Deformation and Failure Behaviour of Coal-Backfilling Composite Structure Under Double Shearing

The interface is the weakest region of the coal-backfilling composite structure (CBCS) for underground mines, the sawtooth angles of which is one of the most critical concerns to be accounted. However, limited studies have focused on the deformation and failure behaviours of CBCS with different interface sawtooth angles under double shearing. To investigate the effect of interface sawtooth angles on the damage characteristics of CBCS under double shearing, CBCS with different interface sawtooth angles (i.e., 0°, 15°, 30°, 45°, 60°) were prepared and double shearing tests were carried out in the present research. Macroscopic failure characteristics (i.e., failure mode, failure process, and shear strength) of the CBCS were experimentally investigated via the digital image correlation (DIC), acoustic emission (AE), and 3D scanner system, whereas microscopic feature of which, including the contact force–microcrack evolution, and displacement vector of CBCS were numerical simulated by the discrete element modelling (DEM) method. The results showed that the failure modes of the CBCS can be classified into three types (e.g., climbing failure, cutting failure, and sliding failure). Among them, the climbing failure was only occurred when the loads on the sawtooth surfaces exceeded its shear strength, while the cutting failure occurred if the load on the sawtooth end of the coal element or backfilling element exceeded its shear strength. The sliding failure occurred, because the load applied to the interface exceeded the shear strength of the specimen interface. Moreover, the peak shear stress, the degree of strain concentration, and the extent of the strain concentration zone of the CBCS increased with the enlarged sawtooth angle. The research outcomes will contribute to obtain an in-depth understanding about the mechanical response of the coal-backfill composites under double shear loading. Highlights Double shearing tests were carried out on coal-backfilling composite structures and the shear behaviour of CBCS was observed by DIC and AE monitoring systems. The contact force–microcrack distribution and displacement vector of CBCS during double shearing were investigated using DEM. Deformation and failure characteristics of CBCS were revealed from a microscopic and macroscopic perspective. The mechanisms of the double shearing failure process of CBCS with different interface sawtooth angles were revealed.