Simulation and On-Site Monitoring of Deformation Characteristics of Roadway Excavation along Goaf in Soft and Thick Coal Seams in Western Mining Areas

Abstract

In the western mining region, weakly cemented rock layers above the coal seams often lead to frequent catastrophic accidents during mining due to their instability. To address this, this paper analyzes the movement characteristics of surrounding rock in the recovery roadway and the effectiveness of from nearby large coal pillar roadways. A mechanical model for the failure of weakly cemented roadways was established, and numerical simulations were used to verify the feasibility of leaving small coal pillars along soft, thick coal seams. Additionally, existing measurements were used to evaluate the impact of leaving small coal pillars on the deformation of the surrounding rock in the recovery roadway. The results show that after changing the coal pillar retention to 5 m in the 130,205 working face of the Yangchangwan mining area, the roadway is in a low-stress zone, with minimal surrounding rock deformation, meeting safety requirements for production.

1. Introduction

Most of the rock formations in the mining areas of western China are mainly Jurassic and Cretaceous, and the bottom plate of coal seams is mostly weakly cemented soft rocks such as silty mudstone, carbonic mudstone, and medium-fine sandstone [1,2]. This type of rock mass is poorly cemented, has poor interlayer adhesion, and shows disintegration due to sandification and muddification. A roadway excavated in such a rock mass is prone to instability in the surrounding rock, and roadway maintenance is difficult, which is a problem that urgently needs to be solved in order to safely mine coal in western China.

Accidents along empty mining lanes account for more than 90% of coal mine accidents [3,4]. Along the empty mining roadway, the upper section and lateral sections of the working surface can be affected by pressures and be physically deteriorated, so roadway accidents such as roofing collapse, vertical support pillar damage, and pillar sinking can easily occur. What is more serious is that during the recovery of the working surface, the walls of the roadway are affected not only by the lateral support pressure of the adjacent goaf area but also the added support pressure of the working surface that is necessary during recovery, which further increases the possibility of accidents and disasters. Therefore, the stability of the rock surrounding soft and thick coal seams is particularly important, and many experts and scholars have conducted in-depth research on the stability and deformation characteristics of the rock surrounding roadways.

Jeromel Gregor used the longwall mining method to conduct in-depth geomechanical analysis of underground mining, which can identify the relationship between the physical and mechanical parameters of geological materials based on coal mining strength [5]. Milan Medved analyzed a small period of time during the deformation of the surrounding rock based on the changes in the surrounding rock stress and found that the rock structure is mainly damaged by tectonic motion [6]. Majdi Abbas et al. analyzed the lateral limiting compressive strength of rocks in the surrounding rocks of a roadway to establish artificial neural network (ANN) and multivariate regression analysis (MVRA) models [7]. Raghwendra Singh et al. constructed mathematical models to predict the parting stability of adjacent coal seams or cross-sections as they disengaged at the same time [8].

In addition, Sun Lihui et al. studied the physical and mechanical properties of weak cement formations in the western mining area and believed that the weakly cemented rock layer was prone to disintegration when exposed to water, resulting in the height of the stope collapse zone and the fissure zone being greater than that of the same mining conditions in central and eastern China [9]. Weiming Wang et al. established a constitutive model of damage in western soft rock formations that considered initial damage, which accurately reflected the relationship between elastic modulus and confining pressure in the prepeak stage [10]. Zenghui Zhao et al. proposed a calculation method for the width and regional stress distribution of the coal-side stress limit equilibrium zone, considering the coupling effect of weakly cemented rock layers of soft rock, and established a numerical model of different anchoring methods applied in weakly consolidated soft rocks [11,12,13]. Jinlong Cai introduced the correction coefficient of the damage variable when considering the residual strength of soft rock and established a mathematical expression for a three-dimensional statistical constitutive model of damage [14]. Beiju Du used numerical methods to analyze the evolution of the surrounding rock stress field in a roadway in the western mining area and revealed the distribution characteristics of the support pressure of the coal column [15]. Liu Zhu et al. proposed a support technology system of “strengthening support for weak structural parts”, which effectively controlled the instability deformation of the surrounding rock of the roadway [16]. Yiran Yang et al. optimized the support methods and parameters of the roadway of a sharply inclined coal seam and effectively controlled the dynamic instability of a large inclined coal seam [17]. Wenkai Ru et al. studied the failure mechanism of the roof of a roadway under the shallowly buried close coal seam residue coal column [18]. Arasteh, H. et al. analyzed and assessed geological conditions, technical parameters, and support methods in longwall mining to determine the causes and patterns of roof collapses and falls in fully mechanized longwall mining [19]. Vu, T.T. used a synthetic rock mass model and PFC2D numerical simulations to predict roof collapses and coal seam dilution and evaluated the risk of these events [20]. Smoliński, A. et al. investigated the impact on the geomechanical state of rocks around the longwall face when abandoned drill cuttings were left in the mined-out area instead of being transported to the surface [21]. Malashkevych, D. et al. studied the formation of quantitative and qualitative indicators of coal mining using a new mining technology with waste rock accumulation under dynamic spatial and temporal conditions and developed an algorithm for predicting the operational ash content and quality of coal using this selective mining technology [22]. Zhang Wei. Analyzed the current state of coal technology in China, combined with existing production methods, introduced the development directions of coal technology in China, and explored the trends in the development of coal technology in China [23]. Gui, Bing. et al. By observing the deformation of surrounding rock through conventional mine pressure measurements, the deformation patterns of the surrounding rock were identified [24].

The above scholars carried out mechanical analysis and support stability tests on return mining roadways from the perspective of surrounding rock control, mainly considering the support pressure distribution and support mode, but there are relatively few studies on the stability of the surrounding rock along the empty excavation lanes of the western mining area, where there are loose and weakly cemented thick coal seams. For a comprehensive analysis, only considering the perspective of small coal pillar support technology in a large section of a thick coal seam in the western mining area is insufficient. So, here, working surface 130,205 of the Yangchangwan Coal Mine, with characteristics typical of loose and weakly cemented rock layers, was selected for study. In addition, the pressure distribution law of the working surface support and the mechanical characteristics of the surrounding rock deformation were compared and analyzed. The feasibility of the small coal column guarding the alley of the 130,205 working surface was determined, and the stress distribution and deformation characteristics of the 5 m small coal column were simulated by FLAC3D 6.0 numerical simulation software. The deformation of the surrounding rock along the empty excavation lane and the advanced support pressure distribution of the working surface were further summarized and analyzed. The research results provide theoretical and technical guidance for tunneling in regions of soft and thick coal seams in western mining areas and will help advance the technology necessary to control the surrounding rock for large sections of thick coal seams with small coal columns.

2. Project Overview

The Yangchangwan Coal Mine is located in Ningdong town, Lingwu city, Ningxia Hui Autonomous Region, western China, and the location of the coal mine is shown in Figure 1. The working surface 130,205 is located in the east of the Yangchangwan wellfield. The surface is covered by sand dunes, the terrain is relatively flat, the ground is not undulating, the vertical distance between the coal seam and the surface is between 587.1 and 726.7 m, and the average vertical depth is approximately 650 m. There are two coal seams mined at the 130,205 working surface, with a burial depth of 650 m, a thickness of 8.2–10.7 m, and an average coal seam thickness of 8.4 m. The specific details are shown in Table 1. The layer directly above is composed of a combination of siltstone and fine sandstone, and the layer above that is composed of a combination of medium sandstone and fine sandstone; directly below is siltstone and the second coal seam, and the layer below that is siltstone.

3. Theoretical Analysis

3.1. The Support Pressure Distribution Law of the Goaf Area of the Previous Working Surface

After the completion of face mining, the surrounding rock stresses near the goaf will be redistributed, and the support pressure distribution [19] curve is shown in Figure 2. Among them, the low-stress area includes locations where the stress redistribution is small and the stress after plastic destruction is less than the stress of the original rock. The high-stress area consists of parts of the redistribution where the stress exceeds the original stress and parts of the original stress area that are not affected by the recovery.

It can be clearly seen that when the roadway is excavated near the goaf area, the roadway position directly affects the stability of the roadway, and if the roadway is excavated in the low-stress area of the surrounding rock, then the roadway will be conducive to maintenance. If the roadway is in the high-stress area of the surrounding rock, then the stability of the roadway will be greatly affected. If the roadway is excavated in the original rock stress area, then the coal column needs to be left wide, leading to a considerable waste of resources.

The ventilation return lane is excavated on the side of the goaf in working area 130,205. After monitoring and analysis of the ore pressure of working surface 130,203, the impact range of the support pressure is approximately 175 m, the peak supporting pressure is located at 20 m to 40 m on both sides of the working surface, the internal stress field range is within 10 to 12 m, and the external stress field range is outside of 12 m, as shown in Figure 2. The Yangchangwan Coal Mine preliminarily designed the ventilation return lane with a 35 m coal column; the width of the roadway is 5 m, and the roadway is in the range of 35~40 m. Since it is close to the peak area of support pressure, the roadway will have large deformation during excavation and maintenance.

In area 130,205 of the Yangchangwan Coal Mine, the excavation work was conducted in the previous period of work and a 35 m large coal column is still in used to protect the alley. The deformation of the weakly cemented surrounding rock is large (as described in Figure 3 and Figure 4), and although the tunnel floor has been reinforced with grouting, there is still a bulging phenomenon occurring on the tunnel floor. The roadway is difficult to maintain, the multiexpansion effect is not good, and the ore pressure is obvious, so it is necessary to reasonably optimize the width of the coal column along the empty excavation alley.

3.2. Weakly Cemented Return Mining Roadway Width Retention Design

• 1.. Mechanical model of weak cementation return mining roadway failure

In the elastoplastic deformation state, the mechanical model of the ultimate equilibrium of the coal column [20,21] is as shown in Figure 5. The stress is concentrated in the coal column on the side of the goaf area. Close to the edge of the coal column, the vertical stress gradually increases, and within a certain distance from the edge, the supporting pressure will be in a state of balance with the bearing capacity of the coal column.

This can be seen from the mechanical model in Figure 5.

(1)$\{\begin{array}{c}{\sigma }_y={\displaystyle \frac{1}{A}}\left(P_0+{\displaystyle \frac{C_0}{\tan {\phi }_0}}\right)e^{{\displaystyle \frac{2\tan {\phi }_0}{mA}}+{\displaystyle \frac{{\tan }^2{\phi }_0}{A}}}\\ {\tau }_{xy}=-\left(C_0+{\sigma }_y\tan {\phi }_0\right)\\ X_0={\displaystyle \frac{mA}{2\tan {\phi }_0}}\ln \left({\displaystyle \frac{K\gamma H\tan {\phi }_0+C_0}{C_0+P_0}}\right)\end{array}$

In the formula, σ_y—vertical stress of the coal body; A—lateral pressure coefficient; P₀—roadway support strength; C₀—coal body cohesion; φ₀—friction angle in coal; m—coal seam thickness; τ_xy—shear stress between the coal body and top bottom plate; K—goaf-side stress concentration coefficient; H—roadway burial depth; γ—average bulk density of the weakly cemented rock layer.

The minimum theoretical width of the section coal column can be written as follows:

B = k(X₀ + X₁)(2)

where X₁ is the effective length of the anchor rod of the narrow coal column and k is the surplus coefficient with a value from 1.15 to 1.45.

Here, m = 8.4 m; A = 1.3; K = 4; H = 650 m; φ₀ = 32; C₀ = 5 MPa; P₀ = 0.3 MPa; and γ = 0.025 MN/m³. These can be substituted into (1), to obtain X₀ = 5.6 m. X₁ = 1.8 m and k = 1.15 can be substituted into Formula (2) to obtain B = 8.51 m, and the width of the actual small coal column is approximately 5 m.

• 2.. Analysis of mine pressure monitoring results on adjacent working surfaces

According to the mine pressure monitoring results of working surface 130,203, the stresses affected by different-width coal columns are analyzed. The support pressure first decreases and then increases with the increase in the width of the coal column, and finally tends to the original rock stress level. The stress when the coal column is 5 m in width is only 15 MPa, the stress in the coal column with a width of 5–8 m does not change much, and when the width of the coal column reaches 20 m, the stress reaches the peak 50 MPa; when the width of the coal column is 35 m, the stress is 33 MPa, which is still a high stress level.

Considering the comprehensive placement of working surface 130,205 and the geological conditions, the optimal width of the coal column is determined to be 5 m.

4. Numerical Simulation Calculation

4.1. Numerical Simulation Analysis of Coal Column Stability in a Weakly Cemented Environment

• 1.. Numerical model establishment and roadway excavation design

To verify whether it is feasible in working area 130,205 to leave a 5 m wide coal column in the ventilation return lane, a numerical model is created with the help of FLAC3D, and the deformation, stress distribution, and plastic zone range of the weakly cemented surrounding rock of the 5 m wide coal column are analyzed. In this way, the 5 m wide coal column is further justified.

To analyze the geological conditions of working surface 130,205 in the Yangchangwan Coal Mine, four sets of numerical models of the ventilation return lane were first constructed. The four groups of models uniformly maintained a burial depth of 650 m, with the same support conditions, but the width of the coal column was 3 m, 5 m, 8 m, and 10 m in turn. After the model balance, the ventilation return lane was excavated, and the stability of the 3 m, 5 m, 8 m, and 10 m width coal pillars was analyzed from the vertical stress distribution, the plastic zone range, and the deformation of the weak cement surrounding rock.

• 2.. Support scheme

The working surface roadway support conditions were as follows: a number of sets of rebar anchor rod supported the top, with an anchor rod combined with two sections of resin cartridges; the ends were supported by multiple sets of threaded steel rock bolts. The rock anchor rod used was model 20#-M22-2300mm-BHRB500, with mesh specifications of 6000 × 1000 mm, a steel strand anchor cable anchor, and 300 × 300 × 16 mm arch plates, filled with four sections of Φ23 × 700 mm resin powder roll (including two with ultrafast roll and two with fast roll).

• 3.. Simulation scenario

The model is based on the actual mining conditions. The burial depth is 650 m; 100 m is taken along the direction of the working surface tendency, 200 m is taken along the direction of the working surface, the inclination angle is 5°, and the coal seam endowment and rock formation structure are established according to the actual geological conditions. The size of the final numerical calculation model is 400 m × 100 m × 215 m, and the entire model is divided into 122,039 units and 132,902 nodes, as shown in Figure 6.

Both the X and Y boundaries of the model apply horizontal displacement constraints, and the bottom boundary of the model applies vertical (Z-direction) displacement constraints. The upper part of the model is a free boundary, the stress boundary condition is applied, the self-weight of the upper rock layer is simulated, and the simulation calculates the application of vertical stress σ_z = 13.75. The Mohr–Coulomb criterion was selected for the model failure criterion, the stress softening model for coal seam selection, and the Mohr–Coulomb criterion was also selected for other stratigraphic failure criteria. The weak cement formation in the simulation scheme is taken from borehole number 1711 at the coal mine site, and the physical and mechanical parameters of the related rock formation are shown in Table 2.

4.2. Analysis of Numerical Simulation Results

• 1.. Stress distribution of roadways with coal pillars of different widths

After the excavation model reaches equilibrium, the stress cloud diagram of the coal column of different widths (as shown in Figure 7) is analyzed. The stress concentration of the coal column near the goaf area is clear, and the maximum stress of the coal column with widths of 3 m, 5 m, 8 m, and 10 m is 27.7 MPa, 22.3 MPa, 23.6 MPa, and 24.9 MPa, respectively. When the width of the coal column is greater than 5 m, the maximum principal stress gradually increases with the width. The maximum principal stress is much greater when the width of the coal column is 3 m than when it is 5 m, and the stress on the solid coal side is relatively small, while the stress on the goaf side is relatively large. When, in area 130,205, the width of the coal column in the ventilation return lane is 5 m, the stress peak area is not connected with the stress peak area of the goaf area in the upper section and the coal column is in a position with low stress, which does not cause instability.

• 2.. Displacement analysis for coal column roadways of different widths

The vertical and horizontal displacements of the surrounding rocks of coal columns of different widths (3 m, 5 m, 8 m, and 10 m) were simulated, and the deformation cloud map of the surrounding rock of the roadway was obtained, as shown in Figure 8. According to the measurement point monitoring data, for coal column widths of 3 m, 5 m, 8 m, and 10 m, the displacement of the top and bottom plates of the roadway is 117 mm, 87 mm, 93 mm and 97 mm, respectively, and the displacement of the two vertical supports is 259 mm, 174 mm, 183 mm, and 192 mm, respectively.

For any given column width, the displacement of the top and bottom plate of the roadway is less than that of the two vertical supports moving closer; as the width increases (>5 m), there is a decreasing trend in the amount of horizontal approach of the surrounding rock of the roadway and the displacement of the top and bottom plate. Among the four sets of wide coal pillars, the surrounding rock of the 3 m coal column condition moved closer, and the maximum vertical displacement occurred at the junction of the top plate of the small coal column side of the roadway and the vertical support, while the maximum horizontal displacement occurred in the middle of the alley.

Through the numerical simulation results of the plastic zone and the deformation of the surrounding rock of the roadway, it can be found that, comparing coal column widths of 3 m, 5 m, 8 m, and 10 m, the stress concentration of the 5 m width coal column and the degree of deformation of the roadway are the lowest, making it the most reasonable design to protect the ventilation return lane of working area 130,205.

5. Field Monitoring and Analysis

In working area 130,205, the original plan was to mine a 35 m large coal column to protect the lane, but after the team optimized the design, this was changed to a 5 m small coal column. To scientifically evaluate the effect of the small coal pillar guard along the empty excavation alley, the ventilation roadway is monitored during recovery according to the relative positions of comprehensive mining work surfaces 130,203 and 130,205, the arrangement and coal mining method along the empty roadway, and the roof management measures. The actual problems encountered in the production process for other working surfaces of the mine are also considered.

5.1. Deformation Analysis of the Surrounding Rock along the Empty Excavation Alley

The cross-point method [22] is used to monitor the amount of surrounding rock in the 130,205 ventilation return lane. The equipment used is a mining intrinsically safe laser distance meter, which automatically transmits measurement data to the surface via the internet. The measurement frequency ranges from 10 to 50 times per second due to environmental influences underground. The roadway section is divided into four sections for measurement, and five groups of surrounding rock deformation observation stations are set up in the 130,205 ventilation return lane. The deformation characteristics are analyzed by comparing the statistical data for each time.

The data monitoring of return air began on 24 February 2019, and according to the design requirements, the relevant data were collected regularly, and the relationship between the distance from the observation point and the working surface distance was drawn on the observation section 100 m and 150 m from the open eye of the working surface (see Figure 9 and Figure 10).

Combined with the location, it was found that the support drum on the side of the 5 m coal column in the goaf area was larger than that at other positions in the roadway, which was the reason why the roof plate pressed the 5 m coal column and protruded backward to the goaf area. The roof plate of the goaf area behind the working surface is fully dropped, the overall cutting of the top beam of the bracket is good, and there is no bracket crushing phenomenon.

Based on the deformation analysis of the surrounding rock of the roadway in area 130,205, the following characteristics of the ore pressure of the ventilation return lane are presented.

• 1.. First item:

Within the range of 0 to 25 m ahead of the working surface, the roadway deformation is significant, and under the influence of the advanced support pressure, the gravity of the overlying rock layer is applied to the two vertical supports and transmitted to the bottom plate of the roadway, causing the supports and the bottom plate to bulge. According to the monitoring of the advanced support pressure of the working surface, the roadway within 50 to 70 m of the working surface is affected by the advanced support pressure, the plastic area of the roadway within 20 to 30 m of the working surface is obvious, and the mineral pressure is obvious.

• 2.. Second item:

The two gangs of the roadway showed asymmetrical displacement, the surrounding rock on the solid coal side was significantly greater than that of the 5 m coal column side, and the coal body collapsed. Due to the influence of the mine pressure distribution, the solid coal side is closer to the peak of the advanced support pressure of the working surface and the stress in the coal body is larger, resulting in the deformation of the solid coal-side alley gang being larger, and the local help drum appears. While the 5 m coal column side is far from the peak of the support pressure and is in a stable internal stress field, the 5 m coal column has been grouted and anchored, and its mechanical properties are enhanced, so the deformation is small, and the strength of the mine pressure is not obvious.

5.2. Analysis of the Upper Support Pressure in Front of the Work during the Recovery Period

During the mining process of the working face, the ZYC-10 flexible hydraulic coal body stress sensor (calibrated) and an infrared data collector were used to monitor the distribution of the advance support pressure. The sensors were arranged in the solid coal side of the 130,205 ventilation return lane. The flexible detection units were installed at distances of 310 m, 312 m, and 316 m from the working surface open cut eye to monitor the change of the advanced support pressure. The drill hole for the stress measurement point is numbered outward from the opening eye position of the working surface: IV-1, IV-2, and IV-3. The depth of the measuring hole is 10 m, and the drilling diameter is 48 mm. The equipment installation is shown in Figure 11, and the force of the flexible detection unit is shown in Figure 12, Figure 13 and Figure 14.

Combined with the observation and analysis of the surrounding rock deformation of the advanced roadway and the force of the anchor rod, the observation of the ore pressure of the 130,205 ventilation return lane is consistent with the deformation observation. The analysis shows that starting from 60 m to 80 m in front of the work, the support pressure begins to gradually increase, the stress reaches its peak at a position 20 m from the working surface, the plastic zone appears in the range of 20 to 30 m, and the fractures in the coal body develop obviously, are more broken, and have no impact failure effect. Until the goaf position stress gradually decreases, the front of the peak is in a state of elastic compression, the back of the peak is in a state of plastic deformation, and the stress concentration coefficient is approximately 2.5 to 3. The deformation of the roadway in the plastic area is large, and the interior of the coal rock mass is broken. The stope is in a stress-reduced area with no impact damage, and the rock cover movement of the roadway will cause ore pressure to occur, but it will be within the controllable range.

6. Conclusions

According to the structural characteristics of the surrounding rock along the empty excavation alley, with the tunnel boring, the surrounding rock stress is redistributed, forming three areas: the low-stress area, high-stress area, and original-stress area. The 130,205 roadway of the original 35 m position of the working surface is in the high-stress area of the stress peak, and the stability of the surrounding rock of the roadway is poor. Based on the calculation results for the created weakly cemented return mining roadway failure mechanical model, combined with the mine pressure monitoring results of the adjacent working surface, the roadway position behind the 5 m width coal column is in the low-stress area with relatively small stress. Combined with the improved support reinforcement technology, the stability of the surrounding rock of the roadway is greatly improved.

Through numerical simulation analysis, when the width of the coal column is 5 m, the range of the two plastic areas of the ventilation return roadway is relatively small, the roadway is in the low-stress area of the support pressure, the displacements of the two vertical supports of the roadway and the top and bottom plate are relatively small, and the coal column support ability is strong, which meets the stability requirements of the roadway.

The advanced influence range of support pressure during the recovery of working surface 130,205 is 70 m, and the peak area of the roadway support pressure is located approximately 20 m in front of the work. Affected by the advanced support pressure, the protective coal column in the weakly cemented surrounding rock environment stores a large compressive elastic energy, and the elasticity can reach a certain limit after the two vertical supports along the roadway transmit to the roadway bottom plate and are released, mainly manifested as two supports moving closer and the bottom plate bulging. In the 5 m protected coal column, although a plastic area has appeared, the cracks in the coal body have developed, but there is no obvious mineral pressure. This scheme can meet the requirements for safe production and provide theoretical and technical guidance for mining nearby soft and thick coal seams along the hollow tunnel in the western mining area.

In the context of protecting roadways with large coal pillars in adjacent lanes, numerical simulations were applied to predict the conditions of roadways protected by smaller coal pillars. The findings concluded that roadways protected by smaller coal pillars offer higher safety. Additionally, for roadways that intersect with coal seams, this approach contributes to coal resource conservation and plays a role in promoting the sustainable development of coal resources.

Author Contributions

Conceptualization, B.Z. and S.Z.; methodology, B.S.; software, B.S. and X.H.; validation, S.S. and J.D.; formal analysis, Y.L.; investigation, B.Z.; resources, B.Z. and J.D.; data curation, X.H. and S.S.; writing—original draft preparation, B.Z.; writing—review and editing, B.Z.; visualization, S.Z. and Y.L.; supervision, B.S.; project administration, S.Z. and Y.L.; funding acquisition, S.Z. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author due to privacy.

Conflicts of Interest

The authors declare that they have no conflicts of interest. This article does not report any studies involving animals performed by any of the authors.

Footnotes

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Figures and Tables

Figure 1. Location of the Yangchangwan Coal Mine.

Figure 2. Mechanical model of the force of the tunnel at different locations.

Figure 3. Distribution of stress fields on the working surface.

Figure 4. Deformation of the roadway in the peak area of the support pressure of the large coal column roadway in the adjacent mining area: (a) bending of steel beam; (b) anchor cable failure; (c) pillars sinking; (d) serious deformation of coal pillar side.

Figure 5. Mechanical model of the ultimate equilibrium of the coal column [20,21].

Figure 6. Simulation scheme: (a) modeling schemes; (b) numerical models.

Figure 7. Displacement analysis of coal column roadways of different widths: (a) 3 m coal pillars; (b) 5 m coal pillars; (c) 8 m coal pillars; (d) 10 m coal pillars.

Figure 8. Cloud diagram of different-width coal column displacement distributions: (a) vertical displacement distribution of the 3 m coal column; (b) horizontal displacement distribution of the 3 m coal column; (c) vertical displacement distribution of the 5 m coal column; (d) horizontal displacement distribution of the 5 m coal column; (e) vertical displacement distribution of the 8 m coal column; (f) horizontal displacement distribution of the 8 m coal column; (g) vertical displacement distribution of the 10 m coal column; (h) horizontal displacement distribution of the 10 m coal column.

Figure 9. Deformation map of the surrounding rock of the 130,205 ventilation return lane 100 m from the open cut eye.

Figure 10. Deformation map of the surrounding rock of the 130,205 ventilation return lane 150 m from the open cut eye.

Figure 11. Installation of the flexible detection unit equipment.

Figure 12. Force curve for the IV-1 flexible detection unit.

Figure 13. Force curve for the IV-2 flexible detection unit.

Figure 14. Force curve for the IV-3 flexible detection unit.

Table 1

Working surface: top plate and bottom plate.

Location	Lithology	Thickness/[m]	Lithological Characteristics
1 coal	A layer of coal	0.92	Black, semidark briquettes, eye-shaped fracture strip structure, layered structure. The roof is a coarse sandstone aquifer at the base of the Giro Formation.
Main roof	Fine sandstone	4.96	Black-gray and composed mainly of quartz; the middle and lower parts of the development have wavy, lens-like, and small staggered layers.
	Medium sandstone	12.7	The color is off-white, containing long-term minerals and mud chips, mainly gelatinous calcium; good sorting; semicircular, developmental level layer.
Immediate roof	Fine sandstone	3.23	Dark gray, mainly composed of long-term minerals and mud chips, mud calcareous cementation.
	Siltstone	4.62	Dark gray, with a thin layer of fine sandstone in the upper part and a high argilla content in the middle.
False roof	Carbonaceous peat	0.4	Black, containing a small amount of silt and pyrite, with brittleness, angular fractures, and stratigraphic development, relatively broken.
2 coal	Second layer of coal	8.4	Black, blocky, nonstick coal, mainly composed of semibright coal and silk charcoal, stepped fractures, striped structure, layered structure, stable deposition, simple structure.
Direct bottom	Siltstone	1.44	Black, black-gray, striped structure, block structure; the boundary line with the lower depression formation is obvious, and the coal line is partially thinned.
	Two layers under the coal	0.93	Black, striped structure, broken step-like, with a clear boundary with the lower strata.
Basic base	Siltstone	10.4	Light gray, silty structure, block structure, thin coal line.

Table 2

Parameter table for each rock formation.

Numbering	Rock Name	Thickness [m]	Depth [m]	Compressive Strength [MPa]	Tensile Strength [MPa]	Elastic Modulus [GPa]	Poisson’s Ratio	Cohesion [MPa]	Unit Weight[kg/m3]
M10	Siltstone	14.68	474.90	42.2	4.22	13.6	0.24	4.66	2.35
M9	Medium sandstone	22.40	497.30	38.3	3.83	14.4	0.23	4.83	2.35
M8	Siltstone	35.29	532.59	45.2	4.52	14.6	0.22	4.66	2.35
M7	Medium sandstone	3.70	536.29	38.6	3.86	14.4	0.23	4.83	2.35
M6	Siltstone	7.59	543.88	45.4	4.54	14.6	0.22	4.66	2.35
M5	Coarse sandstone	39.27	583.15	37.3	3.73	14.4	0.23	4.83	2.35
M4	1 coal	1.15	584.30	15.0	1.5	1.86	0.25	0.75	1.5
M3	Siltstone	5.24	589.54	45.2	4.52	14.6	0.23	4.66	2.35
M2	Medium sandstone	12.00	601.54	38.6	4.6	14.4	0.23	4.83	2.35
M1	Siltstone	8.56	610.10	35.7	3.57	12.5	0.22	4.27	2.2
M0	2 coal	8.70	618.80	13.0	1.3	1.86	0.25	0.87	1.45
F1	Siltstone	1.45	620.45	45.2	4.52	14.6	0.23	4.66	2.35

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