Monitoring of Overburden Failure with a Large Fractured-Height Working Face in a Deep Jurassic Coal Seam Based on the Electric Method

Abstract

The development height of a water-flowing fractured zone is the key parameter to consider when carrying out mining under water pressure and coal mining with water conservation. In this paper, Jurassic coal seam 3-1 in the Menkeqing Coal Mine was taken as the research target, and a three-dimensional mining geological model was established by using FLAC^3D to study the deformation and failure rules of overburden. Three roof boreholes were drilled in the auxiliary transportation roadway of adjacent working faces for dynamic monitoring by the resistivity method, which can better observe the whole process from failure to stability of the overburden. The results show that due to the complex sedimentary environment and large buried depth of coal seams in western China, there is a large deviation between the calculation results of the empirical formula of the fractured zone height under the “Regulations of buildings, water, railway and main well lane leaving coal pillar and press coal mining” (three regulations) and the simulation and on-site measurement. Based on the comprehensive analysis, the influence range of mining advance abutment pressure is approximately 60 m. The height of the water-flowing fractured zone is approximately 106 m, and it is located at the interface between sandy mudstone and mudstone. The height of the caving zone is approximately 22 m, and it is located at the interface between fine sandstone and medium sandstone. The ratio of the fractured height and coal seam thickness (R_f) reached 24.4, which was basically consistent with the test result of the adjacent Yushenfu mining area (which was 26 on average). There is no obvious change in the development height of the caving zone and water-flowing fracture zone from the working face to the drilling borehole position of more than 120 m, which reflects that the height of the overburden failure zone is related to the control of lithological combination.

1. Introduction

Western China is rich in coal resources, accounting for approximately 70% of the total coal resources in China [1]. Among these coal resources, the Jurassic coal field in northern Shaanxi is one of the areas with the best development prospects, with a high degree of coal resource enrichment, good coal quality and relatively simple mining conditions [2]. However, most of the resources in the western region are located in fragile ecological environment areas, and groundwater leakage and ecological environment damage are easily caused by mining [3]. Accurately detecting the development height of the water-flowing fractured zone in the working face is an effective way to solve the problem of coal mining.

At present, there are many research methods for investigating overburden deformation and failure of the working face, including theoretical analysis [4,5,6,7], empirical statistics [8,9,10,11], simulation calculations [12,13,14,15,16] and field measurements [17,18,19,20,21,22]. The application of the above methods has played a guiding role in the safe, efficient and green mining of coal resources in a specific period. However, there are still numerous shortcomings, which mainly manifest as follows: (1) Methods and technologies such as theoretical calculation and numerical simulation typically simplify the complex rock mass in the working area and make approximate selections of relevant parameters. As a result, the evaluation results often deviate to a certain extent from the actual situation, and the change process of overlying rock cannot be simulated accurately. (2) Currently, the results of rock deformation and failure obtained through in situ testing are mostly judged by a single method and based on a single parameter. Thus, the resolution, accuracy, and the degree of identification of multiple disaster sources are limited. (3) Due to the influence of mining, the deformation and failure characteristics of the surrounding rock after coal face mining mostly focus on transient effects. Moreover, monitoring methods that combine the background field and ground electric field of rock formation are rarely studied. Therefore, it is essential to employ a multi-means and multi-attribute joint interpretation method to accurately detect the deformation development law of overburdened rock.

At present, due to the “westward migration” of China’s coal layout, the mining and utilization of deep coal seams in western China are facing significant challenges. In particular, the deeply buried coal seam in the western ecologically fragile area has typical geological structure characteristics, which makes the overburden fracture mechanism caused by working face mining obviously different from that of the conventional development law. The development of the caving zone and water-flowing fractured zone shows the characteristics of a large fracture height, widespread range and poor regularity. The main reason for the difference is that the Jurassic coal seam is deeply buried, the strata structure is not consistent with the Carboniferous Permian in north China, and the key strata are mostly located in the middle and upper parts. Therefore, it is of great significance to accurately grasp the law of overburden fracture development of deep coal seams in the ecologically fragile area of western China in order to carry out water conservation mining, green mining and damage reduction mining in the later stage.

The mining area studied in this paper is Menkeqing coal mine, which belongs to Ordos Basin and is a typical ecologically fragile area. The occurrence characteristics are as follows: (1) the embedded depth is approximately 750 m; (2) the thickness of the Quaternary loose layer is relatively large; (3) the water content in the loose layer is large, which leads to a risk of water inrush; (4) there is a large mining height, where the thickness of the coal seam is more than 4 m; and (5) there is no stable distribution of aquiclude in the whole area. To deeply study the law of overburden deformation and fracture development in Jurassic coal seam mining, the conventional calculation formula and analogous analysis are used to predict the height of the caving zone and the water-flowing fractured zone, and then the law of overburden failure development in the process of coal seam mining is simulated by numerical calculation, combined with the dynamic monitoring of the underground roof borehole electrical method. The development of a water-flowing fractured zone and the range of influence of mining are comprehensively analyzed and judged.

2. Overview of the Study Area

2.1. Geological Overview

The Menkeqing Coal Mine belongs to the Hujierte mining area of the Dongsheng coalfield, located in Wushen Banner, Ordos City. Among these mining areas, working face 11-3101 is the first working face of Menkeqing Coal Mine, the ground elevation is +1293~+1309 m, the working face elevation is +596~+612 m, the coal seam is 3-1 coal, the coal thickness is 3.83~5.45 m, and the average is 4.35 m. Coal seam dip angle 1~4°, average 2°. The strike length of the working face is 3904.3 m, and the dip length is 260.4 m.

According to the relevant exploration data and hydrological supplementary survey data of the mining area, the roof aquifer of the working face 11-3101 is highly water-rich, and the roof strata are mainly medium sandstone, fine sandstone, gritstone and sandy mudstone, and there are sandy mudstone and mudstone interbedding phenomena in some areas. Due to the large mining height, high ore pressure, high water pressure and high temperature in the Menkeqing mining area, the development height of the fracture zone may be inconsistent with the expected development height. In particular, there is basically no effective water barrier layer in the 3-1 coal roof of the working face, and the development height of the fracture zone caused by mining may be high. Therefore, it is necessary to conduct a fine study on the development law of the fracture zone in the roof of the working face. The geographical location and geological profile of the study area is shown in Figure 1.

2.2. Height Prediction of Water-Flowing Fractured Zone

Generally, empirical formulas and analogy methods are used to estimate the height of the water-flowing fractured zone of the working face. Based on 3-1 coal geological data, two kinds of representative formulas are selected. One is the calculation formula in “Regulations of buildings, water, railway and main well lane leaving coal pillar and press coal mining”. Second, the formula summarized by Wang et al. [1] is deemed suitable for high-intensity mining in western China. The specific results are shown in

Table 1. The predicted result of experience formula of working face fractured zone height.

Formula Type	Caving Zone Formula	Water-Flowing Fractured Zone Formula	Hc/(m)	Hf/(m)	Rc	Rf
First category	$100M/\left(4.7M+19\right)\pm 2.2$	$100M/\left(1.6M+3.6\right)\pm 5.6$	13.2	46.8	3.0	10.8
Second category	$4.3665M-4.8462$	$100M/\left(0.095M+7.28\right)\pm 8.62$	14.1	65.2	3.2	15.0

Note: In the formula, M is 4.35 m, and the calculated value is the maximum. H_c is the height of the caving zone, H_f is the height of the water-flowing fractured zone, R_c is the ratio between the caving zone and coal seam thickness, and R_f is the ratio between the water-flowing fractured zone and coal seam thickness. The same applies below.

.

At the same time, based on the proven height of water-flowing fractured zone data of adjacent mining areas, an analogy analysis of this working face was carried out. Fan et al. [9] sorted the ratio data of R_f of fully mechanized mining faces in the Yushenfu mining area and Shendong mining area, with an average of approximately 26 times. Wang et al. [1] suggested that the development H_f in the Yushenfu shallow coal seam should be 18~28 times the mining height. A large number of measured results show that the H_f in the western region is more than 20 times higher, and close to 30 times higher, while the results based on the empirical formula of “Regulations of buildings, water, railway and main well lane leaving coal pillar and press coal mining” are basically less than 15 times higher, which is quite different.

3. Three-Dimensional Numerical Simulation

3.1. Modeling Parameter Testing and Selection

In this paper, FLAC^3D numerical simulation software (version 5.0) is used to establish a three-dimensional numerical model of coal seam mining combined with the physical and mechanical parameters of the rock core obtained from the rock mechanical property test. The development scale and spatial distribution characteristics of the coal seam roof caving zone and water-flowing fractured zone after coal seam mining are studied under the conditions of a huge thick sandstone layer, high ore pressure and high water pressure.

The key to numerical modeling is to accurately obtain the rock mechanics parameters of each stratum. There are two main ways to obtain rock mechanical parameters. One is to carry out rock core in the roof of the study area and use relevant instruments to carry out indoor test experiments, as shown in Figure 2. The second is to collect and organize the mechanical parameters data near the working face or near the mining area. Finally, the obtained rock mechanical parameters are shown in

Table 2. Physical and mechanical parameters of the working face rock strata.

Rock Strata	Elastic Modulus /GPa	Bulk Modulus /GPa	Shear Modulus /GPa	Tensile Strength /MPa	Poisson’s Ratio	Friction Angle/°	Cohesion/MPa	Bulk Density/kg/m3
Siltstone	18.00	15.00	6.92	3.00	0.30	22.50	1.24	2430
Gritstone	18.20	18.96	6.79	2.85	0.34	32.05	1.78	2420
Coal seam 2-2	12.00	10.00	4.62	2.00	0.30	28.00	1.10	2300
Medium sandstone	20.10	17.01	7.71	3.41	0.30	31.78	2.11	2540
Fine sandstone	21.60	20.00	8.18	0.77	0.32	38.47	2.86	2550
Sandy mudstone	15.30	15.00	5.75	2.28	0.33	32.78	1.47	2410
Mudstone	14.00	15.56	5.19	2.20	0.35	28.03	1.17	2340
Coal seam 3-1	12.00	10.00	4.62	2.00	0.30	28.00	1.10	2300

.

3.2. Numerical Model Construction and Parameter Selection

To obtain a more comprehensive understanding of the overlying rock failure and development law of coal seam mining in Menkeqing mining area, based on the actual parameters of the working face, the model is set with a 400 m strike length, a 300 m tilt width, and 240 m height and is 400 m × 300 m × 240 m size. The model includes 2-2 and 3-1 coal seams. Since the actual dip angle of the coal seam is 2° on average and the dip angle is small, the average dip angle of the coal seam is 0° in the simulation, the simulated excavation width of the working face is 200 m, the mining length is 200 m, and the mining thickness of the coal seam is 5 m. The 3D model is divided into 1,200,000 3D units with a total of 1,241,391 nodes. The numerical model adopts the Moore–Coulomb rock strength criterion. Figure 3 depicts the 3D numerical model diagram.

To eliminate the influence of boundary effects on the accuracy of numerical simulation results, certain parameters need to be set for the model. Based on the geometric scale of the model, mining depth, and actual on-site conditions, a 100 m protective coal pillar was left at each end of the working face direction, and a 50 m protective coal pillar was left at each end of the inclination direction. Usually, to limit the range of mechanical motion in the model, it is necessary to constrain the boundaries of the X, Y, and Z axes separately. Once the boundaries are constrained and fixed, there will be no displacement or velocity in the fixed direction. The horizontal displacement of all nodes on the fixed boundary surfaces is as follows: X = 0 m and X = 400 m. The displacement in the Y direction of all nodes on the fixed boundary surfaces is as follows: Y = 0 m and Y = 300 m. There is fixed displacement in the Z direction of the bottom boundary (Z = 0 m). To generate gravity, a gravity acceleration of 9.8 m/s² is usually loaded in the negative Z-axis direction and the density of the model is set. The entire model is divided into 18 layers of rock, with an average density of 2500 kg/m³ for the overlying rock model. To simulate the real stress environment, a certain load needs to be applied to the model to accurately reflect its stress situation, with a compensation load of 12.5 MPa. The single excavation step is 20 m.

3.3. Maximum Principal Stress Analysis

Coal seam mining in the working face will lead to the fracture, fracturing, and caving of the overlying strata, causing the overburden failure to exhibit a certain regular change. When coal is mined, stress increases, decreases, and concentrates in the roof strata. The development range and characteristics of overlying rock failure can be determined according to the characteristics of stress changes. To more clearly reflect the stress distribution and evolution process in the simulation, the maximum principal stress distribution cloud map with obvious stress changes is selected for analysis, and the stress evolution law and development form are judged when the overburden rock is damaged. Figure 4 shows the cloud map of maximum principal stress distribution at different mining distances.

In FLAC simulation, tensile stress is positive and compressive stress is negative. The maximum principal stress can determine whether the rock formation has tensile failure. As can be seen from the cloud diagram of maximum principal stress distribution in Figure 4, as the working face advances from left to right, a parabolic stress zone is generated in the overburden, and a certain stress concentration occurs in front of the coal wall and behind the working face. The tensile stress values of the maximum principal stress are 0.21 MPa, 1.06 MPa, 1.15 MPa, 0.99 MPa, 1.16 MPa, 1.27 MPa, 1.41 MPa, 1.42 MPa, 1.31 MPa, and 1.45 MPa, respectively. In the early stage of coal seam mining, the overlying rock strata above the goaf are under the combined action of compressive stress and tensile stress, and the trend of compression breakage and shear failure of the rock strata is relatively obvious. With the increase in mining distance, the goaf becomes larger and larger. Due to the action of coal walls on both sides of the working face and surrounding rock, the stress distribution of the overlying rock in the working face gradually forms a stress-bearing structure.

3.4. Plastic Zone Analysis

The failure profile of the plastic zone of the 3D mining geological model is shown in Figure 5. With the excavation of the working face, the rock strata around the stope are gradually subjected to tensile shear failure, and the scope of failure expands. However, when the working face is excavated to 160 m, the failure height of the overburden plastic zone no longer increases and expands only in the transverse range in the later stage. Based on the analysis and judgment of the excavation results after 200 m, the development height of the water-flowing fractured zone is approximately 105 m, and the R_f is 21. At the same time, the plastic zone section reflects that the overburden failure is saddle-shaped, with a high middle and a low middle at both ends.

4. Borehole Resistivity Monitoring of Overburden

4.1. Basis of Physical Property Exploration

The overburden of the working face will form a caving zone, a water-flowing fractured zone and a bending subsidence zone after mining (hereinafter referred to as “three zones”). Under the condition in which there is no water-filled fracture, different zones have different resistivity characteristics. Based on the previous research results, the caving zone resistivity is the highest, and is usually more than three times greater than the original rock strata resistivity. The fractured zone is above the caving zone, and the resistivity is usually more than 1.5 times greater than the resistivity of the original rock strata. The bending subsidence zone is above the fracture zone, the fractures are separated locally, and the resistivity value increases slightly [22].

Single-hole resistivity detection is carried out by constructing electrode device holes in the roof of the coal seam [23,24]. Electrodes with different spacing are buried for single-hole measurement. Based on the electrode points in a single hole, two-dimensional geo-electric cross-section measurement is conducted, providing a large amount of data and extensive information. The single-hole electrical method uses the high-density resistivity method for corresponding treatment. Its physical premise is the electrical conductivity difference between underground media. Similarly to the conventional resistivity method, it supplies current I to the ground through the A and B power supply electrodes and then measures the potential difference ∆V between the M and N measurement electrodes to obtain the apparent resistivity value of each point.

In practical applications, the single-hole resistivity test has been improved to form a three-dimensional test space for boreholes. Usually, two boreholes are constructed in the roof, and a hole–hole CT test system is designed to obtain corresponding potential data. Since the data inversion of the hole-to-hole resistivity method increases the constraint effect of the electrode in the borehole, the result is more accurate than that of the single-hole resistivity test system. This is convenient for directly comparing the resistivity difference in rock deformation and failure, thereby improving the judgment accuracy [25,26].

Generally, when the coal strata are not affected by mining, a resistivity sensing unit is installed in the roof borehole, grouting is carried out to seal the borehole, and the background resistivity value is collected. In the later stage, with the mining of the working face, parameters such as apparent resistivity in the borehole are dynamically tested regularly, which can reflect the dynamic change in rock strata failure near each electrode.

4.2. Monitoring Scheme

Based on the analysis of the geological conditions of the working face, the observation system is arranged near point Y12 of the auxiliary transportation roadway of the working face to observe the dynamic change process of the “three zones” of the overburden after the gradual advance of the working face and to determine the height of the caving zone and the water-flowing fractured zone. The layout diagram of the observation system is shown in Figure 6. The three boreholes are located at the same position, which is the auxiliary transportation roadway of the adjacent working face. The position is 620 m away from the working face, which can make it easier to observe the whole overburden process, from failure to stability. The final position of borehole #1 is located at the edge of the working face, and the final positions of borehole #2 and borehole #3 are located inside the working face, which can facilitate accurate detection of the height of the water-flowing fractured zone with a saddle-shaped fracture.

The installation of the working face overburden resistivity monitoring system began on 4 March 2018, and all of the systems were installed and grouted until 11 March 2018. After the installation of the whole monitoring system, the cable in the borehole was tested first. After the cement slurry in the borehole was fully consolidated and stable, the background value was collected, which provided an effective reference for later monitoring data. The resistivity data acquisition in the whole monitoring period is shown in Figure 7.

4.3. Analysis of Monitoring Results

4.3.1. Analysis of Single-Borehole Resistivity Results

Borehole #1 is located in the protective coal pillar of the auxiliary transportation roadway, which is not in the same section as borehole #2 and borehole #3, so only a single-borehole resistivity inversion can be carried out. To understand the location of resistivity change and the development height of the water-flowing fracture zone, the apparent resistivity change map is pasted onto the geological section map.

As shown in Figure 8, the resistivity values are mainly concentrated at 200~300 Ω·m, and the whole section is distributed in this resistivity range. Therefore, the apparent resistivity profile can be used as the background value for analysis, which can be used for comparative analysis of subsequent detection results.

Figure 9a shows the resistivity characteristics during the initial stage of mining. At this time, the working face does not enter the control level range of the borehole. Within the range of 0~80 m above the overburden, due to the influence of advanced abutment pressure, the resistivity of local strata decreases and increases within the range of 80~100 m. With the advance of the working face, the mining position gradually enters the control range of the borehole, as shown in Figure 9b, and the resistivity value of the local area of borehole #1 at a vertical height of approximately 100 m continues to increase, which is caused by the abutment pressure that occurs during coal seam mining. As shown in Figure 9c, the resistivity increases significantly in a range of 90~110 m in the horizontal direction and 80~105 m for the vertical height of borehole #1, and the resistivity reaches 800 Ω·m, showing that the coal seam is affected by mining disturbance and that the fractured zone has begun to develop. As shown in Figure 9d, most of the horizontal control range of borehole #1 is in the goaf, and the resistivity value continues to increase in the area with a horizontal direction of 90~110 m and vertical height of 40~105 m, reaching 1200 Ω·m, showing that fractures have developed widely in this area, which is the fractured zone, and the rest of the area is the mining stress-affected area. As shown in Figure 9e, at this time, the mining position of the working face has reached the orifice, its resistivity value continues to increase, and the fractures continue to develop, but they may still undergo dynamic change.

As shown in Figure 10a, the control range of borehole #1 is within the influence range of the goaf, in which the resistivity value increases significantly between the horizontal direction of 80~110 m and the vertical height of 50~106 m, and the resistivity value reaches 1400 Ω·m. As shown in Figure 10b, at this time, the working face is 79.5 m across the orifice, and the resistivity value of borehole #1 is relatively different near the vertical height of 106 m, reaching 1600 Ω·m. At this height, the fractures are well developed. As shown in Figure 10c, the stopping position of the working face is 120 m across the orifice, and the resistivity change in borehole #1 tends to be stable below the vertical height of 106 m and forms an obvious interface with other areas. The resistivity value is 5–6 times different from the background value. The height is the development height of the fractured zone.

In summary, because borehole #1 is in the protective coal pillar, its resistivity value changes slowly in the early stage of coal seam mining. After the mining position passes through the borehole, the resistivity value of borehole #1 changes drastically, and the fracture is fully developed. At the same time, Figure 6, Figure 7 and Figure 8 show that the change in resistivity has an increasing downward influence, and a vertical height of 106 m is the characteristic interface of resistivity. Combined with geological data, the height is the development height of the water-flowing fractured zone.

4.3.2. Analysis of Cross-Borehole Resistivity Results

Borehole #2 and borehole #3 are in the same vertical section, and cross-borehole data are used for constrained inversion. The results are shown in Figure 10. In Figure 10, 0~50 m in the horizontal direction is the protective coal pillar, and the overall change in its resistivity is small. The horizontal direction of 50~110 m is the overburden, and the overburden damage is mainly concentrated in this range. Most of the resistivity values in Figure 11a are at 100 Ω·m, which is the variation range of the normal resistivity value of the rock mass and can be regarded as the background value.

The difference in resistivity values in Figure 11b is small, indicating that the rock mass maintains relative integrity at this time. When the working face is 87.1 m away from the orifice, as shown in Figure 11c, the local regional resistivity increases between the vertical height of 20~70 m. The analysis shows that the change feature is mainly affected by the advance stress, and the advance influence distance is approximately 60 m. When the working face is 54.8 m away from the orifice, as shown in Figure 11d, the resistivity value between the vertical height of 11~20 m increases significantly, and the resistivity value reaches 800 Ω· m, showing that the rock mass in this area has been broken and has begun to collapse. When the working face is 20 m away from the orifice, as shown in Figure 11e, the rock fragmentation intensifies, and the scope of the caving zone expands further to 5~20 m, initially forming the characteristics of the caving zone. At the same time, the resistivity value increases at vertical heights of 90~100 m, indicating that fractures begin to develop. When the working face continues to be mined, the roof rock mass is increasingly affected by the mining effect, as shown in Figure 11f. The fracture development range is expanded to 50~100 m and the resistivity value reaches 1400 Ω·m.

When the working face crosses the orifice, the change range of the resistivity value decreases gradually, as shown in Figure 11g. The resistivity value with a vertical height of 5–22 m reaches 1600 Ω·m, which indicates that the rock mass in this area is relatively broken, which is characterized by the development of the caving zone. The range of resistivity in a range of 40~60 m of vertical height continues to expand, indicating that the fractured zone has undergone further expansion. When the working face is over 79.5 m from the orifice, as shown in Figure 11h, the movement and resistivity of the roof rock stratum are basically stable. The resistivity value changes greatly compared with the background value within the range of vertical height of 40~102 m, and the resistivity value reaches 1400 Ω·m, which is 5–6 times the difference from the background value, and the analysis shows that the range is typical for fractured zone development. The resistivity of the rock strata in a range of 5~22 m vertical height is high, and the local area reaches 1900 Ω·m, which is approximately 8–9 times the background value. According to the analysis of the characteristics of resistivity change, the range is judged to be the caving zone. According to the analysis of the resistivity variation characteristics of boreholes #2 and #3, a vertical height of 102 m is the upper limit of the development height of the water-flowing fractured zone, and 22 m is the upper limit of the development height of the caving zone.

4.4. Comprehensive Analysis

4.4.1. Analysis of Development Height of “Two Zones” of Overburden

According to the geological data of the Menkeqing mining area and the indoor rock loading experiment, the overburden type of the test area is medium hard-type rock strata. The failure range of overburden “two zones” is estimated by using the “three regulations” and the empirical formula summarized by Wang et al. [11]. The maximum R_f and R_c obtained by the two formulas are 15 times and 3.2 times, respectively. Fan et al. [9] summarized and analyzed the water-flowing fractured zone and caving zone under similar geological conditions in the Yushenfu mining area. The average R_f was 26 and the R_c was 5. At the same time, based on the established three-dimensional mining geological model, the failure and development law of overburden during coal seam 3-1 mining was simulated and measured on site. The results of various research methods are shown in

Table 3. Contrastive analysis results of “two zones” of overburden.

	Empirical Formula	Numerical Simulation	Method of Analogy	Field Test
Rf	15	21	26	24.4
Rc	3.2		5	5.1

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Table 3 shows that the empirical formula and numerical calculation results are lower than the measured values, and there is a large difference between the empirical formula and the actual formula. The main reasons for this are as follows: ① The mining length of the 11-3101 working face is large, the coal seam is thick, and the advancing speed is fast, so the empirical formula is no longer applicable. At the same time, in the simulation calculation, due to the differences between the selection of modeling parameters and the actual conditions, as well as the influence of the later artificial parameter adjustment and other factors, the results also have deviations. ② The sedimentary history of Jurassic strata is more complex. Compared with the sedimentary environment of the Carboniferous Permian in north China, the thickness of the Quaternary loose layer is larger, composed mainly of aeolian sand and fine sand, and the key layer is located mostly above the middle. However, the empirical formula in the “three regulations” is derived mainly from the mining of coal seams under the geological conditions of the Carboniferous Permian coal formation, so there are significant differences. Although the coal seam in the study area is buried shallowly, its coal seam-forming geological environment is consistent with that in the Menkeqing mining area [9]. Therefore, the analogy results have good guiding significance for field measurements, and better illustrate the success of resistivity method in monitoring overburden deformation and failure in coal mining.

According to the results of numerical simulation and field measurement, the development characteristics of overburden failure in the 3-1 coal seam are comprehensively analyzed. Figure 2 shows that the failure of the surrounding rock mainly takes the form of shear and tensile failure, and the failure area presents a “saddle” shape. According to Figure 10 and Figure 11, the actual measured water-flowing fractured zone of borehole #1 in the protective coal pillar area outside the working face is 106 m, while the joint test results of borehole #2 and borehole #3 in the working face reflect that the water-flowing fractured zone is 102 m, indicating that the saddle height of the water-flowing fracture zone is slightly lower than the edge height, which conforms to the “saddle” development rule in overburden failure.

4.4.2. Analysis of Overburden Stress Characteristics

The advancement of the coal mining working face is a dynamic process, and the deformation and failure of the overburden is also a dynamic evolution process. To capture the law of strata movement throughout the whole process, the monitoring starts from 393 m in front of the orifice to 120 m behind the orifice. According to Figure 8 and Figure 10, during the monitoring period, the strata from initial stability to secondary equilibrium are well reflected in the resistivity profile. When the coal seam working face enters the control range of the borehole, it will lead to the development of overburden fracture under the influence of advance abutment pressure and periodic weighting. According to Figure 10c, when the working face is 87.1 m away from the orifice, the resistivity of the local area increases in a range of 20~70 m in the vertical height of 20~30 m in front of the orifice. This analysis is based on the influence of advance abutment pressure, and the influence range of advance abutment pressure is approximately 60 m. At the same time, fractures begin to develop at the interface of different lithologic combinations, in which fractures of hard roofs are less developed, and weak rock and the interface of soft and hard rock easily produce cracks, separation and other phenomena. The strata in the caving zone sustain significant damage and the resistivity is high. Fractures mainly develop during the process of overburden failure. The rock strata structure maintains good integrity in a range of 106~120 m control height of boreholes, and the development characteristics of the separated-layer fractured zone are found to be in the range of 120~127 m. The development height of the caving zone and the water-flowing fracture zone exhibits no obvious change from the working face to the 120 m borehole orifice, which indicates that the development height of overburden failure zone is related to the control of the lithologic combination.

5. Conclusions

In this paper, the deformation and failure characteristics of overburden in deep buried thick Jurassic coal seam mining in western China have been investigated by using different approaches.

(1)

The overburden structure of the Jurassic deep coal seam in western China is quite different from the overburden structure of the Carboniferous Permian in north China. At the same time, the key layer of overburden is located mostly in the middle and upper parts, resulting in large fracture height characteristics. The empirical formula based on the conventional “three regulations” is not suitable for the calculation of the “two zones” height, so a different approach is needed.

(2)

The average thickness of the coal seam in the test area is 4.35 m, the maximum height of the measured water-flowing fractured zone is 106 m, and the development height of the caving zone is 22 m, so the R_f is 24.4 times, slightly larger than 21 times the three-dimensional numerical simulation, which is basically consistent with the comprehensive test results of the adjacent mining area (with an average of 26 times). At the same time, the overburden failure mode is a saddle-shaped failure section with a high middle and a low middle at both ends. In the range of 20~70 m of overburden vertical height, the influence range of advance abutment pressure of the working face can reach approximately 60 m, and the separation fracture development zone is in a range of 120~127 m.

(3)

The field resistivity measurement period is complete, and the effective electric field data are obtained from 393 m in front of the orifice to 120 m behind the orifice. This effectively captures information about the overburden process from initial stability to deformation, then during fracture development, fracture and secondary stability after mining, which has a certain guiding role in the analysis of overburden deformation and fracture development rules.