# The Criteria of Underground Rock Structure Failure and Its Implication on Rockburst in Roadway: A Numerical Method

Authors: Xiaofei Guo; Zhiqiang Zhao; Xu Gao; Zhenkai Ma; Nianjie Ma
Journal: Shock and Vibration
 (2019)

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## Abstract

Rockburst in roadway happened along with a large-scale destruction of the surrounding rock. To study the failure laws of the surrounding rock in the process of rockburst in roadway, the evolution behaviors of the plastic zone and the criteria of large-scope failure were studied by using FLAC numerical simulation. Meanwhile, the stress response laws of the plastic zone were studied by loading or unloading in a single direction. The results showed that, in the 20 MPa stress environment, large-scale failure zone would appear when the maximum confining pressure was loaded to 50 MPa or the minimum confining pressure was unloaded to 6 MPa. Loading in the direction of maximum confining pressure or unloading in the direction of minimum confining pressure, when the stresses reached a certain limit, could lead to a large-scale expansion to the failure zone of the surrounding rock a roadway. Meanwhile, the stress response of the plastic zone became more sensitive, which might easily trigger rockburst in roadway. In addition, two sine qua nonstress conditions for rockburst in roadway were determined: high stress ratio and high stress level. This might provide a theoretical basis for the stress source mechanism of roadway rockburst.


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## Body

## 1. Introduction

Rockburst in mining displays the characteristics of suddenness, uncertainty, instantaneity, and destructiveness, and it is one of the greatest threats to the underground engineering [1–3]. Due to the complex geological conditions, China has become one of the worst affected countries by the rockburst and its related geohazards [4–7]. It is expected that the risk of rockburst will continuously increase due to the high mining intensity and increase of the stress with mining depth deeper and deeper.To date, extensive efforts have been devoted to explain the mechanisms of rockburst [8–11], and different classifications were interpreted based on various mechanisms [12–14]. The classification method based on different stress sources has been widely accepted [15], namely, rock failure type, fault slip type, and roof fracture type. According to the classification method [16, 17], rock failure typed rockburst was caused by the coal and rock failure under overloaded stresses. Fault slip typed rockburst was induced by the fault slip near the geological structure belts. Roof fracture typed rockburst was caused by the roof fracture in a long span roof. Due to the limited void space of the roadway section in underground mines, the large structural instability events such as fault slip and roof break were unlikely to happen [11–15]. Many rockbursts in roadway were caused by the failure of the surrounding rock under a certain stress condition. Because high level stresses easily accumulate a high elastic energy in the surrounding rock, it has been recognized as a primary mechanism of rockburst in the mine [18, 19]. Many studies on the mechanism of rockburst in the roadway were essential to analyse the source of high stress [20–23]. However, the failure laws of the surrounding rock under specific stress conditions have not been adequately studied.Many previous studies summarized that rockbursts frequently occurred near the regions of exiting geological faults, coal seam variation belt, folded area of coal seam, and high tectonic stress zone [24–26]. Based on a large number of in situ measurements, it was found that the stress distributions in these regions showed a strong nonuniformity [27, 28], resulting in an increase or reduction of stresses in one direction. The rock failure typed rockburst was caused by the coal and rock failure under a specific stress condition. Therefore, the occurrence of rock failure typed rockburst may be attributed to the nonuniform stress distribution. Recently, Ma et al. [29, 30] studied the failure mode around a roadway in the nonuniform stress field. It was found that the butterfly-shaped failure was commonly observed under the nonuniform stress field. On the basis of the butterfly-shaped failure, the mechanism of roadway roof fall [31, 32] and the large deformation mechanism were proposal and discussed [33, 34].In this work, we studied the failure laws of the surrounding rock a roadway under different stress paths and tried to find the criteria of large-scope failure. Meanwhile, the stress response laws of the plastic zone were studied by loading or unloading in a single direction.


## 2. Numerical Simulation Methods

To study the failure behaviors of the surrounding rock a roadway under different stress paths, the FLAC numerical simulation software was used. As we all know, the void space of the roadway has an important influence on the redistribution of stress and the formation of the failure zone. Therefore, the stress, surrounding rock, and free space of roadway should be regarded as a unit when a stress model was built. To facilitate research and find rules more easily, we need to make the following assumptions: the length of the roadway is much larger than its diameter and is considered to be infinite, the horizontal and vertical stresses do not change along the length, and the surrounding rock is a homogeneous continuous medium without creep and viscous behavior. Therefore, the plane strain method can be used and take any section as a research representative of the infinite roadway [35]. The influence of the section shape a roadway on the rockburst was not the research content of this study, and, therefore, the section shape of roadway in the stress model was designed to be a rectangular section commonly used in the field at present. In this study, the stress model of the surrounding rock a roadway in the regional stress field was built, as shown in Figure 1, where P1 and P3 are the maximum and minimum confining pressures.Figure 1
Stress model of the surrounding rock a roadway in the regional stress field.In engineering research, the plastic zone is a theoretical method for measuring the rock failure. In this work, we used the FLAC3D numerical simulation software to study the plastic zone of the rock surrounding a roadway under different stress paths. The roadway width and height were designed to be 4 m and 3 m, respectively, as shown in Figure 1. With the improvement of support technology, to improve work efficiency and save cost, most roadways are excavated in coal seam in China [36]. Therefore, the coal mechanical parameters were referred when choosing the surrounding rock mechanical parameters [29, 34]. Coal is a low-strength rock, and the compressive strength of the coal is approximately 10 MPa [29, 34]. The detailed parameters are shown in Table 1. The plastic zone evolutions of the rock surrounding a roadway under different stress paths were studied, and the stress conditions of large-scale failure zone formation were explored.Table 1
Rock mechanics parameters in numerical simulation.
Compressive strength (MPa)
Tensile strength (MPa)
Elastic modulus (GPa)
Poisson’s ratio
Cohesion (MPa)
Internal friction angle (deg.)
9.42
0.35
22.96
0.23
3
25In order to study the failure laws of the surrounding rock a roadway under different stress models, three numerical simulation schemes were designed, as shown in Figure2.Figure 2
Simulation schemes.Scheme 1.
Keep the minimum confining pressure unchanged and load gradually in the direction of maximum confining pressure, as shown in Figure2(a). Then observe and record the shapes and scope of the plastic zone of the surrounding rock a roadway.Scheme 2.
Keep the maximum confining pressure unchanged and unload gradually in the direction of minimum confining pressure, as shown in Figure2(b). Then observe and record the shapes and scope of the plastic zone of the surrounding rock a roadway.Scheme 3.
Keep the biaxial confining pressure ratio (P1/P3) unchanged and load gradually in the biaxial directions, as shown in Figure 2(c). Then observe and record the shape and scope of the plastic zone of the surrounding rock a roadway.


## 3. Numerical Simulation Results

### 3.1. Response of Plastic Zone of the Rock Surrounding a Roadway Loading in the Direction of Maximum Confining Pressure

The stress conditions of the roadway with 800 m depth were simulated in this study. Keeping the minimum confining pressure unchanged as 20 MPa, the maximum confining pressure was gradually increased from 20 MPa. We recorded the plastic zone distribution of the surrounding rock a roadway under various stress conditions and also calculated the maximum radius of the plastic zoneRmax (the furthest distance from the plastic zone boundary to the roadway centre). Figure 3 is an evolution of the plastic zone loading in the direction of maximum confining pressure.(1)
Loading in the direction of the maximum confining pressure, the plastic zone of surrounding rock a roadway expanded constantly, and its shape also changed. When the maximum confining pressure was 20 MPa, the model was subjected to bidirectional isobaric stresses, and the plastic zone had a small range. The both sides of the roadway were relatively larger with a depth of 0.6 m. With the increase of the maximum confining pressure, the plastic zone of the surrounding rock a roadway expanded continuously, and the maximum depth of the damage was changed from two sides to the corners of the roadway. As the maximum confining pressure continued to increase, the plastic zone shape would not change and the plastic zone would expand in one direction, as shown in Figures3(d)–3(f).(2)
At the initial stage, the plastic zone of the surrounding rock a roadway was small and had a dull response to stress; namely, the change rate of the plastic zone range caused by increasing a unit stress was small, as shown in Figures3(a)–3(c). With the stress increase of 10 MPa, the maximum radius of the plastic zone only extended 3.2 m. At the later stage, the plastic zone of the surrounding rock a roadway was larger and had a sensitive response to stress; namely, the change rate of the plastic zone range caused by increasing a unit stress was large, as shown in Figures 3(d)–3(f). With the stress increase of 5 MPa, the maximum radius of the plastic zone extended 26.9 m.Figure 3
Evolution laws of the plastic zone loading in the direction of maximum confining pressure. (a)Rmax = 2.6 m. (b) Rmax = 3.6 m. (c) Rmax = 6.8 m. (d) Rmax = 12.1 m. (e) Rmax = 21.5 m. (f) Rmax = 48.4 m.Taking the maximum confining pressure as thex-axis and the maximum radius of the plastic zone as the y-axis, we established the relationships between the maximum radius of the plastic zone and the maximum confining pressure, as illustrated in Figure 4.(1)
With the increase of the maximum confining pressure, the curve showed an increasing trend and the curve slope increased gradually. The maximum radius of the plastic zone underwent a slow growth state to an accelerated growth state until it reached infinity. In particular, the curve slope represented the stress response characteristics of the surrounding rock, namely, the plastic zone change laws caused by one unit stress change.(2)
According to the curve trend, the maximum confining pressures were divided into three different loading areas: slow growth loading area, accelerated growth loading area, and extreme loading area. When the maximum confining pressure was in the slow growth loading area, the plastic zone of the surrounding rock a roadway was small, and it had a dull response to stress; at this stage, the surrounding rock structure of the roadway was stable, and there was no risk of rockburst. When the maximum confining pressure was in the accelerated growth loading area, the plastic zone of the surrounding rock a roadway was large, and it had a sensitive response to stress; at this stage, the stability of the surrounding rock structure became deteriorated, and the stress changes could lead to the surge in the scope of the destruction area, which easily induced the surrounding rock structure instability. When the maximum confining pressure increased to a certain limit, the maximum radius of the plastic zone would reach infinity and the surrounding rock structure of the roadway would destabilize; when the maximum confining pressure was in the extreme loading area, there was a high risk of rockburst in the roadway.Figure 4
Relationship between maximum radius of plastic zone and maximum confining pressure.Therefore, loading in the direction of maximum confining pressure, the bigger the stress was, the larger the surrounding rock plastic zone was, the more sensitive the stress response was, the worse the stability of the surrounding rock was, and the greater the risk of rockburst was. The stability of the surrounding rock structure became worse, and the risk of rockburst would be greater.


### 3.2. Response of Plastic Zone of the Rock Surrounding a Roadway Unloading in the Direction of Minimum Confining Pressure

According to the above research methods, keeping the maximum confining pressure unchanged as 20 MPa, the minimum confining pressure gradually decreased from 20 MPa. The distribution of the plastic zone of the surrounding rock a roadway was calculated, and the maximum radius of the plastic zoneRmax under each stress state was recorded. Figure 5 is the evolution of the plastic zone unloading in the direction of minimum confining pressure.(1)
Unloading in the direction of the minimum confining pressure, the expansion law of the plastic zone was similar to that loading in the direction of maximum confining pressure. With the decrease of the minimum confining pressure, the plastic zone of the surrounding rock a roadway expanded continuously, and the maximum depth of the damage changed from two sides to the corners of the roadway. As the minimum confining pressure continued to decrease, the shape of the plastic zone would not change and the plastic zone would expand in one direction, as shown in Figures5(d)–5(f).(2)
At the initial stage, the plastic zone of the surrounding rock a roadway was smaller and it had a dull response to stress; as shown in Figures5(a)–5(c), with the stress decrease of 5 MPa, the maximum radius of the plastic zone only extended 0.26 m. At the later stage, the plastic zone of the surrounding rock a roadway was larger and had a sensitive response to stress; as shown in Figures 5(d)–5(f), with the stress decrease of 1 MPa, the maximum radius of the plastic zone extended 17.8 m.(3)
The stress response of the plastic zone was more sensitive unloading in the direction of the minimum confining pressure than that loading in the direction of the maximum confining pressure. This showed that stress concentration and high stress were not sufficient conditions for rockburst in the roadway. On the contrary, unloading in the direction of minimum confining pressure could easily lead to the plastic zone rapid expansion and induce rockburst.Figure 5
Evolution laws of plastic zone unloading in the direction of minimum confining pressure. (a)Rmax = 2.6 m. (b) Rmax = 2.74 m. (c) Rmax = 3 m. (d) Rmax = 7.1 m. (e) Rmax = 12.9 m. (f) Rmax = 30.7 m.Taking the minimum confining pressure as thex-axis and the maximum radius of the plastic zone as the y-axis, we established the relationship between the maximum radius of the plastic zone and the minimum confining pressure, as indicated in Figure 6.(1)
The curve trend unloading in the direction of the minimum confining pressure was consistent with that loading in the direction of the maximum confining pressure. With the decrease of the minimum confining pressure, the curve showed an increasing trend and the curve slope increased gradually. The maximum radius of the plastic zone underwent a slow growth state to an accelerated growth state until it reached infinity.(2)
According to the curve trend, the minimum confining pressures were divided into three different unloading areas: slow growth unloading area, accelerated growth unloading area, and extreme unloading area. When the minimum confining pressure was in the slow growth unloading area, the plastic zone of the surrounding rock a roadway was small and it had a dull response to stress; at this stage, the surrounding rock structure of the roadway was stable, and there was no risk of rockburst. When the minimum confining pressure was in the accelerated growth unloading area, the plastic zone of the surrounding rock a roadway was large and it had a sensitive response to stress; at this state, the stability of the surrounding rock structure a roadway became deteriorated, and the stress changes would lead to the surge in the scope of the destruction area which easily induced instability of the surrounding rock structure a roadway. When the minimum confining pressure decreased to a certain limit, the maximum radius of the plastic zone would reach infinity and the surrounding rock structure of the roadway would destabilize; when the minimum confining pressure was in the extreme unloading area, there was a high risk of rockburst in the roadway.Figure 6
Relationship between the maximum radius of plastic zone and minimum confining pressure.Therefore, unloading in the direction of minimum confining pressure, the smaller the stress was, the larger the surrounding rock plastic zone was, the more sensitive the stress response was, the worse the stability of the surrounding rock was, and the greater the risk of rockburst was. The stability of the surrounding rock structure a roadway became worse, and the risk of rockburst would be greater.Loading in the direction of maximum confining pressure or unloading in the direction of minimum confining pressure could lead to the expansion of the plastic zone. During the loading or unloading, the plastic zone of the surrounding rock a roadway underwent the slow expansion stage to the accelerated development stage until the complete destruction. The stress concentration and high stress were not sufficient conditions for rockburst in the roadway. Both the high stress in the direction of maximum confining pressure and the low stress in the direction of minimum confining pressure could lead to the plastic zone rapid expansion and induce rockburst.


### 3.3. Response of Plastic Zone of Rock Surrounding a Roadway under the Conditions of Different Biaxial Confining Pressure Ratios

Loading in the direction of the maximum confining pressure and unloading in the direction of the minimum confining pressure had the same effect that both increase the biaxial confining pressure difference. To study the influence of the biaxial confining pressure difference on the stability of the surrounding rock, we studied the relationship between the plastic zone and the maximum confining pressure under different biaxial confining pressure ratios (P1/P3), as shown in Figure 7. Each curve in the figure corresponded to a confining pressure ratio. Although the abscissa was the maximum confining pressure, due to the confining pressure ratio fixed, the curve also represented the trend of the maximum radius of the plastic zone with the minimum confining pressure to a certain extent.(1)
When the biaxial confining pressure ratio was small, as shown in the curves of ratios 1 and 2 in Figure7, even if the stress reached the level of 100 MPa which was impossible to reach at present, the plastic zone of the surrounding rock was still small. The curves changed almost linearly with the change of stress; namely, the stress response was dull. When the surrounding rock structure of roadway was in such a stress environment, the failure zone was small and the surrounding rock structure of roadway was stable without the risk of rockburst.(2)
When the biaxial confining pressure ratio was large, as shown in the curves of ratios 3 and 4 in Figure7, in the case of lower stress value, the plastic zone of the surrounding rock a roadway would have a wide range of expansion. As the stress increased, the curves showed an exponential growth, and the curve slope increased continuously; namely, the stress response sensitivity of the plastic zone continued to increase. When the surrounding rock structure of the roadway was in such a stress environment, the failure zone was large and the surrounding rock structure of the roadway was instable.(3)
When the confining pressure ratio was constant, the higher the stress value was, the larger the plastic zone of the surrounding rock was, and the more unstable the surrounding rock structure was, as shown in the curves of ratios 3 and 4 in Figure7. When the stress was low, the surrounding rock plastic zone was small and the surrounding rock structure was stable. Only when the stress level reached a certain value would the plastic zone of the surrounding rock expand greatly. Therefore, not only the occurrence of rockburst does need a certain biaxial confining pressure ratio but also needs a certain value of stress.Figure 7
Relationship between maximum radius of plastic zone and maximum confining pressure under different confining pressure ratios.Both loading in the direction of the maximum confining pressure and unloading in the direction of the minimum confining pressure cause the increase of the biaxial confining pressure ratio. On the basis of the above analysis, two necessary stress conditions should be met in the occurrence of rockburst in the roadway: high stress level and high stress ratio. The traditional understanding of the high stress level in the stress condition of the rockburst in the roadway was one-sided. Without the condition of the high stress ratio, even if there is a large stress level, the surrounding rock structure of the roadway is still stable, and the rockburst will not occur.


### 3.4. Stress Conditions of Rockburst in Roadway

We studied the stability of the surrounding rock structure a roadway by means of loading and unloading in one direction and changing biaxial confining pressure ratio. The necessary stress conditions for rockburst in the roadway were determined: high stress ratio and high stress level. These two basic conditions are indispensable: only having the high stress ratio but not enough high stress level is not enough to destroy the surrounding rock and only high stress level without high stress ratio will not cause extensive failure zone to the surrounding rock.In the actual site, the regional stress field around the roadway is often affected by such factors as earthquakes, fault activation, period pressure of coalface, moving abutment pressure in front of coal mining face, and mining activities. All of these activities will lead to loading or unloading in one certain direction to the roadway regional stress field. If it is loaded in the direction of the maximum confining pressure or unloaded in the direction of the minimum confining pressure, the biaxial confining pressure ratio will increase. When the stress reaches a certain limit, the roadway surrounding rock will have a large-scale failure zone, causing the surrounding rock structure of roadway instability, and the rockburst could be triggered, as shown in Figure8.Figure 8
Mechanism of roadway rockburst.


## 3.1. Response of Plastic Zone of the Rock Surrounding a Roadway Loading in the Direction of Maximum Confining Pressure

The stress conditions of the roadway with 800 m depth were simulated in this study. Keeping the minimum confining pressure unchanged as 20 MPa, the maximum confining pressure was gradually increased from 20 MPa. We recorded the plastic zone distribution of the surrounding rock a roadway under various stress conditions and also calculated the maximum radius of the plastic zoneRmax (the furthest distance from the plastic zone boundary to the roadway centre). Figure 3 is an evolution of the plastic zone loading in the direction of maximum confining pressure.(1)
Loading in the direction of the maximum confining pressure, the plastic zone of surrounding rock a roadway expanded constantly, and its shape also changed. When the maximum confining pressure was 20 MPa, the model was subjected to bidirectional isobaric stresses, and the plastic zone had a small range. The both sides of the roadway were relatively larger with a depth of 0.6 m. With the increase of the maximum confining pressure, the plastic zone of the surrounding rock a roadway expanded continuously, and the maximum depth of the damage was changed from two sides to the corners of the roadway. As the maximum confining pressure continued to increase, the plastic zone shape would not change and the plastic zone would expand in one direction, as shown in Figures3(d)–3(f).(2)
At the initial stage, the plastic zone of the surrounding rock a roadway was small and had a dull response to stress; namely, the change rate of the plastic zone range caused by increasing a unit stress was small, as shown in Figures3(a)–3(c). With the stress increase of 10 MPa, the maximum radius of the plastic zone only extended 3.2 m. At the later stage, the plastic zone of the surrounding rock a roadway was larger and had a sensitive response to stress; namely, the change rate of the plastic zone range caused by increasing a unit stress was large, as shown in Figures 3(d)–3(f). With the stress increase of 5 MPa, the maximum radius of the plastic zone extended 26.9 m.Figure 3
Evolution laws of the plastic zone loading in the direction of maximum confining pressure. (a)Rmax = 2.6 m. (b) Rmax = 3.6 m. (c) Rmax = 6.8 m. (d) Rmax = 12.1 m. (e) Rmax = 21.5 m. (f) Rmax = 48.4 m.Taking the maximum confining pressure as thex-axis and the maximum radius of the plastic zone as the y-axis, we established the relationships between the maximum radius of the plastic zone and the maximum confining pressure, as illustrated in Figure 4.(1)
With the increase of the maximum confining pressure, the curve showed an increasing trend and the curve slope increased gradually. The maximum radius of the plastic zone underwent a slow growth state to an accelerated growth state until it reached infinity. In particular, the curve slope represented the stress response characteristics of the surrounding rock, namely, the plastic zone change laws caused by one unit stress change.(2)
According to the curve trend, the maximum confining pressures were divided into three different loading areas: slow growth loading area, accelerated growth loading area, and extreme loading area. When the maximum confining pressure was in the slow growth loading area, the plastic zone of the surrounding rock a roadway was small, and it had a dull response to stress; at this stage, the surrounding rock structure of the roadway was stable, and there was no risk of rockburst. When the maximum confining pressure was in the accelerated growth loading area, the plastic zone of the surrounding rock a roadway was large, and it had a sensitive response to stress; at this stage, the stability of the surrounding rock structure became deteriorated, and the stress changes could lead to the surge in the scope of the destruction area, which easily induced the surrounding rock structure instability. When the maximum confining pressure increased to a certain limit, the maximum radius of the plastic zone would reach infinity and the surrounding rock structure of the roadway would destabilize; when the maximum confining pressure was in the extreme loading area, there was a high risk of rockburst in the roadway.Figure 4
Relationship between maximum radius of plastic zone and maximum confining pressure.Therefore, loading in the direction of maximum confining pressure, the bigger the stress was, the larger the surrounding rock plastic zone was, the more sensitive the stress response was, the worse the stability of the surrounding rock was, and the greater the risk of rockburst was. The stability of the surrounding rock structure became worse, and the risk of rockburst would be greater.


## 3.2. Response of Plastic Zone of the Rock Surrounding a Roadway Unloading in the Direction of Minimum Confining Pressure

According to the above research methods, keeping the maximum confining pressure unchanged as 20 MPa, the minimum confining pressure gradually decreased from 20 MPa. The distribution of the plastic zone of the surrounding rock a roadway was calculated, and the maximum radius of the plastic zoneRmax under each stress state was recorded. Figure 5 is the evolution of the plastic zone unloading in the direction of minimum confining pressure.(1)
Unloading in the direction of the minimum confining pressure, the expansion law of the plastic zone was similar to that loading in the direction of maximum confining pressure. With the decrease of the minimum confining pressure, the plastic zone of the surrounding rock a roadway expanded continuously, and the maximum depth of the damage changed from two sides to the corners of the roadway. As the minimum confining pressure continued to decrease, the shape of the plastic zone would not change and the plastic zone would expand in one direction, as shown in Figures5(d)–5(f).(2)
At the initial stage, the plastic zone of the surrounding rock a roadway was smaller and it had a dull response to stress; as shown in Figures5(a)–5(c), with the stress decrease of 5 MPa, the maximum radius of the plastic zone only extended 0.26 m. At the later stage, the plastic zone of the surrounding rock a roadway was larger and had a sensitive response to stress; as shown in Figures 5(d)–5(f), with the stress decrease of 1 MPa, the maximum radius of the plastic zone extended 17.8 m.(3)
The stress response of the plastic zone was more sensitive unloading in the direction of the minimum confining pressure than that loading in the direction of the maximum confining pressure. This showed that stress concentration and high stress were not sufficient conditions for rockburst in the roadway. On the contrary, unloading in the direction of minimum confining pressure could easily lead to the plastic zone rapid expansion and induce rockburst.Figure 5
Evolution laws of plastic zone unloading in the direction of minimum confining pressure. (a)Rmax = 2.6 m. (b) Rmax = 2.74 m. (c) Rmax = 3 m. (d) Rmax = 7.1 m. (e) Rmax = 12.9 m. (f) Rmax = 30.7 m.Taking the minimum confining pressure as thex-axis and the maximum radius of the plastic zone as the y-axis, we established the relationship between the maximum radius of the plastic zone and the minimum confining pressure, as indicated in Figure 6.(1)
The curve trend unloading in the direction of the minimum confining pressure was consistent with that loading in the direction of the maximum confining pressure. With the decrease of the minimum confining pressure, the curve showed an increasing trend and the curve slope increased gradually. The maximum radius of the plastic zone underwent a slow growth state to an accelerated growth state until it reached infinity.(2)
According to the curve trend, the minimum confining pressures were divided into three different unloading areas: slow growth unloading area, accelerated growth unloading area, and extreme unloading area. When the minimum confining pressure was in the slow growth unloading area, the plastic zone of the surrounding rock a roadway was small and it had a dull response to stress; at this stage, the surrounding rock structure of the roadway was stable, and there was no risk of rockburst. When the minimum confining pressure was in the accelerated growth unloading area, the plastic zone of the surrounding rock a roadway was large and it had a sensitive response to stress; at this state, the stability of the surrounding rock structure a roadway became deteriorated, and the stress changes would lead to the surge in the scope of the destruction area which easily induced instability of the surrounding rock structure a roadway. When the minimum confining pressure decreased to a certain limit, the maximum radius of the plastic zone would reach infinity and the surrounding rock structure of the roadway would destabilize; when the minimum confining pressure was in the extreme unloading area, there was a high risk of rockburst in the roadway.Figure 6
Relationship between the maximum radius of plastic zone and minimum confining pressure.Therefore, unloading in the direction of minimum confining pressure, the smaller the stress was, the larger the surrounding rock plastic zone was, the more sensitive the stress response was, the worse the stability of the surrounding rock was, and the greater the risk of rockburst was. The stability of the surrounding rock structure a roadway became worse, and the risk of rockburst would be greater.Loading in the direction of maximum confining pressure or unloading in the direction of minimum confining pressure could lead to the expansion of the plastic zone. During the loading or unloading, the plastic zone of the surrounding rock a roadway underwent the slow expansion stage to the accelerated development stage until the complete destruction. The stress concentration and high stress were not sufficient conditions for rockburst in the roadway. Both the high stress in the direction of maximum confining pressure and the low stress in the direction of minimum confining pressure could lead to the plastic zone rapid expansion and induce rockburst.


## 3.3. Response of Plastic Zone of Rock Surrounding a Roadway under the Conditions of Different Biaxial Confining Pressure Ratios

Loading in the direction of the maximum confining pressure and unloading in the direction of the minimum confining pressure had the same effect that both increase the biaxial confining pressure difference. To study the influence of the biaxial confining pressure difference on the stability of the surrounding rock, we studied the relationship between the plastic zone and the maximum confining pressure under different biaxial confining pressure ratios (P1/P3), as shown in Figure 7. Each curve in the figure corresponded to a confining pressure ratio. Although the abscissa was the maximum confining pressure, due to the confining pressure ratio fixed, the curve also represented the trend of the maximum radius of the plastic zone with the minimum confining pressure to a certain extent.(1)
When the biaxial confining pressure ratio was small, as shown in the curves of ratios 1 and 2 in Figure7, even if the stress reached the level of 100 MPa which was impossible to reach at present, the plastic zone of the surrounding rock was still small. The curves changed almost linearly with the change of stress; namely, the stress response was dull. When the surrounding rock structure of roadway was in such a stress environment, the failure zone was small and the surrounding rock structure of roadway was stable without the risk of rockburst.(2)
When the biaxial confining pressure ratio was large, as shown in the curves of ratios 3 and 4 in Figure7, in the case of lower stress value, the plastic zone of the surrounding rock a roadway would have a wide range of expansion. As the stress increased, the curves showed an exponential growth, and the curve slope increased continuously; namely, the stress response sensitivity of the plastic zone continued to increase. When the surrounding rock structure of the roadway was in such a stress environment, the failure zone was large and the surrounding rock structure of the roadway was instable.(3)
When the confining pressure ratio was constant, the higher the stress value was, the larger the plastic zone of the surrounding rock was, and the more unstable the surrounding rock structure was, as shown in the curves of ratios 3 and 4 in Figure7. When the stress was low, the surrounding rock plastic zone was small and the surrounding rock structure was stable. Only when the stress level reached a certain value would the plastic zone of the surrounding rock expand greatly. Therefore, not only the occurrence of rockburst does need a certain biaxial confining pressure ratio but also needs a certain value of stress.Figure 7
Relationship between maximum radius of plastic zone and maximum confining pressure under different confining pressure ratios.Both loading in the direction of the maximum confining pressure and unloading in the direction of the minimum confining pressure cause the increase of the biaxial confining pressure ratio. On the basis of the above analysis, two necessary stress conditions should be met in the occurrence of rockburst in the roadway: high stress level and high stress ratio. The traditional understanding of the high stress level in the stress condition of the rockburst in the roadway was one-sided. Without the condition of the high stress ratio, even if there is a large stress level, the surrounding rock structure of the roadway is still stable, and the rockburst will not occur.


## 3.4. Stress Conditions of Rockburst in Roadway

We studied the stability of the surrounding rock structure a roadway by means of loading and unloading in one direction and changing biaxial confining pressure ratio. The necessary stress conditions for rockburst in the roadway were determined: high stress ratio and high stress level. These two basic conditions are indispensable: only having the high stress ratio but not enough high stress level is not enough to destroy the surrounding rock and only high stress level without high stress ratio will not cause extensive failure zone to the surrounding rock.In the actual site, the regional stress field around the roadway is often affected by such factors as earthquakes, fault activation, period pressure of coalface, moving abutment pressure in front of coal mining face, and mining activities. All of these activities will lead to loading or unloading in one certain direction to the roadway regional stress field. If it is loaded in the direction of the maximum confining pressure or unloaded in the direction of the minimum confining pressure, the biaxial confining pressure ratio will increase. When the stress reaches a certain limit, the roadway surrounding rock will have a large-scale failure zone, causing the surrounding rock structure of roadway instability, and the rockburst could be triggered, as shown in Figure8.Figure 8
Mechanism of roadway rockburst.


## 4. Discussion

### 4.1. A Case Analysis of Rockburst in Roadway Induced by Loading in the Direction of Maximum Confining Pressure

#### 4.1.1. Engineering Background

Gengcun Coal Mine is one of the main mines of Henan Energy and Chemical Group. In this mine, a single horizontal up and down way in a slanting shaft was adopted. In the 13230 work face, with an average depth of 622 m, the strike longwall retreating mining and fully mechanized sublevel caving were adopted. The natural caving method was adopted to manage the roof, with a recoverable length of 971 m and a propensity of 189 m. On the north side of the working face, there were five goafs. On the south side was the unmined coal seam. The work faces layout of the mining area is shown in Figure9.Figure 9
Work faces layout of mining area in Gengcun mine in China.The 13230 work face began to be recovered in December 2015. During the mining, there have been several large and small rockbursts in the 13230 haulage roadway. A large rockburst accident occurred in December 22, 2015, at the beginning of the mining roadway, causing significant economic losses and casualties. With a length of about 150 m outside the working face safe exit, there happened severe bottoming, contraction of the cross section, and damage to equipment such as electromechanical, transportation, support, and other equipment in the roadway.


#### 4.1.2. Mechanism Analysis of Roadway Rockburst Induced by Loading in the Direction of Maximum Confining Pressure

(1) Stress Condition Analysis. On the north side of the 13230 working face, there were five goafs. The large area roof collapse in the goafs leads to the transfer of vertical stress to the solid coal, which makes the vertical stress around the 13230 haulage roadway increase. In the direction of the roadway section, the horizontal stress becomes the minimum confining pressure, and the vertical stress becomes the maximum confining pressure (P1 = Pz, P3 = Px). In front of the 13230 work face, the vertical stress could be increased to more than 3 times the initial stresses under the influence of the advancing abutment pressure during the period of work face cutting coal [37, 38]. The stresses distribution of the surrounding rock in the 13230 haulage roadway is shown in Figure 10. As Figure shows, the stress in the direction of the maximum confining pressure was gradually loaded during work face mining.Figure 10
Stresses distribution of 13230 haulage roadway surrounding rock.(2) Numerical Simulation and Result Analysis. On the basis of the field stress test results [37], the horizontal stress was the minimum confining pressure of the roadway surrounding rock, which was 19.5 MPa, and the vertical stress was the maximum confining pressure, as the vertical stress could be increased to more than 3 times the initial stress during the work face mining. In the simulation, the vertical stress was increased from 19.5 MPa in 5 MPa up to 55 MPa, and the simulated stress conditions are shown in Table 2. The rock mechanics parameters used in the simulation were the mechanical parameters of the coal actually tested [37], as shown in Table 3.Table 2
Numerical simulation stress conditions.
P
1 (MPa)
19.5
25
30
35
40
45
50
55
P
3 (MPa)
19.5Table 3
Rock mechanics parameters for numerical simulation.
Compressive strength (MPa)
Tensile strength (MPa)
Elastic modulus (GPa)
Poisson’s ratio
Cohesion (MPa)
Internal friction angle (°)
9.42
0.75
22.96
0.23
3.0
25.1Figure11 was the plastic zone change curve in the 13230 haulage roadway during mining. During mining, vertically loading would cause the severe expansion of the plastic zone of the surrounding rock a roadway. When the vertical stress was raised to 2.8 times the initial stress (55 MPa), the maximum radius of the roadway surrounding rock plastic zone was reached 37 m, and the maximum confining pressure was in the accelerated growth loading area. The plastic zone of the surrounding rock a roadway was large, and it had a sensitive response to stress. The stability of the surrounding rock structure a roadway became deteriorated, and it easily induced rockburst.Figure 11
Plastic zone change curve in the 13230 haulage roadway during work face mining.The large area roof collapse of five work face goafs on the north side of the 13230 working face led to the increase of the vertical stress around the haulage roadway. Influenced by the advancing abutment pressure during work face mining, the haulage roadway was loaded with several times the initial stress in the direction of the maximum confining pressure, which resulted in a sharp increase of the biaxial confining pressure ratio. At this time, the plastic zone of the surrounding rock a roadway was large, and the stability of the surrounding rock structure a roadway became deteriorated. Therefore, there occurred rockbursts many times in the 13230 haulage roadway during mining.


### 4.2. A Case Analysis of Roadway Rockburst Induced by Unloading in the Direction of Minimum Confining Pressure

#### 4.2.1. Engineering Background

Wudong Coal Mine is located in the northeast of Urumqi city in China. The mine surface elevation was +850 m, and there were two major minable seams (B3 + 6 and B1 + 2). The average dip angles of the two coal seams were about 87°, and the thickness of two coal seams was 40 m and 30 m. A hard rock column dominated by siltstone with an average thickness of nearly 100 m was between two coal seams. According to the different mining level, the working faces were arranged in two minable seams from top to bottom. The length of the work faces was in accordance with the thickness of the coal seams, and work faces moved along the coal seams trend. The two work faces were both mining with the top-coal caving method, in which the extraction-caving ratio was 1 : 7, and the mining height was about 25 m. The general situation of mining engineering in Wudong Mine is illustrated in Figure12.Figure 12
Mining arrangement in Wudong mine in China.From the beginning of rockburst in roadway in September 2009, there have been more than 20 times of large and small rockburst in Wudong Mine, and the degree of disaster is becoming more and more serious. At 5 : 12 on March 24, 2014, during the B3 + 6 work face mining in the +475 m level, a large rockburst occurred in the B3 + 6 preparation roadway, causing the heavy floor lifts in the 150 m area, accompanied by a loud noise.


#### 4.2.2. Mechanism Analysis of Roadway Rockburst Induced by Unloading in the Direction of Minimum Confining Pressure

(1) Stress Condition Analysis. Wudong Mine was worked by the approach of layering mining from top to bottom in accordance with different mining levels. So, the upper area was a goaf during next level coal mining. In view of the hard rock columns on both sides of the coal seams, the pressure was almost completely unloaded in the vertical direction, and the confining pressure was very small in the vertical direction of the roadway. The horizontal stress was the maximum confining pressure, and the vertical stress was the minimum confining pressure (P1 = Px, P3 = Pz). The process of the B3 + 6 work face mining in the +475 m level was also the process of preparation roadway unloading in the direction of minimum confining pressure.(2) Numerical Simulation and Result Analysis. Based on the in situ stress test results in the +475 m level [39], in the simulation, the horizontal stress was the maximum confining pressure of the roadway, which was 15.58 MPa, and the vertical stress was the minimum confining pressure. In this paper, the shapes of the plastic zone of the surrounding rock a roadway were simulated under different stress conditions of the vertical stress from 7 MPa to 0. The simulated stress conditions are shown in Table 4. The rock mechanics parameters used in the simulation were the mechanical parameters of the coal actually tested in Wudong Mine [39], as shown in Table 5.Table 4
Numerical simulation stress conditions.
P
1 (MPa)
14.58
P
3 (MPa)
7
6
5
4
3
2
1
0.5
0.3
0.1
0Table 5
Rock mechanics parameters for numerical simulation.
Compressive strength (MPa)
Tensile strength (MPa)
Elastic modulus (GPa)
Poisson’s ratio
Cohesion (MPa)
Internal friction angle (°)
9.42
1.421
2.037
0.21
2.4
57Figure13 is the plastic zone change curve of the surrounding rock in the B3 + 6 preparation roadway during work face mining. Vertically unloading during mining would cause the severe expansion of the plastic zone of the surrounding rock a roadway. When the vertical stress was unloaded to 0, the maximum radius of the roadway surrounding rock plastic zone was reached 28.03 m, and the minimum confining pressure was in the accelerated growth loading area. The plastic zone of the surrounding rock a roadway was large, and it had a sensitive response to stress. The stability of the surrounding rock structure a roadway became deteriorated and easily induced rockburst in the roadway.Figure 13
Plastic zone change curve in the B3 + 6 preparation roadway during work face mining.The mining approach of layering mining from top to bottom in Wudong Mine led to the large unloading in the vertical direction of roadway surrounding rock. During the working face recovery, the preparatory roadway below was further unloading in the direction of minimum confining pressure, which resulted in a sharp increase of the biaxial confining pressure ratio. At this time, the plastic zone of the surrounding rock a roadway was large, and the stability of the surrounding rock structure a roadway became deteriorated. Therefore, there occurred rockbursts many times during work face mining.


### 4.3. A Stress Preventive Method of Rockburst in Roadway

Occurrence of rockburst in the roadway might cause great casualties and huge economic losses to the mine. Many scholars have proposed many engineering control measures [40–43], such as cutting roof, hydraulic slotting, mining liberated seam, borehole pressure relief, protective pillar design, and support measures. At the same time, these engineering measures have achieved good control effect in the field.Both loading in the direction of the maximum confining pressure and unloading in the direction of the minimum confining pressure cause the increase of the biaxial confining pressure ratio. Two necessary stress conditions should be met in the occurrence of rockburst in roadway: high stress and high stress ratio. The high stress and high stress ratio would lead to a large-scale failure zone, cause the surrounding rock structure of roadway instability, and then trigger the occurrence of rockburst. Under the conditions of low stress level and low stress ratio, the roadway surrounding rock failure zone was small, and the surrounding rock structure of roadway was stable, and there was no risk of roadway rockburst. The stress preventive methods of rockburst in roadway are to take some certain engineering measures to destroy the two necessary stress conditions: reducing the stress level and reducing the stress differences in different directions. Unloading in the direction of the maximum confining pressure by measures of cutting roof, hydraulic slotting, mining liberated seam, and borehole pressure relief could reduce the stress level and reduce the stress differences in different directions; loading in the direction of the minimum confining pressure by measures of strengthening support on the weak side of roadway and the reasonable design of the protective coal pillar size could reduce the stress differences in different directions. The stress preventive methods of rockburst in roadway are shown in Figure14.Figure 14
Stress preventive methods of rockburst in roadway.In summary, the stress preventive methods of rockburst in roadway were determined: reducing the stress level and reducing the stress differences in different directions.


## 4.1. A Case Analysis of Rockburst in Roadway Induced by Loading in the Direction of Maximum Confining Pressure

### 4.1.1. Engineering Background

Gengcun Coal Mine is one of the main mines of Henan Energy and Chemical Group. In this mine, a single horizontal up and down way in a slanting shaft was adopted. In the 13230 work face, with an average depth of 622 m, the strike longwall retreating mining and fully mechanized sublevel caving were adopted. The natural caving method was adopted to manage the roof, with a recoverable length of 971 m and a propensity of 189 m. On the north side of the working face, there were five goafs. On the south side was the unmined coal seam. The work faces layout of the mining area is shown in Figure9.Figure 9
Work faces layout of mining area in Gengcun mine in China.The 13230 work face began to be recovered in December 2015. During the mining, there have been several large and small rockbursts in the 13230 haulage roadway. A large rockburst accident occurred in December 22, 2015, at the beginning of the mining roadway, causing significant economic losses and casualties. With a length of about 150 m outside the working face safe exit, there happened severe bottoming, contraction of the cross section, and damage to equipment such as electromechanical, transportation, support, and other equipment in the roadway.


### 4.1.2. Mechanism Analysis of Roadway Rockburst Induced by Loading in the Direction of Maximum Confining Pressure

(1) Stress Condition Analysis. On the north side of the 13230 working face, there were five goafs. The large area roof collapse in the goafs leads to the transfer of vertical stress to the solid coal, which makes the vertical stress around the 13230 haulage roadway increase. In the direction of the roadway section, the horizontal stress becomes the minimum confining pressure, and the vertical stress becomes the maximum confining pressure (P1 = Pz, P3 = Px). In front of the 13230 work face, the vertical stress could be increased to more than 3 times the initial stresses under the influence of the advancing abutment pressure during the period of work face cutting coal [37, 38]. The stresses distribution of the surrounding rock in the 13230 haulage roadway is shown in Figure 10. As Figure shows, the stress in the direction of the maximum confining pressure was gradually loaded during work face mining.Figure 10
Stresses distribution of 13230 haulage roadway surrounding rock.(2) Numerical Simulation and Result Analysis. On the basis of the field stress test results [37], the horizontal stress was the minimum confining pressure of the roadway surrounding rock, which was 19.5 MPa, and the vertical stress was the maximum confining pressure, as the vertical stress could be increased to more than 3 times the initial stress during the work face mining. In the simulation, the vertical stress was increased from 19.5 MPa in 5 MPa up to 55 MPa, and the simulated stress conditions are shown in Table 2. The rock mechanics parameters used in the simulation were the mechanical parameters of the coal actually tested [37], as shown in Table 3.Table 2
Numerical simulation stress conditions.
P
1 (MPa)
19.5
25
30
35
40
45
50
55
P
3 (MPa)
19.5Table 3
Rock mechanics parameters for numerical simulation.
Compressive strength (MPa)
Tensile strength (MPa)
Elastic modulus (GPa)
Poisson’s ratio
Cohesion (MPa)
Internal friction angle (°)
9.42
0.75
22.96
0.23
3.0
25.1Figure11 was the plastic zone change curve in the 13230 haulage roadway during mining. During mining, vertically loading would cause the severe expansion of the plastic zone of the surrounding rock a roadway. When the vertical stress was raised to 2.8 times the initial stress (55 MPa), the maximum radius of the roadway surrounding rock plastic zone was reached 37 m, and the maximum confining pressure was in the accelerated growth loading area. The plastic zone of the surrounding rock a roadway was large, and it had a sensitive response to stress. The stability of the surrounding rock structure a roadway became deteriorated, and it easily induced rockburst.Figure 11
Plastic zone change curve in the 13230 haulage roadway during work face mining.The large area roof collapse of five work face goafs on the north side of the 13230 working face led to the increase of the vertical stress around the haulage roadway. Influenced by the advancing abutment pressure during work face mining, the haulage roadway was loaded with several times the initial stress in the direction of the maximum confining pressure, which resulted in a sharp increase of the biaxial confining pressure ratio. At this time, the plastic zone of the surrounding rock a roadway was large, and the stability of the surrounding rock structure a roadway became deteriorated. Therefore, there occurred rockbursts many times in the 13230 haulage roadway during mining.


## 4.1.1. Engineering Background

Gengcun Coal Mine is one of the main mines of Henan Energy and Chemical Group. In this mine, a single horizontal up and down way in a slanting shaft was adopted. In the 13230 work face, with an average depth of 622 m, the strike longwall retreating mining and fully mechanized sublevel caving were adopted. The natural caving method was adopted to manage the roof, with a recoverable length of 971 m and a propensity of 189 m. On the north side of the working face, there were five goafs. On the south side was the unmined coal seam. The work faces layout of the mining area is shown in Figure9.Figure 9
Work faces layout of mining area in Gengcun mine in China.The 13230 work face began to be recovered in December 2015. During the mining, there have been several large and small rockbursts in the 13230 haulage roadway. A large rockburst accident occurred in December 22, 2015, at the beginning of the mining roadway, causing significant economic losses and casualties. With a length of about 150 m outside the working face safe exit, there happened severe bottoming, contraction of the cross section, and damage to equipment such as electromechanical, transportation, support, and other equipment in the roadway.


## 4.1.2. Mechanism Analysis of Roadway Rockburst Induced by Loading in the Direction of Maximum Confining Pressure

(1) Stress Condition Analysis. On the north side of the 13230 working face, there were five goafs. The large area roof collapse in the goafs leads to the transfer of vertical stress to the solid coal, which makes the vertical stress around the 13230 haulage roadway increase. In the direction of the roadway section, the horizontal stress becomes the minimum confining pressure, and the vertical stress becomes the maximum confining pressure (P1 = Pz, P3 = Px). In front of the 13230 work face, the vertical stress could be increased to more than 3 times the initial stresses under the influence of the advancing abutment pressure during the period of work face cutting coal [37, 38]. The stresses distribution of the surrounding rock in the 13230 haulage roadway is shown in Figure 10. As Figure shows, the stress in the direction of the maximum confining pressure was gradually loaded during work face mining.Figure 10
Stresses distribution of 13230 haulage roadway surrounding rock.(2) Numerical Simulation and Result Analysis. On the basis of the field stress test results [37], the horizontal stress was the minimum confining pressure of the roadway surrounding rock, which was 19.5 MPa, and the vertical stress was the maximum confining pressure, as the vertical stress could be increased to more than 3 times the initial stress during the work face mining. In the simulation, the vertical stress was increased from 19.5 MPa in 5 MPa up to 55 MPa, and the simulated stress conditions are shown in Table 2. The rock mechanics parameters used in the simulation were the mechanical parameters of the coal actually tested [37], as shown in Table 3.Table 2
Numerical simulation stress conditions.
P
1 (MPa)
19.5
25
30
35
40
45
50
55
P
3 (MPa)
19.5Table 3
Rock mechanics parameters for numerical simulation.
Compressive strength (MPa)
Tensile strength (MPa)
Elastic modulus (GPa)
Poisson’s ratio
Cohesion (MPa)
Internal friction angle (°)
9.42
0.75
22.96
0.23
3.0
25.1Figure11 was the plastic zone change curve in the 13230 haulage roadway during mining. During mining, vertically loading would cause the severe expansion of the plastic zone of the surrounding rock a roadway. When the vertical stress was raised to 2.8 times the initial stress (55 MPa), the maximum radius of the roadway surrounding rock plastic zone was reached 37 m, and the maximum confining pressure was in the accelerated growth loading area. The plastic zone of the surrounding rock a roadway was large, and it had a sensitive response to stress. The stability of the surrounding rock structure a roadway became deteriorated, and it easily induced rockburst.Figure 11
Plastic zone change curve in the 13230 haulage roadway during work face mining.The large area roof collapse of five work face goafs on the north side of the 13230 working face led to the increase of the vertical stress around the haulage roadway. Influenced by the advancing abutment pressure during work face mining, the haulage roadway was loaded with several times the initial stress in the direction of the maximum confining pressure, which resulted in a sharp increase of the biaxial confining pressure ratio. At this time, the plastic zone of the surrounding rock a roadway was large, and the stability of the surrounding rock structure a roadway became deteriorated. Therefore, there occurred rockbursts many times in the 13230 haulage roadway during mining.


## 4.2. A Case Analysis of Roadway Rockburst Induced by Unloading in the Direction of Minimum Confining Pressure

### 4.2.1. Engineering Background

Wudong Coal Mine is located in the northeast of Urumqi city in China. The mine surface elevation was +850 m, and there were two major minable seams (B3 + 6 and B1 + 2). The average dip angles of the two coal seams were about 87°, and the thickness of two coal seams was 40 m and 30 m. A hard rock column dominated by siltstone with an average thickness of nearly 100 m was between two coal seams. According to the different mining level, the working faces were arranged in two minable seams from top to bottom. The length of the work faces was in accordance with the thickness of the coal seams, and work faces moved along the coal seams trend. The two work faces were both mining with the top-coal caving method, in which the extraction-caving ratio was 1 : 7, and the mining height was about 25 m. The general situation of mining engineering in Wudong Mine is illustrated in Figure12.Figure 12
Mining arrangement in Wudong mine in China.From the beginning of rockburst in roadway in September 2009, there have been more than 20 times of large and small rockburst in Wudong Mine, and the degree of disaster is becoming more and more serious. At 5 : 12 on March 24, 2014, during the B3 + 6 work face mining in the +475 m level, a large rockburst occurred in the B3 + 6 preparation roadway, causing the heavy floor lifts in the 150 m area, accompanied by a loud noise.


### 4.2.2. Mechanism Analysis of Roadway Rockburst Induced by Unloading in the Direction of Minimum Confining Pressure

(1) Stress Condition Analysis. Wudong Mine was worked by the approach of layering mining from top to bottom in accordance with different mining levels. So, the upper area was a goaf during next level coal mining. In view of the hard rock columns on both sides of the coal seams, the pressure was almost completely unloaded in the vertical direction, and the confining pressure was very small in the vertical direction of the roadway. The horizontal stress was the maximum confining pressure, and the vertical stress was the minimum confining pressure (P1 = Px, P3 = Pz). The process of the B3 + 6 work face mining in the +475 m level was also the process of preparation roadway unloading in the direction of minimum confining pressure.(2) Numerical Simulation and Result Analysis. Based on the in situ stress test results in the +475 m level [39], in the simulation, the horizontal stress was the maximum confining pressure of the roadway, which was 15.58 MPa, and the vertical stress was the minimum confining pressure. In this paper, the shapes of the plastic zone of the surrounding rock a roadway were simulated under different stress conditions of the vertical stress from 7 MPa to 0. The simulated stress conditions are shown in Table 4. The rock mechanics parameters used in the simulation were the mechanical parameters of the coal actually tested in Wudong Mine [39], as shown in Table 5.Table 4
Numerical simulation stress conditions.
P
1 (MPa)
14.58
P
3 (MPa)
7
6
5
4
3
2
1
0.5
0.3
0.1
0Table 5
Rock mechanics parameters for numerical simulation.
Compressive strength (MPa)
Tensile strength (MPa)
Elastic modulus (GPa)
Poisson’s ratio
Cohesion (MPa)
Internal friction angle (°)
9.42
1.421
2.037
0.21
2.4
57Figure13 is the plastic zone change curve of the surrounding rock in the B3 + 6 preparation roadway during work face mining. Vertically unloading during mining would cause the severe expansion of the plastic zone of the surrounding rock a roadway. When the vertical stress was unloaded to 0, the maximum radius of the roadway surrounding rock plastic zone was reached 28.03 m, and the minimum confining pressure was in the accelerated growth loading area. The plastic zone of the surrounding rock a roadway was large, and it had a sensitive response to stress. The stability of the surrounding rock structure a roadway became deteriorated and easily induced rockburst in the roadway.Figure 13
Plastic zone change curve in the B3 + 6 preparation roadway during work face mining.The mining approach of layering mining from top to bottom in Wudong Mine led to the large unloading in the vertical direction of roadway surrounding rock. During the working face recovery, the preparatory roadway below was further unloading in the direction of minimum confining pressure, which resulted in a sharp increase of the biaxial confining pressure ratio. At this time, the plastic zone of the surrounding rock a roadway was large, and the stability of the surrounding rock structure a roadway became deteriorated. Therefore, there occurred rockbursts many times during work face mining.


## 4.2.1. Engineering Background

Wudong Coal Mine is located in the northeast of Urumqi city in China. The mine surface elevation was +850 m, and there were two major minable seams (B3 + 6 and B1 + 2). The average dip angles of the two coal seams were about 87°, and the thickness of two coal seams was 40 m and 30 m. A hard rock column dominated by siltstone with an average thickness of nearly 100 m was between two coal seams. According to the different mining level, the working faces were arranged in two minable seams from top to bottom. The length of the work faces was in accordance with the thickness of the coal seams, and work faces moved along the coal seams trend. The two work faces were both mining with the top-coal caving method, in which the extraction-caving ratio was 1 : 7, and the mining height was about 25 m. The general situation of mining engineering in Wudong Mine is illustrated in Figure12.Figure 12
Mining arrangement in Wudong mine in China.From the beginning of rockburst in roadway in September 2009, there have been more than 20 times of large and small rockburst in Wudong Mine, and the degree of disaster is becoming more and more serious. At 5 : 12 on March 24, 2014, during the B3 + 6 work face mining in the +475 m level, a large rockburst occurred in the B3 + 6 preparation roadway, causing the heavy floor lifts in the 150 m area, accompanied by a loud noise.


## 4.2.2. Mechanism Analysis of Roadway Rockburst Induced by Unloading in the Direction of Minimum Confining Pressure

(1) Stress Condition Analysis. Wudong Mine was worked by the approach of layering mining from top to bottom in accordance with different mining levels. So, the upper area was a goaf during next level coal mining. In view of the hard rock columns on both sides of the coal seams, the pressure was almost completely unloaded in the vertical direction, and the confining pressure was very small in the vertical direction of the roadway. The horizontal stress was the maximum confining pressure, and the vertical stress was the minimum confining pressure (P1 = Px, P3 = Pz). The process of the B3 + 6 work face mining in the +475 m level was also the process of preparation roadway unloading in the direction of minimum confining pressure.(2) Numerical Simulation and Result Analysis. Based on the in situ stress test results in the +475 m level [39], in the simulation, the horizontal stress was the maximum confining pressure of the roadway, which was 15.58 MPa, and the vertical stress was the minimum confining pressure. In this paper, the shapes of the plastic zone of the surrounding rock a roadway were simulated under different stress conditions of the vertical stress from 7 MPa to 0. The simulated stress conditions are shown in Table 4. The rock mechanics parameters used in the simulation were the mechanical parameters of the coal actually tested in Wudong Mine [39], as shown in Table 5.Table 4
Numerical simulation stress conditions.
P
1 (MPa)
14.58
P
3 (MPa)
7
6
5
4
3
2
1
0.5
0.3
0.1
0Table 5
Rock mechanics parameters for numerical simulation.
Compressive strength (MPa)
Tensile strength (MPa)
Elastic modulus (GPa)
Poisson’s ratio
Cohesion (MPa)
Internal friction angle (°)
9.42
1.421
2.037
0.21
2.4
57Figure13 is the plastic zone change curve of the surrounding rock in the B3 + 6 preparation roadway during work face mining. Vertically unloading during mining would cause the severe expansion of the plastic zone of the surrounding rock a roadway. When the vertical stress was unloaded to 0, the maximum radius of the roadway surrounding rock plastic zone was reached 28.03 m, and the minimum confining pressure was in the accelerated growth loading area. The plastic zone of the surrounding rock a roadway was large, and it had a sensitive response to stress. The stability of the surrounding rock structure a roadway became deteriorated and easily induced rockburst in the roadway.Figure 13
Plastic zone change curve in the B3 + 6 preparation roadway during work face mining.The mining approach of layering mining from top to bottom in Wudong Mine led to the large unloading in the vertical direction of roadway surrounding rock. During the working face recovery, the preparatory roadway below was further unloading in the direction of minimum confining pressure, which resulted in a sharp increase of the biaxial confining pressure ratio. At this time, the plastic zone of the surrounding rock a roadway was large, and the stability of the surrounding rock structure a roadway became deteriorated. Therefore, there occurred rockbursts many times during work face mining.


## 4.3. A Stress Preventive Method of Rockburst in Roadway

Occurrence of rockburst in the roadway might cause great casualties and huge economic losses to the mine. Many scholars have proposed many engineering control measures [40–43], such as cutting roof, hydraulic slotting, mining liberated seam, borehole pressure relief, protective pillar design, and support measures. At the same time, these engineering measures have achieved good control effect in the field.Both loading in the direction of the maximum confining pressure and unloading in the direction of the minimum confining pressure cause the increase of the biaxial confining pressure ratio. Two necessary stress conditions should be met in the occurrence of rockburst in roadway: high stress and high stress ratio. The high stress and high stress ratio would lead to a large-scale failure zone, cause the surrounding rock structure of roadway instability, and then trigger the occurrence of rockburst. Under the conditions of low stress level and low stress ratio, the roadway surrounding rock failure zone was small, and the surrounding rock structure of roadway was stable, and there was no risk of roadway rockburst. The stress preventive methods of rockburst in roadway are to take some certain engineering measures to destroy the two necessary stress conditions: reducing the stress level and reducing the stress differences in different directions. Unloading in the direction of the maximum confining pressure by measures of cutting roof, hydraulic slotting, mining liberated seam, and borehole pressure relief could reduce the stress level and reduce the stress differences in different directions; loading in the direction of the minimum confining pressure by measures of strengthening support on the weak side of roadway and the reasonable design of the protective coal pillar size could reduce the stress differences in different directions. The stress preventive methods of rockburst in roadway are shown in Figure14.Figure 14
Stress preventive methods of rockburst in roadway.In summary, the stress preventive methods of rockburst in roadway were determined: reducing the stress level and reducing the stress differences in different directions.


## 5. Conclusions

To study the failure laws of the surrounding rock in the process of rockburst in the roadway, the stress response laws of the failure zone surrounding rock were studied by loading and unloading in a single direction and changing biaxial stress models. On the basis of the work presented in this study, the following conclusions were made:(1)
Loading in the direction of maximum confining pressure or unloading in the direction of minimum confining pressure, when the stress reaches a certain limit, will lead to a large-scale expansion to the failure zone of the surrounding rock a roadway, and the stress response of the failure zone becomes more sensitive, which easily induce rockburst in the roadway.(2)
The occurrence of rockburst in the roadway was related to both the stress magnitude and the stress mode. High stress ratio and high stress level are two sine qua nonstress conditions for rockburst in the roadway.(3)
The stress preventive methods of rockburst in the roadway were determined: reducing the stress level and reducing the stress differences in different directions.



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*Source: 7509690-2019-02-19.xml*