Three-Dimensional Reconstruction and Evolutionary Distribution Characteristics of Coal Wall Fractures in a Large-Mining-Height Longwall Face

Technical Summary: Three-Dimensional Reconstruction and Evolutionary Distribution Characteristics of Coal Wall Fractures in a Large-Mining-Height Longwall Face

Problem Statement
Rib spalling is a prevalent manifestation of strata pressure behavior in large-mining-height working faces, posing significant risks to coal wall stability and production safety. This phenomenon is driven by a complex interplay of factors, including coal strength, mining height, roof pressure, and the distribution of advance abutment pressure. Crucially, coal damage is not solely a result of mining-induced stress but is also governed by the inherent, concealed, and heterogeneous fracture networks within the coal mass. As the working face advances, stress concentration promotes the propagation and development of these fractures, potentially leading to instability. However, the concealed nature and complex three-dimensional distribution of these fractures make their evolution characteristics difficult to characterize using traditional methods, creating a gap in understanding the specific mechanisms of fracture development in large-mining-height environments.

Methodology
To address these challenges, the study utilized the 22523 working face of Halagou Coal Mine as an engineering background, employing a multi-stage approach combining theoretical analysis, numerical simulation, and advanced imaging:

1. Stress Analysis and Sampling Strategy:

   • Theoretical Analysis: The study applied limit equilibrium theory to calculate the distribution of advance abutment pressure, determining the theoretical peak position and influence range.
   • Numerical Simulation: A FLAC3D model was constructed based on the specific geological conditions of the 22523 face (5m mining height, 150m burial depth). The model simulated excavation lengths of 20m to 100m to observe the evolution of vertical stress and abutment pressure peaks.
   • Sampling: Based on the combined theoretical and simulation results, undisturbed coal samples were collected at four distinct distances from the coal wall: 1 m (post-peak region), 4 m (near peak pressure), 6 m (within influence range), and 40 m (original rock stress zone, minimally affected).

2. Industrial CT Scanning and Reconstruction:

   • Scanning: Collected cylindrical specimens (φ50 mm × 100 mm) were subjected to industrial computed tomography (CT) scanning using a flat-panel detector at 360 kV and 1.3 mA, achieving a resolution of 52 μm.
   • Preprocessing: CT images underwent batch processing using ImageJ, including contrast adjustment to enhance fracture-matrix distinction and Gaussian filtering to reduce system noise.
   • Extraction and 3D Reconstruction: A threshold-based binarization method was applied to extract fracture features (assigning pixels to 0 or 255). The processed data was imported into Avizo software for three-dimensional reconstruction, allowing for the visualization of the fracture network's spatial morphology.

3. Quantitative Analysis:

   • Fracture Ratio: Calculated as the ratio of fracture area to total slice area in representative 2D slices to quantify fracture density.
   • Orientation Analysis: Fracture rose diagrams were generated using 10° intervals to analyze the dominant orientations and spatial distribution of fractures.

Key Results

• Stress Distribution: Both theoretical calculations and FLAC3D simulations indicated that the peak advance abutment pressure occurs approximately 4.85–5.2 m ahead of the working face, with an influence range extending roughly 32.65–35 m.
• Fracture Density Evolution: A clear inverse relationship was observed between the distance from the coal wall and fracture density. The CT-derived fracture ratios were:
  • 1 m from wall: 1.19%
  • 4 m from wall: 0.63%
  • 6 m from wall: 0.56%
  • 40 m from wall: 0.01%
    This confirms that mining-induced disturbance significantly increases internal fracture development. The sample at 40 m contained only primary fractures, while samples closer to the face exhibited complex, connected, and intersecting fracture networks.
• Spatial Morphology and Orientation: The study revealed significant differences in three-dimensional fracture distribution patterns based on the degree of mining influence. As mining disturbance intensified, not only did the number of fractures increase, but the dominant fracture orientations also shifted. For instance, the dominant orientations varied from concentrated ranges of 0°–10° in the 4 m sample to −40° to −50° and 40°–50° in the 1 m sample. This indicates that mining stress promotes the opening, propagation, and recombination of primary fractures, altering their spatial alignment.

Significance and Claims
The paper claims that its findings provide a reference for analyzing the fracture development mechanism of coal walls in large-mining-height working faces. By revealing the distance-dependent evolution of fracture density and orientation, the study offers a basis for evaluating coal wall stability and assessing rib-spalling risks under similar geological conditions.

The authors maintain a modest stance regarding the broader applicability of their results. They explicitly acknowledge limitations, noting that the CT-derived fracture ratio represents a relative indicator based on representative slices rather than absolute volumetric porosity. Furthermore, they state that the conclusions are primarily applicable to the 22523 working face and similar large-mining-height longwall faces, and that further field validation is required before the results can be directly utilized for quantitative rib-spalling prediction. The study emphasizes that the observed evolution from a weakly developed primary state to a mining-induced damage-enhanced state is a critical factor in understanding the mechanical behavior of coal walls in these specific mining environments.