Abstract
To clarify the nonlinear transformation of coal from elastic energy storage to macroscopic fracture under borehole unloading, this study develops a multistage loading–unloading experimental system, incorporates acoustic emission monitoring, and utilises techniques such as moment tensor inversion, time–frequency analysis, and stress field inversion to examine the fracture propagation mechanisms and stress response characteristics of coal. The experimental findings indicate that borehole unloading causes local stress disturbances, which significantly influenced the fracture propagation paths and failure modes, thereby governing the macro-damage evolution process. The three-stage source mechanism analysis reveals that shear failure predominates throughout, while tensile failure notably increases during the secondary loading stage, indicating stage-dependent restructuring of stress distributions within the coal. Stress field inversion results reveal that in the initial loading stage, the maximum principal stress aligns southwest–northeast. During the secondary loading stage, the stress field shifts to a southeast–northwest inclined shear-dominated configuration, promoting shear-tension coupled fracturing. Borehole diameter affects fracturing behaviour: a 10 mm-diameter borehole initiates local high-energy events that inhibit crack propagation, whereas a 12-mm-diameter borehole enables energy cascade release and chain-like destruction. Time–frequency analysis shows that shear fractures mainly concentrate at 80–90 kHz with short-term energy aggregation; tensile fractures occur at 40–50 kHz, displaying mid-frequency expansion characteristics; and compression fractures are characterised by continuous low-frequency outputs at 10–20 kHz. Through theoretical analysis, a borehole pressure relief coal elastic–plastic damage model was developed to elucidate the control mechanisms of lateral pressure coefficient, borehole diameter, and stress level on the evolution of the plastic zone. This study enhances the understanding of crack propagation mechanisms in coal subjected to drilling-induced pressure relief in mines.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgements
We gratefully wish to acknowledge the collaborative funding support from the National Natural Science Foundation of China (52104102).
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This work was supported by the National Natural Science Foundation of China (Grant No. 52104102).
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Kun Liu was responsible for data curation, software development, visualization, and writing of the original draft. Yang Liu contributed to funding acquisition, methodology development, resource provision, and writing – review and editing. Cai-Ping Lu performed formal analysis, supervision, and validation. Lu-Hao Zhou contributed to the conceptualization of the study. Qi-Xiang Zhou conducted the investigation, and Chao Wang was responsible for project administration.
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Liu, K., Liu, Y., Lu, CP. et al. Mechanism of coal mass fracture expansion under drilling and pressure relief. Sci Rep (2026). https://doi.org/10.1038/s41598-026-44731-x
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DOI: https://doi.org/10.1038/s41598-026-44731-x
