are significant factors influencing the propagation behavior of fractures in coal rocks. In this study, we conducted uniaxial compression tests, CT electron scanning, and nuclear magnetic resonance experiments to analyze the mechanical properties and pore-fracture structure of coal rocks. We characterized the fractal dimension of internal fractures in coal rocks by analyzing their distribution characteristics along with fracture inclination angles. Additionally, numerical simulations were employed to analyze the propagation and damage evolution laws of fractures with different inclination angles in coal rocks. The research findings demonstrate that: ① The degree and distribution characteristics of fracture inclination angles significantly impact the complexity and fracturing behavior of coal rock; when a single dominant fracture exists, steeply inclined coal rock exhibits higher compressive strength compared to gently inclined or nearly vertical ones; compressive strength is higher for single-fractured coals than complex-fractured ones. ② Porosity is the primary factor influencing the compressive strength of coal rock of the same type. As porosity increases, the compressive strength of coal rock decreases and exhibits a direct correlation with the presence of large pores. ③ Single-fracture coal and rock exhibit a lower level of complexity compared to complex fractured ones. This results in reduced stress concentration during compression, leading to less initial damage. However, during accelerated crack propagation, there is a rapid release of stored strain energy, resulting in higher levels of shear failure. ④ By establishing a variable for coal and rock damage based on the number of developed fractures, it can be observed that the evolution process follows an exponential growth pattern. This process can be divided into stages including approximate intactness, initial damage, stable development of damage, accelerated development of damage, and residual damage. Such observations provide theoretical support for predicting mining disasters.