Analysis of failure modes in coal–sandstone composite structures with varying height ratios using denoising-based clustering

Coal–rock composite structures are prevalent in deep coal mining operations, and their mechanical behavior plays a crucial role in maintaining tunnel stability and understanding the mechanisms of dynamic disasters, such as rock bursts. In this context, the present study investigates coal–sandstone composite specimens through systematic laboratory uniaxial compression tests. The experimental results reveal that failure predominantly occurs in the coal region, which is the key factor governing the overall mechanical performance of the composite. Specifically, under uniaxial compression, as the coal–sandstone height ratio increases from 1:1 to 2:1 and 3:1, the peak strength drops from that of pure sandstone (46.17 MPa) to 25.17/21.77/19.52 MPa, corresponding to reductions of 45.47%, 52.83%, and 57.75%, respectively. Meanwhile, the peak strain increases overall; for reference, pure sandstone exhibits a peak strain of 0.7965%, while the composites display a deformation response closer to that of coal. In addition, in terms of acoustic emission (AE), cumulative ringing counts at 2:1 and 3:1 are reduced by about 80.23% and 90.05% relative to 1:1. To further analyze AE data, the proposed “denoise-then-cluster” workflow markedly outperforms the direct clustering example; the silhouette coefficient improves from −0.32 to 0.53 for specimen 1:1-1 and from 0.63 to 0.88 for 2:1-1. Furthermore, under the 3:1 condition, shear cracks account for 75.33% of the total, indicating a coal-dominated brittle shear failure mechanism. Collectively, these findings contribute to a deeper understanding of the rock burst initiation mechanism in coal–sandstone composites and provide theoretical support for the stability evaluation and disaster prevention of coal–rock composite structures.