Open AccessArticle

Study on Coal Pillar Setting and Stability in Downward Mining Section of Close Distance Coal Seam

Longpei Ma

^1,2,

Chongyan Liu

^1,3,* and

Guangming Zhao

^1,3

Joint National-Local Engineering Research Centre for Safe and Precise Coal Mining, Anhui University of Science and Technology, Huainan 232000, China

School of Mining Engineering, Anhui University of Science and Technology, Huainan 232000, China

Coal Mine Safety Mining Equipment Innovation Center of Anhui Province, Anhui University of Science and Technology, Huainan 232000, China

Author to whom correspondence should be addressed.

Energies 2024, 17(21), 5441; https://doi.org/10.3390/en17215441

Submission received: 11 October 2024 / Revised: 26 October 2024 / Accepted: 28 October 2024 / Published: 31 October 2024

(This article belongs to the Section H: Geo-Energy)

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Abstract

To investigate the reasonable width of a coal pillar in the downward mining section of close-distance coal seams, the stress state of any point below the residual coal pillar in the overlying goaf and the width of a small coal pillar were studied by theoretical calculation, numerical simulation, similar simulation and field monitoring. The findings indicate that the width range of the small coal pillar is 7.92~11.42 m. The 4-1 coal seam is in the stress reduction zone when it is more than 16.6 m horizontally from the border of the residual coal pillar above it. In addition, the peak stress is situated inside the elastic zone of the coal pillar and is lower than the coal pillar’s bearing limit when a small coal pillar of 8 m is maintained. With the help of distributed optical fiber monitoring to model the coal pillars’ stress distribution, it is found that 8 m simulated coal pillars have a certain bearing capacity. The practical findings demonstrate that the 8 m small coal pillar that was left on the site satisfies the demand, and the convergence of the roadway’s floor and roof, and its two sides fall within the controllable range. The findings of the study offer a reference for the location of a return air roadway and the width of section coal pillars in the downward mining of close-distance coal seams.

Keywords:

close distance coal seam; residual coal pillar; coal pillar width; distributed optical fiber monitoring; stability of surrounding rock

1. Introduction

During the close-distance coal seam group’s downward mining procedure, the stress concentration phenomenon of the coal pillar left in the overlying coal seam and the width of the coal pillar left in the underlying coal seam working face will significantly influence the stability of the mining roadway in the underlying coal seam working face [1,2,3]. Particularly in light of small coal seam spacing, complex coal seam conditions [4,5,6], gob-side entry retaining and retaining small coal pillars for roadway protection [7,8,9,10], the law of mine pressure behavior will be more obvious.

The failure depth of stope floor is one of the main issues in the downward mining of the close distance coal seam group. Numerous academics have used simulated tests to investigate the influence of overlying working face mining on underlying coal strata, which ensures mining production is both safe and effective [11,12,13,14,15]. Additionally, Yang et al. investigated the caving features of the roof during the mining of several coal seams and conducted simulation tests on close-distance coal seams using the digital image correlation method, and revealed the evolution law of roof-stress when close-distance coal seams are being mined [16]. Cui et al. used a numerical modeling technique to examine the fracture growth height of a nearby coal seam under the impact of repeated mining [17]. Li et al. investigated the mine pressure behavior law and features of the roof breaking of close-distance underlaying coal seam mining in order to prevent roof disasters [18].

To guarantee the safe mining of the working face, a reasonable coal pillar width is essential. Numerous academics have optimized the coal pillar size through simulation experiments [19,20,21]. Using the thickness of the underlying coal seam and the breadth of the remnant coal pillar as variables in an orthogonal experiment, Zang et al. demonstrated how the variables affected the close-distance coal seam group’s mining under the residual coal pillar [22]. In addition to exposing the process and mechanism of the dynamic instability of broad coal pillars under dynamic and static load disturbance, Jiang et al. established the mechanical and energy criteria of the dynamic instability of wide coal pillars [23]. To comprehensively analyze the influence of roof rock mass on the stability of coal pillars, Gao suggested using a mechanical approach to create a composite rock mass [24]. Arka Jyoti Das et al. explained how to using numerical simulation tools to determine the slanted coal pillars’ strength, and used the extensively employed joint model for simulating the shearing properties of inclined rock strata [25].

Many studies on the mining of close-range coal seam groups have been conducted recently, but different coal seam spacing, mining technology and overlying working face layout mean that the retention and application of small coal pillars in lower coal seams still need to be further studied. The research background for this work is the 4-1 close-distance coal seam of Selian No. 2 Coal Mine. This study independently designs the mold, and uses the test piece’s distributed optical fiber response curve to describe the coal pillar’s internal failure characteristics, and applies it to the field monitoring, which offers a reference for the problem of coal pillar retention under similar conditions.

2. Engineering Situations

Selian No. 2 Mine’s primary 4-1 coal seam is almost horizontal, with an average inclination angle of 1° and a range of 0° to 2°. The average buried depth of the 12409 working face is 394.3 m, the lengths of the strike and dip are, respectively, 2930 and 242 m, and the average coal thickness is 4.0 m. Figure 1 displays the working face layout diagram.

The working faces 12409 and 12310 are separated by an average of 43.3 m, and there are 44.8 m on average between the 12409 and 12310 working faces. Between the two coal seams, the types of rocks include semi-hard rock such as fine sandstone and sandy mudstone. Following the mining of the working faces 12309 and 12310, this will result in some level of mining influence on the floor. Consequently, the ceiling of the underlying coal seam’s initial rock stress state is destroyed. Meanwhile, a stress concentration may develop around the coal body due to the 30 m residual coal pillar that remains in the coal seam above. The stress concentration is transmitted to the underlying coal seam roof through the coal rock mass. It will have a major impact on the coal pillar’s stability in the coal seam operating face underneath.

Three anchor cables, each measuring 21.6 × 8300 mm and spaced 1600 × 1800 mm apart, and seven bolts with a row spacing of 730 × 900 mm between 20 mm × 2500 mm are arranged on the roof of the 12409 return air roadway. Anchor cables have a pre-tightening force of at least 250 kN, as well as the bolt’s pre-tightening force being at least 70 kN. Five bolts, each measuring 20 × 2500 mm and spaced 750 × 900 mm, are set up along the return air roadway’s side.

3. Theoretical Calculation of Overlying Coal Seam Floor Failure Depth and Underlying Coal Seam Coal Pillar Setting

3.1. Theoretical Calculation of Failure Depth of Overlying Coal Seam Floor

When the 12309 and 12310 working faces complete the production work, due to the roof weight of the mined-out area, the residual coal pillars will be in a concentrated state of stress. This stress concentration effect will demolish the initial rock stress condition of the coal and rock mass beneath the floor within its effect range due to the load operating on the coal pillars. To research the impact range of the residual coal pillar’s stress concentration, the stress calculation equation at any location beneath the residual coal pillar’s floor in the mined-out area is obtained as follows [26]:

\{\begin{cases} σ_{z} = \frac{P}{π} [\arctan \frac{x}{z} - \arctan \frac{x - B}{z} - \frac{z (x - B)}{{(x - B)}^{2} + z^{2}} + \frac{B (x - z)}{x^{2} + z^{2}}] \\ σ_{x} = \frac{P}{π} [\arctan \frac{x}{z} - \arctan \frac{x - B}{z} - \frac{z (x - B)}{{(x - B)}^{2} + z^{2}} + \frac{x z}{x^{2} + z^{2}}] \\ τ_{x z} = \frac{P}{π} [\frac{z^{2}}{x^{2} + z^{2}} - \frac{z^{2}}{{(x - B)}^{2} + z^{2}}] \end{cases}

(1)

where the independent variables x, y, and z refer to the spatial coordinates of any point below the floor, the independent variable B denotes the breadth of the coal pillar value of 30 m, the independent variable P refers to the uniform load value of the coal pillar of 50 MPa, the dependent variable

σ_{z}

denotes the floor rock layer’s vertical stress, the dependent variable

σ_{x}

denotes the horizontal stress in the floor and the dependent variable

σ_{x z}

denotes the shear stress in floor strata.

Substituting Equation (1) into the code written in matrix laboratory (MATLAB R2019b), it is concluded that when the coal pillar’s breadth reaches 30 m, the distribution state of stress in the coal strata below the residual coal pillar is as seen in Figure 2.

Figure 2 shows that the vertical stress forms a structure similar to a “stress bubble” in space, and its range of action exceeds 100 m. The horizontal stress is shown as a “butterfly symmetry” distribution, and its range of action is restricted to the 45 m region near the coal pillar’s base. The shear stress is distributed in an “axisymmetric” manner, and its range of action covers about 58 m below the coal pillar.

Based on the return air roadway’s design in the 12409 working face, the vertical distance between the return air roadway and the residual coal pillar is approximately 43.7 m, and the return air roadway is 3.6 m high. From Figure 2a, it is concluded that the vertical stress presents a “unimodal” distribution at 43.7 m beneath the residual coal pillar, and 20 MPa is the maximum stress. When the residual coal pillar’s edge is 19 m from the return air roadway, it will no longer be affected by the vertical stress.

3.2. Theoretical Calculation of Underlying Coal Seam Pillar Setting

Generally speaking, the basic condition for the coal pillar’s integrity is that both sides of the coal pillar now have the plastic zone, while the middle part is still in the elastic state. For close-distance coal seams with two different layers, when the overlying coal seam is mined, the plastic zone breadth of the coal pillar is obtained [27] as follows:

B^{'} = \frac{(m_{u} + m_{d}) A}{2 \tan φ} \ln \frac{\frac{C_{0}}{\tan φ} + K γ H}{\frac{C_{0}}{\tan φ} + \frac{P_{S}}{A}}

(2)

where the independent variable

m_{u}

refers to the height of mining in the overlying coal seam and its value is 2.6 m, the independent variable

m_{d}

denotes the underlying coal seam mining height and its value is 3.5 m, the side pressure coefficient’s value A is 0.2, the independent variable φ means internal friction angle and its value is 24° and the value of small coal pillar cohesion

C_{0}

is 1.6 MPa. The value of stress concentration factor K is 1.47, the value of overlying rock bulk density γ is 25 kN·m⁻³ and the value of the supporting strength of the coal pillar side wall

P_{s}

is 0.1 MPa.

Through Equation (2), the width of the plastic zone on both sides of the coal pillar is 2.19 m and 2.23 m, respectively. In this time, the minimum width B of the underlying coal pillar to maintain stability is shown [28] in Equation (3):

B = (2.19 + 2.23) + (1 ~ 2) \times (m_{u} + m_{d})

(3)

Through Equation (3), the critical width of small coal pillar stability in the 12409 working face is 7.92~11.42 m.

4. Numerical Simulation of Coal Pillar Setting in Close-Distance Coal Seam

To investigate the stability of the small coal pillars with varying widths in the underlying coal seam and the range of influence that the residual coal pillars in the above mined-out area have on it, the stress distribution law and plastic zone distribution characteristics of coal pillars were analyzed by fast Lagrangian analysis of continua three-dimension (FLAC^3D6.0) numerical simulation software.

Figure 3 illustrates that the model’s size is: X(length) × Y(width) × Z(height) = 400 m × 320 m × 160 m. Both the 3-1 and 4-1 coal seams are encrypted. The model is divided into 1,041,000 units and 1,121,807 nodes. Mohr–Coulomb yield is the criterion that the model uses, and sets the working face to advance along the Y-axis direction. Except for the model’s top, the other five faces of the model are set as fixed boundaries, and zero displacement boundary conditions are used. There is a free boundary at the top of the model. According to the buried depth of the coal seam, the model’s top uses a 9.9 MPa load, and its lateral pressure coefficient is 0.8. Table 1 displays the mechanical and physical characteristics of coal layers.

The two working faces of the 3-1 coal seam are separated by the 30 m coal pillar. Once each unit’s initial stress has been balanced, the 12309 and 12310 working faces of the 3-1 coal seam are excavated first, and then the 12408 and 12409 working faces are excavated in turn. A distance of 20 m is set as an excavation step distance. After each step distance is excavated, the stress is calculated until the stress is balanced, and then the next step distance is excavated.

4.1. Study on the Influence Range of Residual Coal Pillar Stress

Utilizing numerical computation software, following the mining of the three coal working faces of the Selian No. 2 Mine, the stress action zone of the residual coal pillar on the underlying 4-1 coal seam is examined. After numerical calculation and graphic processing, the residual coal pillar’s stress distribution in the 3-1 goaf in the underlying coal seam is obtained (see Figure 4).

Based on the findings of the numerical computation, the vertical stress influence depth of the residual coal pillar reaches 65.3 m, which exceeds the 43.3 m coal seam spacing. The residual coal pillar in the overlying goaf will have a great influence on the surrounding rock of the roadway in the 12409 working face. When there is a horizontal stagger distance of more than 16.6 m between the edge of the residual coal pillar in the overlying mined-out area and the 4-1 coal seam beneath the coal pillar, the 4-1 coal seam is appropriate for the roadway layout because it is in the stress reduction zone. Considering the theoretical calculation results, the overlaying residual coal pillar and the 12409 return air roadway are intended to be 20 m apart horizontally.

4.2. Stability Analysis of Coal Pillars with Different Widths During Mining Period

The small coal pillar between the 12409 working face and 12408 working face should keep the elastic zone at a specific width after bearing the influence of secondary mining disturbance, and its stress level will not exceed its bearing limit and damage will occur.

In order to keep an eye on the integrity of coal pillars of varying widths when mining, a cross section was made in the vertical Y-axis direction at 60 m from the open-off cut of the return air roadway in the 12409 working face, and the coal pillars’ plastic zone distribution features as well as the small coal pillars’ stress distribution law were observed.

Figure 5 illustrates that the sky blue area is the elastic zone with the highest carrying capacity, and the other colors are damaged. The 8 m small coal pillar has a full elastic zone and the length of the elastic core zone is about 2 m, accounting for 25%. The percentage of the elastic zone inside the small coal pillar grows as the coal pillar’s width increases.

Figure 6 shows the small coal pillar’s vertical stress distribution curve demonstrates that the stress of small coal pillars with varying widths basically exhibits a trend of increasing first and then decreasing. When the width of the small coal pillar is 6 m, the internal maximum stress is 19.29 MPa. At this time, the coal pillar is seriously broken under the influence of the static pressure of the overlying strata and the dynamic pressure of the working face. The peak stress rises to 21.59 MPa when the small coal pillar’s width is 7 m as opposed to 6 m, and the plastic zone inside the 7 m small coal pillar accounts for a relatively small proportion, and large deformation may occur due to mining disturbance. Therefore, it is not appropriate to arrange 6 m and 7 m small coal pillars.

The peak stress is 4.2 m from the coal wall, which is situated in the elastic core area of the small coal pillar, when its breadth is 8 m. The peak value of stress is 18.62 MPa, and the compressive strength of 4-1 coal measured by a uniaxial compression test is 17.14 MPa. Since the integrity of the small coal pillar is good and the actual coal pillar’s bearing limit is better than the indoor test findings, there will not be any significant deformation brought on by mining disturbance.

When the small coal pillar is 9 m broad instead of 8 m, the maximum stress is reduced by 7.63%. The 9 m small coal pillar is stable because the highest stress is currently below its bearing limit. However, it causes a waste of some coal resources.

Small coal pillars with a width of 8 or 9 m have a greater bearing capacity when viewed from the standpoint of stress and plastic zone distribution. The width of small coal pillars is intended to be 8 m based on the findings of numerical simulations and theoretical analysis. Additionally, there is a 20 m horizontal offset between the return air roadway and the edge of the residual coal pillar in the coal seam overlying it.

5. Similar Simulation Test of Section Coal Pillar Retention

Stemming from the relevant similar rules (when two phenomena are comparable, their similarity criteria must be numerically same), miniature test pieces of coal pillars with different sizes were made. Through the response curve of the distributed optical fiber in test pieces during the uniaxial compression test, the stress distribution in coal pillars, as well as the deformation and failure characteristics of coal pillars with varying widths, were examined.

5.1. Test Pieces Preparation

A similar simulation model was constructed with the 12409 working face of Selian No. 2 Mine as the background for the experiment. The similarity parameters of the model were determined based on the relevant similar rules. The ratio of geometric similarity was 1:50, the bulk density similarity ratio was 0.93:1, and the mechanical strength similarity ratio was 0.019:1. According to the geometric similarity ratio, four kinds of molds with different widths of 6, 7, 8 and 9 m were designed. Taking the 8 m coal pillar as an example, Figure 7 displays the mold’s schematic diagram.

The breadth of the small coal pillar is represented by the mold’s X-axis. There are two square holes with a side length of 5 mm in the middle of the mold, which are used to place a single-mode fiber for monitoring the internal strain of the test piece. The thickness of the overlying coal strata that the test failed to simulate is 394 m, the bulk density of the overlying strata is 2657 N·m⁻³, and the gravity load that the model needs to apply is 20 kPa. The uniaxial compression test of 4 coal samples in Selian No. 2 Coal Mine was carried out, and the coal samples’ uniaxial compressive strength was 17.1 MPa. Based on the mechanical strength similarity ratio, the compressive strength of the model should be 0.3249 MPa, and the ingredients of the test pieces are shown in Table 2.

The uniaxial compression test loading equipment uses an electronic universal testing machine, and the internal stress monitoring equipment uses an optical frequency domain reflectometry (OFDR) demodulator. Displacement loading is the universal testing machine’s loading mode, and the loading speed is 0.02 mm·min⁻¹. The indoor test ignores the influence of temperature change on the monitoring data.

5.2. OFDR-Based Distributed Optical Fiber Sensing Technology

OFDR is a distributed optical fiber monitoring technology with high spatial resolution [29]. Its spatial resolution can reach up to 1 mm. It is mainly composed of a linear sweep light source, coupler, fixed mirror, sensing fiber, photodetector and receiver, as depicted in Figure 8.

When the fiber experiences strain variations due to an external force or the ambient temperature, the refractive index inside the fiber will change, causing the reflection spectrum to drift. The relationship between the frequency drift in the fiber and the strain [30] is shown in Equation (4):

\{\begin{cases} ε_{0} = \frac{Δ l}{l} = \frac{C \times (t_{2} - t_{1})}{l \times n} \times α_{1} \\ ε_{s} = (ε_{0} - ε_{t}) \times α_{2} \\ η = \frac{ε_{s}}{λ} \end{cases}

(4)

where

ε_{0}

refers to the total strain of the fiber;

ε_{s}

refers to the strain formed by tension or compression;

ε_{t}

refers to strain caused by temperature change; the independent variable l denotes the original length of the fiber; the independent variable

∆ l

denotes fiber length deformation; the independent variable t₁ denotes delay time before fiber deformation; the independent variable t₂ denotes delay time after fiber deformation; the constant C denotes the speed of light; the constant η denotes the optical fiber refractive index; the constant α₁ denotes stress correction factor and the constant α₂ denotes temperature correction factor.

The fiber is placed in the center of the test piece in a horizontal orientation to simplify the failure state of the test piece in the uniaxial compression test, and the correlation between the horizontal optical fiber strain and the test piece fracture angle in the similar simulation material is obtained, as shown in Figure 9.

Prior to the test piece under uniaxial compression, the optical fiber’s strain curve represents the starting value, which is approximately a horizontal straight line. After the test piece is broken, the exposed optical fiber between the two broken test pieces will be in an obvious tensile state. The optical fiber strain value monitored by OFDR technology (Suzhou Nanzhi Sensing Technology company, Suzhou, China) at the corresponding position will increase. Meanwhile, the distributed fiber strain curve will have a strain peak, and an inflection point closest to the peak point will be taken on both sides of the peak point, and the length between the two sites of inflection corresponds to the distance between the two broken test pieces. The test piece’s height is H, the angle α is formed by the shattered test pieces 1 and 2 and the distance between broken test pieces is L. Then, α can be expressed as Equation (5):

\sin \frac{α}{2} = \frac{L / 2}{H / 2}

(5)

Since the angle between the two broken test pieces is small, simplify

\sin (α / 2)

into

α / 2

, then Equation (5) can be expressed as follows:

α = 2 L / H

(6)

It can be seen from Equation (6) that, when the height of the similar material is constant, as the fracture angle of the test piece increases during the uniaxial compression process, the distance between the two test pieces increases, causing the tensile strain of the distributed fiber to grow.

5.3. Analysis of Deformation and Failure Characteristics of Test Pieces

Following the uniaxial compression test, the test pieces’ fracture properties with different widths are shown in Figure 10. Export data from OFDR monitoring instruments, and the data are integrated and processed. The response curve of the internal distributed optical fiber after uniaxial compression is drawn as shown in Figure 11.

Figure 11 illustrates that the 120 mm test piece (corresponding to the 6 m small coal pillar) has a clear tensile state in the range of 0~10 mm, 30~50 mm, 90~110 mm and 110~120 mm, and the peak strain reaches 14,580. Both the 10~30 mm and 50~90 mm are compressed. The interior of the test piece presents a state of “rupture–compaction–rupture–compaction–rupture” from shallow to deep. The interior of the test piece is relatively broken and the bearing capacity is weak.

The strain curve of the 160 mm test piece (corresponding to the 8 m small coal pillar) is obviously distinct from that of the previous test pieces. The strain value is negative in the range of 20~120 mm in the middle of the strain curve, which indicates that the test piece is in a relatively stable compression state in this range. Except for a certain broken area in the range of 20~120 mm and 120~135 mm, the residual parts of the test piece are in a state of compression. The thorough investigation demonstrates that the 8 m small coal pillar has a certain bearing capacity.

6. Field Monitoring of Coal Pillar Stability

To verify the feasibility of leaving an 8 m small coal pillar in the 12409 working face, through field measurement, with the help of high-precision equipment such as a laser range finder and distributed optical fiber demodulator in the 12409 working face, the internal stress of the coal pillar and the surrounding rock deformation are monitored.

6.1. Monitoring Scheme Design

In order to monitor the return air roadway’s two sides, as well as its roof and floor, six surface displacement measuring points were set up in the 12409 return air roadway. As shown in Figure 12a, the surface displacement measuring point 1 is located 800 m in front of the working face, and one measuring point is set up every 5 m along the advancing direction of the working face. The height of the roof and floor of the roadway and the width of the two sides are measured by the “Cross Method” (the displacement of the return air roadway surface can be determined by placing cross points on its surface and measuring them using instruments like total stations).

As shown in Figure 12b, an optical fiber measuring point is set at 788 m of the 12409 return air roadway, and the deformation and stress distribution characteristics of the surrounding rock on the side of the coal pillar are monitored by means of a Brillouin optical time domain reflectometry (BOTDR) distributed optical fiber demodulator [31]. After the fiber is installed in the borehole, the hole is filled with 425# ordinary Portland cement and the hole is sealed, so that the distributed optical fiber is coupled with the coal pillar.

6.2. Surface Displacement Monitoring Analysis

Figure 13 shows that the deformation law of the nearby rock at each measurement point is essentially the same. With the working face in operation, the cumulative displacement of the surrounding rock increases, and the total displacement of the roadway’s roof and floor is more than the total displacement of the roadside. The roof and floor of the roadway are more obvious than the mine pressure of the side. The sides’ rate of deformation progressively rises as they get 60 m away from the operating face, but the increase is less than that of the roof and floor.

The surface displacement monitoring results indicate that the maximum displacement of the two sides during the mining period is 132 mm. The floor and roof’s peak displacement is 270 mm, and the displacement of the roof and floor of the roadway and the two sides is within the controllable range.

6.3. Distributed Fiber Optic Monitoring Analysis

As shown in the optical fiber monitoring curve of the small coal pillar side in Figure 14, the optical fiber strain increases from 0 to 7938 in the range of 0.32~1.54 m at the measuring point 74 m in front of the working face. It suggests that the distributed optical fiber strain is always in a tensile state [32], and the strain fluctuation range is mainly in the range of 0.32~1.54 m, and there is some breakage in the coal pillar’s shallow section.

As the working face reaches 20 m in front of the measurement point, the strain of the optical fiber at 1.34 m changes from positive to negative, and the stress of the fiber at this position changes from tension to compression. It is considered that 1.34 m in the small coal pillar is the intersection of the elastic and plastic zones. The small coal pillar’s 1.34~5.9 m fiber strain is constantly under compression, indicating that the bearing area is produced within the pillar. The stress concentration of the surrounding rock mainly appears in the deep part of 1.34~5.9 m.

7. Conclusions

In this paper, the close-distance coal seam in Selian No. 2 Mine is taken as the research background. Theoretical study, numerical simulation, similar simulation and field monitoring yield the following findings. The conclusion of this paper is consistent with other authors [33].

(1): The small coal pillar, which drives into the close-distance coal seam, is intended to be 8 m wide, and the overlaying residual coal pillar and the 12409 return air roadway are intended to be 20 m apart horizontally.
(2): When the 8 m small coal pillar is left, the small coal pillar’s plastic zone has not been penetrated, and the maximum stress is less than the bearing limit of the coal pillar. After the similar compression test, the strain in the range of 20~120 mm in the middle of the test piece is stable to a negative number, which demonstrates the sufficient bearing capacity of the 8 m coal pillar.
(3): The field monitoring results show that when the 8 m small coal pillar is left, the distributed optical fiber in the range of 1.34~5.9 m in the small coal pillar is in a relatively stable compression state, demonstrating that the coal pillar forms the bearing area, which is in line with the findings of numerical simulation and similar simulations. The two sides’ convergence during the mining process is 132 mm, and the roof and floor’s convergence is 270 mm, both of which satisfy the standards for safe production.

Author Contributions

Conceptualization, C.L. and L.M.; methodology, G.Z. and C.L.; software, L.M. and C.L.; validation, G.Z. and C.L.; data curation, L.M. and C.L.; writing—original draft preparation, L.M.; writing—review and editing, C.L. All authors have read and agreed to the published version of the manuscript.

Funding

The study was funded by the Open Research Grant of the Joint National-Local Engineering Research Centre for Safe and Precise Coal Mining, China (No. EC2023026), the Scientific Research Foundation for High-level Talents of Anhui University of Science and Technology, China (No. 2023yjrc131), the National Key Research and Development Project of China (No. 2023YFC2907602), and the National Natural Science Foundation of China (Nos. 52404068 and 52004006).

Data Availability Statement

Data to support the findings of this study are available from the first author upon request.

Acknowledgments

The authors of this paper would like to express their gratitude for the Coal Mine Safety Mining Equipment Innovation Center of Anhui Province (CMSMEICAP2024006).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Working face layout diagram.

Figure 2. Underlying coal rock stress distribution nephogram. (a) Vertical stress curve. (b) Horizontal stress curve. (c) Shear stress curve.

Figure 3. Numerical simulator model.

Figure 4. Vertical stress cloud diagram of the residual coal pillar.

Figure 5. Features of the plastic zone distribution of coal pillars with varying widths. (a) A 6 m coal pillar plastic zone distribution. (b) A 7 m coal pillar plastic zone distribution. (c) An 8 m coal pillar plastic zone distribution. (d) A 9 m coal pillar plastic zone distribution.

Figure 6. Vertical stress distribution curve of a small coal pillar.

Figure 7. Schematic diagram of the test die.

Figure 8. OFDR working schematic diagram.

Figure 9. Test piece breaking fiber characterization model.

Figure 10. Failure characteristics of test pieces. (a) A 120 mm test piece. (b) A 140 mm test piece. (c) A 160 mm test piece. (d) A 180 mm test piece.

Figure 11. Distributed optical fiber response curve.

Figure 12. Diagram of measuring point arrangement. (a) Measuring point diagram. (b) Optical fiber measuring point.

Figure 13. Surrounding rock displacement curve of air roadway.

Figure 14. Distributed fiber optic monitoring curve of coal pillar.

Table 1. Mechanical parameters of coal strata.

Rock Type	Density (kg·m⁻³)	Bulk Modulus (MPa)	Shear Modulus (GPa)	Cohesion (MPa)	Internal Friction Angle (°)	Tensile Strength (MPa)
Coal	1390	1.1	0.8	1.5	28	0.5
Fine sandstone	2670	2.46	2.26	2.4	33	3.5
Siltstone	2690	2.56	2.51	2.4	33	3.5
Post office box stone	2670	2.71	2.49	2.1	26	3.45
Medium grained sandstone	2590	2.84	2.71	2.3	36	3.56
Kern stone	2752	2.87	2.79	2.5	30	3.5
Sandy mudstone	2570	1.79	2.43	2.9	34	3.04

Table 2. Test pieces material ratio table.

Test Raw Material	Distribution Proportion (%)	Remark
Cement	6.4	425# ordinary Portland cement
River sand	9.6	Grain size 20 mesh
Water	9.0	Ordinary tap water
Coal dust	75.0	particle size 40~30 mesh, 30~20 mesh, 1:1 mixed

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Ma, L.; Liu, C.; Zhao, G. Study on Coal Pillar Setting and Stability in Downward Mining Section of Close Distance Coal Seam. Energies 2024, 17, 5441. https://doi.org/10.3390/en17215441

AMA Style

Ma L, Liu C, Zhao G. Study on Coal Pillar Setting and Stability in Downward Mining Section of Close Distance Coal Seam. Energies. 2024; 17(21):5441. https://doi.org/10.3390/en17215441

Chicago/Turabian Style

Ma, Longpei, Chongyan Liu, and Guangming Zhao. 2024. "Study on Coal Pillar Setting and Stability in Downward Mining Section of Close Distance Coal Seam" Energies 17, no. 21: 5441. https://doi.org/10.3390/en17215441

APA Style

Ma, L., Liu, C., & Zhao, G. (2024). Study on Coal Pillar Setting and Stability in Downward Mining Section of Close Distance Coal Seam. Energies, 17(21), 5441. https://doi.org/10.3390/en17215441

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