Dynamic mechanisms of coal and gas outbursts based on a multi-zone combined coal model

Coal and gas outbursts (hereafter referred to as outbursts) severely affect coal mining safety. However, many scientific problems underlying the outbursts remain unsolved. To minimize the influence of anisotropy in the mechanical properties of coals on research into outbursts, this study constructed a multi-zone combined coal model. Based on the stress distribution of overlying strata and the stress wave propagation mechanisms, this study located the weak zone of coals, shrinking the research object from the outburst center to the weak zone. Given that outbursts are triggered by the disturbance of external dynamic load, this study examined the propagation patterns of stress waves in layered multi-zone combined coals through impact tests and constructed a stress-strain constitutive model for the coals. Furthermore, it ascertained that under the action of unloading waves, the axial multi-layered spalling of coals resulted from the tensile stress waves formed by the reflection of loading shock waves, formed by impact within coal mass points, on the free surface. Within the radial plane of coals, unloading waves pursued plastic loading waves due to Poisson’s effect, leading to the formation of multiple radial and circumferential fractures. Accordingly, this study derived the dynamic evolutionary patterns of the spalling thickness of coals and further defined the three-dimensional damage path of coals under the disturbance of external dynamic load. Specifically, the damage path consisted of the earliest destruction of the weak zone, multi-layered spalling in the axial direction, and the formation of multiple radial and circumferential fractures within the radial plane sequentially. Accordingly, this study proposed a dynamic mechanism of outbursts based on the multi-zone combined coal model, segmenting the outburst process into four stages: preparation, commencement, development, and termination. In the preparation stage, stress transfer and concentration in the overlying strata of coals result in a high gas pressure gradient, creating conditions for subsequent instability and failure of coals. In the commencement stage, coals in the axial direction within the weak zone are destroyed the earliest under the disturbance of external dynamic load, resulting in multi-layered spalling and the formation of multiple radial and circumferential fractures within the radial plane. In the development stage, the desorption of adsorbed gas and the accumulation of free gas form high-pressure gas ejected from coals. As a result, outbursts expand toward the deep part, forming secondary damage. In the termination stage, the accumulated gas pressure is below the tensile strength of coals, leading to the formation of a stable spindle-shaped outburst cavity, signaling outburst termination. This mechanism preliminarily accounts for the causes of dynamic phenomena like coal burst sound and outburst cavities with a small opening and a large body during outbursts, providing a novel philosophy for preventing outbursts in mines.