Reasonable Working-Face Size Based on Full Mining of Overburden Failure

Abstract

To improve production efficiency and prevent potential disasters in coal mines, comprehensive research methods such as similar simulation, numerical simulation, theoretical analysis, and on-site detection were used in this study. The migration characteristics of overburden rock under different working face lengths and development heights of water-conducting fracture zones were investigated via these methods in order to determine the reasonable length of the working face. The results show that the regularity of the development height of water-conducting fracture zones in similar simulations and numerical simulations is highly consistent, and the final stable values are 48 and 50 m. When the working face length was 300 m, the error between the simulated value and the value calculated from the formula dropped below 10% and exhibited a further decreasing trend; as a result, the working face length of 300 m was found to be the turning point for the development height of the water-conducting fracture zone to become stable. Based on the simulation results and mining damage theory, the critical size of the working face was 307.6 m, and the height of the water-conducting fracture zone was determined to be in the range of 45.5–60.5 m. The actual detection result of the height of the water-conducting fracture zone under the critical size of the working face was 55 m, which conforms to the law obtained from the simulation. Finally, the reasonable working face length under the geological conditions of a coal mine was determined to be 300–400 m. This study offers important reference value for determining the reasonable working face length under similar geological conditions, and may have significance for the sustainable development of coal resource mining.

1. Introduction

The continuous development of internal defects and fissures of the rock mass leads to the formation of an important channel for water from the aquifer to the working face, i.e., the “water-conducting fracture zone”. As a result, water flows easily into the goaf and the mining face through the fracture in the rock mass, which is not conducive to the safe and efficient mining of coal resources and the coordinated development of the ecological environment. Therefore, systematic exploration of the development of the water-conducting zone is the basic strategy used for the prevention and control of water disasters; thus, water-retaining mining technology has been put forward [1,2,3]. Notably, the length of the mining face has been increasing year after year. On the one hand, it has effectively improved production efficiency and resource recovery rates. On the other hand, it can increase the impact on the deformation and failure of the overburden rock, so that the developed height of the water-conducting fracture zone remains unknown. Changes have caused unknown safety hazards, such as problems in roof management, water-induced damage, and increases in the working face gas. Therefore, investigation of the influence of working-face length on overburden deformation and failure is of great practical significance.

At present, the empirical formula method in the “three-underground” mining specification is the most commonly used method to calculate the height of water-conducting fracture zones [4]. Through physical similarity simulation experiments and numerical simulation, Lai et al. analyzed the development law of overburden rock migration and water-conducting fracture zones, which provided an important reference for water-preserved mining at three soft coal seams [5,6]. The model of engineering geomechanics was established by Lu et al. The laws of stress, displacement, and crack propagation were analyzed using the simulation method. The development law of actual fracture zones in Jurassic and Carboniferous coal seams was obtained [7]. Su et al. proposed a new model that was able to predict the height of the water-conducting fracture zone based on the direction of propagation of the fracture [8]. Jiang et al. carried out an experimental study and theoretical analysis and calculated the development height of water-conducting fracture zones in combined rock strata, and verified its accuracy when predicting the height of water-conducting fracture zones under these geological conditions [9]. Feng et al. studied the three-dimensional (3D) development form of water-conducting fracture zones by combining drilling and 3D seismic methods. The research results could accurately predict the 3D shape and development characteristics of water-conducting fracture zones [10]. Du et al. used a numerical simulation method to study the development height of the water-conducting fracture zone under two mining methods and verified the difference between the two through field measurement, which provided an important theoretical basis for preventing mine-roof water inrush disasters [11]. Liu et al. detected the height of the water-conducting fracture zone in the overburden soil–rock composite structure, and studied the influence of the soil layer on the height of the water-conducting fracture zone [12]. Hu et al. carried out multiple regression on the height of water-conducting fracture zones in many mines and considered many factors to predict the height [13]. Kang et al. studied the effects of mining height and advance rate on the height of water-conducting fracture zones and determined the mechanism for the development of fractures [14]. Zhang et al. carried out MATLAB multivariate nonlinear regression fitting for five influencing factors of water-conducting fracture zones, and predicted the height of water-conducting fracture zones in extra-thick coal seam mining [15]. Yin et al. established a multiple regression model using different influencing factors of multiple working faces and accurately predicted the height of water-conducting fracture zones [16]. Yu et al. simulated the field in the laboratory and conducted a comprehensive analysis of the height of the water-conducting fracture zone in combination with numerical calculations [17]. Through the calculation of key strata combined with simulation software, Li et al. determined how the overburden was deformed and could reliably determine the development height of the water-conducting fractured zone [18]. Ren et al. analyzed the collapsing of the rock above the working face and also predicted the height of the water-conducting fractured zone based on a combination of laboratory simulation, software simulation, and formula-based calculation [19]. Zhai et al. analyzed the influence of water-bearing strata through software simulations and summaries of data laws, which were found to be very conducive to the smooth production of coal mines [20]. Yuan summarized the fracture development model according to the height of the water-conducting fracture zone. The model could learn independently to infer the fracture expansion [21].

The above-mentioned studies adopted different methods and considered the impact of different influencing factors on the height of water-conducting fracture zones; nonetheless, studies from the perspective of overburden failure have rarely been carried out to date. These studies have certain limitations; therefore, the present study is based on the actual geological conditions of a coal mine, fully considering the quantitative influence of working face length. Physical similarity simulation experiments and Dimension Distinct Element Code (3DEC) discrete element software were used to study the influence of different working-face sizes on the height development of water-conducting fractured zones under this particular geological condition. According to the comparative analysis of laboratory experiments and software simulations, the boundary size of the working face under the full mining of overburden failure was determined, which provides reasonable suggestions about the size of the mine working face. Mine production efficiency can thus be improved further.

2. Engineering Outline

A coal mine main mining 2 # coal seam was studied herein. The coal seam structure was simple: the coal seam angle was 0–4°, with an average of 2°; the coal seam thickness was 0.82–6.05 m, with an average thickness of 3.45 m; and the buried depth was 465–707 m, with an average depth of 586 m. The direct roof of the No. 2 coal seam is mainly siltstone and fine-grained sandstone, with a thickness of 1.50–8.00 m, and the main roof is mainly medium-fine-grained sandstone, with a thickness of 5.00–10.00 m. The lithology is hard, thick, and not easy to fall. The floor of the 2 # coal seam is mostly mudstone, sandy mudstone, or siltstone, with a stable horizon and a thickness of 1.0–2.0 m. The physical and mechanical parameters of the overlying rock are listed in

Table 1. Physical and mechanical parameters of overlying strata.

Serial No.	Lithology of Overlying Rock	Thickness (m)	Bulk Density (kN·m−3)	Tensile Strength (MPa)	Elastic Modulus (GPa)
1	Mudstone	25.0	25.57	2.13	2.64
2	Fine-grained sandstone	6.0	25.52	2.95	6.35
3	Sandy mudstone	12.0	26.39	1.84	4.02
4	Fine-grained sandstone	7.0	25.52	2.95	6.35
5	Mudstone	15.0	25.57	2.13	2.64
6	Fine-grained sandstone	7.0	25.52	2.95	6.35
7	Medium-grained sandstone	3.0	23.39	2.69	4.46
8	Siltstone	7.0	26.18	1.35	4.33
9	Mudstone	4.5	25.57	2.13	2.64
10	Fine-grained sandstone	13.0	25.52	2.95	6.35
11	Siltstone	11.0	26.18	1.35	4.33
12	Coal no. 2	3.5	13.70	0.81	0.89

.

3. Development Characteristics of Overburden Failure Height in Different Working Face Lengths

3.1. Physical Similarity Experiment

(1)

Model design and scheme

This research adopted a plane simulation experiment frame with dimensions of 3 m × 0.2 m × 1.8 m to simulate the overlying strata structure. River sand was selected as aggregate; gypsum white powder was mainly used for cementation; separation between various rock layers was maintained with mica powder; and the lower letters $Y$ and $M$ denote prototype and model, respectively, as follows:

Geometric similarity ratio: $C_l=l_Y/l_M=200$, bulk density similarity ratio: $C_{\gamma }={\gamma }_Y/{\gamma }_M=1.6$, stress similitude ratio: $C_{\sigma }={\sigma }_Y/{\sigma }_M=320$, load similitude ratio: $C_q=q_Y/q_M=3.2\times {10}^6$, and time Similarity Ratio: $C_t=t_Y/t_M=14$.

The size of the model pavement was 2 m × 0.2 m × 1.3 m. The position at 0.5 m from the left boundary of the model was the setup entry, and the position at 0.5 m from the right boundary of the model was the end of the model mining. A total of 2.0 m was mined, and the 0.5 m boundary coal pillar was retained to eliminate the influence of the boundary on the experiment. Figure 1 shows the layout after the model was built. Measuring points were arranged in a total of 12 rows, their perpendicular direction is A–L, from left to right to 1, 2, 3…, during the experiment; the total station was used to monitor the displacement of 270 measuring points.

(2)

Analysis of simulation results

In the tendency similar simulation experiment, the falling phenomenon in the mining process was recorded using a camera at 50 m for each excavation, as shown in Figure 2. The figure illustrates that when the dip length of the working face was 150, 200, and 250 m, the direct caving height was 10 m. When the dip length of the working face was 300 m, the height of the caving zone increased from 10 to 14 m. Previously, the caving was not sufficient due to the thick direct roof. When the dip length of the working face was 300 m, it completely collapsed due to more severe disturbance. At this time, the mining degree of the working face was more sufficient, and there was no room for the caving zone to develop upward. The final height was 14 m.

The rock above the falling zone was hinged to form a “voussoir beam” structure, and numerous axial fissures developed. The larger the length of the working face, the greater the weight of the overburden rock to be carried. At this time, it was mainly carried by the key stratum; thus, the axial fracture extended to the key stratum, which is also called the water-conducting fracture zone. When the inclined length of the working face was 150, 200, 250, and 300 m, the development height was 22, 30, 40, and 46 m, respectively. When the inclined length of the working face was 300, 350, and 400 m, the development height was 46, 47, and 48 m, respectively.

(3)

Discussion

Figure 3 demonstrates that under working faces of six different lengths, the variation characteristics of the height of the water-conducting fracture zones were divided into two obvious stages, and the inflection point of the two stages was the working face with a length of 300 m. The height of the water-conducting fracture zone was 46 m. Compared with the working face with a length of 150 m, the growth rate was 4.54%. The growth rate dropped sharply and became stable, and the influence degree of the working face length was reduced. The height of the caving zone was 14 m, and the growth rate was 40%. Owing to the full collapse of the immediate roof, it no longer continued to increase. The results indicate that the trend of the roof failure height of 300 m working face is abrupt, which is the key point for the developing height to stabilize.

Multivariate linear regression analysis is widely used in various fields. Multiple linear regression analysis was used to verify the accuracy of the above-mentioned research [13]. The formula is as follows:

$$H_f=3.74{\displaystyle \sum M}+37.52b+1.95\ln L+6.84$$

(1)

where $H_f$ is the height of the water-conducting fracture zone; ${\displaystyle \sum M}$ is the cumulative mining thickness of the coal seam, 3.5 m; $L$ is the working face length, 250, 300, 350, and 400 m; $s$ is the mining depth, 586 m; and $b$ is the coefficient of hard rock lithology ratio, $b=\frac{{\displaystyle \sum h}}{\left(15~20\right){\displaystyle \sum M}}$, ${\displaystyle \sum h}$ is the sum of hard rock thickness, calculated value 36.3 m, find $b=0.52$.

According to Formula (1), the heights of 250, 300, 350, and 400 m water-conducting fracture zones are 50.21, 50.56, 50.86, and 51.12 m, respectively. A comparison of experimental and calculated values is shown in Figure 4.

Figure 4 demonstrates that when the length of the working face is less than 300 m, the error between the two is more than 25%. When the length of the working face is 300 m, the error between the two is less than 10%, indicating that the coincidence between the two is high at this time. Moreover, the error between the calculated value and the experimental value is significantly reduced, and a further reduction in trend is observed. It indicates that as the length increases, the degree of mining adequacy also increases, and the development height of the water-conducting fracture zone tends to become more stable.

3.2. DEC Numerical Simulation

(1)

Model construction

The discrete element numerical simulation software 3DEC can be used in various structural forms of overburden strata migration, fracture, and complex mechanical problems. In particular, it is ideal for working-face mining overlying strata migration and water-conducting fracture zone development-law problem simulations. Therefore, according to the research objectives of this study, a numerical model was established as shown in Figure 5. The model size was 500 m × 135 m. The working faces of different lengths (150, 200, 250, 300, 350, 400 m) were excavated within the excavation range, and the monitoring lines were arranged at the heights of 10, 20, 30, 40, 50, and 60 m, respectively, above the coal seam to analyze the quasi-dynamic change of overburden rock caving with the length of the working face.

(2)

Analysis of simulation result

The variation characteristics of the falling height of different strata above the coal seam with the length of the working face are shown in Figure 6. The figure illustrates that the increase in working face length causes different degrees of deformation of overlying strata, and the vertical displacement of overlying strata increases with the increase in working face length; however, the overall increasing trend starts decreasing, and the formed ‘U’ type caving range also increases equidistantly. The subsidence value always tends to stabilize under the length of a 300 m working face, and the maximum value of each layer is 3.43–3.48 m. The maximum length of a 400 m working face is 3.44–3.50 m, indicating that when the length of the working face reaches 300 m, the falling height of the rock layer has reached stability. From the beginning steady state to the final collapse, the change in the subsidence value between the layers does not exceed 0.03, and the increase is only 0.8%. This further proves that the overlying rock falls completely and reaches a stable state.

Figure 7 shows the simulation results, exhibiting that the overburden structure at both ends of the working face produces more cracks. The overall transport range of the bedrock shows a gradient reduction in length and height, and the final shape is a trapezoid. The overburden rock in the middle of the tendency is re-compacted when the surface length is 150 m, the crack propagation height on both sides is 27 m, and the shape is trapezoidal. When mining continues, the crack propagation height is increased to 32 and 42 m, and the overburden rock in the middle of the tendency exhibits a higher compaction width. Some differences are observed in the expansion of fractures on both sides affected by mining disturbances. The expansion of fractures on the right side is obvious, and that on the left side is compressed by the continuous compaction of overlying rock. At the key point of 300 m, the left crack propagation does not change significantly, while the right crack propagation is 50 m and is gradually stabilized. In the subsequent 350 m and 400 m working faces, the right crack propagation height is still 50 m, and the height of the water-conducting fracture zone does not increase significantly.

3.3. Discussion

The development law of the water-conducting fracture zone in Figure 8 can be described as presented below. In the formation stage, the fracture expansion height is $H_1$. In the development stage, the fracture expansion height gradually increases to $H_3$. After reaching the key point of 300 m, the fracture expands to the maximum value $H_4$. When the tendency length continues to increase, it is only reflected in the width, and there is no significant change in the height, with an obvious stable value. Moreover, with the increase in propensity length, there always exist three stages of formation–development–stability.

4. Discrimination and Verification of Working Face Boundary Size under Full Mining of Overburden Rock Damage

4.1. Determination of Full Mining Degree of Overburden Rock Damage

According to the subsidence degree of the surface moving basin, the mining degree can be divided into insufficient mining, sufficient mining, and super sufficient mining [22]. The development of water-conducting fracture zones can also aid in determining the degree of mining in the formation and development stagesis the insufficient mining stage of overburden failure. When the maximum height is reached, it is the sufficient mining stage. In the stable stage, it is the super-sufficient mining stage of overburden failure. The length at maximum height is the working face boundary size, calculated by using the following formula:

Figure 9 shows the relationship between the dip length $L$ and the limit span $l_{jL}$ of the $j$ overburden rock:

$$L=l_{jL}+{\displaystyle \sum _{i=1}^{j-1}{h}_i}\cot {\alpha }_1+{\displaystyle \sum _{i=1}^{j-1}{h}_i}\cot {\alpha }_2$$

(2)

where $h_i$ is the thickness of the $i$ layer; and ${\alpha }_1$, ${\alpha }_2$ denote the front and rear fracture angle of the overburden, respectively. In this experiment, the difference between the fracture angles before and after the overburden is small, thus they are approximately equal, that is, $\alpha ={\alpha }_1={\alpha }_2$, then

$$L=l_{jL}+2{\displaystyle \sum _{i=1}^{j-1}{h}_i}\cot \alpha$$

(3)

When the bulking factor of caving overburden rock approaches the residual bulking factor, the height of free space below the caving strata [23] is expressed as follows:

$$\Delta =M-{\displaystyle \sum _{i=1}^{j-1}{h}_i}\left(k_i-1\right)$$

(4)

where $M$ is mining height and $k_i$ is the residual bulking coefficient of the $i$ stratum.

The fractured rock structure is regarded as a fixed beam, and the maximum deflection is calculated by using Formula (5) as follows:

$${\omega }_{\max }=\frac{5q{l}^4}{384EI}$$

(5)

where $q$ is the load applied to the rock strata; $l$ is the hanging distance; $E$ is the elastic modulus; and $I$ is the section distance.

The fracture of the layer $j$ rock formation in the overlying strata must meet the following conditions [24]. The parameters of the layer $j$ rock formation are brought into Formulas (5) and (6) is obtained, as follows:

$$\{\begin{array}{l}{l}_j>l_{jL}\\ \frac{5q{l}_j^4}{384E_j{I}_j}<M-{\displaystyle \sum _{i=1}^{j-1}{h}_i}\left(k_i-1\right)\end{array}$$

(6)

When coal seam mining thickness and overlying strata lithology and structure are certain, for the layer $j$ rock formation, when its limit span is $l_{jL}$, as long as the following equation is satisfied [25], $l_{jL}$ is the span when the overburden failure reaches full mining:

$$\frac{5q{l}_{jL}^4}{384E_j{I}_j}=M-{\displaystyle \sum _{i=1}^{j-1}{h}_i}\left(k_i-1\right)$$

(7)

By combining Formulas (3) and (7), the working-face size of overburden failure to achieve full mining can be obtained as follows:

$$L=\sqrt[4]{\frac{384E_j{I}_j}{5q}\left[M-{\displaystyle \sum _{i=1}^{j-1}{h}_i}\left(k_i-1\right)\right]}+2{\displaystyle \sum _{i=1}^{j-1}{h}_i}\cot \alpha$$

(8)

4.2. Determination of Boundary Size of Working Face

In order to validate the work-face boundary dimensions, the results presented in

Table 2. Key layer calculation results.

Serial No.	Overburden Lithology	Thickness (m)	Key Layer	Broken Length (m)	Height from Coal Seam (m)
1	Mudstone	25	Main key layer	47.02	85.5
5	Mudstone	15	Key layer 1	42.08	60.5
10	Fine-grained sandstone	13	Key layer 2	38.23	11
11	Siltstone	11	Key layer 3	33.68	0

were calculated using the critical layer discrimination method.

The following discussion can judge the damage of overlying rock, and the front and rear caving angles of overburden correspond to 60°. The residual bulking coefficients of mudstone, fine-grained sandstone, medium-grained sandstone, siltstone, and sandy mudstone are 1.05, 1.08, 1.09, 1.08, and 1.06, respectively. The result calculated by using Formula (8) is 307.6 m. That is, the crack can extend to the maximum height at this length. According to Formulas (3) and (4), the expansion height of the cracks on both sides can be close to the main key layer; however, the influence of the thicker rock layer 1 leads to the final expansion height of 45.5–60.5 m.

5. In Situ Measurement Verification

The accuracy of the experimental results was verified by considering the 207 working face with a length of 300 m. The observation location of the borehole was 3850 m in the 209 return airway. Four boreholes were set up in total. The lengths of the 1 #, 2 #, 3 #, and 4 # boreholes were 120, 106, 110, and 120 m, respectively. The direction of the 1 # borehole pointed to the 209 working face. The monitoring results were mainly used for comparative analysis. The directions of the 2 #, 3 #, and 4 # boreholes pointed to the 207 working face, which was used to detect the leakage of water injection after mining. Figure 10 presents the details of borehole arrangement.

After the processes including water injection–monitoring–extraction of data were carried out according to the construction design, the monitoring data set of drilling water injection leakage was drawn as the leakage curve change diagram, as shown in Figure 11. The monitoring results indicate that the leakage of water injection in the 1 # borehole was small on the whole, and the maximum leakage was 2.501 L·min⁻¹ at the height of 18 m; at this time, the overburden fracture was not developed. The maximum leakage height of the 2 # borehole was 4.556 L·min⁻¹ at 31 m. At this time, the leakage amount increased obviously; that is, the fracture development at this position was obvious and located in the fracture zone. The higher the height of the 3 # hole, the greater the leakage, mainly in the range of 30-50 m where the leakage reached 4.000 L·min⁻¹. Moreover, at the maximum height of 41 m, the leakage was 4.874 L·min⁻¹. Affected by mining, the cracks in this range still developed. The overall water injection leakage of the 4 # borehole exhibited two trends with the increase in vertical height. The maximum value was 4.117 L·min⁻¹ in the height range of 20–66 m, and gradually decreased in the height range of 50–60 m. The average value of 55 m was the final height of the water-conducting fracture zone.

The research condition presented in this study is the geological condition of hard rock below 3.5 m mining height. The critical dimension is 300 m by experiment and 307.6 m by mechanical model, about 0.5 (average mining depth), overburden failure to fully mining. The critical size of overburden failure reaching full mining can be calculated by using Formula (8). According to the research results presented herein, the recommended value of reasonable size of working face is 0.5–0.7 $H$.

6. Conclusions

(1)

Physical similarity simulation experiments and 3DEC numerical simulation results indicate that the development height of the water-conducting fracture zone is 46 and 50 m when the working face length is 300 m. Moreover, the development height is gradually stabilized with the increase in the working face length, and their regularity is highly consistent. Multiple linear regression analysis indicates that when the length of the working face is 300 m, the error value suddenly drops to less than 10%, thus, this length is an important turning point.

(2)

The experimental results and mining damage theory reveal that when the water-conducting fracture zone develops to the highest stage, the overburden failure is the fully mining stage. According to the established calculation formula of working-face boundary size, the working-face boundary size is 307.6 m, which is highly consistent with the experimental results, and the development range of height is determined to be 45.5–60.5 m.

(3)

The height development of actual water-conducting fracture zones in mines is detected. The detection result is 55 m, which conforms to the calculation results and the simulation results. The reasonable working face size under the local conditions is in the range of 0.5–0.7 $H$.