Mechanical Behaviour of Strata under the Coupled Influence of Fluid Flow and Geo-Stress in Underground Mining

<p dir="ltr">Groundwater significantly affects the safety and stability of underground mining. It can activate strata fractures and weaken rock masses. Mining activities may trigger the release of water from high-pressure water-bearing strata, leading to stress in overburden strata, increasing risks of mining disasters. This study addresses the engineering challenges related to the coupling of high-pressure water and geo-stress in underground coal mines, focusing on the hydro-mechanical interactions that govern fracture development, water inflow, and strata instability. The effects of water on weakening coal and rock, along with the influence of pore water pressure on their mechanical behaviour, were investigated. The mechanisms of high-pressure water release in overburden aquifers during mining, and its interactions with the stress and fracture fields were examined. This research aims to enhance the understanding of groundwater and geo-stress coupling mechanisms and offers valuable insights for addressing real-world challenges in actual engineering cases. The study primarily focuses on the following aspects:</p><p dir="ltr"><br></p><ol><li>Using three-dimensional Computed Tomography (3D-CT) scanning technology, the water distribution characteristics within coal specimens during the saturation process were studied. Uniaxial compression tests were then conducted to assess the mechanical properties and bump proneness of coal specimens at various degrees of water saturation. Additionally, overburden sandstone was subjected to water immersion and uniaxial compression tests. The results show that coal specimens exhibited greater strength and higher bump proneness in a dry state. Saturated coal retained significant bump proneness despite reductions in strength and Young’s modulus. Sandstone specimens show higher sensitivity to water, with pronounced changes in mechanical properties upon saturation.</li><li>An improved discrete element numerical (DEM) method was developed to simulate the hydro-mechanical coupling process in fractured rock masses. This enhanced modelling framework incorporates fracture development, dynamic evolution of pore pressure, and two-way interactions between seepage and stress fields, enables more accurate simulations of internal seepage behaviour in rock masses with complex fracture networks. An engineering model based on one of the Australian coal mines was then constructed to investigate the effects of high-pressure water on the stress field and hydro-mechanical coupling behaviour of the overburden rock as the longwall coal face advances. The results indicate that high-pressure water in the water-bearing strata increases tensile stress in the lower overburden rock, potentially promoting coal roof instability and fracture development. However, water pressure influence decreases as confined water is progressively released during mining.</li><li>Based on actual engineering cases, a mining model was developed for the longwall working face with a high-pressure water-bearing strata, and the evolution of the overburden stress field, flow field, fracture field, and water inflow during the advancement of the longwall face was studied. The results indicate that high-pressure water intensifies stress concentrations in the strata and accelerates fracture development. In turn, the formation of fractures facilitates the release of confined water, significantly lowering water pressure in the water-bearing strata near the fracture zone. Numerical trends closely matched field observations, with water inflow initially low but increasing significantly after periodic overburden failure.</li><li>A 2D numerical model examined fault activation mechanisms and their impacts on strata stability and water inflow. The research reveals that when the working face nears the fault, fractures caused by high compressive stress concentration in front of the face interact with the fault, activating its water-conducting capacity. This creates a continuous water-discharge path between the fault and the overlying water-bearing strata, resulting in a surge in water inflow at the working face and forming the peak water inflow during the mining process.</li><li>The outburst propensity of coal seams under coupled gas pressure and geo-stress was investigated using DEM simulations for both “normal” and “rapid” longwall drivage conditions in an Australian coal mine. The study incorporated variations in gas pressure, presence of gas pockets, and geological faults. Results indicate that elevated gas pressures significantly weakened coal-rock masses, increasing outburst risks, especially near faults and gas pockets. Rapid mining reduced roof deformation but still showed instabilities near high-pressure gas zones.</li></ol><p dir="ltr">This study advances critical knowledge on the coupling of high-pressure water and geo-stress in underground coal mines, their effects on strata stability, water release, and fracture development. The research offers valuable insights for improving mining safety, managing groundwater inflow, and optimising mining operations to mitigate potential mining hazards.</p>