Characteristics of Grouting-Induced Microfractures in Fractured Rock Masses: Numerical Simulation, Microseismic Monitoring, and Laboratory Tests

In deep mining engineering, grouting operations, although designed for reinforcement, inevitably induce microfracturing and associated microseismicity. Investigating the characteristics of grouting-induced microfractures in fractured rock masses is crucial for evaluating the grouting process and its effectiveness. Using the Wutongzhuang Mine as a case study, this paper first establishes mechanical criteria covering three stages—fracture filling, coupled permeation, and fracturing propagation—to analyze the process characteristics of grouting-induced microfractures. It reveals the mechanisms by which grouting pressure, in situ stress, and rock mass strength control fracture initiation and propagation. Furthermore, a grouting simulation method based on the Particle Flow Code (PFC) is proposed and summarized, constructing a “pipe-domain” fluid network considering fluid–solid coupling, thereby achieving a refined numerical reproduction of the entire grouting process. Addressing the complex geological conditions of the mine, three typical grouting modes are simulated and analyzed: grouting under conventional geological conditions, grouting under densely fractured conditions, and grouting near fault structures. The simulation results unveil their core influencing factors and behavioral characteristics: under conventional conditions, microfractures exhibit a “three-stage” evolution with the grouting process; under densely fractured conditions, the density of pre-existing fractures dominates the formation of complex fracture networks; and finally, fault structures guide fracture propagation, causing microfractures to cluster nearby. Subsequently, the development trends of microfractures under different grouting effects are clarified: after effective reinforcement, the rock mass strength increases, and the scope and quantity of fractures induced by subsequent grouting significantly decrease. The behavioral patterns under these different grouting modes are effectively validated through field microseismic monitoring, confirming the intrinsic relationship between the spatio-temporal evolution of grouting-induced microfractures and geological structures/grouting techniques. Finally, laboratory tests are conducted using a self-developed experimental apparatus, selecting grouting pressure, pore water pressure in the rock mass, and matrix grain size as variables. The mapping relationships between these variables and microseismic waveform characteristics, amplitude, etc., under different schemes are obtained, providing a basis for inverting the microfracturing process and evaluating grouting effectiveness. The research results provide multi-faceted references for characterizing the stability of fractured rock masses via microseismic monitoring and for optimizing grouting effectiveness.