Study on Height Development Characteristics of Water-Conducting Fracture Zone in Fully Mechanized Mining of Shallow Thick Coal Seam under Water

Abstract

The height of water-conducting fracture zone (HWCFZ) is one of the important technical parameters for water-preserved coal mining. The purpose of this paper is to acquire information about the height development characteristics of water-conducting fracture zone (WCFZ) in fully mechanized mining of shallow thick coal seam under water body in western mining area of China. The 91,105 fully mechanized mining face of Daheng coal mine under composite water body was taken as the research object, the development height, morphological characteristics, development and evolution process of WCFZ in working face mining were studied through underground up-hole water injection method by intervals, borehole TV and numerical simulation. The results show that the HWCFZ in 91,105 fully mechanized mining face is 52.7~53.6 m, and the fracture mining ratio is 12.55~12.76. The final development form is saddle-shaped with “large at both ends and small in the middle”. It is accurate and reliable to determine the development characteristics of overburden fractures and the HWCFZ by the field measurement of the combination of underground upward hole segmented water injection method and borehole TV. The development height of the water-conducting fracture zone obtained by numerical simulation is consistent with the field measured results. The development and evolution of the height of WCFZ presents four stages: “development–slow increase–sudden increase–stability”. When the WCFZ develops to a certain layer, the cracks generated by the weak strata in the fracture zone of overlying strata on the working face will automatically close with the advancement of the working face, resulting in “bridging phenomenon”, which inhibits the further development of the WCFZ. That is, the existence of soft rock with a certain thickness in overburden will become the key inhibiting layer for the development of WCFZ, effectively blocking the communication between water-conducting fracture and overlying aquifer. The research results are intended to provide guidance for the implementation of water preserving mining and ecological environment protection in ecologically fragile areas in western China.

1. Introduction

With the westward shift of China’s coal industry strategy, the coal production in the western part of China has increased year by year. However, the western mining areas suffer from water shortage and fragile ecological environment. The negative impact of large-scale and high-intensity coal mining on water resources and ecological environment is much greater than that of the eastern mining areas [1,2]. Maintaining the coordinated development relationship between secure and efficient coal mining and water conservation has become the main task in achieving green mining in western mining areas of China [3,4,5,6]. Accordingly, the western mining areas should give priority to water preserving mining and focus on the protection of ecological environment. The development height of water-conducting fracture zone (HWCFZ) is one of the important preconditions for the safety analysis of water preserving mining [7,8]. Therefore, it is essential for scientific guidance on the implementation of water preserving mining and ecological protection in western mining areas of China.

Predecessors have done a great deal of research on the development characteristics of WCFZ in coal seam mining in western mining areas of China. Based on the achievements of water preserving mining technology in western mining area, Fan et al. [9] proposed the prediction model of the development HWCFZ under certain mining height conditions in Yushenfu mining area. Xu et al. [10,11] proposed a method to predict the HWCFZ based on the location of key strata, which has been verified by the practice of water preserving mining in the mining areas such as Shendong and Yushenfu. Lai et al. [12] used physical similarity simulation to analyze the fracture evolution characteristics of overburden fracture and failure height of overlying strata in three soft coal seams from Dananhu No.1 coal mine. According to the occurrence conditions of coal water in shallow coal seam in western China, Huang [13] developed the development law of “upward cracks” and “downward cracks” of overlying water-resisting rock group in shallow coal seam. Xu et al. [14] used FLAC 3D numerical simulation, similar material simulation and other methods to study the development and evolution characteristics of WCFZ of coal mining roof in Dananhu mining area, and evaluated the feasibility of water preserving mining in the area studied. Geng et al. [15] conducted studies on the prevention and control technology of water disaster in shallow coal seam mining around large reservoir in Shenfu mining area by combining borehole water injection, theoretical calculation, numerical simulation and other means to determine the development HWCFZ.

The above research results have greatly enriched the theory and technological system of water preserving mining in western mining areas of China. However, the research on the height development characteristics of WCFZ in fully mechanized mining of shallow thick coal seam under water in western mining areas is quite limited. The 91,105 working face of Daheng Coal Mine belongs to the coal mining under composite water body, which adopts comprehensive mechanized full-height mining with the mining height of 4.2 m. There are few studies on the development characteristics of WCFZ under similar mining conditions. Firstly, the authors used the underground up-hole water injection method by intervals to detect the development height of overburden fracture, then combined with the borehole television to visually detected the development situation of mining fractures, and studied the height development characteristics of the WCFZ in the 91,105 working face. Finally, the authors used numerical simulations to invert the dynamic evolution process of WCFZ in 91,105 working face, with the intention of providing a reference for water preserving mining under similar engineering geological conditions in ecologically fragile areas in western China.

2. Project Profile

Daheng Coal Mine is located in the semi-arid area of Loess Plateau in northwest Shanxi Province, with vertical and horizontal distribution of gullies and sparse vegetation, which is a typical ecological fragile area in western China. In the western part of the well field, there is a seasonal surface water body Wanghuolang River (main tributary of Maguan River). In the dry season, the river is typically dry and waterless, while in the rainy season, a brief flood can be formed in the gully, which is characterized by duration and large flow.

The 91,105 working face of Daheng coal mine study area is located in the northwest of the mine field. The north is the protective coal pillar of the mine field boundary (Xingtao Coal Mine). The west side is the F1 fault and its protective coal pillar and the 91,103 working face that has been mined. The east side is the 91,106 planned working face. The south is the protective coal pillar of the three main roadways of the belt, track and return air in the west wing of the lower coal group and the auxiliary shaft industrial square. The location of coal mine and the plane layout of 91,105 working face are shown in Figure 1. The strike length of the working face is 876.3 m, and the dip length is 145.5 m. The 9-1 coal seam is mined in the working face, of which the upper 4 and 8 coal seams are not mined. The thickness of coal seam is 4.1~5.1 m, with an average thickness of 4.2 m, an average dip angle of 3°, and a buried depth about 185 m, which is classified as shallow buried thick coal seam. The working face is laid out along the longwall, with comprehensive mechanized one-time full-height mining and the full caving method for managing the roof.

The overlying strata on the working face are mainly composed of siltstone, coal seam, sandy mudstone, fine sandstone, mudstone, etc., whose lithology is comprehensively assessed as medium hard rock. There are 7 aquifers in the upper part of 9-1 coal seam, among which aquifer I is Quaternary and Neogene porous aquifer, which is mainly recharged by atmospheric precipitation and surface water infiltration. The water level changes greatly with weak to medium water-abundance. Aquifers II~IV are sandstone fissure aquifers of Shihezi Formation, with a small amount of bedrock exposed in valleys, which are limited in recharge by atmospheric precipitation infiltration, with weak water-abundance; Aquifers V~VII are sandstone fractured aquifers of Shanxi Formation and Taiyuan Formation, which are directly water-filled aquifers exploited by the working face, with weak water-abundance. There is a stable water-resisting layer between aquifers with weak hydraulic connections. The geological profile and hydrogeological column in the well field are shown in Figure 2. After analysis, the water body above 91,105 working face includes multi-layer aquifer and seasonal surface Wanghuolang river water, which is classified as coal mining under compound water body. In order to realize secure and efficient water preserving mining in working face, the HWCFZ must be accurately predicted.

3. Subsection Water Injection Observation in Underground Up-Hole

3.1. Observation Method and Principle

At present, the detection methods of the HWCFZ mainly include ground drilling flushing fluid leakage method [16], underground upward hole segmented water injection observation method [17], borehole TV method [18], geophysical prospecting method [19], optical fiber sensing [20] and microseismic monitoring [21]. The method of flushing fluid leakage in ground drilling is to determine the HWCFZ by measuring the change of flushing fluid leakage during drilling, which is the most commonly used and reliable method at present. The underground upward hole segmented water injection observation method is to arrange a certain number and angle of upward holes in the appropriate position of the goaf. By measuring the change of leakage of drilling water, the failure height of overlying strata is analyzed. Compared with the ground drilling observation method, it has the advantages of fast observation speed, simple process and low engineering cost. At the same time, combined with borehole TV method, sound velocity logging and other methods, it lays a foundation for quantitative characterization of mining rock fractures. Geophysical prospecting method is to judge the height of “two zones” according to the change of resistivity of fractured rock mass, but it is easy to be affected by geological factors, and there are some errors and difficulties in interpretation. At present, new methods such as well-ground combined microseismic monitoring, optical fiber sensing monitoring and tracer gas method are also widely used in field measurement, but the observation accuracy needs to be further studied. Therefore, the author used the underground upward hole segmented water injection observation method combined with the borehole TV method to detect the overburden failure height and fracture development characteristics of the 91,105 working face.

The drilling field is dug in Sectional alley of adjacent working face of underground coal mining face or a roadway other than the stop line or open-off cut at the measured working face, and then the inclined borehole is drilled above the goaf area according to the observation plan. The length of the borehole should exceed a certain distance above the expected height of the top boundary of the fracture zone, and the borehole should not cross the collapse zone. The double end water shutoff device is used to plug the water injection section by section, and the leakage amount of each section after pressurized water injection is observed, so as to find out the cracking and loosening of the overlying strata and determine the upper bound height of the fracture zone. The working principle of observation instrument is shown in Figure 3.

3.2. Construction of the Numerical Model

The average dip angle of coal seam in the 91,105 working face is 3°, which is a near-horizontal coal seam. It is expected that the fracture zone of overlying strata will be saddle-shaped after coal seam mining [22] with its starting point inside the coal body, and the boundary gradually shift to the coal body, and the highest point is slightly inside the mined-out area of the mining boundary.

Considering the stability of the surrounding rock, the convenience of ventilation and pedestrians, and the limitation of construction conditions at the observation position, the drilling site was placed in the transportation roadway about 41 m away from the terminal line of the working face. For this observation, three boreholes were designed and constructed. The borehole CQ1 was a pre-mining comparison borehole, which was used to observe the development of primary fissures in coal seam roof strata that were not affected by mining. Boreholes CH1 and CH2 were post-mining boreholes to observe the development of mining cracks in the roof strata of coal seam, with borehole CH1 used to observe the middle position of saddle top, and borehole CH2 used to observe the highest point of saddle-shaped top. The borehole location is shown in Figure 4.

Due to the influence of mining, the overlying strata move and deform, making it difficult to form the boreholes. Considering the influence of overlying rock lithology and mining thickness, the reasonable observation time of HWCFZ should be one month after the mining of the working face.

In order to determine the observation depth of boreholes, the approximate height of overburden failure should be predicted theoretically. Then on the basis of obtaining detailed geological data such as geological structure, histogram of boreholes and physical properties of rock strata in the mining area, the HWCFZ is predicted based on the calculation of tensile deformation of rock strata in the literature [23]:

$$\epsilon =\left(l_1-l_0\right)/l_0$$

(1)

In the formula, $\epsilon$ is the tensile rate of the middle layer of the rock layer; $l_0$ and $l_1$ are the lengths of intermediate stratum before and after the tensile deformation, m.

Assuming that the $i$th stratum is taken as the analysis stratum, $l_0$ and $l_1$ can be calculated by Equations (2) and (3) [23]:

$$l_0=H\left(\cot \delta +\cot \psi \right)$$

(2)

$$l_1=\left(w_i^2-l_0^2\right)\arcsin \left[2w_i{l}_0/\left(w_i^2+l_0^2\right)\right]\pi /\left(180\cdot 2w_i\right)$$

(3)

where $H$ and $w_i$ can be calculated by Equations (4) and (5) [24]:

$$H={\displaystyle \sum _{i=1}^n{h}_i}+h_0/2$$

(4)

$$w_i=M-{\displaystyle \sum _{j=1}^n{h}_j}\left(k_j-1\right)-{\displaystyle \sum _{l=1}^n{h}_l}\left(k_l-1\right)$$

(5)

In the formula, $H$ is the horizon height of the middle layer of coal seam roof rock, m; $h_i\left(i=1,2,\dots ,n\right)$ is the thickness of the th stratum above the coal seam, m; $h_0$ is the layer thickness of the rock stratum analyzed, m; $\delta$ is the boundary angle of coal seam mining; $\psi$ is the angle of full subsidence; $w_i$ is the maximum subsidence of the interlayer stratum, m; $M$ is mining thickness, m; $h_j\left(j=1,2,\dots ,n\right)$ is the thickness of the immediate roof strata of the lower $j$th layer, m; $k_j$ is the bulking coefficient of the immediate roof strata of the lower $j$th layer; $h_l\left(l=1,2,\dots ,n\right)$ is the thickness of the basic roof of the lower $l$th layer, m; $k_l$ is the bulking coefficient of the basic roof of the lower $l$th layer.

According to the literature [25], in the vertical direction, the relationship between the average bulking coefficient of overlying strata in goaf $k$ and the distance from coal seam roof $h_m$ is

$$k=k_z-0.017\ln h_m ,\left(h_m<100\right)$$

(6)

In the formula, $k_z$ is the bulking coefficient of the lower immediate roof.

According to the above calculation Equations (1)–(6), the tensile rate of the middle layer of the rock layer can be obtained. Combined with the criterion value of the critical tensile rate of rock stratum given in the reference [23]: soft rock stratum is more than 0.4%, medium hard rock stratum is 0.1~0.24% and hard rock stratum is less than 0.04%, the development HWCFZ is finally determined.

With reference to the empirical values of the parameters in this mining area and in combination with the field, it is concluded that the $k_z$ of 91,105 working face is 1.08, $cot\delta$ and $cot\psi$ are both 0.577. The values of the calculation parameters of the tensile rate of the interlayer stratum with different horizon heights are shown in

Table 1. Calculation parameters of rock tensile rate.

Serial Number	Rock Stratum	Bulking Coefficient	The Horizon Height of Interlayer Stratum/m	The Subsidence of the Interlayer Stratum/m	Tensile Rate/%
13	Mudstone	1.01	54.37	2.71	0.07
12	Medium sandstone	1.01	48.13	2.74	0.11
11	Sandy mudstone	1.01	43.92	2.79	0.15
10	4-1 coal	1.02	37.34	2.95	0.26
9	Mudstone	1.02	32.49	2.98	0.37
8	Fine sandstone	1.02	28.51	3.11	0.54
7	Sandy mudstone	1.03	24.40	3.17	0.79
6	4-2 coal	1.03	20.89	3.32	1.21
5	Fine sandstone	1.03	15.87	3.47	2.32
4	Sandy mudstone	1.04	12.44	3.54	3.96
3	Siltstone	1.04	8.64	3.77	9.23
2	8 coal	1.05	5.19	3.83	25.32
1	Siltstone	1.08	2.30	4.2	24.63

.

As shown in Table 1, the tensile rate of rock layer gradually decreased as the increased of horizon height of interlayer. The lithology of overlying strata in 91,105 working face of Daheng Coal Mine is medium hard, and the critical tensile rate is 0.1~0.24%, while the tensile rate between No.12 medium sandstone and No.13 mudstone decreases from 0.11% to 0.07%. Therefore, No.12 middle sandstone is classified as the category of WCFZ, and No.13 mudstone as the category of bending subsidence zone.

The estimated height of the WCFZ at this point was 49.79 m. Three observation boreholes with different dip angles were designed accordingly. To ensure that the observation top boundary exceeds the actual overburden failure height, the boreholes depth was designed to be extended by 10~15 m. The borehole profile layout parameters are shown in

Table 2. Borehole profile layout parameters.

Borehole Number	Borehole Diameter/mm	Pitch Angle/(°)	Azimuth Angle/(°)	Borehole Length/m
CQ1	89	89	55	79
CH1	89	348	45	89
CH2	89	350	50	82

.

3.3. Observation Results and Discussion

The data of water injection leakage observed in each borehole are drawn in Figure 5. Then the development HWCFZ was judged by comparing the changes of water injection leakage in each hole section.

(1)

CQ1 pre-mining borehole

It can be seen from Figure 5 that before mining in the working face, the overlying strata were not affected by mining. The average leakage of water injection in the borehole CQ1 was 2.5 L/min, indicating that the roof overlying rock mass structure was relatively complete before mining in the working face. The local water injection leakage was 5.2~8.3 L/min, indicating that there were well-developed primary fractures in local rock strata.

(2)

CH1 post-mining borehole

Compared with the pre-mining borehole CQ1, the fluctuation of water injection leakage in different horizons of CH1 borehole observation section was more dramatic. In the range of borehole depth of 74.5~89 m, the leakage of borehole section was mostly stable at 2.1~3.5 L/min, and the leakage was small. This indicated that the integrity of this section of rock stratum was good, and the fracture zone was not developed to this height. However, there were two borehole sections with large leakage, namely, the borehole depth of 82 m (vertical height of 58.0 m) and the borehole depth of 86.5 m (vertical height of 61.2 m), with the leakage of 11.8~13.4 L/min. It was considered that there were primary micro-fractures in this section, not mining fractures. In the borehole depth of 55~73 m borehole area, the leakage increased to 19.8~32.5 L/min, and the fluctuation range of leakage was larger, significantly higher than the previous section. it showed that a large number of secondary fractures were produced in the rock stratum of this hole section due to mining, and the drilling hole entered the WCFZ. Therefore, the observation results of CH1 post-mining hole show that the hole depth is 74.5m, which is the top boundary of the WCFZ (combined with the borehole histogram (Figure 2B), it can be seen that the corresponding rock stratum here is mudstone with a thickness of 9.2 m), and the vertical height from the roof of the working face is 52.7m.

(3)

CH2 post-mining borehole

The variation curve of injection water leakage in different horizons of the post-mining borehole CH2 was similar to that of borehole CH1. In the borehole depth of 70~82 m, the water injection leakage was 1.9~3.6 L/min. Compared with the water injection leakage of the same borehole section of the pre-mining borehole, it showed that the rock stratum of this borehole section was not damaged. In the borehole depth of 61~68.5 m, the water injection leakage increased significantly, reaching 15.8~28.3 L/min, indicating that this section was the top of the fracture zone. Therefore, the results of observation of CH2 post-mining borehole showed that the borehole depth of 70 m is the top boundary of the WCFZ, and the vertical height from the roof of the working face is 53.6 m, which is similar to the measurement results of the borehole CH1.

To sum up, it can be seen that the maximum height of the WCFZ measured in 91,105 working face is 52.7~53.6 m, the fracture mining ratio is 12.55~12.76 (ratio of HWCFZ to coal mining thickness), and the corresponding rock stratum is a mudstone with a thickness of 9.2 m. According to the drilling location and observation results, the development form of WCFZ in 91,105 working face presents saddle-shaped, as shown in Figure 6.

According to the comparison between the detection results and the spatial position of the overlying aquifer in the working face, it can be seen that the WCFZ has developed and communicated with the V~VII aquifers with weak water abundance, but it has not communicated with the overlying IV and above aquifers and surface water. This is because there are many layers of soft rock such as sandy mudstone and mudstone with large thickness in the range of WCFZ, and this kind of soft rock is easy to soften, expand and disintegrate when encountering water. When the fracture develops to a certain layer, under the action of the gravity of the overlying strata, the fracture is filled and closed by it, and it is not easy to form a seepage water inrush channel. This kind of soft rock becomes the key inhibiting layer to inhibit the continuous upward development of the WCFZ. According to the field investigation, the roof of 91,105 working face has basically no seepage and water drenching during the mining process, which shows that the development of WCFZ on the roof has little disturbance to the overlying aquifer after coal mining, which will not pose a threat to the mining of 9-1 coal seam.

4. Borehole TV Observation

After the completion of the water injection test, the distribution and development of mining fissures in the overlying strata in the borehole were visually observed by borehole TV. The borehole CH1 was selected as the observation borehole, and the results of detection of injection water leakage in the borehole were verified.

The development of overburden fractures in the borehole is shown in Figure 7. Within the range of 45.2 m before the borehole depth, it was less affected by mining, the rock stratum was dense and complete, with no obvious mining fractures (Figure 7A). The rock stratum in the local hole section was wet (Figure 7B), but no seepage and dripping occurred. Combined with the analysis of the borehole histogram, it could be seen that this phenomenon was caused by the existence of the V and VI aquifers in this borehole section, which also reflected the poor water abundance of those aquifers, and the fractures had little impact on the working face when it penetrated those aquifers. From the borehole depth of 45.2 m, the overburden began to produce longitudinal cracks (Figure 7C), but the crack is short in extension and small in width, depth and quantity, indicating that the borehole section was obviously affected by mining and had entered the fracture zone. With the increase of the borehole depth (Figure 7D,E), the mining cracks expanded upward, and the overburden rock was seriously damaged. The horizontal and vertical cracks were densely interlaced, with the longitudinal cracks dominating and extending longer. At the borehole depth of 58.5 m, that is, the position 41.4 m away from the coal seam roof was sandy mudstone. The softening and expansion properties of mudstone would lead to the phenomenon of crack closure (Figure 7F). At the borehole depth of 65.8 m, that is, the position 46.5 m away from the coal seam roof was the medium sandstone aquifer, and a small amount of water seepage occurred in the rock stratum (Figure 7G), which indicated that the aquifer was weak in water abundance and had little impact on the working face. When reached the borehole depth of 75.2 m, that is, the position 53.2 m away from the coal seam roof, the crack basically disappeared (Figure 7H). The cracks propagation tended to be stable, and the final HWCFZ is 53.2 m.

The HWCFZ in the borehole CH1 is determined to be 53.2 m by the borehole TV observation, which is basically consistent with the HWCFZ determined by the borehole injection water leakage observation. With the increase of borehole depth, the development and evolution of overburden fracture in the borehole experienced the process of beginning to produce, expanding upward, gradually closing and tending to stability.

5. Numerical Simulation Analysis

5.1. Model Building

According to the actual geological conditions of mining at 91,105 working face, a numerical model was established by the software pf FLAC^3D to simulate the process of overburden caving, breaking and fracture development in the whole process of working face advancing from open-off cut to full mining in this mining condition, and the dynamic failure evolution characteristics, failure development height and shape of overburden rock in the process of working face mining are revealed. The working face has a strike length of 300 m and a tendency length of 145 m. In order to reduce the influence of the model boundary, 100 m boundary coal pillars were set around the working face, and the model size was 500 m × 245 m × 215 m (length × width × height), as shown in Figure 8. The height of coal seam excavation is 4.2 m, and the working face mining is simulated by step excavation. Each excavation is 25 m, and the total excavation is 300 m. The bottom of the model was fixed boundary, surrounded by roller boundary, and the top was free boundary. The Mohr Coulomb criterion was used as the unit failure criterion. The specific physical and mechanical parameters of coal and rock mass are shown in

Table 3. Rock mechanics parameters of working face.

Lithology	Density (kg/m³)	Cohesion (MPa)	Tensile Strength (MPa)	Friction Angle (°)	Bulk Modulus (GPa)	Shear Modulus (GPa)
Loess	1970	0.96	0.80	20	3.73	1.56
Gritstone	2750	1.10	1.90	35	7.92	5.42
Medium sandstone	2700	1.43	1.52	34	8.38	5.75
Fine sandstone	2830	1.30	1.40	36	8.54	5.82
Sand-mud interbed	2510	1.65	1.10	34	6.81	3.65
Mudstone	2420	2.10	0.89	29	5.20	3.43
Sandy mudstone	2470	1.52	0.95	32	6.37	3.94
Siltstone	2530	2.00	1.50	33	7.33	4.60
Coal seam	1400	1.20	0.50	31	3.89	2.96

.

5.2. Simulation Results and Discussion

After the coal seam is mined, the stress of the surrounding rock of the working face is redistributed, and the height of the rock layer with shear failure or plastic deformation caused by stress exceeding the shear strength or yield strength could be taken as the upper limit of the WCFZ [26] (The actual maximum development height of WCFZ may be slightly lower than this height). Therefore, the development HWCFZ (The height of the mining overburden failure) could be analogized to the height range of the plastic zone in numerical simulation.

The plastic zone evolution nephogram of the working face at different advancing distances is shown in Figure 9. The development height of the plastic zone of the roof showed a non-linear expansion trend with the advancing of the working face. The overburden failure was affected by the interaction of tensile deformation and shear slip, with the plastic zone expanding dynamically upwards and forwards.

When the working face advanced 25 m (Figure 9A), the internal tensile stress of the immediate roof (siltstone) exceeded the tensile strength of the rock stratum and tensile failure occurs, and shear failure occurred in front of the open cut and the coal wall, causing the immediate roof to break and collapse along the edge of the coal wall. The roof rock mass formed an arched plastic zone and expanded upward dynamically, with a plastic zone height of 4.6 m. When the working face advanced 125 m (Figure 9B), the height and area of the plastic zone were expanded. Under the influence of mining, the overburden failure height at both ends of the goaf was obviously higher than that in the middle, and dominated by the shear failure. The stress was distributed in an asymmetric saddle shape, and it was transmitted upward to the bottom interface of hard fine sandstone with large layer thickness. The height of the plastic zone was 25 m. As the working face continued to advance, the plastic zone penetrated the hard fine sandstone with a sharp increase in the height of the plastic zone. When the working face advances 225 m (Figure 9C), the plastic zone reaches the maximum height of 56 m. After that, the maximum HWCFZ no longer increase with the advancing of the working face, and finally stabilizes at about 56 m, showing the saddle-shaped failure characteristics of “big at both ends and small in the middle”.

According to the borehole histogram, the rock stratum at 56 m in the WCFZ is mudstone with a thickness of 9.2 m. Because the mudstone expands and muddies when it meets water, the fracture is compacted and bridged, which inhibits the development height and shape of the plastic zone. That is, the soft rock with large thickness in the overlying strata will become the key inhibiting layer that restricts the continuous upward development of the height of water-conducting fracture zone. With the mining of the subsequent working face, the inhibiting layer may be broken. At this time, the sandstone-mudstone interbed with the thickness of 14.9 m in the overlying rock layer (Figure 2B) will become the key rock layer to inhibit the upward development of the water-conducting fracture, effectively preventing the water-conducting fracture from communicating with the overlying aquifer.

The dynamic evolution process of the HWCFZ in the advancing process of the working face is shown in Figure 10. The WCFZ formed by the overlying strata developed to the maximum value with the advancing distance of the working face, and then tended to be stable. The development and evolution of the HWCFZ presented four stages of “development–slow increase–sudden increase–stability”.

(1)

Development stage: When the working face advanced 25 m from the open cut, the immediate roof (siltstone) reached the limit span and collapse, causing overburden failure to begin to develop upward.

(2)

Slow increase stage: As the working face continued to advance, the rock falling into the goaf supported the overlying strata to maintain temporary stability to a certain extent due to its own dilatancy and bearing capacity, thus increasing the damage height slowly.

(3)

Sudden increase stage: When the working face advanced to 125 m, the overburden failure developed to the bottom interface of the overlying fine sandstone. Because the thick fine sandstone was hard and difficult to break, the overburden failure was inhibited to expand upward. With the increase of advancing distance, the thick hard fine sandstone broke and lost its bearing capacity. It drove the upper weak rock layer to destroy, thus the height of overburden failure increased sharply with the periodic fracture of the overlying hard strata.

(4)

Stability stage: When the working face advanced to 225 m, the full mining was achieved. The overburden failure height did not increase with the advancing of the working face, and the failure height was basically stable at 56 m. Therefore, the maximum HWCFZ is 56 m.

To sum up, it can be seen from numerical simulation that the maximum HWCFZ is 56 m, and its development shape is saddle-shaped, which is basically consistent with the field measured results, indicating that the measured HWCFZ is accurate and reliable. The analysis shows that when the WCFZ develops to a certain layer, the cracks generated by the weak strata in the fracture zone of overlying strata on the working face will automatically close with the advance of the working face, resulting in “bridging phenomenon”, which inhibits the further development of the WCFZ. That is, the existence of soft rock with a certain thickness in overburden will become the key inhibiting layer for the development of WCFZ, effectively blocking the communication between water-conducting fracture and overlying aquifer.

6. Conclusions

(1)

The HWCFZ can be accurately identified by detection and analysis of two technical means of underground up-hole water injection by intervals and borehole TV. Under the condition of fully mechanized mining of shallow buried thick coal seam under water body in Daheng Coal Mine, when the once mining height is 4.2 m, the HWCFZ is 52.7~53.6 m, and the fracture mining ratio is 12.55~12.76, with a saddle-shaped feature of “big at two ends and small in the middle”. The fracture only penetrates the V~VII aquifer with weak water abundance, and does not communicate with other aquifers or surface water.

(2)

The numerical simulation shows that with the advancing of the working face, the development and evolution of the HWCFZ presents four stages: “development–slow increase–sudden increase–stability”.

(3)

When the WCFZ develops to a certain layer, the cracks generated by the weak strata in the fracture zone of overlying strata on the working face will automatically close with the advance of the working face, resulting in “bridging phenomenon”, which inhibits the further development of the WCFZ. That is, the existence of soft rock with a certain thickness in overburden will become the key inhibiting layer for the development of WCFZ, effectively blocking the communication between water-conducting fracture and overlying aquifer.