Evolution Characteristics of Bulking Factor in the Multi-field Loading of Broken Coal: An Experimental Study

From the perspective of spontaneous combustion control in coal mining, this study conducted a progressive axial loading test of broken coal using a self-designed multi-field coupling oxidation test system. The evolutionary characteristics of the bulking factor under different loading conditions of stress, stress–temperature, and stress–moisture were obtained. Subsequently, factors affecting the bulking properties of broken coal in the synchronous compression and deformation stage were analyzed theoretically using the four-particle accumulation model and Hertzian contact deformation principle. The research results are summarized as follows: (1) in the process of axial loading, the bulking factor of broken coal generally decreased, but showing three stages; (2) under the stress-moisture coupling effect, when the axial force was 0 kN and the external water content increased from 0 to 6.75%, the water wedge effect increased the bulking factor of broken coal by 2.17%; (3) under the stress-temperature coupling effect, when the axial force was 47.1 kN and the temperature increased from room temperature (23.3 °C) to 100 °C, the bulking factor of broken coal decreased by 5.08%; and (4) in the synchronous compression and deformation stage (at the axial stress of 94.2 kN), the theoretical analysis revealed that the bulking factor decreased gradually with increasing stress, temperature, and external water content, which was consistent with the test results. The abovementioned test results established the basic theory and provide practical support for subsequent recognition of the spontaneous combustion of confined coal.