The coal mining face, comprising the goaf gangue and the regenerated roof, undergoes a continuous mechanical process involving compression, lateral confinement, unloading, deformation, and failure. Conventional triaxial testing equipment is primarily utilized for closed loading conditions. The lateral boundaries of the system are typically fixed, and the formation of a free surface during unloading is challenging to replicate. Consequently, it is challenging to simulate the entire mechanical trajectory from gangue compaction to single-sided unloading and instability failure. In experimental teaching methods, direct observation of this process can be challenging for students. The stress path, boundary transformation, crack development, and failure mode are often abstract and difficult to comprehend. To address these challenges, a novel apparatus for the compression and unloading of goaf gangue has been developed. The apparatus was utilized to formulate a teaching experiment that replicated the comprehensive process of gangue compression, lateral pressure loading, single-sided unloading, and failure.

The apparatus is composed of a top-loading system, a hydraulic pumping apparatus, a pressure-unloading chamber, a lateral confining pressure loading system, a loading monitoring and control apparatus, and an external monitoring system. The top-loading system exerts axial load and provides vertical support. The hydraulic pumping apparatus is responsible for the storage and transportation of hydraulic oil, in addition to providing power to the actuating cylinders. The pressure-unloading chamber is utilized for specimen placement, side boundary control, and single-sided unloading. The lateral confining pressure loading system exerts lateral pressure on the specimen, thereby simulating a three-dimensional stress state. The loading monitoring and control box is designed to facilitate connectivity among disparate loading channels, meticulously record pressure and displacement signals, and regulate the pressurization and pressure relief of each oil circuit. The external monitoring system is designed to record acoustic emission signals, surface deformation, and crack evolution during the compression and unloading processes. Mudstone and sandy mudstone from a typical mining area in Huainan were selected as aggregates, and local yellow mud was used as the cementing material. Considering the chamber size, size effect, and particle-size characteristics of broken gangue, the gangue aggregates were divided into five distinct particle-size ranges. The range of sizes includes 0–4, 4–8, 8–12, 12–16, and 16–20 mm. The mass ratio of gangue, yellow mud, and water was 20:6:3. The experiment comprised four stages: side-confined compression, initial stress loading, dynamic unloading, and failure loading. Throughout the process, the stress-strain response, acoustic emission characteristics, and full-field deformation of the unloading surface were meticulously documented.

The apparatus demonstrated its efficacy by successfully recording the complete loading and unloading process of cemented gangue specimens. In the side-confined compression stage, the stress-displacement curves exhibited three phases: pore compaction, initial compression, and dense compression. The displacement is defined as the axial loading displacement recorded from the initial filling height of 150 mm. In the phase of pore compaction, the stress increased gradually and remained below 0.5 MPa. In the initial compression phase, the stress increased from 0.26–0.45 MPa to 0.46–1.19 MPa. In the dense compression phase, the stress increased rapidly, and the differences in bearing among the various particle-size groups became apparent. In the unloading and failure stage, the specimens exhibited four mechanical response phases: compaction elasticity, yield, plastic strengthening, and post-peak failure. The maximum tensile strength exhibited a decline from 3.8 MPa in the 0–4 mm group to 2.7 MPa in the 16–20 mm group. The acoustic emission monitoring system captured high-amplitude pulse clusters and high-energy release events during the peak stress drop. The detachable side plate facilitated observation of the unloading surface through digital image correlation. The small-particle specimens primarily exhibited relatively uniform mesh-like cracks, while the large-particle specimens displayed localized inclined shear bands, structural collapse, and substantial angular fragments.

The developed compression-unloading apparatus is capable of reproducing the mechanical process of goaf gangue from axial compression to lateral confinement, single-sided unloading, and final failure. This method effectively addresses the limitations of conventional triaxial equipment in simulating the formation of a free surface during unloading processes. The apparatus demonstrates notable compatibility with acoustic emission monitoring and digital image correlation techniques. The teaching experiment, based on this apparatus, includes specimen preparation, loading path design, monitoring system installation, data processing, result analysis, and group discussion. This approach assists students in comprehending the mechanical behavior of goaf gangue and the regenerated roof from the perspective of stress response, acoustic damage, and visible deformation. The experiment enhances the practical teaching content of mine pressure and strata control, improves students’ ability to analyze complex mining engineering problems, and provides a useful reference for transforming research equipment into undergraduate experimental teaching resources.