Study of the Evolution of Water-Conducting Fracture Zones in Overlying Rock of a Fully Mechanized Caving Face in Gently Inclined Extra-Thick Coal Seams

Abstract

To study the caving of thick hard overburdens and evolution of water-conducting fracture zones in fully mechanized top-coal caving faces of gently inclined extra-thick coal seams, we comprehensively analyzed the 8103 working face of the Beixinyao Coal Mine. We investigated to the caving characteristics of thick hard overburden in fully mechanized top-coal caving faces, fracture information of the internal structure of overburden, and development heights of the “two zones” of overburden after coal mining. Our research methods included those of similarity simulation experiments, such as the use of microseismic monitoring systems, numerical simulations, theoretical analysis, and engineering practice. The results showed that the overlying strata generally experienced stages of roof caving, crack formation, delamination, crack development, and surface subsidence. Due to the influence of overlying strata movement and mining, the separation layer experienced an evolution process called “emergence-development-closure”, where the height of the overlying strata caving envelope increases with the advancing of the working face. When full mining was achieved, the overlying strata caving height was stable, and the height development range of the water-conducting fracture zone was 100–120 m, which is consistent with the height of the overlying strata caving envelope. Most microseismic events occurred near the water-conducting fracture zone, and the water-conducting fracture zone was formed in an area with concentrated energy density. In our numerical simulation, the concentrated distribution area of the fracture field was characterized by a “bridge arch”. The fracture development model in the middle of the goaf was higher than at both ends of the working face, and roof strata deformation was obvious. When the energy value of microseismic event reaches 108.708 J, cracks are produced, and these cracks gradually penetrate to form water-conducting fracture zones. Engineering practice showed that the height range of the water-conducting fracture zone was 98–123 m, and caving of the thick hard overburden and evolution of the water-conducting fracture zone in a fully mechanized top-coal caving face provide a scientific basis for water prevention and control.

1. Introduction

The exploitation of underground mine resources will cause the collapse and destruction of overlying strata in the working face, thus forming water-conducting cracks in overlying strata [1,2], increasing water inrush from the mine roof [3,4,5], and seriously affecting the safety and production of the mine [6,7,8]. Therefore, it is urgent to study the development of overburden fractures caused by fully mechanized top-coal caving mining.

For the scientific formulation of safety measures, it is important to determine the laws of overburden caving and water-conducting fracture zone development [9]. Xu Jialin et al. [10] studied the height prediction method of water-conducting fractured zones based on the key stratum theory and observed the structural characteristics of the key stratum of the overburden under specific mining conditions. Hu Xiaojuan et al. [11] analyzed the relationship between the height of water-conducting fracture zone development and various influencing factors. Wang Xiaozhen et al. [12] argued that different structural stabilities of the main key stratum would lead to different forms of water-conducting fractures in the upper part of the main key stratum. Liu Yingfeng et al. [13] investigated the development of water-conducting fracture zones using borehole TV combined with a simple hydrological observation method. Gao Baobin et al. [14] observed the development of water-conducting fracture zones by combining theoretical calculations with field measurements. Lai Xingping [15] measured the characteristics of water conduction in overlying strata fissures and fissures in the working face through the changes of overlying strata in fully mechanized top-coal caving mining. Many scholars have studied the laws of fracture development caused by coal seam mining and other influencing factors. However, the existing research focuses on theoretical analysis. We use various methods of physical and numerical simulations for a comprehensive understanding of the development of overlying rock fractures and conduct field verification.

A water-conducting fracture zone is formed due to a water inrush phenomenon caused by the collapse of overlying strata in the mining face [16]. The height of the water-conducting fracture zone is affected by many factors, including coal seam buried depth, coal seam dip angle, working face width, coal seam thickness, physical and mechanical properties of coal rock, etc. The greater the buried depth, dip angle, working face width, coal seam thickness, and coal rock hardness, the more obvious the fracture development is. It is difficult for a single evaluation to describe the developmental process of water-conducting fractures due to mining. These fractures will affect the safe mining of coal mines [17,18,19]. In recent years, with the continuous maturing of microseismic monitoring methods in China [20], microseismic monitoring technology has been widely used in the coal mine industry [21,22,23]. Microseismic technology was used for early warnings of rock bursts in the past [24,25,26], and now, microseismic technology can be used to monitor the formation of water channels in the roof and floor [27,28].

As noted above, scholars have made a series of achievements in microseismic applications [29,30,31,32,33,34,35]. However, there is little correlation between microseismic monitoring and development of internal fractures in water-conducting fracture zones. Based on the study of the 8103 working face in the Beixinyao Coal Mine, we conducted a similarity simulation experiment to analyze the characteristics of overburden caving and its development of delamination. The characteristics of overburden fissure development were inversed by the distribution laws of energy events obtained from microseismic monitoring. Using a numerical simulation experiment, we clarified the distribution of the overburden fracture field and fracture development characteristics. Then, quantitative analyses of the microseismic event energy and water-conducting fractures were carried out, and the height of fracture development was verified. The purpose of this study was to provide a scientific basis for water prevention and control in a fully mechanized top-coal caving face under conditions of thick hard overburden caving and continuous crack development.

2. Engineering Background

The Beixinyao Coal Mine adopts a section upward fully mechanized top-coal caving mining method, mainly mining No. 2 coal. The first mining face in the mine is numbered 8103, with an inclination of 19–25° and an average inclination of 22°. The total thickness of the coal seam is 4.3–7.0 m, with an average thickness of 5.6 m and an average buried depth of 322.85 m, of which the bedrock thickness is 310.81 m and the loose soil layer thickness is 47.69 m According to the hydrogeological exploration report, the Ordovician limestone water level elevation of this lane is about 1060 m, the thickness of Ordovician limestone water and the water-resisting layer of No. 2 coal seam is 120 m, and the water inrush coefficient is 0–44 KPa/m. Ordovician limestone water generally does not affect the No. 2 coal seam, but there is a possibility of structural water inrush in incomplete areas of the coal seam floor. The normal water inflow of the 8103 working face is 260 m³/h, and the maximum water inflow is 380 m³/h. There are several aquifer groups in the upper part of the No. 2 coal seam, and the positional relationship among the main coal seam, aquifer, and water-resisting layer is shown in Figure 1.

The main aquifer in the upper part of No. 2 coal is K2, K3, and K4 sandstone-fractured aquifer, with weak to medium water abundance, which is a direct water-filled aquifer of No. 2 coal. There is a stable water-resisting layer between aquifers, and the runoff conditions are generally stable. Affected by the roof sandstone-fractured aquifer, the mine water inflow from the roof has reached 100 m³/h from mine development, which seriously affects mine safety and production.

The direct roof or main roof of No. 2 coal is medium-fine and medium-coarse grained sandstone, with a thickness of 1.10–22.81 m, and a gray and hard lithology. Its compressive strength is 45.60–98.40 MPa, with an average of 76.32 MPa, and it is a semi-hard hard rock. The direct roof or pseudo-roof is mudstone and sandy mudstone, with a thickness of 0–17.25 m, a gray and gray-black soft stone, and its compressive strength is 19.60–30.00 MPa, with an average of 24.80 MPa; this means it is a weak rock.

3. Similarity Simulation Experimental Study on the Law of Overlying Strata Caving

3.1. Similarity Simulation Experimental Design

Underground rock mass is a complex mechanical medium. During the mining process, the surrounding rocks at different positions will have different deformations, as well as discontinuous fracture and caving processes, which makes theoretical calculation extremely difficult. Field measurements are also limited by manpower, material resources, and actual conditions. The essence of the similar model test is to make use of materials with similar mechanical properties to the prototype, shrink them into a model according to a certain proportion, test them, and observe the stress, strain, displacement, and damage of the model to analyze and make predictions for the actual situation of the prototype.

Based on the stratum characteristics and mining parameters of the No. 2 coal seam 8103 working face in the Beiyao Coal Mine, the physical similarity simulation experiment was designed using model experiment technology based on similarity theory. The elevation of the 8103 working face was 918.7–1063.8 m, ground elevation was 1275.5–1352.7 m, and the buried depth of the working face was 211.7–434 m. To fully reflect the developmental height of overlying rock fractures and characteristics of surface subsidence of the first mining face of No. 2 coal seam, the experiment was carried out according to the average buried depth of the No. 2 coal seam. The 11.96 m thick siltstone at 8.63 m above the coal seam with a 17.15 m loose soil layer was taken as the main roof, the 11.23 m thick fine-grained sandstone at 9.6 m above the main roof was the sub-key layer of overburden, and the 23.32 m thick fine-grained sandstone at 42.02 m above that was the main key layer of overburden. The geometric similarity ratio of the model was 1:200 (model: actual). In the experiment, a plane strain model frame with overall dimensions (length × width × height) of 3.0 m × 0.2 m × 2 m was adopted, and the paving dimension (length × width × height) of the model was 3.0 m × 0.2 m × 1.70 m.

The geometric similarity ratio α_L = L_H/L_M of the simulation experiment was determined to be 1:200. According to the similarity theorem [36], the time similarity ratio α_t = α_L^1/2 was 1/14, Poisson’s ratio or the internal friction angle similarity ratio was 1, the stress similarity ratio α_σ = γ_H/γ_M × α_l was 1/300, and the pressure similarity ratio α_p = γ_H/γ_M × α_l³ was 1/1.2 × 10⁷. According to the physical and mechanical parameters of the coal and rock at the working face, the material ratios of the physical model rock stratum simulation was calculated and prepared accordingly.

3.2. Microseismic Monitoring Scheme

Microseismic monitoring is widely used as a means to better monitor overlying strata failures and energy evolution under the influence of mining in the working face. It is useful for the accurate identification of rock fracture signals in the advancing of the working face, and monitoring of the temporal and spatial distributions of overlying strata fractures and the change in energy released. For this reason, microseismic equipment was arranged for the physical simulation experiment to monitor the whole mining process. The equipment included a new generation microseismic monitor (SOS microseismic monitor) designed and manufactured by the Institute of Mining Seismology, Polish Institute of Mining Research. The SOS microseismic monitoring system adopts a 0.1–600 Hz vibration velocity type mine seismic measuring probe with an embedded signal transmission module, and the sensor sensitivity was 50–15,000 mas/m; The maximum sampling frequency was 2500 Hz.

Using an independent trunk data transmission system for bidirectional control transmission can help remote monitoring and debugging of the working state of the measuring probe. The system can record the complete waveform of the mine earthquake signal, analyze the fracture information of the overlying strata on the working face, and describe the spatial rock structure movement and the migration and evolution laws of the stress field. The microseismic monitoring system was used to monitor the magnitude, frequency, and location of energy released when the overburden was damaged. Six microseismic monitoring sensors were laid in the model, and the sensors were numbered #16, 3, 9, 13, 7, and 5, counterclockwise. The arrangement of microseismic monitoring sensors is shown in Figure 2.

3.3. Characteristics of Overlying Strata Caving and Delamination Evolution

Similar experiments have observed the caving characteristics of overlying strata after mining in the 8103 working face, and then analyzed the development of the water-conducting fracture zone by studying the caving process of overlying strata. Figure 3 shows the caving characteristics of the overlying rock after mining of the 8103 working face. After mining of the working face, overburden caving forms a breaking line and reaches a critical point, and the overburden migration height is 160 cm (320 m). The overburden rock of the 8103 working face was disturbed by mining, resulting in delamination and fractures, which led to the formation of a water-conducting fracture zone.

During mining in the working face, the overburden separation layer mainly showed a change in shape and development height. As shown in Figure 4, the evolution of the mining separation layer in the 8103 working face shows the height of the water-conducting fracture zone in the working face reaches about 50 cm from the open cut, and the water-conducting fracture zone above the working face continues to develop after mining ends, stopping when the height reaches about 60 cm. With the advancing of the working face, the upper roof cannot completely fill the goaf, and the upper strata of the goaf move asynchronously, resulting in bed separation. With the advancing of the working face, the mined-out area is continuously filled, and the range of the height and length of the crack increases. The middle part of the broken rock mass is hinged, forming a “triangular” separation zone with the upper unbroken rock mass. Compared with the previous separation, the separation volume is obviously increased. As the working face continues to advance, the development height of the “triangle” separation zone extends to 73.4 cm, and begins to decrease gradually. As the hard main key layer plays a good supporting role, the separation layer does not continue to develop upwards, but develops into a “basin” at the bottom of the main key layer. At the bottom of the main key layer, the abscission layer experiences the evolution of “emergence-development-closure”.

The evolution of overlying strata caving is shown in Figure 5. The height and range of overlying strata caving are constantly expanding with periodic weighting, broken rock blocks are transformed into piled rock blocks, and undamaged rock strata form a new envelope of overlying strata caving. With development of the mining face, the range of the overlying strata caving envelope increases. When the working face advances to 51.8 cm, the caving height of overlying strata reaches 10.3 cm; when the working face advances to 73.4 cm, the overlying strata continue to develop upwards and collapse violently, and the development height of fissures reaches 43.8 cm. When the working face is 141.8 cm, the development height of the water-conducting fracture zone reaches the maximum, and does not continue to develop upwards, with a height of 73.4 cm. When full mining is achieved, the height of overburden fracture stability is 73.4 cm, which is similar to the height of abscission layer development shown in Figure 4.

3.4. Location and Energy Characteristics of Microseismic Events in the Process of Overlying Strata Caving

The microseismic monitoring system monitors microseismic events in the entire excavation process in real time and reveals the gestation process of the water channel through research on microseismic events and energy distribution.

Figure 6 shows the microseismic events and the cloud image of energy distribution in the entire excavation process. As shown in Figure 6a, the dots represent microseismic events, and their colors represent the energy of the microseismic events. Cracks are continuously formed as mining takes place at the working face. In this process, microseismic events are gradually generated and mainly concentrated in the key strata above the mining face (including sub-key strata and main key strata), and a large number of microseismic energy events are mainly concentrated in the range of 50–60 cm above the coal seam, indicating that mining of the working face has a great impact on the disturbance in this area, and most of the microseismic energy are green and blue events. The energy of a single microseismic event is mainly distributed in the range of 0–100 J. There are also a few yellow and red events in the whole mining process, and most of the high-energy microseismic events (red events) are distributed in the later stages of the mining process. Therefore, special attention should be paid to microseismic monitoring in later stages of mining. As shown in Figure 6b, microseismic events are concentrated in energy-intensive areas from the mining of the working face to the end of the mining face, forming a downward semi-arch. In the process of overburden caving in the working face, the key stratum plays a key role in bearing the abutment pressure of the upper strata, and it is precisely because of the existence of the key stratum that the abutment pressure of the working face is reduced. As shown in the figure, the focal line with concentrated energy is 50–60 cm in height. As a result of this large amount of energy accumulation, the rock stratum fractures, and the fractures develop and penetrate the rock, eventually forming a water-conducting fracture zone.

With the fracturing of key strata, the phenomenon of weighting in the working face is obvious. In the actual mining process, attention should be paid to the influence of key strata caving on the working face. The water-conducting fractured zone develops upwards with the expansion of the mining area, and with the advancing of working face, the mining area expands. When the mining reaches a certain range, the height of the water-conducting fractured zone reaches a maximum. After that, the height of fractured zone tends to become stable and generally does not develop upwards.

4. Numerical Simulation of Developmental Range of Water-Conducting Fracture Zone

4.1. Numerical Calculation Model Construction

In order to understand the development of water-conducting fracture zones considering the dip angle effect, 3DEC numerical simulation software was used to simulate fracture and overburden evolution during mining. The numerical simulation model is shown in Figure 7. The overall dimensions of the model (length × width × height) were 400 m × 600 m × 340 m. The inclined strata from bottom to top in the model are defined as the 1st to 24th strata in turn, with the 3rd floor being the No. 2 coal seam of 5.6 m. The first and second layers at the bottom of the model are the bottom strata of the coal seam, which are the bottom medium-grained sandstone and mudstone of the coal seam floor, respectively. See

Table 1. Mechanical parameters of the coal seam and its roof and floor.

Rock Character	Density/(kg/m3)	Bulk Modulus/GPa	Shear Modulus/GPa	Internal Friction Angle/(°)	Cohesion/MPa	Tensile Strength/MPa	Compressive Strength/MPa
Siltstone	2637	12.6	1.98	36.20	4.2	1.16	18.9
Coarse-grained sandstone	2621	23.2	3.75	28.45	14.6	1.68	71.3
Sandy mudstone	2572	5.8	1.37	33.32	9.4	0.97	15.8
Medium-grained sandstone	2619	13.2	1.98	36.29	11.6	1.28	39.9
Fine grained sandstone	2526	17.8	2.53	36.29	10.1	1.67	63.0
Mudstone	2631	10.6	2.03	43.02	0.4	1.13	7.5
No. 2 coal seam	1360	5.44	2.37	30.37	2.35	1.98	13.26

for the physical and mechanical parameters of each stratum. The initial boundary design of the model is very important in the numerical simulation experiment. We referred to the initial boundary setting elements of the models used in many studies [37,38,39]. A certain gradient stress was applied in the horizontal direction of the model to make it meet the initial condition of a measured horizontal stress 0.45 times that of the vertical stress. As this numerical calculation model was simulated to the earth’s surface, no boundary stress was applied to the upper surface of the model. The initial conditions set the gravity to be 0 and the acceleration of gravity to be 9.8 m/s². Except for the upper surface of the model which was not bounded, the allowable deformation of the other five boundary surfaces was set to be 0.1 m to limit the lateral and bottom displacements of the model.

After the model was built, the mechanical parameters of each rock layer were inputted, and the coal seam in the working face was continuously excavated in a distributed manner. The left and right coal pillars of 40 m each would be excavated at 40 m from the left boundary, and the mining would be finished at 40 m from the right boundary.

4.2. Analysis of Overburden Fracture Field

By analyzing overburden movement and the caving process in the 8103 working face of a 3D model, the developmental height and range of the water-conducting fracture zone in the working face were studied. The evolution of the 3D-model overburden is shown in Figure 8.

Figure 8a shows the distribution characteristics of the fissure field when the working face was mined to 167 m. At this point, the upward expansion of the fissure field in the roof strata was particularly obvious, and the concentrated distribution area of the fissure field formed a “bridge arch”. The fissure development model in the middle of the goaf was higher than both ends of the working face, and the deformation of the roof strata was obvious, mainly concentrated within the model widths of 22.5–177.5 m and lengths of 10–177 m. The height of the water-conducting fissure zone in the middle of the goaf was the highest, about 97.2 m, and the lower fissure field at both ends of the goaf was about 94.4 m Although there were tiny fissures above the model, a continuous fissure field was not formed.

As shown in Figure 8b, the evolution of overlying strata at the end of mining shows that the height of the fracture field in roof strata is stable, and the concentrated distribution area of the fracture field is still in the shape of a “bridge arch”, mainly within model widths of 22.5–177.5 m and lengths of 10–344 m. The height of the water-conducting fracture zone in the middle area of the goaf was the highest, about 122.3 m. At this point, the water-conducting fracture zone was stable.

4.3. Analysis of the Developmental Process of the Water-Conducting Fracture Zone

Figure 9 shows the evolution of the water-conducting fracture zones in the 8103 mining face, in which the three red envelope curves are water-conducting fracture zones with the first square, second square, and third square, in turn, and the other black envelope curves are water-conducting fracture zones with the mining times of 50, 100, 200, 300, 400, and 520 m, in turn. Analysis of the development height and range of the water-conducting fracture zone in the process of mining of the 8103 working face revealed that the roof of the L₀ working face collapsed for the first time when it was mined to 50 m, and the cracks in overlying strata visibly developed into the water-conducting fracture zone for the first time with a development height ranging from 24.3–28.1 m, which is about 4.34 to 5.02 times the mining height.

During the advancement from 0–200 m, the development of the water-conducting fractured zone gradually progressed with the advancing of the working face, and the overlying strata visibly expanded upwards, especially during mining from 50–100 m, with the maximum development height gradually increasing from 28.1 to 89.4 m. In this stage, the upward expansion speed of the fractured zone was 1.23 m, that is, the average fractured zone was the largest per 1 m advancement during the mining process from 50–100 m. L₁ was pushed 300 m in the working face, and the development height of the water-conducting fracture zone was basically stable at this time. After the working face was fully mined at 300 m, the water-conducting fracture zone was basically stable, with a height of 101.3–120.7 m, which is about 18.09–21.55 times the mining height. From L₂ to the end of mining in the working face, the development height of water-conducting fracture zone is 101.5–122.3 m, which is about 18.13–21.84 times the mining height.

The development trends of the water-conducting fracture zone in the mining process are shown in Figure 10. During the mining process of 50–100 m in the first mining face, the fracture expands rapidly, and the maximum development height gradually increases from 28.1 to 89.4 m. In the process of mining between 100–300 m, the upward propagation speed of fractures decreased, and the maximum development height gradually increased from 89.4 to 120.7 m. In the process mining between 300–520 m, the upward propagation speed of fractures was almost zero, and the maximum development height gradually increased from 120.7 to 122.3 m.

5. Quantitative Relationship between Microseismic Event Energy and Water-Conducting Fractures

The distribution law of microseismic events was obtained by microseismic monitoring and numerical simulation. In order to obtain the quantitative analysis of the water-conducting fracture zone, according to reference [40], the stress intensity factor K₁ was obtained as follows:

$$K_1={\sigma }_{\mathrm{y}}\sqrt{\pi a}$$

(1)

In the formula, σ_y is the stress component perpendicular to the crack direction at the crack front end, according to the numerical simulation; a is half the length of the crack.

Numerical simulation calculations show that the strength of σ_y was 19.8 MPa when the fracture occurred; by measuring the crack length, we know that the length of a is 20 mm. Therefore, the stress intensity factor K₁ can be obtained as:

$$K_1=156.915\left(MPa\cdot \sqrt{\mathrm{mm}}\right)$$

(2)

The relationship between stress intensity factor and energy release rate is:

$$G_1=\frac{K_1{}^2}{\overline{E}}$$

(3)

In the formula, Ē is the elastic modulus of the rock formation when cracks occur. The previous sections of this paper determined the range of the water-conducting fracture zone. When the height ranges from the coal seam were 40–70 cm (rock layer 16–rock layer 20), the average value of the elastic modulus was 9.06 GPa.

The energy release rate is calculated as:

$$G_1=2.7177\left(\mathrm{N}/\mathrm{mm}\right)$$

(4)

Finally, the energy E₁ generated by the crack tip (that is, the crack in the rock layer) was obtained as:

$$E_1=108.708J$$

(5)

It can be concluded that when the energy of microseismic events is greater than 108.708 J in similar simulation experiments, cracks will be produced, that is, E₁ = 108.708 J, and cracks will continue to develop into water-conducting fracture zones.

To better study the location of the cracks and judge the development height of the water-conducting fracture zones,

Table 2. Statistical table of crack position in physical model.

Advancing Degree/cm	Distance from Coal Seam Height/cm	Distance from Open Cut/cm	Microseismic Energy/J
51.8	3.41	2.45	131.036
73.4	163.64	45.38	176.599
	125.69	60.24	171.957
	48.09	58.55	159.414
87.8	69.14	51.61	388.292
91.4	235.98	20.5	388.243
102.2	40.81	31.09	341.704
120.2	192.14	61.2	186.669
138.2	102.41	59.43	152.561
149	178.27	58.78	150.0
	174.74	56.07	453.769
156.2	174.85	51.66	334.945
159.8	122.75	54.93	338.023
195.8	189.71	60.42	113.029
199.4	224.12	24.52	168.143
	237.54	5.08	145.049
	174.54	46.43	125.752
203	145.75	53.14	431.985
228.2	88.02	59.14	195.340
	220.79	23.8	167.633
	81.39	55.02	359.605
235.4	17.17	28.42	123.098
	244.15	38.92	338.829
	217.4	58.89	345.065
239	221.27	27.52	169.879
249.8	166.95	59.84	146.569
	234.81	38.35	414.688

shows the specific locations of the cracks and the magnitudes of microseismic energy. It can be seen from Table 2 that the propulsion degree is between 51.8 and 249.8 cm, and all the microseismic events meet the conditions for cracking, of which 50% are big events [41]. From the location of microseismic events the development height of water-conducting fracture zone was determined to be between 51 and 61 cm.

To study the specific location of cracks in overlying rock structures, the microseismic events of cracks in the physical model are summarized. The microseismic energy envelope is drawn as shown in Figure 11. The microseismic events in the figure are connected to form the semi-arch shown in the figure, and the cracks are generated and continuously penetrate to form the water-conducting channel, that is, the formation of the water-conducting fissure zone. Compared with the energy concentrated source line, the development position and height of the water-conducting fissure zone were better verified.

According to the real-time monitoring of microseismic monitoring equipment, the total energy and frequency of microseismic events when cracks are generated in each working face excavation are summarized. There were 19 cycles of weighting at the end of mining in the working face, as shown in Figure 12. These were measured when the excavation was 51.8, 73.4, 87.8, 102.2, 120.2, 138.2, 149, 249.8, 203, 228.2, 235.4, and 138.2 cm.

6. Engineering Practice Verification

6.1. Empirical Formula Calculation

The roof of No. 2 coal seam is hard rock, and there are two empirical formulas for the maximum height (H_f) of the water-conducting fracture zone. Empirical Formula (6):

$$H_f=\frac{100\Sigma m}{1.2\Sigma m+2.0}\pm 8.9$$

(6)

Empirical Formula (7):

$$H_f=30\sqrt{\Sigma m}+10$$

(7)

In the formula: m is the mining thickness of coal seam, and the unit is m; H_f is m, and the average thickness of coal seam is 5.6 m.

According to Formula (6), the height range of the water-conducting fracture zone is 58–73 m; according to Formula (7), the height of the water-conducting fracture zone is 81 m. The height of the water-conducting fracture zone calculated by empirical formula is relatively low compared with the height studied above, which is due to the complexity of the stratum space, persistence of mining disturbance, influence of non-isotropic body of mine rock mass and its complex structure, differences in physical and mechanical properties, and effects of geological structure. Therefore, the height of the water-conducting fracture zone of overlying rock was different from the actual site.

6.2. Engineering Practice Verification

For on-site monitoring of the 8103 working face, a borehole TV was used to preliminarily analyze peeping in the borehole, and borehole position was determined according to topographic conditions, construction conditions, borehole protection requirements, and factors that did not affect coal mine safety production. The development height of the two zones was explored. It was planned to start drilling hole D2 into the ground of the 8103 working face for post-mining observation. As mining started in the 8103 working face, the exploration drilling hole was arranged in the second half of the working face, and the final hole depth was 296.75 m.

The observation characteristics of drilling leakage could be roughly described in four stages, as shown in Figure 13. In the first stage, the hole depth was 0–55.6 m, drilling leakage was relatively small, and drilling fluid returned normally. In the second stage, the hole depth was 55.6–170.15 m, drilling leakage suddenly increased, and the return water of drilling fluid visibly decreased. In the third stage, the hole depth was 170.15–238.76 m, and drilling leakage increased by leaps and bounds. The drilling fluid did not return water at 177.02 and 194.06 m, and the leakage and footage per unit time increased four times compared with the second stage. With the continuous increase of drilling footage, the leakage per footage per unit time fluctuated constantly, showing a law of oscillation. In the fourth stage, the hole depth was 238.76–296.75 m, drilling leakage reached a maximum, and the drilling fluid did not return water. According to the drilling leakage and return water of drilling fluid, the height range of the water-conducting fracture zone was within 125.6 m. This is consistent with the results of microseismic monitoring and the numerical simulation.

In order to better analyze the height of the water-conducting fracture zone in the Beixinyao Coal Mine and study the development of internal rock fractures, the peephole diagram of the D2 borehole was analyzed. The layout of the D2 borehole is shown in Figure 14 below, with boxes a, b, c, and d indicating the heights above the coal seam. Affected by the mining of the first mining face, different levels of overlying strata of No. 2 coal were damaged to different degrees: the hole wall was seriously damaged, and the coring rate was decreased. There were many vertical and oblique fractures in the pore strata, which are wide, and the primary horizontal bedding had different development, expansion, and even delamination. There was a serious block drop in the hole. According to the peephole diagram of overburden failure at about 80 m above the coal seam, it was observed that there were many cracks in the hole and the cracks tended to develop upwards. When the hole reached 123 m above the coal seam, the cracks gradually decreased, and when the hole reached 128 m above the coal seam, the cracks were completely closed and the hole wall was relatively complete. When the borehole reached 123 m above the coal seam, the fracture gradually closed, and the water-conducting fracture zone stopped developing. The results show that the development height of the water-conducting fracture zone is in the range of 98–123 m, which is consistent with the research results and actual measurements.

To sum up, the height range of the water-conducting fractured zone was 98–123 m, and the height of the water-conducting fractured zone was 17–22 times that of the mining height, which proved the accuracy of our results from microseismic monitoring, the 3DEC numerical simulation, and theoretical formula derivation. The combination of similarity simulations, microseismic monitoring, numerical simulations, and theoretical formulas, compared with field practice, made accurate predictions on the development height and prevention measures of the two zones, and effectively realized the safe mining of coal mines.

6.3. Analysis on the Difference of Development Range of the Water-Conducting Fracture Zone

To explore the development of water-conducting fractured zones, different height ranges of water-conducting fractured zones were obtained by comparing different methods, and the height comparison of various water-conducting fractured zones is shown in Figure 15. In this paper, the collapse of overlying strata was seen by using the real photos of similar simulation experiments, and the height range of the water-conducting fracture zone was obtained. The location and time of microseismic events were obtained by the microseismic monitoring system, and the height of the water-conducting fracture zone was 100–120 m from the microseismic energy distribution cloud chart. A three-dimensional model was established by using the numerical simulation software 3DEC. By observing fracture development, it was judged that the development height of water-conducting fracture zone was between 101.5 and 122.3 m; the energy absorbed during crack development could also be used to locate the crack, and the height range of the water-conducting crack zone was 102–118 m. According to the empirical formula of the water-conducting fracture zone, the height was calculated to be 58–81 m; on-the-spot monitoring showed that the height of the water-conducting fracture zone was between 98 and 123 m, based on the TV peep chart of the borehole and the monitoring and analysis of water inflow. Through the analysis of the height of the water-conducting fracture zone by the above research methods, the reliability of similarity simulation experiments, microseismic monitoring, numerical simulations, and theoretical analysis was validated, and these methods can be used to provide scientific guidance for field work.

In this paper, aiming at the prediction of mine water inrush and the prevention and control of mine water disaster, through comprehensive analysis using various methods, the aim was to investigate the evolution of a water-conducting fracture zone in a fully mechanized top-coal caving face of a gently inclined extra-thick coal seam. First, through the comprehensive application of a physical simulation experiment, numerical simulation experiment, and theoretical analysis, we observed the characteristics of overlying strata caving, the distribution law of the fracture field, and characteristics of fracture development. Then, we obtained the critical energy value of overburden cracks through theoretical derivation and formed a quantitative analysis method based on microseismic event energy inversion of water-conducting cracks. These research and quantitative analysis methods can be applied to the research of water prevention and control in similar coal seams with thick and hard overburden caving and continuous cracks.

7. Conclusions

(1)

In the physical simulation experiment, through microseismic monitoring, the magnitude and location of microseismic event energy were analyzed. Based on the microseismic energy distribution cloud chart, the area of water-conducting fracture zone was divided. After fully mining, the overburden caving height was stable, and the height development range of the water-conducting fracture zone was 100–120 m, which is consistent with the height of the overburden caving envelope.

(2)

Using 3DEC numerical calculation and similarity simulation theory and technology, we found that the concentrated distribution area of the fracture field formed a “bridge arch”, with the fracture development model in the middle of the goaf being higher than at both ends of working face, and roof strata deformation was obvious. The development height of the water-conducting fracture zone was 101.5–122.3 m, which is about 18.13–21.84 times that of mining height.

(3)

From the stress intensity factor, we deduced that when the energy of crack expansion and development reaches 108.708 J, the crack will form and continuously penetrate to form a water-conducting channel, that is, a water-conducting fissure zone. Compared with the focal line with concentrated energy, the development position and height of the water-conducting fissure zone were better verified.

(4)

We performed on-the-spot monitoring of the 8103 working face. A preliminary analysis was made by peeping through the hole with a drilling TV, and the hole position was comprehensively determined according to the topographic conditions, construction conditions, drilling protection requirements, and factors that did not affect the safety production of coal mine. The development heights of the two zones were explored. According to the drilling leakage and the return water of drilling fluid, the height of the water-conducting fracture zone was found to be between 98 and 123 m, which was consistent with the results of microseismic monitoring and numerical simulation.