Abstract
Accurate characterisation of the three-dimensional (3D) fracture network of coal is of vital significance to enhancing coal seam permeability during simultaneous extraction of deep coal and methane resources. The limitations of traditional experimental methods prevent direct in situ observation and characterisation of the 3D fracture network and its evolution during loading processes. This study presents a novel approach that incorporates computed tomography and servo-controlled triaxial loading techniques to accomplish the laboratory in situ observation of the continuous evolution of 3D fracture networks inside coal samples which were subject to confining pressures and axial compressive loads. Spatial growth and morphologies of the interior fractures at various loading stages were captured in situ and extracted using imaging processing algorithms. The 3D fracture networks observed at different loading stages were quantitatively characterised using fractal theory and compared to evaluate the influences of confining pressures and vertical loads on their evolution. The results indicated that the original existing fractures of coal closed when the specimens were subject to confining pressures and vertical compressive deformation were in the linear elastic stage. Load-induced fractures expanded notably only when the axial compressive load reached the maximum value. The fractal dimension of the 3D fracture network tended to decrease initially and subsequently increased during the loading process, which reflects the evolutionary characteristics of coal fractures from a closed to an expanded state.
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Abbreviations
- F :
-
Fractal sets
- D B :
-
Box-counting fractal dimensions
- δ :
-
The side length of covering boxes
- η k :
-
Grid size of the kth covering
- η k+1 :
-
Grid size of the (k + 1)th covering
- N δ (F):
-
Number of boxes covering the fractal set F
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Acknowledgements
The authors sincerely appreciate the constructive and valuable comments from the editors and anonymous reviewers to help improve this manuscript. The authors also gratefully acknowledge financial support from the State Key Research Development Program of China (Grant no. 2016YFC0600705), the National Natural Science Foundation of China (Grant nos. 51674251, 51727807, 51374213), the National Major Project for Science and Technology (Grant no. 2017ZX05049003-006), the National Natural Science Fund for Distinguished Young Scholars of China (Grant no. 51125017), Fund for Innovative Research and Development Group Program of Jiangsu Province (Grant no. 2014-27), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (Grant no. PAPD 2014).
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Ju, Y., Xi, C., Zhang, Y. et al. Laboratory In Situ CT Observation of the Evolution of 3D Fracture Networks in Coal Subjected to Confining Pressures and Axial Compressive Loads: A Novel Approach. Rock Mech Rock Eng 51, 3361–3375 (2018). https://doi.org/10.1007/s00603-018-1459-4
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DOI: https://doi.org/10.1007/s00603-018-1459-4