Experimental study of the deformation characteristics and bearing mechanism of gravel compaction in goafs

The stability of gob-side roadways, especially that of narrow coal pillars, is critically influenced by the compaction behavior, crushing expansion characteristics, and lateral stress of crushed gangue in the goaf. Understanding the deformation characteristics and bearing mechanism of this crushed material is essential for ensuring mining safety and optimizing support designs in complex geological conditions. This study investigates the effects of lithology and particle size on the compaction deformation and load-bearing behavior of crushed rocks from a typical coal mine goaf, providing a foundational basis for analyzing the cooperative bearing mechanism between coal pillars and crushed gangue.

A self-developed gravel compression test system was used, which consists of an electro-hydraulic servo pressure testing machine, an axial compression device, and resistance strain instruments. The axial compression device included a pressure piston, a cylindrical wall, and a base. Crushed rock samples were prepared from three lithologies—mudstone (immediate roof), siltstone (direct roof), and muddy siltstone (basic roof)—obtained from the No. 7 coal seam of a Panjiang mine. The samples were crushed using a jaw crusher and sieved into four particle size ranges: 0–5 mm, 5–10 mm, 10–15 mm, and 15–20 mm. A total of 12 sample groups were prepared, each with a mass of 1000 g, representing single-size distributions for each lithology. The axial compression tests were conducted under a constant axial stress of 15 MPa to simulate the overburden pressure. During testing, strain gauges attached to the outer surface of the cylindrical vessel measured lateral strain, and lateral stress was calculated based on Lamé's solution for thick-walled cylinders under internal pressure. Key parameters, such as the bulking factor, residual bulking factor, stress–strain relationships, and lateral pressure coefficient, were analyzed to evaluate the compaction and bearing characteristics.

The physical and mechanical properties of the three lithologies were determined. Mudstone had a density of 1823 kg/m³, a uniaxial compressive strength of 44.47 MPa, and a tensile strength of 0.85 MPa; siltstone had a density of 2213 kg/m³, a compressive strength of 73.34 MPa, and a tensile strength of 3.47 MPa; muddy siltstone had a density of 2175 kg/m³, a compressive strength of 52.90 MPa, and a tensile strength of 1.84 MPa. The bulking factor ranged from 1.19 to 1.63 and increased with lithological strength and particle size. The residual bulking factor, however, first increased and then decreased with particle size, ranging from 1.11 to 1.30. The maximum compressive strain of the gravel was directly proportional to particle size but showed no clear correlation with lithological strength. For example, mudstone samples with particle sizes of 0–5 mm, 5–10 mm, 10–15 mm, and 15–20 mm exhibited maximum strains of 0.361, 0.428, 0.484, and 0.561, respectively. Under the same conditions, siltstone and muddy siltstone showed lower and similar strain values, respectively. The compaction stress–strain curves were divided into three stages: rapid, stable, and full compaction. Lateral stress was proportional to axial stress, and the lateral pressure coefficient decreased with increasing lithological strength but showed no notable relationship with particle size. The lateral pressure coefficients for mudstone, siltstone, and muddy siltstone were 0.4276, 0.3343, and 0.3448, respectively.

The compaction deformation and bearing mechanism of goaf gravel are significantly influenced by lithology and particle size. Stronger lithologies and larger particle sizes result in higher bulking factors, while the residual bulking factor initially increases and then decreases with particle size. The maximum compressive strain is more dependent on particle size than lithological strength. The lateral pressure coefficient is inversely related to lithological strength but independent of particle size. These findings provide important insights into the load-transfer mechanisms in goaf areas and contribute to the design of stable gob-side roadways. Future research should focus on the cooperative bearing behavior between coal pillars and crushed gangue to further enhance the understanding of overall ground control in mines.