Coherence analysis of the crack strain field in coal rock with borehole-crack composite defects

Abstract

Cracks in the coal mass surrounding boreholes for gas extraction will significantly impact the efficiency of gas extraction. The primary objective of this research is to explore the effects of original cracks and borehole grouting on cracks propagation in the coal mass around the boreholes. Accordingly, this research employs coal mass with borehole, borehole-cracks composite coal mass, and borehole-cracks-grouting coal mass as experimental subjects. The digital image correlation (DIC) platform of the coal deformation and damage was used to carry out the damage experiment of the coal mass around the boreholes. The Matlab GUI crack detection system was used to extract the crack skeleton, and time domain characteristics of strain and strain rate coherence coefficients in the area near the cracks before and after grouting were obtained according to the Pearson product-moment coherence analysis method. The research findings obtained in this article are as follows: (1) The tips of the prefabricated fractures act as stress concentration zones, where the further extension of wing cracks severely degrades the continuity and uniformity of the coal mass. The injection of ultrafine cement significantly enhances the overall deformation resistance of the coal mass. However, the local stiffness under specific stress conditions shows limited improvement due to the presence of micro-damage around the prefabricated cracks. (2) The expansion of the cracks around the borehole and the wing cracks at the tips of the prefabricated cracks is primarily due to tensile forces. As the cracks extend to the top of the specimen, they transition to a mixed mode of tensile-shear, with the extension of Type II wing cracks being faster and more unstable compared to Type I. (3) The correlation coefficient between multiple crack fields and their highly significant characteristics indicate that crack propagation influences each other. This influence diminishes as the specimen progressively damage. (4) The mechanical properties of the specimen show a strong correlation with the average interference coefficient R̅ of the cracks. Better mechanical properties of the coal rock are associated with higher geometric similarity in the strain and strain rate curves across multiple strain fields, the cracks tend to propagate synchronously.