The influence of abrupt changes of coal dip angle on coal and gas outburst and impact evolution characteristics

To investigate the influence of abrupt changes in coal dip angles within deep and complex geological structures on the evolution characteristics of coal and gas outburst and impact evolution characteristics, an interface with an abrupt change in dip angle is established by taking into account the practical situation of coal and gas outbursts. The upper side of the interface consists of virgin coal, while the lower side consists of briquetted coal, which are utilized to simulate the virgin and the tectonic coal, respectively. With monitoring units, such as impact force testers, acoustic emission detectors, and data acquisition systems, coal and gas outburst simulation experiments under three-dimensional stress conditions are conducted considering different conditions of abrupt dip angle (the initial angle of coal dip angle mutation θ_I, the mutation angle θ_M) changes. The influence of coal dip angle mutation on outburst intensity and impact parameters (peak impact force, maximum negative pressure, and duration of sharp change) are analyzed. The research results show that within the range of 10°<θ_I≤20° and 10°≤θ_M≤20°, a critical value of coal dip angle mutation θ_T exists. When both θ_I and θ_M are greater than or equal to θ_T, low-index outbursts are prone to occur. Given a constant θ_I, the critical gas pressure exhibits a negative correlation with θ_M, while the outburst intensity per unit shows a positive correlation with θ_M. The evolution process of impact force in the simulated roadway can be divided into rapid change stage, fluctuation change stage and stable change stage. The number and density of high-frequency pulse A in the positive pressure stage and high-frequency pulse B in the negative pressure stage are closely related to θ_M. With the increase of θ_M, the fluctuation of the impact force in rapid change stage becomes more complex, and the high-frequency pulses A and B gradually show a highly concentrated trend in this stage. The peak impact force and the maximum value of negative pressure stage have a negative linear relationship with the gas concentration factor I_θ, while the duration of the rapid change stage has a positive linear relationship with the gas concentration factor. The peak velocity of impact airflow and the cumulative AE energy show basically the same trend. Both of them have a negative linear relationship with the gas concentration factor, and exhibit more significant changes when θ_M is greater than or equal to θ_T.