Analysis of Seepage Failure and Fluidization Mechanisms in Gas-Containing Tectonic Coal Outbursts

Abstract

This study investigates the mechanisms of gas-containing tectonic coal outbursts by modeling tectonic coal and gas as analogous to soil and pore water. Analytical methods from soil mechanics, specifically those related to quicksand and seismic liquefaction, are employed to classify these outbursts into two types: “quicksand type” and “fluidization type.” Their formation mechanisms are elucidated based on a fracture network model and a one-dimensional seepage failure criterion developed for tectonic coal. The findings indicate that “quicksand type” outbursts result from the continuous detachment of tectonic coal slices within the pressure relief zone under gas seepage pressure. The thickness-to-radius ratio of these coal slices increases with rising gas pressure but decreases with increasing coal strength and normal geostress. A larger thickness-to-radius ratio signifies a more pronounced granular characteristic and accelerates the development of coal and gas outbursts. “Fluidization type” outbursts occur when the effective stress drops to zero, resulting in a complete loss of coal strength. These outbursts represent a specific case of “quicksand type” outbursts and can be triggered by vibrations. The susceptibility of tectonic coal to outbursts is attributed to its low mechanical strength and the presence of dense fractures, which increase the acting area of seepage pressure and, consequently, raise the overall seepage force. According to this analysis, the depth of outburst cavities is generally less than the width of the pressure relief zone, which can result in delayed outbursts. This study enhances the understanding of quicksand and seismic liquefaction theories in soil mechanics and provides valuable guidance for predicting and mitigating coal and gas outbursts.

1. Introduction

Coal and gas outbursts are significant hazards associated with mining activities. Researchers worldwide have proposed various hypotheses to explain their mechanisms from different perspectives. Soviet scholar B.B. Khodot [1] introduced the “energy hypothesis” and suggested that outbursts resulted from the combined effects of coal seam properties, gas pressure, and geostress. Zheng [2], through comparative calculations, found that the internal energy of gas during an outburst was one to three orders of magnitude greater than the elastic energy of the coal. Zhou et al. [3] proposed the “rheological hypothesis” and examined the temporal and spatial dynamics of gas-bearing coal throughout the outburst process. Zhang et al. [4] developed a unified instability model that linked dynamic pressure to coal and gas outbursts. They suggested that both phenomena were dynamic instability processes triggered by disturbances within the unstable deformation system of coal (or rock). Yu [5] proposed that outbursts originated at a certain depth within the coal seam and propagated from the inside outward. Jiang et al. [6] developed a shell instability model and asserted that outbursts occurred when coal seam detached in an outside-to-inside manner under the combined effects of geostress and gas pressure. Hu et al. [7] divided the outburst process into four stages—preparation, initiation, development, and termination—from a mechanical perspective. In recent years, the concept of compound dynamic disasters, which suggests that coal and gas outbursts and rock burst pressures in deep coal mining are interrelated and mutually influential, has attracted increasing attention [8,9,10]. Most of the aforementioned hypotheses analyze coal and gas outbursts from a rock mechanics perspective and treat coal as a continuous medium. However, the coal involved in outbursts is mainly tectonic coal, which exhibits behaviors more similar to that of granular materials. In addition, scholars from different countries have conducted lots of indoor simulation experiments on coal and gas outbursts. Some experiments have found that the coal ejected to the ground during coal and gas outbursts is in the form of flakes [6,11,12,13], while more experiments have shown that the coal ejected is in the form of granules. The above theoretical models for coal and gas outbursts cannot explain this phenomenon well.

Quicksand and seismic liquefaction are common challenges in geotechnical engineering, with well-established theories and mitigation methods. Ding et al. [12] and Hu et al. [14] analyzed the mechanisms of coal and gas outbursts from a seepage failure perspective; however, their seepage failure criteria differed and did not fully consider the effects of fracture networks in tectonic coal or the role of vibrations in triggering outbursts. Therefore, this study applies soil mechanics concepts related to quicksand and seismic liquefaction to classify coal and gas outbursts into two categories: “quicksand type” outbursts and “fluidization type” outbursts. “Fluidization”is a chemical engineering term that refers to a type of coal and gas outburst in this article.

It further explores the mechanisms and characteristics of gas-bearing tectonic coal outbursts.

2. Study Methodology

2.1. Characteristics of Fractures in Tectonic Coal and Gas Occurrence

2.1.1. Characteristics of Tectonic Coal and Fracture Network Model

(1)

Characteristics of tectonic coal

Coal seams undergo deformation and metamorphism throughout geological evolution. In coal geology, seams that retain their original structural characteristics after geological processes are referred to as “primary structural coal,” whereas those that exhibit changes in composition, structure, and texture are known as “tectonic coal.” Tectonic coal can be further classified into four types based on the degree of fragmentation: fractured coal, granular coal, pulverized coal, and mylonitic coal [15]. Fractured coal represents a transitional type between primary structural coal and tectonic coal. Granular coal is characterized by a dominant particle size greater than 1 mm, whereas mylonitic coal has a dominant particle size smaller than 1 mm. In general, tectonic coal is believed to have lower mechanical strength and permeability than primary structural coal [16] while exhibiting enhanced gas desorption properties [17], which makes it more prone to outbursts. Tectonic coal is widely regarded as a necessary condition for the occurrence of coal and gas outbursts [18].

Tectonic coal is also referred to by various other names, including soft coal and outburst coal. The Mining Research Institute of the Soviet Union classified coal seams into five categories (I to V) based on their fracture characteristics and stress-induced failure states. It was concluded that only categories III, IV, and V were prone to outbursts. In these coal seams, the average fracture spacing is typically less than 1 mm, and the average fracture width is below 0.01 mm.

According to China’s classification standards for rock mass structures, rock masses are categorized as blocky, layered, fragmented, or granular. Granular rock masses are typically characterized by low strength and can be easily disaggregated by hand to exhibit engineering properties similar to those of soil. Given these characteristics, tectonic coal is classified as a granular rock mass, and its engineering behavior is treated similarly to that of soil. This classification supports the application of soil mechanics theories—such as those related to quicksand and seismic liquefaction—for analyzing the mechanisms of tectonic coal outbursts.

(2)

Fracture network model

Tectonic coal is an inhomogeneous material composed of a coal matrix interspersed with fracture surfaces. An analysis of the impact of fractures on outbursts requires the development of a fracture network model. The fracture ratio of coal can be obtained by observing coal specimens collected in the laboratory using optical or electron microscopes or by geophysical methods on site.

The area of fractures perpendicular to the seepage direction represents the actual effective area subjected to the seepage pressure of free gas. To analyze the effective fracture area subjected to seepage pressure, all fracture surfaces within a specified spatial range are projected onto planes both perpendicular and parallel to the seepage direction. The areas of these projected fracture surfaces are then calculated. Finally, the average spacing of the fractures can be quantified by defining the area fracture ratios in each direction. This approach enables the construction of a spatial fracture network model for tectonic coal. Directly measuring fracture areas is challenging; thus, the linear fracture lengths and linear fracture ratios on sections parallel to the seepage direction can be used to approximate the fracture areas and area fracture ratios.

Grid lines are drawn at appropriate intervals on the tectonic coal profile along the seepage direction. The fractures intersecting these grid lines are decomposed into components parallel and perpendicular to the seepage direction. The lengths of the fractures in both directions are measured and then plotted along the corresponding grid lines. The resulting fracture network model is illustrated in Figure 1. In this model, a coal pillar with a cross-sectional area of $L\times 2R$ is divided into several smaller pillars, each with an equal cross-sectional area of $l\times 2r$, using the fracture grid.

2.1.2. Gas Occurrence Forms

Gas in coal exists in two forms: adsorbed gas and free gas. Adsorbed gas can be further classified into sorption-state gas and absorption-state gas. Sorption-state gas adheres to the surfaces of micropores, whereas absorption-state gas penetrates the coal’s colloidal structure under high pressure [19]. Free gas mainly occupies fractures and larger pores, where it can flow freely and exert pressure [20], while adsorbed gas remains bound and does not flow freely or generate pressure. Nevertheless, free gas and adsorbed gas remain in a state of dynamic equilibrium. Their interconversion is influenced by factors such as gas pressure, temperature, coal structure, porosity, and the degree of coalification. Existing studies indicate that gas in coal seams is predominantly in the adsorbed state and accounts for 80% to 90% of the total gas content. Liu et al. [21] emphasized that free gas plays a crucial role in triggering coal and gas outbursts.

Water in the soil exists as bound water and unbound water. Bound water can be further categorized into strongly bound and weakly bound water. It adheres to soil particles through various forces and restricts its movement under the influence of gravity. Unbound water includes gravitational water and capillary water. Gravitational water, also known as free water, moves freely under gravity and can transmit pressure. Capillary water represents a transitional state between gravitational water and bound water.

This analysis suggests that adsorbed gas in coal seams is analogous to bound water in soil, while free gas corresponds to gravitational (free) water. This analogy establishes a similarity in pore fluid behavior and provides a theoretical basis for applying quicksand and seismic liquefaction theories from soil mechanics to analyze the mechanisms of tectonic coal outbursts.

The main difference between gas in coal and water in soil lies in gas expansibility. Due to the enormous expansion energy of high-pressure gas in coal, once an outburst occurs, it can cause significant damage to personal safety and laneway facilities.

2.2. Classification of Outburst Types

The preceding comparative analysis of tectonic coal and soil, as well as gas and pore water, highlights significant similarities between these elements. Consequently, analytical methods from soil mechanics—specifically those related to quicksand and seismic liquefaction—can be applied to investigate the mechanisms of coal and gas outbursts. In this study, outbursts are categorized into two types: “quicksand type” and “fluidization type.”

2.3. Mechanism of “Quicksand Type” Outbursts

2.3.1. Quicksand Discrimination Criterion

The flow of pore water exerts pushing, frictional, and dragging forces on soil particles, collectively known as seepage force. Damage to foundations caused by seepage force is referred to as seepage failure. Quicksand is a common form of seepage failure in geotechnical engineering. When upward seepage acts on soil—whether cohesive or non-cohesive—the suspension and movement of soil masses or particle groups occur in localized areas once the hydraulic gradient reaches a critical threshold. This phenomenon is known as quicksand. The criterion for identifying this condition is as follows:

$$i_{cr}={\displaystyle \frac{G_s-1}{1+e}}$$

(1)

where $i_{cr}$ represents the critical hydraulic gradient; $G_s$ denotes the specific gravity of the soil particles; and $e$ indicates the void ratio of the soil.

Figure 2 illustrates a quicksand incident in a foundation pit. As the soil layers are excavated, the hydraulic gradient at the bottom increases. When the gradient reaches the critical threshold, quicksand occurs. This process leads to a significant outflow of sandy soil from the bottom of the pit and causes surface subsidence outside the pit.

2.3.2. One-Dimensional Seepage Failure Criteria for Tectonic Coal

Simulation experiments on coal and gas outbursts conducted by scholars worldwide have shown that, during an outburst, the coal seam exhibits a flake-like delamination phenomenon, as illustrated in Figure 3 [13]. Jiang et al. [6] proposed that this delamination results from the expansion of fractures within the coal body under geostress, followed by the detachment of coal shells along the fracture surfaces owing to gas pressure. Other scholars have analyzed this flake-like delamination observed in laboratory experiments from the perspective of seepage failure [12,14].

Researchers from the former Soviet Union proposed that coal and gas outbursts were triggered by seepage failure. They argued that when the gas pressure differential near the exposed coal surface exceeded the failure strength of the coal layer, the layer could become detached. For structurally uniform coal seams, Ding et al. [12] observed that an outburst occurred when the gas pressure differential reached the tensile strength of the coal seam. The corresponding criterion is expressed as follows:

$$P_i-P_0={\sigma }_t$$

(2)

where $P_i$ represents the gas pressure at a specific point within the coal seam (MPa); $P_0$ is the gas pressure in the roadway (MPa); and ${\sigma }_t$ denotes the tensile strength of the coal body (MPa).

Equation (2) is insufficient for analyzing the delamination of coal slices with a large thickness-to-radius ratio, as it overlooks the surrounding shear resistance and the influence of fractures within tectonic coal.

Hu et al. [14] argued that coal and gas outbursts resulted from seepage pressure generated by gas migration within the coal matrix, which overcame the shear resistance of the solid skeleton of the coal body. Considering this result, they proposed the following criterion for seepage failure:

$$F\ge {\displaystyle {\int }_0^x\tau (x)}dx$$

(3)

where $F$ represents the gas seepage pressure in the coal seam (unit: MN), and $\tau (x)$ denotes the shear strength of the coal body’s solid skeleton in the x-direction (unit: MPa).

For coal slices with a large thickness-to-radius ratio, it is essential to account for both interfacial tension effects and the shear resistance surrounding the coal slice. As illustrated in Figure 4, for a horizontal outburst occurring in a cylindrical coal seam with a radius r, where the vertical fracture spacing (coal slice thickness) is l, the mechanical equilibrium equation governing the delamination of the coal slice under gas seepage pressure can be expressed as follows:

$$\lambda \pi r^2\left(P_i-P_0\right)=\pi r^2{\sigma }_t+2\pi rl\tau$$

(4)

where $p_i$ is the gas pressure at a distance l from the coal wall, with a unit of MPa; $p_0$ is the gas pressure at the coal wall (MPa); $r$ is the radius of the cylindrical coal seam (m); $l$ is the thickness of the coal slice (m); ${\sigma }_t$ is the tensile strength of the coal body (MPa); $\tau$ is the shear strength of the coal body (MPa), which is calculated using $\tau =c+{\sigma }_N\tan \phi$; and $\lambda$ is the area fracture ratio, which is defined as the ratio of the fracture area to the cross-sectional area of the coal body on a specific plane. This parameter reflects the actual effective area subjected to gas pressure within the fractures.

Rearranging Equation (4) yields the following one-dimensional seepage failure criterion for coal seams:

$$P_i-P_0\ge {\displaystyle \frac{{\sigma }_t}{\lambda }}+{\displaystyle \frac{2l\tau }{\lambda r}}$$

(5)

Wang et al. [13] conducted the validation of the rationality of Equation (5) through laboratory simulation experiments on coal and gas outbursts using a custom-designed experimental device. The coal sample used in the experiment is a remolded coal sample, and the particle size of the coal particles used to prepare the coal sample is less than 5 mm. The gas used in the experiment is nitrogen.During the experiment, the gas pressure increases gradually until an outburst occurs, which ensures the observation of coal flakes peeling.The present study aims to further explore the related theoretical issues.

2.3.3. Differences Between Seepage Failure Criteria for Soil and Tectonic Coal

Equations (1) and (5) differ significantly owing to their distinct force conditions. The seepage failure of sandy soil is characterized by vertical upward seepage of pore water in thin soil layers, where the seepage pressure only needs to overcome the buoyant force acting on the sand, without considering the interaction forces between soil particles. In contrast, the seepage failure of coal seams involves the horizontal seepage of gas within the coal layer. Here, the seepage pressure must overcome both the interfacial tension and the lateral shear resistance of the coal body. Furthermore, in the case of vertical outbursts—such as those occurring during shaft excavation—the gravitational force of the coal body must also be considered. Accordingly, Equation (5) can be modified as follows:

$$P_i-P_0={\displaystyle \frac{{\sigma }_t}{\lambda }}+{\displaystyle \frac{2l\tau }{\lambda r}}+{\displaystyle \frac{l{\gamma }^{\prime }}{\lambda }}$$

(6)

where ${\gamma }^{\prime }$ is the buoyant unit weight of the coal body, expressed in KN/m³.

Indeed, seepage failure can occur not only at the ground surface but also within mine shafts and tunnels, potentially resulting in quicksand or mudflow incidents. In cases of horizontal seepage failure of pore water in deep soil layers under three-dimensional high-pressure confinement—conditions analogous to the stress environment during coal and gas outbursts—the interparticle forces must also be considered. Therefore, Equation (6) can be regarded as a universal criterion.

Equation (5) indicates that lower gas pressure results in a smaller thickness-to-radius ratio, leading to more pronounced delamination features. Consequently, delamination phenomena are more readily observed in experiments conducted under lower gas pressure conditions. Figure 3 illustrates the experimental observations as the gas pressure approaches the critical delamination threshold.

According to seepage mechanics theory, equipotential surfaces form during the seepage of pore fluids in porous media. Consequently, seepage failure in such media typically exhibits planar failure characteristics. The delamination of coal slices shown in Figure 3 accurately reflects this behavior. However, delamination failure is not observed in sandy soil seepage experiments, which may be attributed to the following factors: (1) In quicksand experiments, sandy soil has low cohesion and minimal contact areas between particles; (2) the specific gravity of soil particles is higher than that of coal particles; and (3) the viscosity of pore water is greater than that of gas.

2.4. Mechanism of “Fluidization Type” Outbursts

2.4.1. Re-Distribution of Geostress in Coal Seams

Normal stress is a key parameter in calculating the shear resistance around a coal slice. Underground mining operations cause stress redistribution within the coal (rock) mass surrounding and ahead of the roadway, as illustrated in Figure 5. These areas are generally divided, from the outside inward, into the pressure relief zone, the stress concentration zone, and the original stress zone. The stress in the concentration zone exceeds that of the original stress zone, with peak stress typically ranging from 1.5 to 2.5 times the original stress value, ${\sigma }_H$. In contrast, the stress in the pressure relief zone is lower than ${\sigma }_H$, and stress levels near the coal wall are minimal. When a longwall is excavated, the redistribution of geostress in the coal body in front of and on both sides of the working face is shown in the literature [22].

Research by Lin et al. [23] indicated that the width of the pressure relief zone was inversely proportional to the friction coefficient at the coal seam interface and the tensile strength of the coal body. The width is directly proportional to the coal seam thickness, the lateral pressure coefficient, and the mining depth. Data from mines in Cinena show that the widths of both the stress concentration zone and the pressure relief zone are typically around 5 m. Due to the smaller friction coefficient and the tensile strength of the tectonic coal compared with the hard coal, the width of the pressure relief zone of tectonic coal is wider than that of the hard coal.

2.4.2. Discrimination Criterion for “Fluidization Type” Outbursts

Field statistics indicate a correlation between coal and gas outbursts and both natural and mining-induced earthquakes [24,25]. In this study, the seismic liquefaction theory is applied to examine this phenomenon.

Seismic liquefaction is a well-documented phenomenon in geotechnical engineering. During an earthquake, if the pore water within the soil cannot drain rapidly, it causes a sudden rise in pore water pressure and reduces the effective stress to zero. Consequently, sand particles become suspended in the pore water, leading to a complete loss of soil strength. The soil behaves like a fluid, a condition known as seismic liquefaction. In this state, the liquefied soil and water are often expelled to the surface, potentially causing severe engineering failures. The fundamental cause of seismic liquefaction in saturated soils is poor drainage combined with soil contraction. The expression for effective stress is given as follows:

$${\sigma }^{\prime }=\sigma -u$$

(7)

where ${\sigma }^{\prime }$ is the effective stress (MPa), $\sigma$ is the total stress (MPa), and $u$ is the pore water pressure (MPa).

The effective shear strength of tectonic coal can be expressed as follows:

$${\tau }^{\prime }=c^{\prime }+({\sigma }_N-P)\tan {\phi }^{\prime }$$

(8)

where ${\tau }^{\prime }$ is the effective shear strength (MPa); $c^{\prime }$ is the effective cohesion (MPa); ${\sigma }_N$ is the total normal stress perpendicular to the seepage direction (MPa); $P$ is the gas pressure (MPa); and ${\phi }^{\prime }$ is the effective internal friction angle.

In the pressure relief zone, tectonic coal is subjected to relatively low normal geostress ${\sigma }_N$. If the effective cohesion $c^{\prime }$ is also minimal, even a small gas pressure $P$ can reduce the effective shear strength to zero. Under these conditions, coal particles become completely isolated by the gas and lose contact with one another, which causes the coal to behave like “liquefied soil.” This phenomenon is referred to as the fluidization of tectonic coal in this study, and the resulting outbursts are termed “fluidization type” outbursts.

Tectonic coal typically occurs in granular, powdery, and cataclastic forms. It has an area void ratio $\lambda$ close to 1 and exhibits low permeability. Under vibration, the propagation of tensile and compressive stress waves further extends fractures within the tectonic coal and promotes the desorption of adsorbed gas from the voids. This process can lead to a sudden increase in gas pressure, potentially triggering a “fluidization type” outburst. As the outburst progresses, both the stress concentration zone and the pressure relief zone advance deeper into the coal seam and allow the “fluidization type” outburst to extend further, resulting in an event of significant intensity.

2.4.3. Interpretation of Outburst Phenomena

(1)

Explanation of outburst cavity depth

Statistical data indicates that most outburst cavities in coal mines are less than 6 to 7 m deep—approximately corresponding to the width of the pressure relief zone—which makes it difficult for these cavities to extend into the stress concentration zone.

According to Equation (8), when the gas pressure $P$ equals the total normal stress ${\sigma }_N$ perpendicular to the seepage direction, a “fluidization type” outburst may occur within the coal seam. The smaller the total normal geostress ${\sigma }_N$, the lower the gas pressure $P$ required for an outburst, thus increasing the likelihood of outbursts. As shown in Figure 5, the normal geostress within the pressure relief zone is lower than that in the stress concentration zone. Therefore, outbursts mainly occur within the pressure relief zone unless the gas pressure is sufficiently high. This occurrence explains why most outburst cavities in coal mines are less than the width of the pressure relief zone in depth.

(2)

Explanation of delayed outbursts

Some outbursts are characterized by delayed occurrences, which means they happen after a period following blasting, with the delay ranging from a few minutes to several days or even longer. According to the fluidization mechanism model proposed in this study, these delayed outbursts can be explained as follows: an outburst will only occur when the normal stress in the coal seam decreases to a level equal to the gas pressure after blasting.

3. Results

3.1. The “Quicksand Type” Outburst Process

3.1.1. Delamination Process of a Single Coal Slice

Figure 3 illustrates that seepage failure in the coal seam manifests as coal slice delamination. The following example further demonstrates the delamination process of an individual coal slice within the seam.

It is assumed that tectonic coal is buried at a depth of 600 m, with an initial geostress of 15 MPa and a coal seam thickness of 2.5 m. The area fracture ratio is 0.6, and the vertical fracture spacing within the tectonic coal is 1 mm. The tensile strength of the tectonic coal is 0.05 MPa, while its cohesion and internal friction angle are 0.6 MPa and 23°, respectively [5].

Assuming that the normal geostress in the pressure relief zone increases linearly, the average normal geostress at a distance of one-tenth of the width of the pressure relief zone from the coal wall is approximately 0.75 MPa, i.e., ${\sigma }_N$ = 0.75 MPa. Substituting these parameters into Equation (5) yields the calculation results (where the diameter of the outburst cavity is assumed to be equal to the thickness of the coal seam), as presented in

Table 1. Calculation results of thickness-to-radius ratio of coal slice.

Gas Pressure ${\mathit{P}}_{\mathit{i}}$/MPa	Thickness of Coal Slice $\mathit{l}$/mm	Radius of Coal Slice $\mathit{r}$/mm	Thickness-to-Radius Ratio of Coal Slice $\mathit{l}/\mathit{r}$
0.086	1	1250	0.0008
0.3	1	14	0.071
0.3	89	1250	0.071
0.74	1	5	0.214
0.74	267	1250	0.214
1.0	1	3	0.299
1.0	374	1250	0.299

. The analysis of Table 1 leads to the following conclusions:

(1)

As gas pressure increases, the thickness-to-radius ratio of the coal slice also increases. When the gas pressures are 0.3 MPa, 0.74 MPa, and 1.0 MPa, the corresponding thickness-to-radius ratios of the coal slices are 0.071, 0.214, and 0.299, respectively. A gas pressure of 0.74 MPa is sufficient to induce the delamination of a coal slice with a thickness of 267 mm and a radius of 1250 mm. Therefore, it is essential to account for the shear forces acting around coal slices with larger thickness-to-radius ratios.

(2)

The gas pressure required to induce the delamination of a single coal slice is relatively low. In this example, a gas pressure of just 0.086 MPa is sufficient to cause the delamination of a coal slice with a thickness of 1 mm and a radius of 1250 mm. However, such low gas pressure is insufficient to sustain a true outburst, as the gas pressure within the coal seam rapidly decreases during the event. Consequently, the number of coal slices that can be delaminated under this low-pressure condition is inadequate to initiate a full-scale outburst. Therefore, there is a “threshold” gas pressure for coal and gas outbursts, with 0.74 MPa representing this threshold in the present case.

(3)

The degree of fragmentation of the coal slices increases from the interior toward the exterior. A gas pressure of 0.74 MPa is sufficient to induce the delamination of a large coal slice with a thickness of 267 mm and a radius of 1250 mm. This 267 mm thick coal slice contains 267 fractures, and the gas pressure within each fracture remains uniform. Consequently, the gas pressure within each fracture can trigger further delamination of the coal body adjacent to the outburst cavity. The coal slices closer to the cavity are thinner, resulting in a progressive decrease in the radius of these delaminated slices as the thickness-to-radius ratio remains constant. This decrease indicates an increasing degree of fragmentation in the coal body. The most fragmented coal slice is the one delaminated by the gas in the outermost fracture, with a thickness of 1 mm and a radius of just 5 mm. The presence of fractures parallel to the seepage direction causes further fragmentation of the delaminated coal slices during their transport and ejection. This process reduces the radius of the coal slices while increasing their thickness-to-radius ratio by the time they reach the surface, resulting in a granular appearance. As delamination continues to advance through the larger coal slices—such as those with a thickness of 267 mm and a radius of 1250 mm—it leads to the dispersal of coal fragments of various sizes on the ground. The resulting accumulation of coal dust displays a distinct degree of particle size sorting.

(4)

The susceptibility of tectonic coal to outbursts is attributed not only to its low mechanical strength but also to the large effective area subjected to seepage pressure, as indicated by its high area fracture ratio λ. The denser the fractures perpendicular to the seepage direction, the greater the total area over which seepage pressure acts. Consequently, the cumulative seepage pressure increases and raises the likelihood of an outburst. The denser the fractures perpendicular to the seepage direction, the greater the gas content. Under the same gas pressure in coal, the denser the fractures, the higher the gas content, and the greater the total energy of the outburst.

3.1.2. Continuous Delamination Process of Coal Slices

The preceding analysis focuses on the failure process of a single coal slice in tectonic coal. In reality, coal and gas outbursts occur as a continuous failure process. Once a coal slice is delaminated, a new pressure difference is established between the gas pressure within the coal seam and the ambient pressure in the outburst cavity. If this pressure difference satisfies the criteria specified in Equation (5), additional coal slices will undergo delamination. This process leads to continuous seepage failure within the coal seam [26].

During an outburst event, the internal gas pressure within the coal seam decreases owing to seepage, while the pressure inside the outburst cavity rises as coal dust accumulates. This outburst results in a gradual reduction in the pressure difference between the coal seam and the outburst cavity. If this pressure difference no longer meets the criteria specified in Equation (5), the outburst will cease.

According to Table 1, a gas pressure of 0.74 MPa can induce the delamination of coal slices measuring 267 mm in thickness, 1250 mm in radius, and weighing approximately 1.9 tons each. Therefore, an outburst involving 60 tons of coal would require the delamination of roughly 30 such slices. This explains how a substantial amount of coal can be ejected in a short period during an outburst event.

3.2. The “Fluidization Type” Outburst Process

When tectonic coal undergoes fluidization, it loses its strength and naturally satisfies the conditions described in Equation (5). Therefore, “fluidization type” outbursts can be regarded as a special case of “quicksand type” outbursts. Because coal particles become completely isolated by the gas and lose contact with one another, this fluidization process is characterized by the continuous outflow of a coal–gas mixture, without the occurrence of laminar delamination. When an outburst is over, the coal ejected to the ground appears as granular.

3.3. Concept of Coal and Gas Outbursts Prediction Methods

3.3.1. Prediction of “Quicksand Type” Outburst

According to Equation (5), the calculation formula for the ratio of coal seam thickness to radius is:

$${\displaystyle \frac{l}{r}}={\displaystyle \frac{\lambda (P_i-P_0)-{\sigma }_t}{2\tau }}$$

(9)

It can be seen that the thickness-to-radius ratio of the coal slice is related to gas pressure, coal fractures, and coal strength. Collect coal powder from the outburst site of the coal mine working face, measure the thickness-to-radius ratio of the coal slice, and find the maximum value ${\left({\displaystyle \frac{l}{r}}\right)}_{\max }$, which can be considered to correspond to the intensity of this outburst. By substituting ${\left({\displaystyle \frac{l}{r}}\right)}_{\max }$ and the relevant parameters of the coal seam in a new working face into Equation (5), the critical gas pressure value $P_{cr}$ of the new working face can be obtained. By comparing the critical gas pressure value $P_{cr}$ of the new working face with the actual on-site gas pressure value of the working face, it can be determined whether the new working face will occur an outburst.

3.3.2. Prediction of “Fluidization Type” Outburst

According to Equation (8), if the effective cohesion of the coal body can be ignored, then when the gas pressure $P$ is equal to the normal geostress ${\sigma }_N$, a “fluidizationtype” outburst will occur in the working face. Due to the fact that the outburst is a process that progresses from the outside to the inside, the normal geostress ${\sigma }_N$ can be taken as the average value ${\sigma }_{Nave}$ of the normal geostress of a certain width near the coal wall in the pressure relief zone. The critical gas pressure value $P_{cr}$ is equal to ${\sigma }_{Nave}$. The method for determining ${\sigma }_{Nave}$ needs further research.

3.3.3. Comprehensive Prediction

Compare the two critical gas pressure values obtained by the above two methods, and use the smaller value as the criterion for determining whether a new working face is undergoing an outburst.

4. Discussion

There is a view that as the geostress increases, the risk of coal outbursts increases [6]. However, according to Equation (5), the geostress has two opposite effects on coal and gas outbursts:on the one hand, it breaks the coal body, which increases the area fracture ratio $\lambda$ of the coal body, reduces the strength of coal body, and promotes outbursts; on the other hand, an increase in normal geostress ${\sigma }_N$ will enhance the shear strength $\tau$ of coal body and suppress outbursts. Therefore, the impact of geostress on coal and gas outbursts depends on the combined effect of the two aforementioned factors.

The outbursts of coal pillars can be attributed to high geostress, as the high geostress causes the coal pillars to break, increasing the area fracture ratio $\lambda$ of the coal pillars and reducing the strength of coal body. Similarly, coal and gas outbursts caused by rock bursts can also be attributed to high geostress. The sudden rupture of the thick hard roof causes an enormous increase in geostress, leading to a significant increase in coal body fractures and a decrease in coal body strength, thereby inducing a coal and gas outburst.

For the same coal seam at different depths, if the fracture ratio $\lambda$ of the coal seam area does not increase with the depth, the normal stress ${\sigma }_N$ of the deeper coal seam is higher, which increases the shear strength $\tau$ of the coal body and makes the coal seam less likely to occur outbursts. In recent years, as mining depths have increased, the frequency of coal and gas outburst incidents in China has greatly declined. This reduction is largely attributed to improved management practices. Moreover, it supports the conclusion of this study that higher normal geostress ${\sigma }_N$ exerts a suppressive effect on outbursts. Therefore, it is recommended to conduct mining experiments focused on increasing the upper limit of allowable gas pressure, to study new strategies for safe and efficient coal mine production.

5. Conclusions

Quicksand and seismic liquefaction are common challenges in geotechnical engineering, with well-established theories and mitigation strategies. In this study, tectonic coal and gas are analogized to soil and pore water. According to this analogy, the mechanisms of seepage failure and fluidization in tectonic coal are examined via analytical methods from soil mechanics. The following conclusions are drawn:

(1)

The similarities between tectonic coal and gas with soil and pore water are analyzed. The fractures in tectonic coal decompose along directions both parallel and perpendicular to the seepage flow. According to this observation, a fracture network model and a one-dimensional seepage failure criterion are established for tectonic coal. Coal outbursts are classified into two types: “quicksand type” and “fluidization type.”

(2)

“Quicksand type” outbursts result from the continuous detachment of coal near the pressure relief zone under the influence of gas seepage pressure. The thickness-to-radius ratio of the coal slices increases with higher gas pressure but decreases as coal strength and normal geostress increase.

(3)

“Fluidization type” outbursts occur when the effective stress drops to zero, which represents a specific case of the “quicksand type” outburst. These outbursts can be triggered by natural vibrations or those induced by mining activities.

(4)

The proposed mechanism model for “fluidization type” outbursts explains why the depth of the outburst cavities is typically less than the width of the pressure relief zone. It also clarifies the mechanisms behind delayed outbursts.

(5)

The thickness-to-radius ratio of the coal slice at the outburst site, as well as the average normal geostress within a certain width range near the coal wall of the working face, could be used to predict coal and gas outbursts.