# Large-Deformation Failure Mechanism of Coal-Feeder Chamber and Construction of Wall-Mounted Coal Bunker in Underground Coal Mine with Soft, Swelling Floor Rocks

Authors: Xingkai Wang; Wenbing Xie; Jianbiao Bai; Shengguo Jing; Zhili Su
Journal: Advances in Civil Engineering
 (2019)

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## Abstract

In traditional vertical coal bunker systems, a coal-feeder chamber (CFC) must bear the whole weight of the bunker. However, maintenance of CFCs within soft, swelling floor rock is a challenge faced in underground coal mines. Floor-heave control is a complex problem and is still not well-solved. Moreover, there is no report on the construction of bunker without a CFC, especially under such weak floor-rock conditions. Based on the serious CFC collapse case at Xiashijie mine, China, this work analyzed the deformation characteristics, main influencing factors, and failure mechanisms of the CFC using a FLAC numerical model. The results indicate that the intrusion of water weakens the strength of the floor rock and causes significant expansive forces; thus, large deformations and tensile failure occur first in the floor, further causing shearing and tensile damage of the reinforced column and even overall instability of the CFC. Then, a new wall-mounted coal bunker (WMCB), without building the CFC, is proposed. The FLAC3D program was adopted to study the stability of the rocks surrounding the new bunker, and an optimized reinforcement scheme was determined. More importantly, a self-bearing system, which includes self-designed H-steel beams, H-steel brackets, and self-locking anchor cables, was proposed and constructed to bear the whole weight of the bunker. The stability of WMCB was verified by a theoretical safety assessment and field test. The invented WMCB could remain stable in spite of severe floor heave. This work can provide helpful references for the construction of vertical bunkers without CFCs in coal mines with soft, swelling floor rocks.


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## Body

## 1. Introduction

Raw coal, produced at the working face in underground coal mines, should be transported to the surface via a transportation system (Figure1), which commonly includes a coal strap transporting system in the longwall panel entry, coal-storage bunker installed in the mining area, conveyor belt in the main haulage roadway, coal bunker at the bottom of the shaft, and main shaft hoisting system. It is clear that the storage bunker plays an indispensable role in coal transportation. In addition, the use of bunkers can reduce the effect of transportation interruptions and congestion [1, 2], increase mining system availability [3], and improve transportation efficiency [4, 5]. The bunker usually contains a bottom coal bunker, district coal bunker, section coal bunker, ground bunker, and tunnel bunker [4], all of which are generally divided into the three forms: horizontal, vertical, or inclined. Overall, horizontal coal bunkers are used widely in the United Kingdom, the United States, Canada, the former Soviet Union, etc. However, China, one of the largest coal-mining countries, mainly researches and builds vertical coal bunkers [2].Figure 1
Coal transportation system in an underground coal mine.Vertical bunkers have been used widely in China for many years. Figure2(a) shows the structure of a traditional vertical bunker, which includes the coal bunker body and the coal-feeder chamber (CFC), which is constructed in the belt haulage roadway and bears the whole weight of the coal in the bunker, concrete silo, and coal-feeder machine. The belt conveyors feed into the top of the bunker from either vibrofeeders or variable-speed belt feeders loading out at the bottom [6]. Thus, the stability of the CFC is crucial to ensure the bunker works effectively. Here, coal bunker #214 could not be used owing to the severe collapse that occurred in the CFC even though it was repaired annually, and many effective measures were taken to improve the stability of the load-bearing structure of the CFC. Figure 3 shows the large deformation that occurred in the CFC.Figure 2
Structure of (a) traditional vertical bunker #214 and (b) the new designed vertical bunker without CFC.
(a)
(b)Figure 3
Collapse characteristics of the CFC in coal bunker #214. (a) The roof was cracked. (b) The roof was fractured. (c) The reinforced concrete column was inclined. (d) The reinforced concrete column was damaged.
(a)
(b)
(c)
(d)On the one hand, much research has been devoted to study floor heave of chamber in coal mines [7–10]. Nevertheless, floor-heave control is a complex problem in underground coal mines; the solution approaches are commonly a combination of bolts, anchor cables, concrete, and steel inverted arches, and such construction techniques are not only difficult to carry out in the field but the application effect is also poor. On the other hand, the study of the existing literature for coal bunker mainly focuses on (i) the optimum bunker size and location selection in underground coal mine conveyor systems [3, 11], (ii) the construction of the coal bunker with a large diameter and vertical height [12, 13], (iii) the optimization of methods of safe construction under different geological conditions [14, 15], (iv) the deterioration and collapse mechanism of the reinforced concrete bunker [16], (v) the curing technique of blockage and fractures in the walls of the coal bunker body [17–19], and (vi) the maintenance of the coal bunker [20–22]. Those studies have made significant progress. However, there are no generally accepted cases applicable to construction of a new type of vertical coal bunker (Figure 2(b)) without a CFC, especially when the CFC of the traditional coal bunker could not be stable within weak floor rock.This work used field surveys, theoretical analysis, laboratory tests, and numerical simulation to analyze the main factors causing floor heave and the failure mechanism of the CFC. Then, a new coal bunker, called a wall-mounted coal bunker (WMCB), without a CFC was invented and designed, and its key technological bases were investigated. This work focused on the construction method of a self-bearing system to transfer the entire weight borne by the CFC into the rock surrounding the coal bunker. Finally, the security and the reliability of the new coal bunker were discussed, and it was implemented at Xiashijie coal mine.


## 2. Background

### 2.1. Geological Conditions

As shown in Figure4, coal bunkers #214 and #3 (the new bunker), both vertical bunkers, are located in the belt haulage roadway near the 953 sump in Xiashijie coal mine, Tongchuan Coal Mining Group Co. Ltd. Coal bunker #214 is the original coal-storage bunker, with a height and diameter of 8.7 m and 5 m, respectively. It is noted that coal bunker #214 is located entirely within coal seam #4-2, the two walls of CFC-surrounding rock are also part of coal seam #4-2, and the floor is mudstone. Coal bunker #3 is built to replace coal bunker #214 because of the latter’s repeated failures. Coal bunker #3 is 5 m in diameter and 15 m in height, and its upper surrounding rock is coarse sandstone and siltstone, of which the thicknesses are 3.2 m and 5.8 m, respectively. The lower part of the surrounding rock is coal seam #4-2, with a thickness of 6 m.Figure 4
Geological cross section through coal bunkers #214 and #3.


### 2.2. CFC Failure Mechanism

#### 2.2.1. Deformation Characteristics of the CFC

The CFC’s load-bearing structure is composed of the roof, floor, and side walls, which are made of reinforced concrete. Figure3 shows the deformation and collapse of the CFC, even though the CFC was repaired twice and was recently reinforced by a concrete column between the walls. The convergence of the chamber was measured by the crossing method [23] after the latest repair. The deformation data collected after monitoring for three months is shown in Figure 5; the roof-floor and wall-wall convergences in the CFC were 1632 mm and 218 mm, respectively. Based on the onsite damage features and the deformation data, the main deformation characteristics could be concluded as follows:(1)
Large deformation of the floor: the cumulative roof-floor convergence was much greater than the convergence of the walls in the CFC, whereas the onsite deformation shows that the roof subsidence was not obvious (less than 150 mm), but severe floor heave occurred(2)
Collapse and failure constantly emerge after repair in the CFC (especially the floor): the load-bearing structure of the CFC could only remain stable for three months after repair; this was mainly attributed to the mudstone and clay materials of the immediate and hard floors, respectively, which are weak and soft in nature and swell severely when water is encountered.Figure 5
CFC deformation monitoring results.


#### 2.2.2. Main Influencing Factors

According to the in situ survey and rock expansion experiments, the main factors inducing failure of the CFC are as follows:(1)
The swelling property of the CFC floor rocks: the swelling properties of the mudstone and clay were verified by the laboratory tests, i.e., the rock-expansion experiments [24]. Figure 6 shows the dilatational rate of the floor strata mudstone, which increased after immersion in water for 2 h and then reached the maximum of 11700 με after 12 h. Mineral-composition analysis by electron microscope found that the mudstone and clay rocks contain abundant minerals, such as kaolinite, montmorillonite, and illite. These minerals contained in the immediate floor and hard floor account for as much as 82% and 85%, respectively, meaning that the floor rocks are liable to swelling, which is not conducive to chamber stability.(2)
The rich supply of mine water under these geological conditions: with a continuous supply of water from the seepage in the working-face goafs, sump #953 above, and the productive water in the belt roadway (Figure4), the swelling floor rocks would have lower strength and generate a significant expansive force; thus, the concrete walls and floor of the chamber were crushed after repair and reinforcement, further causing the collapse in the chamber roof and inducing instability of the whole bearing structure.Figure 6
Rock expansion experiment results.


#### 2.2.3. Influence of Water on the Floor Heave and Failure of CFC

Numerical simulation, as a basic method, is widely used to determine the stresses or displacements in underground spaces [25, 26], analyze the failure mechanism or stability of engineering rock masses [13, 27–30], optimize support schemes for roadways [23], simulate experimental tests and verify the results thereof, etc. In this study, the influence mechanism of water on floor heave and overall failure occurring in CFC was investigated by using a FLAC2D model. According to the main research of this work, the simplified model was established, as shown in Figure 7. The boundary conditions were set as follows: a vertical stress of 9.8 MPa was applied on the upper boundary of the numerical model to simulate the overlying strata pressure (burial depth was nearly 400 m); a horizontal stress of 6.5 Mpa was applied to the left and right boundaries, which were fixed in displacement in the normal direction; and the lower boundary was fixed in the vertical direction. Based on the laboratory experiments and the deformation information supplied by the coal mine, the properties of the coal and rock mass were determined by considering the Hoek–Brown failure criterion [31, 32]. Additionally, a strain-softening constitutive model (Figure 8) was selected. For simplicity, the friction angle remained unchanged while the cohesion of the rock mass gradually reduces to its residual value (0.1 MPa), at which the plastic shear strain threshold is 0.3%. The coal and rock physical and mechanical parameters used in this model are shown in Table 1. However, the intrusion of water weakens the mechanical connection between rock and mineral particles, which leads to the decrease of the cohesion of rock mass and the weakening of the physical and mechanical properties of rock mass [34]. Figure 6 shows that the swelling of the rock was not obvious after 30 h; thus, the mechanical parameters of the floor strata that were immersed in water for 36 h were measured by experimental tests, as given in Table 2. During numerical modeling, the normal stiffness (kn) and shear stiffness (ks) of the interface between the concrete and surrounding rock could be determined by the following equation [33]:(1)kn=ks=10K+4/3GΔzmin,where K and G are the bulk and shear moduli, respectively, and Δzmin is the smallest width of an adjoining zone in the normal direction.Figure 7
Numerical model.Figure 8
Strain-softening constitutive model [33], where εe is the elastic strain and εp is the plastic strain.Table 1
Physical and mechanical parameters of surrounding rock and concrete.
Strata
Density (kg·m−3)
Bulk modulus (GPa)
Shear modulus (GPa)
Friction angle (°)
Cohesion (MPa)
Coarse-grained sandstone
2600
5.0
4.0
28
2.0
Fine-grained sandstone
2500
4.0
3.0
26
1.7
Siltstone
2550
3.5
2.5
24
1.5
Coal seam #4-2
1400
1.3
0.9
18
0.3
Mudstone
1800
2.0
1.5
25
0.7
Clay
1500
2.0
1.3
23
0.4
Reinforced concrete
2700
5.0
4.0
32
2.2
Interface
k
n = ks = 125 (GPa/m)Table 2
Mechanical parameters of floor strata after immersed in water for 36 h.
Strata
Elasticity modulus (MPa)
Bulk modulus (GPa)
Shear modulus (GPa)
Friction angle (°)
Cohesion (MPa)
Mudstone
1860
1.20
0.75
20
0.36
Clay
1650
1.30
0.64
18
0.22To understand the influence of mine water on the failure of the CFC, the distribution of both plastic zones (Figure9) and displacement (Figure 10) in CFC were simulated under conditions with and without water. Because mine water was encountered, the strength of the floor rocks decreased and swelling rock generated significant expansive forces, and a much larger ranging plastic zone appeared in the rock surrounding the floor and walls in Figure 9(b) (the maximum plastic zone reached 5.8 m in the sides and floor) than that in Figure 9(a) (the maximum plastic zone only reached 2.4 m in the roof). Apparently, tension and shearing damage occurred in the roof (Figure 9(b)), which induced cracking and fracture of the roof (as shown in Figure 3). Moreover, the bottom beam of the CFC was almost completely in a state of tensile failure, and the reinforced column was mainly subject to shearing and tension damage, further causing severe shear dilatancy. The collapse characteristics of the CFC were in line with the field photograph (Figure 3).Figure 9
Comparison of plastic zone distribution under conditions (a) without water and (b) with water.
(a)
(b)Figure 10
Comparison of displacement distribution under conditions (a) without water and (b) with water.
(a)
(b)In addition, comparison of displacement distributions without and with influence of water (Figure10) showed the displacement of the chamber affected by mine water (maximum value of 643 mm) was much greater than that without the influence of water (maximum value of 24 mm). More importantly, without the influence of water, the deformation of the surrounding rock was mainly a result of roof subsidence; the floor and reinforced column deformations were small. However, with the influence of water, a large amount deformation appeared in floor and column, while the deformations of the roof and two sides were small, as shown in Figure 10(b), which was in good agreement with in situ monitoring data (Figure 5). In summary, the floor heave, caused mainly by mine water, is the underlying cause of the instability and repeated failure of concrete in CFC.Even though many measures were taken, such as the drainage of the mine water, installation of U36 steel inverted arches, and reinforcement of the bottom beams, it was still difficult to control the floor heave. Thus, coal bunker #214 could not work properly.


## 2.1. Geological Conditions

As shown in Figure4, coal bunkers #214 and #3 (the new bunker), both vertical bunkers, are located in the belt haulage roadway near the 953 sump in Xiashijie coal mine, Tongchuan Coal Mining Group Co. Ltd. Coal bunker #214 is the original coal-storage bunker, with a height and diameter of 8.7 m and 5 m, respectively. It is noted that coal bunker #214 is located entirely within coal seam #4-2, the two walls of CFC-surrounding rock are also part of coal seam #4-2, and the floor is mudstone. Coal bunker #3 is built to replace coal bunker #214 because of the latter’s repeated failures. Coal bunker #3 is 5 m in diameter and 15 m in height, and its upper surrounding rock is coarse sandstone and siltstone, of which the thicknesses are 3.2 m and 5.8 m, respectively. The lower part of the surrounding rock is coal seam #4-2, with a thickness of 6 m.Figure 4
Geological cross section through coal bunkers #214 and #3.


## 2.2. CFC Failure Mechanism

### 2.2.1. Deformation Characteristics of the CFC

The CFC’s load-bearing structure is composed of the roof, floor, and side walls, which are made of reinforced concrete. Figure3 shows the deformation and collapse of the CFC, even though the CFC was repaired twice and was recently reinforced by a concrete column between the walls. The convergence of the chamber was measured by the crossing method [23] after the latest repair. The deformation data collected after monitoring for three months is shown in Figure 5; the roof-floor and wall-wall convergences in the CFC were 1632 mm and 218 mm, respectively. Based on the onsite damage features and the deformation data, the main deformation characteristics could be concluded as follows:(1)
Large deformation of the floor: the cumulative roof-floor convergence was much greater than the convergence of the walls in the CFC, whereas the onsite deformation shows that the roof subsidence was not obvious (less than 150 mm), but severe floor heave occurred(2)
Collapse and failure constantly emerge after repair in the CFC (especially the floor): the load-bearing structure of the CFC could only remain stable for three months after repair; this was mainly attributed to the mudstone and clay materials of the immediate and hard floors, respectively, which are weak and soft in nature and swell severely when water is encountered.Figure 5
CFC deformation monitoring results.


### 2.2.2. Main Influencing Factors

According to the in situ survey and rock expansion experiments, the main factors inducing failure of the CFC are as follows:(1)
The swelling property of the CFC floor rocks: the swelling properties of the mudstone and clay were verified by the laboratory tests, i.e., the rock-expansion experiments [24]. Figure 6 shows the dilatational rate of the floor strata mudstone, which increased after immersion in water for 2 h and then reached the maximum of 11700 με after 12 h. Mineral-composition analysis by electron microscope found that the mudstone and clay rocks contain abundant minerals, such as kaolinite, montmorillonite, and illite. These minerals contained in the immediate floor and hard floor account for as much as 82% and 85%, respectively, meaning that the floor rocks are liable to swelling, which is not conducive to chamber stability.(2)
The rich supply of mine water under these geological conditions: with a continuous supply of water from the seepage in the working-face goafs, sump #953 above, and the productive water in the belt roadway (Figure4), the swelling floor rocks would have lower strength and generate a significant expansive force; thus, the concrete walls and floor of the chamber were crushed after repair and reinforcement, further causing the collapse in the chamber roof and inducing instability of the whole bearing structure.Figure 6
Rock expansion experiment results.


### 2.2.3. Influence of Water on the Floor Heave and Failure of CFC

Numerical simulation, as a basic method, is widely used to determine the stresses or displacements in underground spaces [25, 26], analyze the failure mechanism or stability of engineering rock masses [13, 27–30], optimize support schemes for roadways [23], simulate experimental tests and verify the results thereof, etc. In this study, the influence mechanism of water on floor heave and overall failure occurring in CFC was investigated by using a FLAC2D model. According to the main research of this work, the simplified model was established, as shown in Figure 7. The boundary conditions were set as follows: a vertical stress of 9.8 MPa was applied on the upper boundary of the numerical model to simulate the overlying strata pressure (burial depth was nearly 400 m); a horizontal stress of 6.5 Mpa was applied to the left and right boundaries, which were fixed in displacement in the normal direction; and the lower boundary was fixed in the vertical direction. Based on the laboratory experiments and the deformation information supplied by the coal mine, the properties of the coal and rock mass were determined by considering the Hoek–Brown failure criterion [31, 32]. Additionally, a strain-softening constitutive model (Figure 8) was selected. For simplicity, the friction angle remained unchanged while the cohesion of the rock mass gradually reduces to its residual value (0.1 MPa), at which the plastic shear strain threshold is 0.3%. The coal and rock physical and mechanical parameters used in this model are shown in Table 1. However, the intrusion of water weakens the mechanical connection between rock and mineral particles, which leads to the decrease of the cohesion of rock mass and the weakening of the physical and mechanical properties of rock mass [34]. Figure 6 shows that the swelling of the rock was not obvious after 30 h; thus, the mechanical parameters of the floor strata that were immersed in water for 36 h were measured by experimental tests, as given in Table 2. During numerical modeling, the normal stiffness (kn) and shear stiffness (ks) of the interface between the concrete and surrounding rock could be determined by the following equation [33]:(1)kn=ks=10K+4/3GΔzmin,where K and G are the bulk and shear moduli, respectively, and Δzmin is the smallest width of an adjoining zone in the normal direction.Figure 7
Numerical model.Figure 8
Strain-softening constitutive model [33], where εe is the elastic strain and εp is the plastic strain.Table 1
Physical and mechanical parameters of surrounding rock and concrete.
Strata
Density (kg·m−3)
Bulk modulus (GPa)
Shear modulus (GPa)
Friction angle (°)
Cohesion (MPa)
Coarse-grained sandstone
2600
5.0
4.0
28
2.0
Fine-grained sandstone
2500
4.0
3.0
26
1.7
Siltstone
2550
3.5
2.5
24
1.5
Coal seam #4-2
1400
1.3
0.9
18
0.3
Mudstone
1800
2.0
1.5
25
0.7
Clay
1500
2.0
1.3
23
0.4
Reinforced concrete
2700
5.0
4.0
32
2.2
Interface
k
n = ks = 125 (GPa/m)Table 2
Mechanical parameters of floor strata after immersed in water for 36 h.
Strata
Elasticity modulus (MPa)
Bulk modulus (GPa)
Shear modulus (GPa)
Friction angle (°)
Cohesion (MPa)
Mudstone
1860
1.20
0.75
20
0.36
Clay
1650
1.30
0.64
18
0.22To understand the influence of mine water on the failure of the CFC, the distribution of both plastic zones (Figure9) and displacement (Figure 10) in CFC were simulated under conditions with and without water. Because mine water was encountered, the strength of the floor rocks decreased and swelling rock generated significant expansive forces, and a much larger ranging plastic zone appeared in the rock surrounding the floor and walls in Figure 9(b) (the maximum plastic zone reached 5.8 m in the sides and floor) than that in Figure 9(a) (the maximum plastic zone only reached 2.4 m in the roof). Apparently, tension and shearing damage occurred in the roof (Figure 9(b)), which induced cracking and fracture of the roof (as shown in Figure 3). Moreover, the bottom beam of the CFC was almost completely in a state of tensile failure, and the reinforced column was mainly subject to shearing and tension damage, further causing severe shear dilatancy. The collapse characteristics of the CFC were in line with the field photograph (Figure 3).Figure 9
Comparison of plastic zone distribution under conditions (a) without water and (b) with water.
(a)
(b)Figure 10
Comparison of displacement distribution under conditions (a) without water and (b) with water.
(a)
(b)In addition, comparison of displacement distributions without and with influence of water (Figure10) showed the displacement of the chamber affected by mine water (maximum value of 643 mm) was much greater than that without the influence of water (maximum value of 24 mm). More importantly, without the influence of water, the deformation of the surrounding rock was mainly a result of roof subsidence; the floor and reinforced column deformations were small. However, with the influence of water, a large amount deformation appeared in floor and column, while the deformations of the roof and two sides were small, as shown in Figure 10(b), which was in good agreement with in situ monitoring data (Figure 5). In summary, the floor heave, caused mainly by mine water, is the underlying cause of the instability and repeated failure of concrete in CFC.Even though many measures were taken, such as the drainage of the mine water, installation of U36 steel inverted arches, and reinforcement of the bottom beams, it was still difficult to control the floor heave. Thus, coal bunker #214 could not work properly.


## 2.2.1. Deformation Characteristics of the CFC

The CFC’s load-bearing structure is composed of the roof, floor, and side walls, which are made of reinforced concrete. Figure3 shows the deformation and collapse of the CFC, even though the CFC was repaired twice and was recently reinforced by a concrete column between the walls. The convergence of the chamber was measured by the crossing method [23] after the latest repair. The deformation data collected after monitoring for three months is shown in Figure 5; the roof-floor and wall-wall convergences in the CFC were 1632 mm and 218 mm, respectively. Based on the onsite damage features and the deformation data, the main deformation characteristics could be concluded as follows:(1)
Large deformation of the floor: the cumulative roof-floor convergence was much greater than the convergence of the walls in the CFC, whereas the onsite deformation shows that the roof subsidence was not obvious (less than 150 mm), but severe floor heave occurred(2)
Collapse and failure constantly emerge after repair in the CFC (especially the floor): the load-bearing structure of the CFC could only remain stable for three months after repair; this was mainly attributed to the mudstone and clay materials of the immediate and hard floors, respectively, which are weak and soft in nature and swell severely when water is encountered.Figure 5
CFC deformation monitoring results.


## 2.2.2. Main Influencing Factors

According to the in situ survey and rock expansion experiments, the main factors inducing failure of the CFC are as follows:(1)
The swelling property of the CFC floor rocks: the swelling properties of the mudstone and clay were verified by the laboratory tests, i.e., the rock-expansion experiments [24]. Figure 6 shows the dilatational rate of the floor strata mudstone, which increased after immersion in water for 2 h and then reached the maximum of 11700 με after 12 h. Mineral-composition analysis by electron microscope found that the mudstone and clay rocks contain abundant minerals, such as kaolinite, montmorillonite, and illite. These minerals contained in the immediate floor and hard floor account for as much as 82% and 85%, respectively, meaning that the floor rocks are liable to swelling, which is not conducive to chamber stability.(2)
The rich supply of mine water under these geological conditions: with a continuous supply of water from the seepage in the working-face goafs, sump #953 above, and the productive water in the belt roadway (Figure4), the swelling floor rocks would have lower strength and generate a significant expansive force; thus, the concrete walls and floor of the chamber were crushed after repair and reinforcement, further causing the collapse in the chamber roof and inducing instability of the whole bearing structure.Figure 6
Rock expansion experiment results.


## 2.2.3. Influence of Water on the Floor Heave and Failure of CFC

Numerical simulation, as a basic method, is widely used to determine the stresses or displacements in underground spaces [25, 26], analyze the failure mechanism or stability of engineering rock masses [13, 27–30], optimize support schemes for roadways [23], simulate experimental tests and verify the results thereof, etc. In this study, the influence mechanism of water on floor heave and overall failure occurring in CFC was investigated by using a FLAC2D model. According to the main research of this work, the simplified model was established, as shown in Figure 7. The boundary conditions were set as follows: a vertical stress of 9.8 MPa was applied on the upper boundary of the numerical model to simulate the overlying strata pressure (burial depth was nearly 400 m); a horizontal stress of 6.5 Mpa was applied to the left and right boundaries, which were fixed in displacement in the normal direction; and the lower boundary was fixed in the vertical direction. Based on the laboratory experiments and the deformation information supplied by the coal mine, the properties of the coal and rock mass were determined by considering the Hoek–Brown failure criterion [31, 32]. Additionally, a strain-softening constitutive model (Figure 8) was selected. For simplicity, the friction angle remained unchanged while the cohesion of the rock mass gradually reduces to its residual value (0.1 MPa), at which the plastic shear strain threshold is 0.3%. The coal and rock physical and mechanical parameters used in this model are shown in Table 1. However, the intrusion of water weakens the mechanical connection between rock and mineral particles, which leads to the decrease of the cohesion of rock mass and the weakening of the physical and mechanical properties of rock mass [34]. Figure 6 shows that the swelling of the rock was not obvious after 30 h; thus, the mechanical parameters of the floor strata that were immersed in water for 36 h were measured by experimental tests, as given in Table 2. During numerical modeling, the normal stiffness (kn) and shear stiffness (ks) of the interface between the concrete and surrounding rock could be determined by the following equation [33]:(1)kn=ks=10K+4/3GΔzmin,where K and G are the bulk and shear moduli, respectively, and Δzmin is the smallest width of an adjoining zone in the normal direction.Figure 7
Numerical model.Figure 8
Strain-softening constitutive model [33], where εe is the elastic strain and εp is the plastic strain.Table 1
Physical and mechanical parameters of surrounding rock and concrete.
Strata
Density (kg·m−3)
Bulk modulus (GPa)
Shear modulus (GPa)
Friction angle (°)
Cohesion (MPa)
Coarse-grained sandstone
2600
5.0
4.0
28
2.0
Fine-grained sandstone
2500
4.0
3.0
26
1.7
Siltstone
2550
3.5
2.5
24
1.5
Coal seam #4-2
1400
1.3
0.9
18
0.3
Mudstone
1800
2.0
1.5
25
0.7
Clay
1500
2.0
1.3
23
0.4
Reinforced concrete
2700
5.0
4.0
32
2.2
Interface
k
n = ks = 125 (GPa/m)Table 2
Mechanical parameters of floor strata after immersed in water for 36 h.
Strata
Elasticity modulus (MPa)
Bulk modulus (GPa)
Shear modulus (GPa)
Friction angle (°)
Cohesion (MPa)
Mudstone
1860
1.20
0.75
20
0.36
Clay
1650
1.30
0.64
18
0.22To understand the influence of mine water on the failure of the CFC, the distribution of both plastic zones (Figure9) and displacement (Figure 10) in CFC were simulated under conditions with and without water. Because mine water was encountered, the strength of the floor rocks decreased and swelling rock generated significant expansive forces, and a much larger ranging plastic zone appeared in the rock surrounding the floor and walls in Figure 9(b) (the maximum plastic zone reached 5.8 m in the sides and floor) than that in Figure 9(a) (the maximum plastic zone only reached 2.4 m in the roof). Apparently, tension and shearing damage occurred in the roof (Figure 9(b)), which induced cracking and fracture of the roof (as shown in Figure 3). Moreover, the bottom beam of the CFC was almost completely in a state of tensile failure, and the reinforced column was mainly subject to shearing and tension damage, further causing severe shear dilatancy. The collapse characteristics of the CFC were in line with the field photograph (Figure 3).Figure 9
Comparison of plastic zone distribution under conditions (a) without water and (b) with water.
(a)
(b)Figure 10
Comparison of displacement distribution under conditions (a) without water and (b) with water.
(a)
(b)In addition, comparison of displacement distributions without and with influence of water (Figure10) showed the displacement of the chamber affected by mine water (maximum value of 643 mm) was much greater than that without the influence of water (maximum value of 24 mm). More importantly, without the influence of water, the deformation of the surrounding rock was mainly a result of roof subsidence; the floor and reinforced column deformations were small. However, with the influence of water, a large amount deformation appeared in floor and column, while the deformations of the roof and two sides were small, as shown in Figure 10(b), which was in good agreement with in situ monitoring data (Figure 5). In summary, the floor heave, caused mainly by mine water, is the underlying cause of the instability and repeated failure of concrete in CFC.Even though many measures were taken, such as the drainage of the mine water, installation of U36 steel inverted arches, and reinforcement of the bottom beams, it was still difficult to control the floor heave. Thus, coal bunker #214 could not work properly.


## 3. Key Techniques of the WMCB

As discussed previously, severe floor heave is the main factor inducing instability of the CFC under this geological condition, and no solution has been proven to be an effective reinforcement method to maintain the traditional coal bunker. It would be better to invent a new type of coal bunker without building the CFC to replace coal bunker #214. Thus, the weight borne by the CFC should be transferred to a newly designed bearing system in the WMCB.


### 3.1. Reinforcement Scheme of the Rock Surrounding the Coal Bunker

First, the rock surrounding the bunker, in which a variety of load-bearing structures of the coal bunker are built, should be controlled effectively so as to develop a WMCB. Therefore, FLAC3D models were established to analyze the stability of the rock surrounding coal bunker #3 based on its geological conditions. Considering the symmetrical design of the coal bunker, the numerical model could be established as shown in Figure11; the model size is 30 m × 15 m × 15 m. Furthermore, as can be seen from Figure 2(b) that the upper boundary of the bunker is the floor of the belt conveyor roadway, the distressed zone, which was free from the underground pressure, was set up at the top of the bunker in the numerical model (Figure 11). The rock parameters and constitutive model adopted in this model were same as those in the previous FLAC model.Figure 11
FLAC3D model of coal bunker #3.The simulation results (Figure12) indicate the following: (1) the surrounding rock in sandstone segments is basically stable, and the cumulative rock deformation is small (less than 25 mm); (2) the surrounding rock in coal seam segments, which moves into the inside of the bunker with a large deformation (80–110 mm) seen, is the main area of deformation in the entire surrounding rock mass; this is consistent with the results of field investigation.Figure 12
Deformation surrounding the body of coal bunker #3.To ensure the long-term stability of the surrounding rock and supporting structures, both the supporting strength and the supporting bearing system’s stability should be taken into account. The bolt-cable combined supporting technology [8, 35] was adopted to control the deformation of the rock surrounding the coal bunker body. At the same time, significant attention should be paid to supporting the rock surrounding coal seam #4-2. Combined with research results of reinforcement mechanism of the bolting with the wire mesh system [36–38], specific reinforcement measures in this work were proposed as follows:(1)
The welded wire mesh, a more rigid screen, was used to increase the stiffness of supporting system for the coal seam segment(2)
The thickness of the support bearing structure formed by bolting, wire mesh, and shallow surrounding rock was increased by increasing the length of the bolt(3)
Apart from the high prestressed bolt and wire mesh system, compensation anchor cables were used to reinforce the support bearing structure formed by bolting, wire mesh, and shallow surrounding rock, which improved the stability of the supporting structure so as to control the deformation of the rock surrounding the coal seam segment, thus ensuring the stability of the overall rock mass around the bunker.Based on the measures discussed above, six reinforcement schemes were designed, and a maximum displacement-monitoring point was set at the center of the coal seam segment (as shown in Figure12) to evaluate their control effects. Additionally, the main parameters of the bolt and cable, which are both cable elements and available from the user’s manual of FLAC3D, are listed in Table 3. It could be found from Table 4 that the maximum displacement of the coal seam segment decreases significantly from 89 mm to 36 mm when the length of the bolt increases from 2.4 m to 3.0 m and that of cable increases from 5.0 m to 6.2 m, respectively; however, the deformation then decreases to an insignificant extent when longer bolts and cables are used. Comparison of Schemes 5 and 6 shows that the reinforcing interval of the grouted anchor cables have a significant effect on the deformation. Therefore, the reinforcing interval was selected as 1600 × 800 mm. Finally, an optimized scheme (Scheme 4) was determined (Figure 13) considering the cost, installation convenience, and technical feasibility. Figure 13 shows that(1)
The bolt and wire mesh were used to reinforce the shallow rock and coal mass at intervals of 800 mm × 800 mm, and the bolt nut’s tightening pretension was not less than 60 kN.(2)
The bearing capacity of the deep surrounding rock was utilized by installation of the anchor cable (Φ17.8 mm × 6200 mm) with the spacing and row spacing being 1600 mm × 800 mm.(3)
The steel ladder bar and set with a diameter of Φ14 mm, along the inner surface of the bunker, were used to combine the bolts and anchors; thus, the overall stability of the supporting system was enhanced.(4)
Additionally, the Φ21.8 mm × 10,000 mm self-locking anchor cables (Figures13–16), which will be explained in detail in a later section, are also used as compensation anchors and were reinforced to improve the stability of the supporting structure formed by the bolting and wire mesh in the coal seam segment. They also serve as the auxiliary carrying system, able to bear the full weight of the coal bunker, which ensured the long-term stability of the coal bunker.Table 3
Parameters of the supporting elements used in the numerical model.
Property
Bolt
Cable
Dimensions (mm)
Ø20 ×L1
Ø17.8 ×L2
Elastic modulus (GPa)
25
200
Yield strength (kN)
150
450
Stiffness of the grout (N/m2)
2 × 107
2 × 108
Cohesive capacity of the grout (N/m)
10 × 105
10 × 105
Grouted length (mm)
0.5L1
0.5L2
Pretension force (kN)
60
120Table 4
Six different reinforcement schemes and their control effect (unit: mm).
Scheme (no.)
Bolt (diameter × length)
Anchor cable (diameter × length)
Cable interval (spacing × row spacing)
Maximum displacement in coal seam segment
1
Φ20 × 2400
Φ17.8 × 5000
800 × 800
73
2
Φ20 × 2400
Φ17.8 × 5000
1600 × 800
89
3
Φ20 × 3000
Φ17.8 × 6200
800 × 800
36
4
Φ20 × 3000
Φ17.8 × 6200
1600 × 800
47
5
Φ20 × 3600
Φ17.8 × 7300
1600 × 800
32
6
Φ20 × 3600
Φ17.8 × 7300
1600 × 1600
61Figure 13
Expanded view of the supporting scheme for the coal bunker.Figure 14
Schematic diagram of the WMCB.Figure 15
3D view of the bearing structure of the WMCB.Figure 16
Structure diagram of the self-locking anchor cable.


### 3.2. Self-Bearing System of the Coal Bunker

#### 3.2.1. Main Bearing System

As stated before, a new type of coal bunker, without the CFC, was invented to replace the traditional vertical coal bunker. The rock surrounding the bunker was controlled by a high-strength supporting scheme, described in a later section. Thus, the following section aims to solve two problems. The first is that a newly designed bearing system should be designed to bear the weight of coal stored in the bunker, concrete, bearing system itself, and feeder (as shown in Figure2(b)); the second is that the hopper should be formed by a newly designed structure in the bearing system.The specific design presented as follows: as the sandstone rock surrounding the upper part of the bunker is stable, the H-steel beams (Figures2(b) and 14), installed in the sandstone rock uniformly and horizontally, bear the whole weight of coal bunker #3 without considering friction and adhesion between the concrete and the surrounding rock, so it was necessary to ensure that the carrying capacity of H-steel beams (Figures 14 and 15) was greater than the total weight of the coal bunker with a certain safety factor. Thus, this computation may be summarized as follows:The bunker total weight is(2)MT=MC1+MC2+MB+MF,where MC1, MC2, MB, and MF are the weight of coal when the coal bunker is filled, the weight of concrete, the total weight of the H-steel brackets, and the weight of the feeder (Figure 2(b)), respectively. The weight borne by each H-steel beam is(3)M=MTn,where n is the number of H-steel beams buried in the bunker wall rock. Then, the shear stress on the section of the H-steel beam is(4)τ=MgS,where S is the cross-sectional area of the H-steel beam.Thus, the designed safety factor can be given by(5)K1=ττ,where τ is the allowable shear stress in the H-steel beam.As discussed above, the lower part of the surrounding rock is coal seam #4-2 with low strength which is unstable and could not be used to bear the weight of the hopper located in the bunker bottom (Figures14 and 15). Then, to form the skeleton of the hopper and enhance the overall stability of the structure of the bunker, the H-steel brackets were designed and installed uniformly along the internal surface rock mass in the bunker (Figures 14 and 15). The upper ends of the H-steel brackets were embedded into the stable surrounding rock in the upper portion of the coal bunker; the bottom parts of brackets were fitted together to form the skeleton of the hopper. Additionally, the upper portions of H-steel brackets were fixed to the rock by the anchor cable, thus not only transferring the bearing capacity of the deep rock and improving the stability of the shallow surrounding rock, but also enhancing the stability of the frame formed by these H-steel brackets.


#### 3.2.2. Assisted Bearing System

As the lower part of the bunker is the weak coal seam, there was a weak interface between the rock and coal seam (Figure14). Furthermore, the bunker bottom was equipped with a hopper and a coal feeder, and the stability of the lower part of the bunker requires special attention. Then, the assisted bearing system, which included 16 self-locking anchor cables (Figure 16) arranged uniformly in the bunker funnel section along the bunker wall rock, was also designed to enhance the long-term stability of the whole bunker. Thus, the weight of the bunker hopper was borne by the sandstone and deep stable coal with the self-locking anchor cable anchored therein (the length of the anchor cables were 10,000 mm, ensuring that its anchoring ends were in the stable sandstone rocks), thereby ensuring the long-term stability of the coal bunker, which is beneficial to the stability of belt roadway under the coal bunker. Figure 16 shows the sample structure of the self-locking anchor cable. The vertical component force of the self-locking anchor cable is(6)Fv=nFsinα,where n is the number of self-locking anchor cables, Fv is the vertical component force provided by the self-locking anchor cable, F is the single anchor bearing capacity, and α is the angle between the cable and the rock face, and given that the whole weight of the bunker is borne by the self-locking anchor cables, the designed safety factor can be given by(7)K2=FvMTg.


#### 3.2.3. Safety Assessment

The bunker body and hopper heights are 11,235 mm and 3,765 mm, respectively. According to theMining Engineering Design Manual [39], the total thickness of concrete around the coal bunker is not less than 400 mm and the concrete strength is 40 MPa.The total weight (concrete, coal, and coal-feeder beam) is calculated as follows:(8)V1=13πH1D1+b122−D1b14,Q=V1+14πD12H2,V2=14πD2H−Q,MC1=ρc1Q,MC2=ρc2V2,MB=PlnBLB,where V1, Q, V2, Mc1, Mc2, and MB are the coal bunker’s funnel volume, bunker volume, concrete volume, coal weight, concrete weight, and total weight of the H-steel brackets, respectively. H is the bunker design height, 15 m; H1 is the coal funnel height, 3.765 m; H2 is the bunker body height, 11.235 m; D is the section diameter of the bunker, 5.8 m; D1 is the net cross-sectional diameter of the bunker, 5 m; b1 is the mouth width of the funnel, 1.23 m; the weight of coal feeder is 4.5 × 103 kg; Pl is the lineal density of the H-steel, 26.05 kg/m; LB is the H-steel bracket length, 15.013 m; nB is the number of H-steel barn brackets, 12; and ρc1 and ρc2 are the density of the coal (1.4 × 103 kg/m3) and concrete (2.71.4 × 103 kg/m3), respectively. These data are substituted into equation (2); thus, the value of MT can be calculated to be 777.1 × 103 kg. With the number of H-steel beams being 60, its cross-sectional area being 0.003318 m2, and its allowable shear stress being 125 MPa, the safety factor can be calculated as(9)K1=12538.25=3.26>3.Unlike surface buildings, for an underground rock engineering project, the long-term property of rocks in underground environment and the carry capacity loss of the supporting element should be considered. However, the related estimates are complex and difficult to quantify. According to theMining Engineering Design Manual [39], the stability coefficient is designed to be 2.5, considering the factors such as variable loads, combined loads (including tension, bending, and torsion), ground pressure, and capacity loss of the bearing components; however, the long-term rock property of sandstone, which is the key bearing strata, should also be considered, so the long-term strength coefficient is set as 0.85. Thus, the rated safety factor in this bunker is assigned to be approximately 3. The calculated factor could meet safety requirements well. More importantly, the actual security needs to be verified by the following field engineering application.The diameter of the self-locking anchor cable is 21.8 mm, and its carrying capacity is 458 kN. Substituting these data into equations (6) and (7), the designed safety factor can be calculated as follows:(10)K2=10361.792777.1×9.8=1.36>1.Here, because the assisted system serves to guarantee the long-term stability of the bunker, assume that all the weight of the bunker was carried by the assisted bearing system, the rated safety factorK2 is assigned to be 1. Furthermore, certain safety reserves should be considered in underground buildings, so this meets safety requirements.


## 3.1. Reinforcement Scheme of the Rock Surrounding the Coal Bunker

First, the rock surrounding the bunker, in which a variety of load-bearing structures of the coal bunker are built, should be controlled effectively so as to develop a WMCB. Therefore, FLAC3D models were established to analyze the stability of the rock surrounding coal bunker #3 based on its geological conditions. Considering the symmetrical design of the coal bunker, the numerical model could be established as shown in Figure11; the model size is 30 m × 15 m × 15 m. Furthermore, as can be seen from Figure 2(b) that the upper boundary of the bunker is the floor of the belt conveyor roadway, the distressed zone, which was free from the underground pressure, was set up at the top of the bunker in the numerical model (Figure 11). The rock parameters and constitutive model adopted in this model were same as those in the previous FLAC model.Figure 11
FLAC3D model of coal bunker #3.The simulation results (Figure12) indicate the following: (1) the surrounding rock in sandstone segments is basically stable, and the cumulative rock deformation is small (less than 25 mm); (2) the surrounding rock in coal seam segments, which moves into the inside of the bunker with a large deformation (80–110 mm) seen, is the main area of deformation in the entire surrounding rock mass; this is consistent with the results of field investigation.Figure 12
Deformation surrounding the body of coal bunker #3.To ensure the long-term stability of the surrounding rock and supporting structures, both the supporting strength and the supporting bearing system’s stability should be taken into account. The bolt-cable combined supporting technology [8, 35] was adopted to control the deformation of the rock surrounding the coal bunker body. At the same time, significant attention should be paid to supporting the rock surrounding coal seam #4-2. Combined with research results of reinforcement mechanism of the bolting with the wire mesh system [36–38], specific reinforcement measures in this work were proposed as follows:(1)
The welded wire mesh, a more rigid screen, was used to increase the stiffness of supporting system for the coal seam segment(2)
The thickness of the support bearing structure formed by bolting, wire mesh, and shallow surrounding rock was increased by increasing the length of the bolt(3)
Apart from the high prestressed bolt and wire mesh system, compensation anchor cables were used to reinforce the support bearing structure formed by bolting, wire mesh, and shallow surrounding rock, which improved the stability of the supporting structure so as to control the deformation of the rock surrounding the coal seam segment, thus ensuring the stability of the overall rock mass around the bunker.Based on the measures discussed above, six reinforcement schemes were designed, and a maximum displacement-monitoring point was set at the center of the coal seam segment (as shown in Figure12) to evaluate their control effects. Additionally, the main parameters of the bolt and cable, which are both cable elements and available from the user’s manual of FLAC3D, are listed in Table 3. It could be found from Table 4 that the maximum displacement of the coal seam segment decreases significantly from 89 mm to 36 mm when the length of the bolt increases from 2.4 m to 3.0 m and that of cable increases from 5.0 m to 6.2 m, respectively; however, the deformation then decreases to an insignificant extent when longer bolts and cables are used. Comparison of Schemes 5 and 6 shows that the reinforcing interval of the grouted anchor cables have a significant effect on the deformation. Therefore, the reinforcing interval was selected as 1600 × 800 mm. Finally, an optimized scheme (Scheme 4) was determined (Figure 13) considering the cost, installation convenience, and technical feasibility. Figure 13 shows that(1)
The bolt and wire mesh were used to reinforce the shallow rock and coal mass at intervals of 800 mm × 800 mm, and the bolt nut’s tightening pretension was not less than 60 kN.(2)
The bearing capacity of the deep surrounding rock was utilized by installation of the anchor cable (Φ17.8 mm × 6200 mm) with the spacing and row spacing being 1600 mm × 800 mm.(3)
The steel ladder bar and set with a diameter of Φ14 mm, along the inner surface of the bunker, were used to combine the bolts and anchors; thus, the overall stability of the supporting system was enhanced.(4)
Additionally, the Φ21.8 mm × 10,000 mm self-locking anchor cables (Figures13–16), which will be explained in detail in a later section, are also used as compensation anchors and were reinforced to improve the stability of the supporting structure formed by the bolting and wire mesh in the coal seam segment. They also serve as the auxiliary carrying system, able to bear the full weight of the coal bunker, which ensured the long-term stability of the coal bunker.Table 3
Parameters of the supporting elements used in the numerical model.
Property
Bolt
Cable
Dimensions (mm)
Ø20 ×L1
Ø17.8 ×L2
Elastic modulus (GPa)
25
200
Yield strength (kN)
150
450
Stiffness of the grout (N/m2)
2 × 107
2 × 108
Cohesive capacity of the grout (N/m)
10 × 105
10 × 105
Grouted length (mm)
0.5L1
0.5L2
Pretension force (kN)
60
120Table 4
Six different reinforcement schemes and their control effect (unit: mm).
Scheme (no.)
Bolt (diameter × length)
Anchor cable (diameter × length)
Cable interval (spacing × row spacing)
Maximum displacement in coal seam segment
1
Φ20 × 2400
Φ17.8 × 5000
800 × 800
73
2
Φ20 × 2400
Φ17.8 × 5000
1600 × 800
89
3
Φ20 × 3000
Φ17.8 × 6200
800 × 800
36
4
Φ20 × 3000
Φ17.8 × 6200
1600 × 800
47
5
Φ20 × 3600
Φ17.8 × 7300
1600 × 800
32
6
Φ20 × 3600
Φ17.8 × 7300
1600 × 1600
61Figure 13
Expanded view of the supporting scheme for the coal bunker.Figure 14
Schematic diagram of the WMCB.Figure 15
3D view of the bearing structure of the WMCB.Figure 16
Structure diagram of the self-locking anchor cable.


## 3.2. Self-Bearing System of the Coal Bunker

### 3.2.1. Main Bearing System

As stated before, a new type of coal bunker, without the CFC, was invented to replace the traditional vertical coal bunker. The rock surrounding the bunker was controlled by a high-strength supporting scheme, described in a later section. Thus, the following section aims to solve two problems. The first is that a newly designed bearing system should be designed to bear the weight of coal stored in the bunker, concrete, bearing system itself, and feeder (as shown in Figure2(b)); the second is that the hopper should be formed by a newly designed structure in the bearing system.The specific design presented as follows: as the sandstone rock surrounding the upper part of the bunker is stable, the H-steel beams (Figures2(b) and 14), installed in the sandstone rock uniformly and horizontally, bear the whole weight of coal bunker #3 without considering friction and adhesion between the concrete and the surrounding rock, so it was necessary to ensure that the carrying capacity of H-steel beams (Figures 14 and 15) was greater than the total weight of the coal bunker with a certain safety factor. Thus, this computation may be summarized as follows:The bunker total weight is(2)MT=MC1+MC2+MB+MF,where MC1, MC2, MB, and MF are the weight of coal when the coal bunker is filled, the weight of concrete, the total weight of the H-steel brackets, and the weight of the feeder (Figure 2(b)), respectively. The weight borne by each H-steel beam is(3)M=MTn,where n is the number of H-steel beams buried in the bunker wall rock. Then, the shear stress on the section of the H-steel beam is(4)τ=MgS,where S is the cross-sectional area of the H-steel beam.Thus, the designed safety factor can be given by(5)K1=ττ,where τ is the allowable shear stress in the H-steel beam.As discussed above, the lower part of the surrounding rock is coal seam #4-2 with low strength which is unstable and could not be used to bear the weight of the hopper located in the bunker bottom (Figures14 and 15). Then, to form the skeleton of the hopper and enhance the overall stability of the structure of the bunker, the H-steel brackets were designed and installed uniformly along the internal surface rock mass in the bunker (Figures 14 and 15). The upper ends of the H-steel brackets were embedded into the stable surrounding rock in the upper portion of the coal bunker; the bottom parts of brackets were fitted together to form the skeleton of the hopper. Additionally, the upper portions of H-steel brackets were fixed to the rock by the anchor cable, thus not only transferring the bearing capacity of the deep rock and improving the stability of the shallow surrounding rock, but also enhancing the stability of the frame formed by these H-steel brackets.


### 3.2.2. Assisted Bearing System

As the lower part of the bunker is the weak coal seam, there was a weak interface between the rock and coal seam (Figure14). Furthermore, the bunker bottom was equipped with a hopper and a coal feeder, and the stability of the lower part of the bunker requires special attention. Then, the assisted bearing system, which included 16 self-locking anchor cables (Figure 16) arranged uniformly in the bunker funnel section along the bunker wall rock, was also designed to enhance the long-term stability of the whole bunker. Thus, the weight of the bunker hopper was borne by the sandstone and deep stable coal with the self-locking anchor cable anchored therein (the length of the anchor cables were 10,000 mm, ensuring that its anchoring ends were in the stable sandstone rocks), thereby ensuring the long-term stability of the coal bunker, which is beneficial to the stability of belt roadway under the coal bunker. Figure 16 shows the sample structure of the self-locking anchor cable. The vertical component force of the self-locking anchor cable is(6)Fv=nFsinα,where n is the number of self-locking anchor cables, Fv is the vertical component force provided by the self-locking anchor cable, F is the single anchor bearing capacity, and α is the angle between the cable and the rock face, and given that the whole weight of the bunker is borne by the self-locking anchor cables, the designed safety factor can be given by(7)K2=FvMTg.


### 3.2.3. Safety Assessment

The bunker body and hopper heights are 11,235 mm and 3,765 mm, respectively. According to theMining Engineering Design Manual [39], the total thickness of concrete around the coal bunker is not less than 400 mm and the concrete strength is 40 MPa.The total weight (concrete, coal, and coal-feeder beam) is calculated as follows:(8)V1=13πH1D1+b122−D1b14,Q=V1+14πD12H2,V2=14πD2H−Q,MC1=ρc1Q,MC2=ρc2V2,MB=PlnBLB,where V1, Q, V2, Mc1, Mc2, and MB are the coal bunker’s funnel volume, bunker volume, concrete volume, coal weight, concrete weight, and total weight of the H-steel brackets, respectively. H is the bunker design height, 15 m; H1 is the coal funnel height, 3.765 m; H2 is the bunker body height, 11.235 m; D is the section diameter of the bunker, 5.8 m; D1 is the net cross-sectional diameter of the bunker, 5 m; b1 is the mouth width of the funnel, 1.23 m; the weight of coal feeder is 4.5 × 103 kg; Pl is the lineal density of the H-steel, 26.05 kg/m; LB is the H-steel bracket length, 15.013 m; nB is the number of H-steel barn brackets, 12; and ρc1 and ρc2 are the density of the coal (1.4 × 103 kg/m3) and concrete (2.71.4 × 103 kg/m3), respectively. These data are substituted into equation (2); thus, the value of MT can be calculated to be 777.1 × 103 kg. With the number of H-steel beams being 60, its cross-sectional area being 0.003318 m2, and its allowable shear stress being 125 MPa, the safety factor can be calculated as(9)K1=12538.25=3.26>3.Unlike surface buildings, for an underground rock engineering project, the long-term property of rocks in underground environment and the carry capacity loss of the supporting element should be considered. However, the related estimates are complex and difficult to quantify. According to theMining Engineering Design Manual [39], the stability coefficient is designed to be 2.5, considering the factors such as variable loads, combined loads (including tension, bending, and torsion), ground pressure, and capacity loss of the bearing components; however, the long-term rock property of sandstone, which is the key bearing strata, should also be considered, so the long-term strength coefficient is set as 0.85. Thus, the rated safety factor in this bunker is assigned to be approximately 3. The calculated factor could meet safety requirements well. More importantly, the actual security needs to be verified by the following field engineering application.The diameter of the self-locking anchor cable is 21.8 mm, and its carrying capacity is 458 kN. Substituting these data into equations (6) and (7), the designed safety factor can be calculated as follows:(10)K2=10361.792777.1×9.8=1.36>1.Here, because the assisted system serves to guarantee the long-term stability of the bunker, assume that all the weight of the bunker was carried by the assisted bearing system, the rated safety factorK2 is assigned to be 1. Furthermore, certain safety reserves should be considered in underground buildings, so this meets safety requirements.


## 3.2.1. Main Bearing System

As stated before, a new type of coal bunker, without the CFC, was invented to replace the traditional vertical coal bunker. The rock surrounding the bunker was controlled by a high-strength supporting scheme, described in a later section. Thus, the following section aims to solve two problems. The first is that a newly designed bearing system should be designed to bear the weight of coal stored in the bunker, concrete, bearing system itself, and feeder (as shown in Figure2(b)); the second is that the hopper should be formed by a newly designed structure in the bearing system.The specific design presented as follows: as the sandstone rock surrounding the upper part of the bunker is stable, the H-steel beams (Figures2(b) and 14), installed in the sandstone rock uniformly and horizontally, bear the whole weight of coal bunker #3 without considering friction and adhesion between the concrete and the surrounding rock, so it was necessary to ensure that the carrying capacity of H-steel beams (Figures 14 and 15) was greater than the total weight of the coal bunker with a certain safety factor. Thus, this computation may be summarized as follows:The bunker total weight is(2)MT=MC1+MC2+MB+MF,where MC1, MC2, MB, and MF are the weight of coal when the coal bunker is filled, the weight of concrete, the total weight of the H-steel brackets, and the weight of the feeder (Figure 2(b)), respectively. The weight borne by each H-steel beam is(3)M=MTn,where n is the number of H-steel beams buried in the bunker wall rock. Then, the shear stress on the section of the H-steel beam is(4)τ=MgS,where S is the cross-sectional area of the H-steel beam.Thus, the designed safety factor can be given by(5)K1=ττ,where τ is the allowable shear stress in the H-steel beam.As discussed above, the lower part of the surrounding rock is coal seam #4-2 with low strength which is unstable and could not be used to bear the weight of the hopper located in the bunker bottom (Figures14 and 15). Then, to form the skeleton of the hopper and enhance the overall stability of the structure of the bunker, the H-steel brackets were designed and installed uniformly along the internal surface rock mass in the bunker (Figures 14 and 15). The upper ends of the H-steel brackets were embedded into the stable surrounding rock in the upper portion of the coal bunker; the bottom parts of brackets were fitted together to form the skeleton of the hopper. Additionally, the upper portions of H-steel brackets were fixed to the rock by the anchor cable, thus not only transferring the bearing capacity of the deep rock and improving the stability of the shallow surrounding rock, but also enhancing the stability of the frame formed by these H-steel brackets.


## 3.2.2. Assisted Bearing System

As the lower part of the bunker is the weak coal seam, there was a weak interface between the rock and coal seam (Figure14). Furthermore, the bunker bottom was equipped with a hopper and a coal feeder, and the stability of the lower part of the bunker requires special attention. Then, the assisted bearing system, which included 16 self-locking anchor cables (Figure 16) arranged uniformly in the bunker funnel section along the bunker wall rock, was also designed to enhance the long-term stability of the whole bunker. Thus, the weight of the bunker hopper was borne by the sandstone and deep stable coal with the self-locking anchor cable anchored therein (the length of the anchor cables were 10,000 mm, ensuring that its anchoring ends were in the stable sandstone rocks), thereby ensuring the long-term stability of the coal bunker, which is beneficial to the stability of belt roadway under the coal bunker. Figure 16 shows the sample structure of the self-locking anchor cable. The vertical component force of the self-locking anchor cable is(6)Fv=nFsinα,where n is the number of self-locking anchor cables, Fv is the vertical component force provided by the self-locking anchor cable, F is the single anchor bearing capacity, and α is the angle between the cable and the rock face, and given that the whole weight of the bunker is borne by the self-locking anchor cables, the designed safety factor can be given by(7)K2=FvMTg.


## 3.2.3. Safety Assessment

The bunker body and hopper heights are 11,235 mm and 3,765 mm, respectively. According to theMining Engineering Design Manual [39], the total thickness of concrete around the coal bunker is not less than 400 mm and the concrete strength is 40 MPa.The total weight (concrete, coal, and coal-feeder beam) is calculated as follows:(8)V1=13πH1D1+b122−D1b14,Q=V1+14πD12H2,V2=14πD2H−Q,MC1=ρc1Q,MC2=ρc2V2,MB=PlnBLB,where V1, Q, V2, Mc1, Mc2, and MB are the coal bunker’s funnel volume, bunker volume, concrete volume, coal weight, concrete weight, and total weight of the H-steel brackets, respectively. H is the bunker design height, 15 m; H1 is the coal funnel height, 3.765 m; H2 is the bunker body height, 11.235 m; D is the section diameter of the bunker, 5.8 m; D1 is the net cross-sectional diameter of the bunker, 5 m; b1 is the mouth width of the funnel, 1.23 m; the weight of coal feeder is 4.5 × 103 kg; Pl is the lineal density of the H-steel, 26.05 kg/m; LB is the H-steel bracket length, 15.013 m; nB is the number of H-steel barn brackets, 12; and ρc1 and ρc2 are the density of the coal (1.4 × 103 kg/m3) and concrete (2.71.4 × 103 kg/m3), respectively. These data are substituted into equation (2); thus, the value of MT can be calculated to be 777.1 × 103 kg. With the number of H-steel beams being 60, its cross-sectional area being 0.003318 m2, and its allowable shear stress being 125 MPa, the safety factor can be calculated as(9)K1=12538.25=3.26>3.Unlike surface buildings, for an underground rock engineering project, the long-term property of rocks in underground environment and the carry capacity loss of the supporting element should be considered. However, the related estimates are complex and difficult to quantify. According to theMining Engineering Design Manual [39], the stability coefficient is designed to be 2.5, considering the factors such as variable loads, combined loads (including tension, bending, and torsion), ground pressure, and capacity loss of the bearing components; however, the long-term rock property of sandstone, which is the key bearing strata, should also be considered, so the long-term strength coefficient is set as 0.85. Thus, the rated safety factor in this bunker is assigned to be approximately 3. The calculated factor could meet safety requirements well. More importantly, the actual security needs to be verified by the following field engineering application.The diameter of the self-locking anchor cable is 21.8 mm, and its carrying capacity is 458 kN. Substituting these data into equations (6) and (7), the designed safety factor can be calculated as follows:(10)K2=10361.792777.1×9.8=1.36>1.Here, because the assisted system serves to guarantee the long-term stability of the bunker, assume that all the weight of the bunker was carried by the assisted bearing system, the rated safety factorK2 is assigned to be 1. Furthermore, certain safety reserves should be considered in underground buildings, so this meets safety requirements.


## 4. WMCB Application

In practice, the new bunker was built according to the following technical procedures (Figure17). First, the roadway below the WMCB should be reinforced before the excavation of the WMCB by the anchor cables (Φ17.8 mm × 6200 mm) with the spacing and row spacing being 1600 mm × 800 mm, which was determined according to the specific reinforcement measures mentioned previously and acted as a compensating support. Second, excavate the circular coal bunker by means of stepwise deletion of the rocks, a total of 19 cycles with the excavating face advancing by 0.8 m from top to the bottom in every cycle, during which the bolt-mesh-cable system was installed immediately based on the supporting scheme (Figure 13). Third, install or fix the self-designed H-steel beams, H-steel brackets, and the self-locking anchor cables based on the design scheme as shown in Figures 13 and 14. Fourth, configure rebars and carry out formwork around the bunker. Fifth, pour concrete into the space between the templates and the sidewall, thus immersing all the bearing elements in the concrete and forming the WMCB (Figures 2(b) and 15). Finally, complete the installation of related components at the top of the bunker, as shown in Figure 14.Figure 17
Construction procedures. (a) Installation of the bolt-mesh-cable system and the self-designed H-steel beams during stepwise excavation of the rocks, (b) installation of the self-designed H-steel brackets, (c) installation of the self-locking anchor cables, (d) fitting the bottom parts of H-steel brackets together to form the skeleton of the coal hopper, (e) configuring rebars around the bunker, (f) pouring concrete and (g) that was vibrated to remove any entrapped air bubbles, and (h) installation of related components at the top of the bunker.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)The wall-mounted bunker had been in operation for five years (2013–2018). Long-term monitoring data (Figure18) and site observation show that the cumulative slippage of the whole bunker body was 64.6 mm, the coal bunker diameter convergence measured by crossing method was 52.0 mm, and no cracking occurred at the reinforced concrete since the application of the WMCB.Figure 18
Monitoring of bunker fall and diameter convergence.The great economic benefits, due to the application of the WMCB, are shown in Table5. As coal bunker #214 could remain stable less than three months after repair, it should be repaired for a month every time. The annual production capacity of the mine is 2.4 million tons, and the use of the WMCB gives rise to a production increase of more than two hundred thousand tons of coal every year (an economic benefit of 23.22 million USD at a unit-price of 38.69 USD per ton). Furthermore, the application of the WMCB reduces costs for the construction and maintenance of the original CFC; even though the construction cost of the WMCB is 0.582 million USD and the construction time is two months (production delay with a financial loss of 15.48 million USD), the coal mine has gained an economic benefit amounting to 101.906 million USD over five years.Table 5
Calculation of the economic benefits (unit: million UD).
Cost of coal bunker #214
Cost of the WMCB
Construction cost
Construction time
Cost of repair of coal bunker #214 every year
Construction cost
Construction time
Delayed production efficiency
Repair cost
Failure time
Delayed production efficiency
A = 0.398
Two months
B = 0.294
Three months
C = 23.22
D = 0.582
Two months
E = 15.48
Net benefit over five years
A + (B + C) × 5 − D − E = 101.906


## 5. Discussion

In this study, the influencing factors and failure mode of the CFC were revealed. Then, the WMCB was designed to avoid the influence of floor heave, during which, a self-bearing system, which included H-steel beams, H-steel brackets, and self-locking anchor cables, formed a replacement for the CFC to bear the whole weight of the bunker; thus, the whole weight was transferred to the stable and deep rocks. The stability of the new type of the coal bunker was verified in Xiashijie coal mine and made coal production therein more efficient.Comparison of the WMCB and the traditional vertical coal bunker shows the differences between them, in that the former is a new vertical coal bunker without the CFC and can remain stable when the floor heave is severe or the strength of the floor rock is especially low; thus, the new coal bunker is a better solution to the floor heave issue. To date, no vertical coal bunkers without CFC have been reported. The new coal bunker is stable and easy to maintain, and it offers great potential for application in mines with similar geological conditions. In addition, the newly designed coal bunker was granted a patent for invention in 2015 by State Intellectual Property Office of the People’s Republic of China [40].The work in this paper is preliminary, and the techniques used herein for stabilizing the coal bunker are mainly focused on field engineering. Further theoretical research should be performed in the future.


## 6. Conclusions

(1)
The failure mechanism of the CFC was that the floor rocks were liable to swelling due to mine water. The strength of the floor rock decreased, and the swelling rock mass generated significant expansive forces. A much larger ranging plastic zone and deformation appeared in rocks surrounding the floor, causing tensile failure that occurred first in the floor concrete, further resulting in shearing damage in the sidewalls and tension and shearing collapse in the reinforced column, and eventually, overall instability of the CFC occurred.
(2)
The WMCB was designed to replace the traditional vertical coal bunker, and the stability of its surrounding rock was simulated and analyzed. The results showed that the surrounding rock in the coal seam segment was the main deformation area. Then, the surrounding rock was controlled by an optimized high-strength bolt-cable support scheme. More importantly, a self-bearing system of the wall-mounted bunker was invented that was composed of H-steel beams, H-steel brackets with fastened anchors, and self-locking anchor cables and was able to bear the whole weight of the coal bunker, thus negating the need for a bearing structure at the bottom.
(3)
The WMCB differs from the traditional coal bunker in that the former, without the CFC, is easier to maintain than the latter, and it could be remain stable in spite of severe floor heave.
(4)
The new coal bunker has been put into practice at Xiashijie coal mine, and its specific construction technology is presented in detail. Field testing proves that the WMCB, being stable and safe, contributes to a significant enhancement in production. The coal mine has accrued economic benefits amounting to 101.906 million USD over three years (2013–2018).
(5)
It is key to construct a WMCB rather than a traditional coal bunker under geological conditions encompassing severe floor heave, loose (or fractured) and weak rock masses, and other similar conditions. The WMCB offers great prospects for application in other mines subject to similar geological conditions.



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*Source: 6519189-2019-08-15.xml*