# Study on Antiweathering Support Technology of Thin Spray-On Liners in Shallowly Buried Coal Seam Roadway under Corrosion Condition

Authors: Jie Zhang; Qing-Song Zhuo; Sen Yang; Tao Yang; Bin Wang; Wen-Yong Bai; Jian-Jun Wu; Yi-Feng He; Hong-Ru Li
Journal: Shock and Vibration
 (2022)

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## Abstract

The thin spray-on liner is an inorganic polymer product that has been widely used to support mine perimeter rock roadways, but it is mainly used in metallic hard rock roadways with high rock strength and is less used in roadways affected by roadway corrosion and weathering in shallowly buried coal seams. Therefore, this study studies the thin spray-on liner support technology under such geological conditions. First, the causes of anchor support failure in this condition are analyzed, and it is concluded that alkaline ionized water corrodes the anchor rods, and chloride ions in the water play a role in accelerating the degree of anchor rod corrosion. Next, microscopic testing was used to determine the content of weathering and swelling minerals contained in the roof rock and the development of tectonic fissures. Third, the loosening circle of the surrounding rock is theoretically calculated and used as a basis to design the anchor mesh rope support parameters for the roadway. Finally, the construction process of a thin spray-on liner anticorrosion and antiweathering support technology is introduced. This technology improves the anchor force environment of anchor rods while incorporating anchor rods (ropes)—rock—metal mesh into the support system to play a coupling support role. On-site monitoring is performed to derive the optimum thickness of the spraying layer in different environments. At the same time, the deformation of the roadway surrounding rock and the anchor force can be improved to meet the requirements of anticorrosion and antiweathering and tighten the roadway surrounding rock. Compared with concrete support technology, the economic and environmental benefits of this support technology are apparent, and it helps to promote the application in shallowly buried coal seam mines.


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## Body

## 1. Introduction

With the deepening of China’s reform and opening up and the deepening of the belt and road, coal plays an important role in its economic development, accounting for about 70% of China’s primary energy consumption structure [1]. The coal mining areas in western China have gradually replaced the eastern coal mining areas as the center of gravity of China’s coal energy structure, which has become the center of coal resource development [2]. The coalfields in the western part of China are formed in the Jurassic and Cretaceous periods, resulting in weakly cemented sandstones at the top and bottom of the coal seams, which are easily weathered, low strength, poorly cemented, and quickly disintegrated, bringing hidden dangers to mine safety production [3]. At the same time, the groundwater in the western mining area is infiltration type, which is alkaline and highly mineralized due to water chemistry, and all these factors bring hidden dangers to the stability of soft rock roadway support in mines [4]. At home and abroad, a variety of support methods are mainly adopted for the support form of soft rock roadway, while combined control technology is mainly adopted under alkaline conditions. The combined control technology is based on the principle of coupled support, physical and mechanical experiments on the surrounding rock, mineral composition analysis, and ground stress test before the support design, optimizes the design after collecting data, and carries out secondary support technology during the best support time, to realize the stability of the surrounding rock in soft rock roadway [5–9]. These studies make the soft rock roadway support technology continuously improve and develop. As a combined control technology for soft rock roadway support, shotcrete plays a coupling role. Shotcrete plays an active role in the management of soft rock roadways against weathering and corrosion, but its economic, social, and environmental benefits are also criticized. In recent years, a thin spray-on liner support technology has emerged at home and abroad for the anticorrosion, antiweathering, and sealing role of the roadway, replacing concrete and part of the metal mesh support, which has achieved good support effect. However, its main research direction is still focused on metal mines with high surrounding rock strength, and there is less research on corrosion and weathering prevention of underground roadways in shallow buried coal seams [10–16]. Therefore, this study first investigates such geological conditions, then studies and analyzes the destabilization mechanism of the roadway support system from multiple angles, and adopts thin spray-on liner auxiliary anchor (cable) support. Finally, mine pressure monitoring analysis verifies the effect of the thin spray-on liner support and comparatively analyzes its economic and environmental benefits. It lays a solid technical foundation for the large-scale application of the spray-on liner support technology in shallowly buried coal seam mines in northern Shaanxi Province, China, and has significant economic and social value.


## 2. Project Overview

The shallow buried coal seam in northern Shaanxi is buried within 150 m, and coal was formed in the Jurassic period. The main mining seam is a 3−1 coal seam, with a thickness of 2.6 m and an average burial depth of 140 m, which is a typical shallow buried close coal seam with mudstone and siltstone upward. The 3−1 coal seam roof rock is affected by water body and ventilation, which is easy to weather off and swell and deform in water, thus causing the anchor support force of the anchor rod tray to have no force point and lose the ability to fasten the rock body. At the same time, in the process of setting back the mining roadway in 3−1 coal seam, the roof plate is drenched with water, and the prestressing anchors and anchor ropes are under a wet environment and tension for a long time, and the metal corrosion reaction occurs, causing the failure of some anchors and anchor ropes support. In addition, due to the long-term ventilation in the roadway, the surface of the rock mass in the roadway is loose and broken, and slag begins to fall. Because the supporting members such as anchor rods, anchor cables, and anchor nets lose their focus points, the supporting points lose their effectiveness and the supporting strength is greatly reduced. The mine adopts shotcrete on the surface of the roadway for control. However, the concrete spraying is poorly formed due to its brittle properties to withstand tensile stress, and the surface cracks and the surrounding rock depart under the mine pressure, which significantly reduces the coupling effect with the anchor support. Its support disadvantages such as not being waterproof, poor economic applicability, and pollution of the environment cannot be applied on a large scale in the underground.


## 3. Corrosion and Weathering Analysis of the Roadway Support System

### 3.1. Investigation of Anchor Corrosion Conditions

Northwest China is primarily an arid and semiarid region, and due to limited atmospheric precipitation and surface water, water bodies mainly use groundwater as their primary water source [17]. The water bodies interact with the surrounding rocks during downward seepage, causing hydrogeochemical reactions, thus changing the physicochemical composition of the groundwater [15]. As measured in the field, the temperature inside the 3−1 coal seam tunnel in the shallowly buried coal seam in China’s north Shaanxi mining area is relatively stable over the year, at about 20°C, and the relative humidity of the air is 93% to 95%. The seepage water from the coal seam roof was sampled and assayed for its pH value of about 8.5. The laboratory assay yielded various physicochemical property indexes, as shown in Figure 1. As seen in Figure 1, the water quality is dominated by Cl−, SO42−, Ca2+, and Mg2+, and the water body is highly mineralized.Figure 1
The chemical performance index of roadway roof shower.


### 3.2. Anchor Rod Corrosion Analysis

The anchor rods and metal nets that maintain the stability of the roadway are metal components, and the corrosion process of these metal components is an electrochemical corrosion process [18, 19]. The surface of metal components appears to have a significant potential difference in an alkaline aqueous environment, and the various reaction processes are as follows:(1)Fe⟶Fe2++2eO2+2H2O+4e⟶OH−Fe2++2OH−⟶FeOH2FeOH2+O2+2H2O⟶4FeOH3(2)Fe2++2Cl−+4H2O⟶FeCl2⋅4H2O(3)FeCl24H2O⟶FeOH2+2Cl−+2H2O+2H+From equations (2) and (3), it can be seen that chloride ions and alkaline ions compete for corrosion to produce divalent iron ions. However, chloride ions will not do like the end product. The corrosion process is not consumed but only plays a catalytic role in corrosion, promoting the generation of corrosion. Hence, the water body has a high chloride ion content, which is reflected in Figure 1. At the same time, oxygen maintains a high concentration at the metal, water, and air interface, and corrosion most obviously occurs at the metal-air interface. Therefore, in an alkaline water environment, anchor rod support must minimize the contact of the anchor rod (cable) with air and moisture.


### 3.3. XRD Experiment

Before conducting rock XRD experiments, the first task is to prepare the rock sample experimental specimens. The top rock sample was naturally dried and ground into a granular powder with a particle size of about 320 mesh or more to ensure strong diffraction, good peak formation, and high resolution during the experiment [20]. The processed powder was baked at a high temperature for 24 hours, grounded again to a particle size of 0.045 mm, and numbered as 1#, 2#, and 3#. The detection spectrum of some of these rock samples is shown in Figure 2.Figure 2
XRD pattern of 2# rock sample.Figure3 shows the XRD results of the rock specimens. From Figures 2 and 3, it can be seen that the main components of the three rock specimens are silica and alumina, among which quartz (SiO2) accounts for 65.3% on average, kaolinite (Al2O3) accounts for 22.7% on average, and rhodochrosite (FeCO3)) accounts for 4.7% on average, while the rock specimens contain a small amount of dibasic kaolinite, although most of the rock specimens are composed of quartz, hard lithology. However, the interlayer contains a large amount of kaolinite, which is a solid hydrophilic clay mineral and can adsorb water molecules on the surface of the crystal layer and within the crystal layer, causing molecular expansion within the rock particles and thickening of the interparticle water film. At the same time, the rock body inhales water through capillary action so that the volume continues to expand until the rock body loses its bearing capacity. Rhodochrosite and muscovite are easily weathered rocks and have low strength, causing the surface of the exposed rock body to fall off. Therefore, under external factors such as water leakage from the roof rock and long-term exposure of the roof rock, the rock expands and weathers, thus destabilizing and deforming the roof rock, leading to the failure of the roof rock support system and thus producing roof disasters.Figure 3
Mineral composition ratio of rock specimens. (a) Specimen 1 mineral composition. (b) Specimen 2 mineral composition. (c) Specimen 3 mineral composition.
(a)(b)(c)


### 3.4. Scanning Electron Microscope Analysis

Current methods that can view the microstructure of the rock are CT scanning, acoustic emission, scanning electron microscopy, and other equipment [21, 22]. The microscopic morphology of a substance and its structure can be observed by irradiating the surface of an object with electron rays generated by a scanning electron microscope. The instrument used in this experiment was a JSM-6460LV scanning electron microscope, which was used to scan the rock’s microstructure on the roof of the roadway [23]. The SEM photographs of the mudstone and siltstone specimens are shown in Figure 4 to study the microscopic morphology of the rock masses at different magnifications.Figure 4
Scanning electron microscopy of mudstone and siltstone. (a) Mudstone magnification 200 times. (b) Mudstone magnification 500 times. (c) Mudstone magnification 2,000 times. (d) Mudstone magnification 5,000 times. (e) Siltstone magnified 200 times. (f) Siltstone magnified 500 times. (g) Siltstone magnified 2,000 times. (h) Siltstone magnified 5,000 times.
(a)(b)(c)(d)(e)(f)(g)(h)When the mudstone specimen is magnified 200 times, it can be observed that the surface of the rock body shows weakly cemented clasts because a good cementation state is not formed. The weak rock masses fall off, leaving the more robust rock masses and appearing as raised blocks. When the specimen was magnified 500 times, the surface of the rock body showed a large number of holes with diameters ranging from 20 to 50μm. These pores are well connected and provide the natural conditions for the flow and erosion of water bodies. When the specimen was enlarged to 1,000–2,000 times, the surface of the rock sample showed a large area of irregular montmorillonite-shaped scale-like fine crystalline flakes. These crystalline flakes were accumulated around the fissures and pores in a large area. When the specimen is enlarged to 5,000 times, the surface of the rock sample mainly shows montmorillonite crystalline flakes, followed by muscovite crystalline flakes developed around the fissures and pores. These crystalline flakes are subject to hydrological and weathering effects, which are the leading cause of rock destabilization and shedding of particles [24].When the siltstone specimen was magnified 200 times, the surface of the rock sample was dense and was without large pores. When the specimen was enlarged 500 times, the surface of the rock sample showed longitudinal and transverse fracture development. When the specimen was enlarged 1,000 times, the surface of the rock sample had longitudinal and transverse fissures, and montmorillonite crystalline flakes began to appear around the pores. When the specimen is magnified to 2,000–5,000 times, it can be observed that the montmorillonite crystalline flakes gather on the edge of the fissure in a large area, cross each other, and stack together. The porosity between these crystalline flakes is significant, and this structure increases the contact area with the water body, which provides the prerequisite for the expansion of the rock body.Therefore, the analysis of anchor rod corrosion shows that the high mineralization of the water body under such geological conditions and the high content of chloride ions in the water body play a catalytic role in accelerating the corrosion of anchor rods, so it is necessary to take anticorrosion treatment measures for anchor rods. Scanning electron microscopy shows that the primary fissures in the top slab and the tectonic fissures produced by subsequent erosion of the rock body and the montmorillonite and crystalline mica flakes accumulated along its edge are the main reasons for the weathering instability of the rock body. Therefore, effective means need to be taken to isolate the rock body and anchor rods from the influence of external factors to maintain the stability of the roadway support system.


## 3.1. Investigation of Anchor Corrosion Conditions

Northwest China is primarily an arid and semiarid region, and due to limited atmospheric precipitation and surface water, water bodies mainly use groundwater as their primary water source [17]. The water bodies interact with the surrounding rocks during downward seepage, causing hydrogeochemical reactions, thus changing the physicochemical composition of the groundwater [15]. As measured in the field, the temperature inside the 3−1 coal seam tunnel in the shallowly buried coal seam in China’s north Shaanxi mining area is relatively stable over the year, at about 20°C, and the relative humidity of the air is 93% to 95%. The seepage water from the coal seam roof was sampled and assayed for its pH value of about 8.5. The laboratory assay yielded various physicochemical property indexes, as shown in Figure 1. As seen in Figure 1, the water quality is dominated by Cl−, SO42−, Ca2+, and Mg2+, and the water body is highly mineralized.Figure 1
The chemical performance index of roadway roof shower.


## 3.2. Anchor Rod Corrosion Analysis

The anchor rods and metal nets that maintain the stability of the roadway are metal components, and the corrosion process of these metal components is an electrochemical corrosion process [18, 19]. The surface of metal components appears to have a significant potential difference in an alkaline aqueous environment, and the various reaction processes are as follows:(1)Fe⟶Fe2++2eO2+2H2O+4e⟶OH−Fe2++2OH−⟶FeOH2FeOH2+O2+2H2O⟶4FeOH3(2)Fe2++2Cl−+4H2O⟶FeCl2⋅4H2O(3)FeCl24H2O⟶FeOH2+2Cl−+2H2O+2H+From equations (2) and (3), it can be seen that chloride ions and alkaline ions compete for corrosion to produce divalent iron ions. However, chloride ions will not do like the end product. The corrosion process is not consumed but only plays a catalytic role in corrosion, promoting the generation of corrosion. Hence, the water body has a high chloride ion content, which is reflected in Figure 1. At the same time, oxygen maintains a high concentration at the metal, water, and air interface, and corrosion most obviously occurs at the metal-air interface. Therefore, in an alkaline water environment, anchor rod support must minimize the contact of the anchor rod (cable) with air and moisture.


## 3.3. XRD Experiment

Before conducting rock XRD experiments, the first task is to prepare the rock sample experimental specimens. The top rock sample was naturally dried and ground into a granular powder with a particle size of about 320 mesh or more to ensure strong diffraction, good peak formation, and high resolution during the experiment [20]. The processed powder was baked at a high temperature for 24 hours, grounded again to a particle size of 0.045 mm, and numbered as 1#, 2#, and 3#. The detection spectrum of some of these rock samples is shown in Figure 2.Figure 2
XRD pattern of 2# rock sample.Figure3 shows the XRD results of the rock specimens. From Figures 2 and 3, it can be seen that the main components of the three rock specimens are silica and alumina, among which quartz (SiO2) accounts for 65.3% on average, kaolinite (Al2O3) accounts for 22.7% on average, and rhodochrosite (FeCO3)) accounts for 4.7% on average, while the rock specimens contain a small amount of dibasic kaolinite, although most of the rock specimens are composed of quartz, hard lithology. However, the interlayer contains a large amount of kaolinite, which is a solid hydrophilic clay mineral and can adsorb water molecules on the surface of the crystal layer and within the crystal layer, causing molecular expansion within the rock particles and thickening of the interparticle water film. At the same time, the rock body inhales water through capillary action so that the volume continues to expand until the rock body loses its bearing capacity. Rhodochrosite and muscovite are easily weathered rocks and have low strength, causing the surface of the exposed rock body to fall off. Therefore, under external factors such as water leakage from the roof rock and long-term exposure of the roof rock, the rock expands and weathers, thus destabilizing and deforming the roof rock, leading to the failure of the roof rock support system and thus producing roof disasters.Figure 3
Mineral composition ratio of rock specimens. (a) Specimen 1 mineral composition. (b) Specimen 2 mineral composition. (c) Specimen 3 mineral composition.
(a)(b)(c)


## 3.4. Scanning Electron Microscope Analysis

Current methods that can view the microstructure of the rock are CT scanning, acoustic emission, scanning electron microscopy, and other equipment [21, 22]. The microscopic morphology of a substance and its structure can be observed by irradiating the surface of an object with electron rays generated by a scanning electron microscope. The instrument used in this experiment was a JSM-6460LV scanning electron microscope, which was used to scan the rock’s microstructure on the roof of the roadway [23]. The SEM photographs of the mudstone and siltstone specimens are shown in Figure 4 to study the microscopic morphology of the rock masses at different magnifications.Figure 4
Scanning electron microscopy of mudstone and siltstone. (a) Mudstone magnification 200 times. (b) Mudstone magnification 500 times. (c) Mudstone magnification 2,000 times. (d) Mudstone magnification 5,000 times. (e) Siltstone magnified 200 times. (f) Siltstone magnified 500 times. (g) Siltstone magnified 2,000 times. (h) Siltstone magnified 5,000 times.
(a)(b)(c)(d)(e)(f)(g)(h)When the mudstone specimen is magnified 200 times, it can be observed that the surface of the rock body shows weakly cemented clasts because a good cementation state is not formed. The weak rock masses fall off, leaving the more robust rock masses and appearing as raised blocks. When the specimen was magnified 500 times, the surface of the rock body showed a large number of holes with diameters ranging from 20 to 50μm. These pores are well connected and provide the natural conditions for the flow and erosion of water bodies. When the specimen was enlarged to 1,000–2,000 times, the surface of the rock sample showed a large area of irregular montmorillonite-shaped scale-like fine crystalline flakes. These crystalline flakes were accumulated around the fissures and pores in a large area. When the specimen is enlarged to 5,000 times, the surface of the rock sample mainly shows montmorillonite crystalline flakes, followed by muscovite crystalline flakes developed around the fissures and pores. These crystalline flakes are subject to hydrological and weathering effects, which are the leading cause of rock destabilization and shedding of particles [24].When the siltstone specimen was magnified 200 times, the surface of the rock sample was dense and was without large pores. When the specimen was enlarged 500 times, the surface of the rock sample showed longitudinal and transverse fracture development. When the specimen was enlarged 1,000 times, the surface of the rock sample had longitudinal and transverse fissures, and montmorillonite crystalline flakes began to appear around the pores. When the specimen is magnified to 2,000–5,000 times, it can be observed that the montmorillonite crystalline flakes gather on the edge of the fissure in a large area, cross each other, and stack together. The porosity between these crystalline flakes is significant, and this structure increases the contact area with the water body, which provides the prerequisite for the expansion of the rock body.Therefore, the analysis of anchor rod corrosion shows that the high mineralization of the water body under such geological conditions and the high content of chloride ions in the water body play a catalytic role in accelerating the corrosion of anchor rods, so it is necessary to take anticorrosion treatment measures for anchor rods. Scanning electron microscopy shows that the primary fissures in the top slab and the tectonic fissures produced by subsequent erosion of the rock body and the montmorillonite and crystalline mica flakes accumulated along its edge are the main reasons for the weathering instability of the rock body. Therefore, effective means need to be taken to isolate the rock body and anchor rods from the influence of external factors to maintain the stability of the roadway support system.


## 4. Loosening Range of Surrounding Rock Measured and Analyzed

### 4.1. Peephole Arrangement

Mastering the size and change law of the loosening circle range of the roadway is important for choosing a reasonable and effective roadway support method and the selection of support parameters. The peephole loosening circle range is the 30206 transport roadway of Sunjiacha Longhua mine in the Shanbei mining area. Five sets of peepholes are arranged at 50 m intervals from the start of the withdrawal channel to the working face, respectively, named No. 1∼5 observatory. Each observatory constructs peepholes in the middle of the roof plate and the middle of the two gangs of the roadway, which are used to compare and analyze the loosening circle range of the roadway surrounding rock under different areas. The diameter of the peephole is 42 mm. The top plate was drilled vertically upward to a depth of 7 m. The gang drilling was drilled at an angle of 5 degrees elevation to a depth of 2 m. The loosening circle peephole location and peephole arrangement cross section are shown in Figure5.Figure 5
Loose ring peep location and peephole arrangement cross-sectional view.


### 4.2. Borehole Peephole Analysis

The CXK12 mining intrinsically safe borehole imager determined the scope of the loosening circle, which adopts DSP image acquisition and processing technology, the panoramic camera of the probe, video-type plus puzzle mode, and the probe advances from outside to inside at a uniform speed of 1.5∼2.0 m/min during the determination. The periodic comparative observation imaging of the same borehole reveals the deformation of the surrounding rock joints, faults, and fissure development. It judges the boundary of the loosening circle of the roadway surrounding rock to predict the development trend of the top plate of the roadway off-layer collapse and to provide a basis for the design of the anchor rod (cable) support parameters. The analysis of the peep diagrams of boreholes 1∼5 of 30206 transportation lane shows that transverse fissures dominate the roof plate of the roadway from the borehole mouth to 1.2 m, and longitudinal cracks are visible in the range of 1.2 m∼1.4 m. The integrity of the roof plate above 1.4 m is good, and only some microfissures of the rock-forming period exist. There are transverse fissures in the range of drill hole opening to 0.6 m on the coal column side of the roadway, longitudinal cracks in the field of 0.6 m∼0.85 m, and the longitudinal and transverse fissures produced after 0.85 m are not obvious. There are transverse fissures in the range from the hole opening to 0.5 m in the working face side of the roadway, mainly longitudinal fissures in the field from 0.5 m to 0.8 m, and the rock body is entirely outside the range of 0.8 m. As shown by the analysis of drilling peephole at each observation point, the damage depth of the plastic zone of the top slab surrounding rock is within 1.5 m, and the plastic damage range of the surrounding rock of both gangs is within 0.9 m, as shown in Figures6 and 7.Figure 6
Loose ring development in the roof of the roadway.Figure 7
Loose ring development in the gang section of the roadway.


## 4.1. Peephole Arrangement

Mastering the size and change law of the loosening circle range of the roadway is important for choosing a reasonable and effective roadway support method and the selection of support parameters. The peephole loosening circle range is the 30206 transport roadway of Sunjiacha Longhua mine in the Shanbei mining area. Five sets of peepholes are arranged at 50 m intervals from the start of the withdrawal channel to the working face, respectively, named No. 1∼5 observatory. Each observatory constructs peepholes in the middle of the roof plate and the middle of the two gangs of the roadway, which are used to compare and analyze the loosening circle range of the roadway surrounding rock under different areas. The diameter of the peephole is 42 mm. The top plate was drilled vertically upward to a depth of 7 m. The gang drilling was drilled at an angle of 5 degrees elevation to a depth of 2 m. The loosening circle peephole location and peephole arrangement cross section are shown in Figure5.Figure 5
Loose ring peep location and peephole arrangement cross-sectional view.


## 4.2. Borehole Peephole Analysis

The CXK12 mining intrinsically safe borehole imager determined the scope of the loosening circle, which adopts DSP image acquisition and processing technology, the panoramic camera of the probe, video-type plus puzzle mode, and the probe advances from outside to inside at a uniform speed of 1.5∼2.0 m/min during the determination. The periodic comparative observation imaging of the same borehole reveals the deformation of the surrounding rock joints, faults, and fissure development. It judges the boundary of the loosening circle of the roadway surrounding rock to predict the development trend of the top plate of the roadway off-layer collapse and to provide a basis for the design of the anchor rod (cable) support parameters. The analysis of the peep diagrams of boreholes 1∼5 of 30206 transportation lane shows that transverse fissures dominate the roof plate of the roadway from the borehole mouth to 1.2 m, and longitudinal cracks are visible in the range of 1.2 m∼1.4 m. The integrity of the roof plate above 1.4 m is good, and only some microfissures of the rock-forming period exist. There are transverse fissures in the range of drill hole opening to 0.6 m on the coal column side of the roadway, longitudinal cracks in the field of 0.6 m∼0.85 m, and the longitudinal and transverse fissures produced after 0.85 m are not obvious. There are transverse fissures in the range from the hole opening to 0.5 m in the working face side of the roadway, mainly longitudinal fissures in the field from 0.5 m to 0.8 m, and the rock body is entirely outside the range of 0.8 m. As shown by the analysis of drilling peephole at each observation point, the damage depth of the plastic zone of the top slab surrounding rock is within 1.5 m, and the plastic damage range of the surrounding rock of both gangs is within 0.9 m, as shown in Figures6 and 7.Figure 6
Loose ring development in the roof of the roadway.Figure 7
Loose ring development in the gang section of the roadway.


## 5. Range of Plastic Zone of Surrounding Rock and Support Design

### 5.1. Range of Plastic Zone of the Surrounding Rock

After the excavation of the roadway, the deformation of the surrounding rock is gradually deformed from the surface rock damage to the internal rock deformation. That is, the surface of the roadway rock for the one-way compression damage state and to the deep part of the roadway development, the compressive strength of the rock block gradually increased, until a certain radius R at the rock body in the elastic deformation state, that is, the ultimate equilibrium state, and this range is called the ultimate equilibrium zone. This range is called the ultimate equilibrium zone. The radiusR of the plastic zone of the roadway when there is no support is as follows:(4)R=rγH+Ccotφ1−sinφCcotφ1−sinφ/2sinφ,where R is the plastic zone radius in m; r is the equivalent circle radius in m; γ is the rock capacity, N/m3; H is the burial depth of the roadway in m; C is the cohesion of the coal body in MPa; and φ is the friction angle within the coal.Because it is a rectangular roadway when applying the principle of ultimate equilibrium to solve the equivalence radius of its above formula, the correction factorf is given when calculating the range of plastic zone of the roadway’s two gangs, and the value is taken as 3.(5)r=b2+h22,where r is the equivalent circle radius in m; b is the half width of the roadway in m; h is the roadway’s height in m.The cores collected from the site of shallow buried coal seam mines in northern Shaanxi Province were subjected to laboratory physical and mechanical experiments on the coal rock body to derive the parameters of the rock body with an average cohesionC = 2.35 MPa, and an internal friction angle is 30 degrees. The burial depth of the roadway H = 140 m, the average capacity of the rock body r = 25 kN/m3, and then, the pressure applied on the roadway is 3.5 MPa. The width of the roadway is 5.6 m, and the height of the roadway is 3 m. The half width is 2.8 m, the equivalent radius is calculated to be 3.2 m, and the radius of the plastic zone R is 3.1 m. So by the following formula can be derived from the top of the roadway plastic zone range of 1.5 m, and the gang range is 0.9 m.(6)Sroof=R−h2=1.5,Sgang=fR−a=0.9.


### 5.2. Anchor Net Support Design

With anchor network support, the first problem to be solved is the anchor rod (cable) corrosion prevention, so it is adopted to use a modified version of chlorinated polyethylene resin with rust remover to coat the anchor rod (cable) body. By analyzing the plastic zone of the surrounding rock of the roadway, the roadway support parameters are designed. The 30206 transport chute roadway digging height is 3 m, digging width is 5.6 m, roadway roof and roadway gang are used anchor (cable) rope support, and the anchor rods were treated with the anticorrosion as mentioned in the above treatment. The specifications of the top plate anchor rods wereφ20 × 2300 mm left-hand threaded steel anchor rods, and the distance between rows of anchor rods was 1200 mm × 1000 mm. The anchor cable adopts φ15.24 × 6000 mm steel strand anchor cable with a row distance of 3000 mm, and the anchor rod of the gang adopts φ18 × 2200 mm left-handed threaded steel anchor rod with the row distance between anchor rods of 1000 mm × 1000 mm, as shown in Figure 8.Figure 8
Cross-sectional view of roadway support.


## 5.1. Range of Plastic Zone of the Surrounding Rock

After the excavation of the roadway, the deformation of the surrounding rock is gradually deformed from the surface rock damage to the internal rock deformation. That is, the surface of the roadway rock for the one-way compression damage state and to the deep part of the roadway development, the compressive strength of the rock block gradually increased, until a certain radius R at the rock body in the elastic deformation state, that is, the ultimate equilibrium state, and this range is called the ultimate equilibrium zone. This range is called the ultimate equilibrium zone. The radiusR of the plastic zone of the roadway when there is no support is as follows:(4)R=rγH+Ccotφ1−sinφCcotφ1−sinφ/2sinφ,where R is the plastic zone radius in m; r is the equivalent circle radius in m; γ is the rock capacity, N/m3; H is the burial depth of the roadway in m; C is the cohesion of the coal body in MPa; and φ is the friction angle within the coal.Because it is a rectangular roadway when applying the principle of ultimate equilibrium to solve the equivalence radius of its above formula, the correction factorf is given when calculating the range of plastic zone of the roadway’s two gangs, and the value is taken as 3.(5)r=b2+h22,where r is the equivalent circle radius in m; b is the half width of the roadway in m; h is the roadway’s height in m.The cores collected from the site of shallow buried coal seam mines in northern Shaanxi Province were subjected to laboratory physical and mechanical experiments on the coal rock body to derive the parameters of the rock body with an average cohesionC = 2.35 MPa, and an internal friction angle is 30 degrees. The burial depth of the roadway H = 140 m, the average capacity of the rock body r = 25 kN/m3, and then, the pressure applied on the roadway is 3.5 MPa. The width of the roadway is 5.6 m, and the height of the roadway is 3 m. The half width is 2.8 m, the equivalent radius is calculated to be 3.2 m, and the radius of the plastic zone R is 3.1 m. So by the following formula can be derived from the top of the roadway plastic zone range of 1.5 m, and the gang range is 0.9 m.(6)Sroof=R−h2=1.5,Sgang=fR−a=0.9.


## 5.2. Anchor Net Support Design

With anchor network support, the first problem to be solved is the anchor rod (cable) corrosion prevention, so it is adopted to use a modified version of chlorinated polyethylene resin with rust remover to coat the anchor rod (cable) body. By analyzing the plastic zone of the surrounding rock of the roadway, the roadway support parameters are designed. The 30206 transport chute roadway digging height is 3 m, digging width is 5.6 m, roadway roof and roadway gang are used anchor (cable) rope support, and the anchor rods were treated with the anticorrosion as mentioned in the above treatment. The specifications of the top plate anchor rods wereφ20 × 2300 mm left-hand threaded steel anchor rods, and the distance between rows of anchor rods was 1200 mm × 1000 mm. The anchor cable adopts φ15.24 × 6000 mm steel strand anchor cable with a row distance of 3000 mm, and the anchor rod of the gang adopts φ18 × 2200 mm left-handed threaded steel anchor rod with the row distance between anchor rods of 1000 mm × 1000 mm, as shown in Figure 8.Figure 8
Cross-sectional view of roadway support.


## 6. Thin Spray-On Liner Construction Process

In order to reduce the corrosion and weathering of the rock at the top plate of the retrieval tunnel, it was decided to use a thin spray-on liner to prevent corrosion and weathering. At the same time, the thin spray-on liner can cover the metal components exposed to the external environment to prevent rust and corrosion. For the anchor rods (ropes) that penetrate into the inner part of the rock, a modified version of chlorinated polyethylene resin with rust remover is applied to the surface of the anchor rods (ropes) before installation to prevent them from rusting. This thin spray-on liner is based on TSBP/3.0 type inorganic material, which can resist corrosion and weathering by spraying the liner after mixing with water, its elasticity and ductility are good, its tensile strength is high, and it can adhere well to the sprayed surface [25–28]. The specific downhole construction process is shown in Figure 9. In order to reduce dust, the wet slurry spraying construction process was selected. The slurry spraying equipment is a TJX3.2 portable mixing and slurry spraying machine. The equipment uses high-pressure wind as the driving energy, and the pressure of the air source is between 0.5 and 0.63 MPa, which enables continuous operation and high stability performance. The specific construction process of the thin spray-on liner is described as follows.(1)
Mixing. High-pressure air is used to scour the roadway’s surface to reduce the dust accumulation on the roadway surface and ensure that the thin spray-on liner is closely bonded to the rock of the roadway roof. Take TSBP/3.0 inorganic material as raw material, add water, and stir according to the ratio of “water/material quality = 0.35～0.45.” The agitator adopts the TJX3.2-type mixing and shotcrete integrated machine, and the agitator is driven by the high-pressure air in the coal mine, which is not affected by the power distribution. Stir the mixture for 3～5 min to form a uniform slurry without powder balls before spraying. Stirring time does not exceed 10 min. Otherwise, it will reach the inflection point of slurry liquidity deterioration after the timeout, which is not conducive to the slurry film effect.(2)
Slurry spraying. The wind pressure is adjusted, the bottom of the tank discharge valve is first opened, and then, the air pipe valve connected to the gun is opened, so that the air is first sprayed out, after opening the gun material pipe valve, until the slurry from the gun sprayed, before the slurry spraying operation. When spraying the slurry gun muzzle and the top plate surrounding vertical rock distance of about 1 m is the best distance, the slurry can be sprayed in the maximum area to achieve the best bonding tightening effect. Too far is the slurry that cannot be bonded to the roof rock, and too close is easy to make the slurry rebound that cannot be filmed.(3)
Equipment cleaning work. At the end of each shift, the equipment and gun are promptly need to be cleaned, and the gun needs to be fully cleaned to avoid residual slurry solidification after blocking the tube and gun, affecting the everyday use of the following equipment. Flowing water is used to rinse the inside of the mixing drum, and then, the tank-gathered water pressure is sprayed out before the material tube and gun are cleaned.Figure 9
Process flow.After the construction of the thin spray-on liner, the initial solidification time of the liner is 45 minutes∼6 hours, and the final solidification of the strength can reach 42 MPa or more. It can play its anticorrosion and antiweathering effect, and the site construction and thin spray-on liner molding effect are shown in Figure10.Figure 10
Slurry spraying equipment and construction effects. (a) Slurry spraying equipment. (b) On-site construction drawings. (c) Initial spray shape. (d) Final spray shape.
(a)(b)(c)(d)


## 7. Surrounding Rock Deformation Monitoring of Roadway

The monitoring of the stability of the roadway envelope can evaluate the roadway deformation in time, analyze its stability, and make an early warning of the roadway destabilization to ensure the safe production of the mine [29, 30]. In order to determine the optimal thickness of the thin spray liner and the final support effect, it is necessary to monitor the deformation of the roadway roof surrounding rock at the construction site. The monitoring instruments used are the KY-82 roof dynamic meter and 10T anchor force meter. The monitoring site is the 30207 transport roadway in Longhua coal mine in Shaanxi Province. The monitoring stations are arranged from the withdrawal channel l 100 m inward. In order to be comparative, 1# measuring station is an unsprayed sections, 2#, 3#, 4#, and 5# measuring stations are sprayed sections, where 2#, 3#, 4#, and 5# measuring stations are monitoring 3 mm, 5 mm, 8 mm, and 11 mm thicknesses of the thin spray-on liner, in turn, to verify the effect of different thicknesses of thin spraying liner support. The location of monitoring the deformation of the roadway enclosure is shown in Figure 11.Figure 11
Location of roadway deformation monitoring.Figure12 shows the top plate sinking curve before and after spraying, and it can be seen that the difference between the top plate sinking at the unsprayed place and the sprayed place is significant, in which the top plate sinking at the unsprayed section is 88 mm, the maximum sinking at the sprayed section is 49 mm, the minimum sinking is 33 mm, the average sinking is 38 mm, and the top plate sinking is reduced by 62%. The top slab sinking performance is not apparent from the different spraying thicknesses, but the top slab rock can quickly reach a stable state, and the time of top slab sinking to stability is reduced by 33%. The top slab sinking for different spraying thicknesses is relatively flat, and there is no jump. Figure 13 shows the monitoring graph of the anchor rod and anchor cable, in which the maximum force of the anchor rod and anchor cable is 75 kN and 148 kN at the unsprayed place. The maximum force of the anchor rod and anchor cable is 53 kN and 119 kN at the sprayed place, indicating that the thin spray-on liner tightens the roof slab rock into one, fully mobilizes the articulation between the rock bodies, improves the anchor force environment of the anchor rod (cable), and thus improves the anchor force of anchor rod (cable). It improves the anchoring force environment of anchor rods (ropes) and thus increases the supporting effect of anchor rods (ropes).Figure 12
Roadway roof sinking curve.Figure 13
Force diagram of roof anchor rod and anchor cable.After the analysis of the above monitoring results, it can be seen that the thin spray-on liner can improve the anticorrosion and antiweathering abilities of the roof rock. The sinking of the roof and the force of the anchor rod have been improved to different degrees. Since the thin spray-on liner can be quickly bonded with the roof rock, it can strengthen the elasticity and ductility of the roof rack and make the roof rock quickly achieve a stable state. At the same time, the thin spray-on liner can incorporate the anchor rod (cable)-metal mesh-rock mass into the support system, coupling support, thereby improving the overall stability of the whole support system. It can isolate the contact between the top rock and the outside water and ventilation, effectively improving its corrosion and weathering resistance. Combined with the site construction of a thin spray-on liner site, the liner thickness of 5 mm is the best, in the convex and concave sections of the roof plate that can be increased to 8 mm, to play a bond and smooth transition.


## 8. Comparison of Economic and Social Benefits

Previously, the mine constructed concrete shotcrete to solve the problem of anticorrosion and antiweathering of the roadway. However, there are many disadvantages in the process of using concrete shotcrete in coal mines [31, 32] : (1) Because the concrete is stirred in a short time after adding water, the dry ingredients such as cement, aggregates, and additives in the concrete are sprayed out before they come fully into contact with water and become wet. As a result, the dust concentration on the construction site is large and the working environment is harsh. (2) The cohesion of the concrete shotcrete material is poor, causing up to 30% to 50% of the concrete grout to fall off and cannot be reused. (3) Affected by the mine pressure after the shotcrete is layered, the shotcrete layer produces a large area of fissures, which are the main channels for water conduction and weathering of the surrounding rock of the roadway, resulting in a reduction in the strength of the roadway rock mass. The use of thin spray-on liner can realize the coupling of inorganic high-strength spraying material and surrounding rock and support system, and the high bonding, high density, and closed isolation of thin spray-on liner can effectively solve the drawbacks of construction concrete, as described in Table 1 for comparison.Table 1
Comparison of economic and social benefits of sprayed concrete and the use of thin spray-on liners.
Object nameConcrete spray layerThin spray-on linersConstruction speedFour person/class 6 mThree person/class 60 mWorking environmentDust concentration up to 100 mg/m3 and poor construction site environmentDustless and good working environmentThickness of lining layer50 m2∼5 mmLabor intensityHigh labor intensity (cement, sand transportation, and manual handling materials)Low labor intensity (stirring directly in mixing drum)Material particles bouncing backLarge springback, more than 30%Almost no reboundCost per square (including labor costs)150 yuan90 yuanTechnical featureEasy to crack, peel off, and peel offHigh viscosity, strong adhesion, good toughness, and compactnessConstruction technicsComplicationEasyConsumption per square163 kg5.5 kgThis time, 30207 back mining roadway is selected as the industrial experiment site, and the construction distance is 500 m. According to 30207 working face roadway width is 5.6 m, the construction of concrete cost is 420,000 yuan, while the construction of thin spray-on liner only cost is 250,000 yuan, and the total saving materials are about 170,000 yuan. Large-scale application in the underground roadway in the later stage can give the mine to save a large amount of considerable money.


## 9. Conclusions

In this study, an antiweathering technology of thin spray-on liner for shallowly buried coal seam roadways under corrosive conditions is proposed to solve the stability problem of the roadway surrounding rock under such geological conditions. The causes of rock instability and anchor support failure are derived through rock physicochemical experimental analysis and anchor corrosion research. A thin spray-on liner anti-corrosion and anti-weathering technology is adopted to effectively solve such problems. The specific research process is shown below. First, site research was conducted to derive the mechanism of roadway destabilization. It includes two aspects: (1) the water leaking out from the top plate is alkaline, and the water is dominated by Ca2+, Mg2+, and Cl− with a high degree of mineralization. The Cl− in the water plays a catalytic role and accelerates the corrosion of anchor rods; (2) the anchor rods (ropes) are in oxygen-rich environmental conditions and produce corrosive chemical reactions after contact with water, thus reducing the strength of the support system. Second, XRD analysis and electron microscope scanning of the rock mass were performed in the laboratory, and it was concluded that the mineral composition of the rock mass and its pore structure were the main reasons for the swelling and weathering of the rock mass. Limit balance theory calculates the range of surrounding rock loosening circles and designs the roadway support parameters. Then, the thin spray-on liner support technology was applied in the field, and the analysis of mine pressure monitoring data concluded that the deformation of the roadway surrounding rock and the anchor rod (cable) force was improved. The following conclusions were drawn: (1) compared with taking shotcrete, the construction of a thin spray-on liner has obvious advantages. The thin spray-on liner can prevent the rock corrosion and weathering and improve the stability of the support system. It can achieve safe, efficient, low-loss, and low-cost construction purposes. It has significant economic benefits. (2) The technology has updated the original traditional support concept of shallowly buried coal seam and improved the operating environment of the underground support site. It has achieved good social benefits and is worth promoting and applying.



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*Source: 7065650-2022-05-24.xml*