Quantifying the mechanical properties of coal matrix and cleat using digital image correlation method

Coal mass consists of matrices and cleats, which exhibits significant difference in mechanical properties, such as uniaxial compressive strength and Young's modulus. Understanding this difference is critical for a number of engineering applications, such as assessing the stability of cleated coal seam gas wellbores, underground excavation stability in coal seams, and estimating cleat aperture response during gas extraction and surface response to reservoir depletion. The conventional method of measuring coal mechanical properties using strain gauges or displacement transducers is impractical and unreliable as it only captures the value for the installed point. This study explores the use of a two-dimensional Digital Image Correlation (2D-DIC) method to quantify the areal deformation of coal matrix and cleat regions and their contribution to the bulk mechanical properties of coal. Cyclic uniaxial compression tests were performed on coal specimens from the Goonyella Middle Seam, Australia. The results from the DIC technique were initially validated against strain gauge and Advanced Video Extensometer (AVE) measurements, showing minimal percentage differences: 5 % with the strain gauge; 16.6 % with the coal cleat region, 12.03 % with the coal matrix region, and 9.28 % with the coal bulk region compared to AVE. These results demonstrate that DIC is a reliable and accurate method for measuring coal deformation. Comparative analysis of cleat, matrix, and overall coal surface regions revealed distinct variations in Young’s modulus, with ratios of E cleat :E matrix :E overall = 0.24:1.60:1.00. The calculated cleat and matrix moduli are 143.6 MPa and 1785.3 MPa respectively. The contributions of E matrix and E cleat to the overall Young’s modulus ( E overall ) were quantified, revealing that the matrix accounts for 56 % (A = 0.56) and the cleat for 44 % (1 - A = 0.44) of the overall modulus. The compressibility of the cleat shows six times that of the coal matrix ( C cleat :C matrix :C overall = 4.24: 0.62: 1.00), highlighting the critical role of cleats in coal deformation and stress-induced permeability changes. Furthermore, Poisson’s ratios computed from the DIC for the tested coal samples range from 0.19 to 0.33, showing strong agreement with reported values in the literature. By integrating DIC analysis with traditional mechanical testing, this study offers a robust approach to evaluating full-field deformation mechanisms in fractured materials. These findings advance the understanding of coal's mechanical properties, which in turn supports more accurate geotechnical modeling, optimizes mining design, and enhances coal seam gas extraction strategies. • Experimental measurements of coal cleat, matrix and full-field deformation by integrating DIC and cyclic uniaxial compression for the first time. • Demonstration of significant Young’s modulus reduction and compressibility increase in cleat regions due to the influence of fracture. • Quantification of cleat and matrix contributions to bulk mechanical properties.