Study on the Dynamic Evolution of Overburden Bed Separation Based on a Bed Separation Water Disaster

Abstract

Water inrush from separation layers is a special type of water inrush disaster that occurs during coal mining. It is characterized by the absence of precursors before the inrush, a sudden and massive influx of water, strong destructive power, and significant difficulty in prevention and control. This disaster causes substantial economic losses and casualties in mines. To study the dynamic evolution characteristics of the overlying rock strata in Guojiahe coal mine, field investigations, theoretical analysis, physical simulations, and numerical simulations were conducted. The development state of the overlying rock strata and the height of the water-conducting fractures in the 1304 working face during continuous mining were analyzed, and the spatial position of the strata where water hazards could form was determined. Through theoretical calculations, the height of the water-conducting fracture zone was found to be approximately 205 m, forming a water-storable separation space about 248 m above the coal seam roof. A physical similarity model was constructed to study the development state of overlying rock strata separation in the 1304 working face. Experimental results indicate that, as the working face advances, the rock strata gradually develop lateral separation spaces from bottom to top, and vertical fractures increase progressively in a similar manner. The spaces and fractures formed in the early stages of the working face advance gradually close and form a compaction zone. However, new fractures develop near the working face side. As mining continues, the asynchronous movement of the overlying rock promotes the development of vertical fractures, providing water-conducting channels for water inrush from the separation. Based on numerical simulation analysis of the development state of overlying rock strata separation and the height of the water-conducting fracture zone, it can be concluded that the water-storable separation space develops between the 8th layer of the formation of sandy mudstone and the 9th layer of the Yijun Formation conglomerate, which is 248 m from the coal seam roof. When excavated to 280 m, the separation space reaches its maximum, with a width of about 19 m and a height of about 1.2 m, and the water storage capacity increases to its maximum. The height of the water-conducting fracture zone eventually increases to 202 m.

1. Introduction

Mine water inrush has become a major disaster facing mines in China. The damage to overlying rock caused by high-intensity mining is becoming increasingly severe, and the complex geological environment makes the mechanism of water inrush more complex and difficult to predict [1]. During the mining process, differences in lithology and the physical–mechanical properties of various rock strata cause adjacent layers to deform and settle at different rates, resulting in separation between the layers. Under the continuous infiltration of surrounding rock water into the separation space, a water-storable separation layer is formed. As the mining area continues to advance, the combined effects of the subsidence of the upper rock layers, increased internal water pressure, and the tendency of the lower rock layers to develop fractures cause vertical fractures to connect with the water-storable separation layer, resulting in water inrush hazards. Separation layer water accumulation is a special manifestation of roof water. During mining activities, overlying rock strata separation occurs, and the accumulated water in the separation layer can break under the influence of water pressure, stress, or mining disturbances, leading to a rapid dispersal of water to the underground, causing disasters. Before the infiltration of accumulated water in the separation layer, there are no warning signs. During a water inrush, the water volume is immense and instantaneous, with strong destructive power, making prevention difficult and causing significant losses to many mines [2]. It can be seen that analyzing the dynamic evolution characteristics of overlying rock strata, studying the development height of water-conducting fractures, and predicting the water inrush volume are key to preventing separation layer water hazards [3,4]. This is also one of the main research hotspots in the field of mine water hazard prevention and control in China.

Qian Minggao, Xu Jialin, and others [5] have conducted a large number of experiments and theoretical analyses on the position, thickness, and influencing factors of the roof strata separation layer. They found that the key stratum mainly controls the migration and failure of the roof strata, and that the separation layer is most likely to occur at the interface between the key stratum and the roof strata, with its maximum development height controlled by the key stratum. Zhang Jianquan, Liao Guohua, et al. [6] theoretically studied the mechanical structure and stress conditions of the formation of overlying rock strata from the mechanical properties and structural state of rocks. They analyzed the occurrence sites of overlying rock strata and provided a maximum delamination prediction method for those overlying rock strata. Qiao Wei [7] studied the fracture laws of the upper rock strata and lower impermeable water-bearing strata of the Cuimu and Zhaoxian mines in the Yonglong mining area and analyzed the conditions for the occurrence of sudden water in layered spaces. They constructed a structural mechanics model of the “sudden water layer zone” in the mining rock mass and proposed a prediction method for water inrush from layers. Skrzypkowski, K. [8] conducted strength tests on sandstone and claystone samples that were heated to temperatures as high as 1200 °C and then exposed to water, determining the range of rock failure zones. The study indicates that claystone layers exhibit significant crack propagation when exposed to water and after being subjected to high temperatures, leading to an increased degree of rock structure damage. Additionally, the mechanical properties of the rocks, such as compressive strength, tensile strength, and Young’s modulus, were significantly reduced. Through a series of experiments and numerical simulations, it was concluded that the presence of water significantly affects the mechanical properties of the rocks, thereby having an important impact on the range of rock failure zones. Savvides, A.-A. [9] improved the estimation method of rock material parameters in geological engineering using a neural network model. This model not only converges quickly and reduces computational costs but also demonstrates excellent predictive capability when optimizing the stiffness and strength of rock materials, providing accurate material parameter estimates at a lower computational cost.

Currently, extensive research has been conducted both domestically and internationally on the formation mechanisms and distribution patterns of separation layers [10,11,12,13,14,15,16,17], but studies on the dynamic evolution patterns of separation layers are relatively scarce. This study takes the 1304 working face of the Guojiahe mine as the research object [18,19]. Through methods such as physical simulation and numerical simulation, the dynamic evolution of the overlying rock strata in the mining area is reproduced, providing technical support for the prevention and control of water inrush in similar mines.

2. Hydrogeological Characteristics of the Study Area

This project takes the 1304 working face of the Guojiahe Mine’s west wing in the first panel area as the engineering background. The mine advances from west to east, with a working face length of 235 m, a strike length of 1374 m, a thickness of 11.60 m, and a burial depth of 646 m. The coal seam dip angle is less than 9°, making it a nearly horizontal coal seam. The immediate roof of the coal seam is mainly composed of medium-grained sandstone and mudstone, with compressive strengths of 24.45 MPa and 38.61 MPa, respectively. It is a relatively stable and minable coal seam. The mining process is comprehensive mechanized caving mining, implementing a “mining one, caving two” scheme. Due to the single-layer mining height of 3.5–3.8 m, the coal seam mining height is 10.5–11.4 m, generally 11 m. The overlying strata include the Yan’an Formation, Zhiluo Formation, Anding Formation, Yijun Formation, and Luohe Formation and the Neogene and Quaternary systems, as shown in Figure 1 [20].

The main aquifers in the field are fourth series loess pore–fissure type groundwater aquifers composed mainly of loess, sandy loess, and ancient soil. The Luohe Formation pore–fissure aquifer is mainly composed of medium-fine sandstone and sandstone gravel, with the aquifer mainly consisting of medium-grain sandstone. The Yijun Formation gravel fissure aquifer is composed of purple and variegated blocky conglomerates, mainly consisting of quartz and pyrite. The Zhiluo Formation sandstone fissure aquifer consists of sandy mudstone, siltstone to medium-coarse sandstone above, medium-coarse sandstone with sandy mudstone and siltstone below, and a massive layer of coarse sandstone at the bottom. The content of medium-to-coarse-grained sandstone ranges between 50–90%. The Yan’an Formation coal seam and roof sandstone aquifer is composed of the 3rd coal seam and the medium-to-coarse-grained sandstone and sandstone conglomerate at its top as the aquifer medium. This aquifer consists of 50–70% medium-to-coarse-grained sandstone and sandy rocks. The thickness of the aquifer is generally between 20–30 m.

The stable layer in the Neogene strata is composed of clay, with the upper part consisting of granular clay and sub clay, and the lower part containing clay rich in calcium components, along with several calcareous nodular layers. Overall, this formation exhibits good impermeability and stability, serving as a stable layer between loose rock layers and aquifers in the field. The waterproofing of the Anding Formation mudstone consists of mudstone interbedded with sandy mudstone and medium-coarse sandstone, with a thick sandstone layer at the bottom. Mudstone and sandy mudstone account for 50–90% of the content, forming a stable waterproof layer.

3. Prediction of Water-Storable Separation Layer Location

As the goaf area continues to expand, the roof separation layer of the goaf continuously develops. The water in the aquifer around the goaf enters the separation layer through fractures. As the water-conducting fracture zone develops close to the separation layer, both the separation layer water and overlying rock deformation increase. The separation layer will form a certain static water pressure under the action of separation layer water, thereby exerting a certain pore pressure and load on the lower rock strata, leading to the destruction of the surrounding rock structure below the separation layer and reducing the effective water-resistant layer thickness. Under the action of separation layer water load, the height of the water-conducting fracture zone increases, penetrating the water-storable separation layer and leading to water inrush in the separation layer.

3.1. Translation to English: Discrimination of Separation Layer Position Based on Key Stratum Theory

The separation layer is caused by the way in which the shear force which acts between the layers—as a result of the weight of the stratified rock mass and the shear force acting on the contact surface—is greater than the bonding force on the contact surface of the rock layer itself under mining activities. The occurrence position of the separation layer is determined using the key stratum theory. The lowest hard and thick layer in the overlying strata is called the main key layer and it plays an important controlling role in the position and state of the separation layer formation. A composite key layer composed of multiple strata with different lithologies can be considered as a whole, and the lithology can be comprehensively judged based on the elastic modulus, specific gravity, and thickness of each stratum. On this basis, the overlying rock structure is abstracted as a plane strain problem, and the composite beam theory is applied to conduct a mechanical analysis. When there is a significant difference in flexural deformation among the composite beams, interlayer separation phenomenon will occur. In each group of composite beams, the load of the nth layer of rock on the bottommost 1st layer of rock in that group is as follows:

$${\mathrm{q}}_{n,1}={\displaystyle \frac{E_1{h}_1^3{\displaystyle \sum _{i=1}^n{\gamma }_i{h}_i}}{{\displaystyle \sum _{i=1}^n{E}_i{h}_i^3}}}={\displaystyle \frac{E_1{h}_1^3({\gamma }_1{h}_1+{\gamma }_2{h}_2+\cdots +{\gamma }_n{h}_n)}{E_1{h}_1^3+E_2{h}_2^3+\cdots +E_n{h}_n^3}}$$

(1)

where E_i represents the elastic modulus of each layer, n is the number of layers in the group, h_i represents the thickness of each layer, and γ_i represents the unit weight of each layer.

When q_n,1 is greater than q_n+1,1, the upper rock stratum no longer exerts a load on the lower rock stratum, which indicates the occurrence of a separation layer between these two layers. The discriminant is q_n,1 > q_n+1,1, as follows:

$${\displaystyle \frac{E_1{h}_1^3{\displaystyle \sum _{i=1}^n{\gamma }_i{h}_i}}{{\displaystyle \sum _{i=1}^n{E}_i{h}_i^3}}}>{\displaystyle \frac{E_1{h}_1^3{\displaystyle \sum _{i=1}^{n+1}{\gamma }_i{h}_i}}{{\displaystyle \sum _{i=1}^{n+1}{E}_i{h}_i^3}}}$$

(2)

After rearranging the formula, we find the following:

$$E_{n+1}{h}_{n+1}^2{\displaystyle \sum _{i=1}^n{\gamma }_i{h}_i>{\gamma }_{n+1}{\displaystyle \sum _{i=1}^n{E}_i{h}_i^3}}$$

(3)

Based on the comprehensive analysis results and the determination formula for separation layers, combined with the overlying rock mechanics parameters [20] (as shown in

Table 1. The main physical and mechanical parameters of the overlying strata of the No. 3 coal seam.

Number	Name of Stratum	Stratigraphic Age	Thickness (m)	Density (kg/m3)	Modulus of Elasticity (103 MPa)	Poisson’s Ratio
11	Loess	Quaternary System	91	1470	0.21	0.37
10	Red earth	Neogene	109	2020	1.04	0.35
9	Gravel rock	Luohe Formation and Yijun Formation	198	2600	16.72	0.30
8	Sandy mudstone	Anding Formation	29	2510	0.81	0.36
7	Mudstone	Anding Formation	87	2590	1.57	0.38
6	Sandy mudstone	Anding Formation	46	2510	0.81	0.36
5	Coarse sandstone	Anding Formation	15	2490	10.01	0.28
4	Medium grained sandstone	Zhiluo Formation	24	2420	12.30	0.28
3	Siltstone	Zhiluo Formation	16	2480	1.87	0.33
2	Coarse sandstone	Yan ‘an Formation	18	2490	10.01	0.28
1	Carbonaceous mudstone	Yan ‘an Formation	13	2480	1.87	0.33
12	No. 3 coal	Yan ‘an Formation	11	1380	1.42	0.45
13	Medium grained sandstone	Fuxian Formation	13	2420	12.30	0.28

), the possible locations of separation layers in the study area are determined.

According to Formula (3), we determine whether a separation occurs between the 1st and 2nd layers. We substitute the data of serial number 1 into the left side of the equation, as follows: 10.01 × 18² × 2480 × 13 ≈ 10⁷. We substitute the data from entry 2 into the right side of the equation, as follows: 2490 × 1.87 × 13³ ≈ 10⁶. When the left side is greater than the right side, a separation layer will form between the first and second layers. However, as mining continues, the first layer will quickly collapse, causing the separation layer to disappear rapidly. According to Formula (3), we determine whether a separation occurs between layer 2 and layer 3. Because separation has already occurred between layer 1 and layer 2, layer 2 will be considered as the first layer of this composite beam. We substitute the data of sequence 2 into the left side of the equation, as follows: 1.87 × 16² × 2490 × 18 ≈ 21 × 10⁶. We substitute the data from sequence number 3 into the right-hand side of the equation, as follows: 2480 × 10.01 × 18³ ≈ 14 × 10⁷. The left side is less than the right side, so no separation will occur between the second and third layers.

According to Equation (3) and following the above sequence of operations, it can be concluded that separation layers will form between layers 1 and 2, between layers 3 and 4, between layers 6 and 7, and between layers 8 and 9.

3.2. Prediction of Water-Conducting Fracture Zone Height Based on Multivariate Nonlinear Regression

By comprehensively considering the quantitative factors of mining thickness, working face oblique length, and coal seam burial depth on the height of the water-conducting fracture zone, a multi-factor nonlinear regression prediction model is constructed under the condition of comprehensive mining with weak overlying strata [21]. This model provides a certain basis for predicting the development height of water-conducting fractures in the 1304 working face of Guojiahe Mine.

Research analysis reveals that there is a nearly linear relationship between the height of developed water-conducting fractures and the mining thickness, a logarithmic relationship with the inclined length of the working face, and an exponential relationship with the burial depth of the coal. Based on the above functional relationships, the development height of water-conducting fractures in soft overlying strata with respect to each factor was established, fitted, and the following regression prediction model was derived:

When considering only the mining thickness, the following regression model was derived:

$$H_{li}={\displaystyle \frac{100{\displaystyle \sum M}}{-0.05{\displaystyle \sum M}+8.34}}+53.49\pm 33.19$$

(4)

where H_li represents the height of the water-conducting fracture zone, measured in meters (m), and ΣM represents the mining thickness, measured in meters (m).

Based on the comprehensive consideration of mining thickness and working face length, a corresponding regression model is derived, as follows:

$$H_{li}={\displaystyle \frac{100{\displaystyle \sum M}}{0.21{\displaystyle \sum M}+3.73}}+80.38\ln L-397.51\pm 24.59$$

(5)

where L represents the inclined length of the working face, measured in meters (m).

Taking into account the three factors of mining thickness, working face inclination length, and coal seam burial depth, a corresponding regression model is derived, as follows:

$$H_{li}={\displaystyle \frac{100{\displaystyle \sum M}}{0.17{\displaystyle \sum M}+0.76}}+58.30\ln L+4.71\sqrt{S}-630.8\pm 20.53$$

(6)

where S represents the burial depth of the coal seam, measured in meters (m).

Using the measured data of the four mines in

Table 2. Measured value of the water-conducting fracture zone height in the Yonglong mining area.

Mine Name	Working Face	Mining Thickness (m)	Strike Length of the Working Face (m)	Depth of Coal Seam (m)	Measured Height of Fracture Zone (m)
Cuimu coal mine	21301	12.0	196	553	239
Guojiahe coal mine	1302	9.0	268	615	209
Zhaoxian coal mine	1307	10.6	155	600	198
Yuanzigou coal mine	1022101	10.7	200	775	253

, we substitute them into Formulas (4)–(6) to determine which formula has the highest prediction accuracy. We then use the formula with the highest accuracy to calculate the height of the water-conducting fracture zone in the 1304 working face of Guojiahe coal mine. Through analysis, it is concluded that, by comprehensively considering multiple influencing factors, the prediction accuracy and rationality can be improved. Therefore, according to Formula (6), we calculate the height of the water-conducting fracture zone for the 1304 working face of Guojiahe coal mine. The burial depth of this working face is 646 m, the strike length is 235 m, and the coal seam thickness is 11 m. The calculated height of the water-conducting fracture zone for the 1304 working face is 205 m. From the calculation results, it can be inferred that the water-conducting fracture zone of the 1304 working face extends to the mudstone in the middle part of the Anding Formation, namely the 7th rock layer. Through the analysis above, we can see that the 9th layer of the 1304 working face is the main aquifer and that the water-conducting fracture zone extends to the 7th rock layer. Therefore, a water storage space can be formed between the 8th and 9th layers, with the water supply coming from the aquifer in the 9th layer.

4. Physical Similarity Test Simulation Process Analysis

Based on the geological conditions of the Guojiahe coal mine and the stratigraphic columns revealed by boreholes, and in conjunction with the objectives and theoretical basis of the similar materials test, the Mohr–Coulomb criterion was used for this test. The experiment was conducted using a two-dimensional similar materials simulation test platform [22,23]. Before laying out the model, tempered glass was installed at the front and back of the experimental frame to provide boundary constraints and was sealed with glass glue. Next, according to the model parameter ratio table (as shown in

Table 3. Model material proportion parameters.

Number	Lithology	Proportion
1	Loess	2231/1584/102/112 (loess/sand/butter/petroleum jelly)
2	Red earth	2231/1584/102/112 (red earth/sand/butter/petroleum jelly)
3	Gravel rock	4276/85 (sand/paraffin wax)
4	Sandy mudstone	4276/65 (sand/paraffin wax)
5	Mudstone	4276/60 (sand/paraffin wax)
6	Coarse sandstone	4276/75 (sand/paraffin wax)
7	Medium grained sandstone	4276/80 (sand/paraffin wax)
8	Siltstone	4276/70 (sand/paraffin wax)
9	Carbonaceous mudstone	4276/60 (sand/paraffin wax)
10	coal	1129/1129/26 (coal powder/sand/gypsum)

), the mass of river sand required for laying different types of bedrock in layers was weighed, and paraffin wax was measured using a balance. Then, the paraffin wax was heated with an electric stove and mixed evenly with the river sand. The evenly mixed material was then poured into the experimental frame, leveled, and compacted. Mica powder was sprinkled between the bedrock layers to achieve stratification.

Using model frame dimensions of 1.2 m × 0.2 m × 1.2 m, and according to the geometric similarity ratio (1:250), when the model is mined, 4 cm coal pillars are left on the left and right sides to eliminate the boundary effect. The mining length of the model is 112 cm, and the advancing distance of the working face of the prototype is 280 m. The working face advances from the left end to the right end, with each 4 cm advancement corresponding to an actual distance of 10 m. According to the design, a cutting eye is set 4 cm from the model boundary, and then the advancement is made from the left side of the model to the right side, as shown in Figure 2.

When the working face advanced to 16 cm (equivalent to 40 m in reality), the immediate roof collapsed for the first time, covering the entire goaf. At a distance of 13 m from the roof, a fracture of about 5 m in length was formed. When the working face advanced to 60 m, periodic collapses occurred in the immediate roof, and the first fracture gradually disappeared with the increase in collapse height. When the working face advanced to 100 m, at a distance of about 45 m from the coal seam roof, sandstone and the overlying strata suddenly fractured at both ends and in the middle, exhibiting combined movement. This created a separation space with a height of approximately 0.6 m and a length of about 8 m.

As shown in Figure 3, as the working face advances to 64 cm, the corresponding actual length is 160 m. At a distance of about 132 m from the top of the coal seam, a high space of about 1 m in height and 15 m in length is formed. Due to mining activities, water-bearing fractures develop towards the lower right of the space away from the seam, and the fractures in the middle of the goaf gradually close as the overlying strata collapse and compact. The collapsed rock mass generally exhibits a trapezoidal structure.

As the work face continues to advance, the overlying strata continue to collapse, and collapse zones, fracture zones, and curved subsidence zones gradually form. With the increase in the distance of the work face advancement, the height of overlying strata collapse and the development of the “three zones” also increase, until the work face advances to 240 m, as shown in Figure 4. The separation space develops to about 248 m above the coal seam roof, reaching below the gravel layer that is above the coal seam roof, with a height of about 0.8 m and a length of about 22 m. At this time, the water-conducting fracture zone has not yet developed to the separation space and remains relatively stable with subsequent mining activities.

In summary, through physical similarity experiments, the evolution law of the separation of overlying strata during the mining period was studied. The research shows that, as the working face advances, the overlying strata of the coal seam gradually develop horizontal separation spaces from bottom to top, appearing between the first and second layers, between the third and fourth layers, between the sixth and seventh layers, and between the eighth and ninth layers. The height of the water-conducting fracture zone increases continuously as the working face advances. The transverse bedding spaces and longitudinal fractures formed in the early stage will gradually compact and form closed areas as the overlying strata collapse. On the side close to the working face, new fracture zones will be generated. In this process, the asynchrony of overlying strata movement continuously promotes the development of longitudinal fractures, providing conditions for sudden water hazards in the water storage separation space.

5. Numerical Simulation Analyses of the Dynamic Evolution Process of Overlying Rock Strata

Universal distinct element code (UDEC, v.7.0) is a numerical simulation software developed by Itasca (Minneapolis, MN, USA). It is used to analyze the mechanical response of geotechnical materials under different stress conditions. The software employs the distinct element method, simulating the discrete particles of rock and soil and their interactions, enabling accurate simulation of complex geological phenomena.

Because the dip angle of the No. 3 coal seam in Guojiahe coal mine is less than 9 degrees, it is considered a gently inclined coal seam, which can be treated as a horizontal coal seam in numerical simulations. According to the actual overlying rock (as shown in

Table 4. Parameters summary table of the Guojiahe coal mine rock mechanics.

Rock Name	Density (kg/m3)	Bulk Modulus (GPa)	Shear Modulus (GPa)	Friction (°)	Cohesion (MPa)	Tensile Strength (MPa)
Gravel rock	2600	9.50	6.05	59.40	1.54	0.67
Mudstone	2590	0.98	0.55	45.56	0.98	0.58
Sandy mudstone	2510	0.55	0.32	45.88	0.29	0.40
Medium grained sandstone	2420	7.38	4.43	58.40	0.78	0.61
Siltstone	2480	2.00	0.62	46.90	0.56	0.25
Coarse sandstone	2490	0.35	0.33	59.40	0.46	0.33
Carbonaceous Mudstone	2480	0.60	0.34	45.56	1.06	0.40
No. 3 coal	1380	0.75	0.52	39.45	0.30	0.12

) characteristics and mining conditions of Guojiahe coal mine 1304 working face, a model is established in the direction along the working face, simulating a height up to the top of the Yijun Formation at 297 m, with a length of 330 m for the working face model, and simulating a thickness of 11 m for the mining of the No. 3 coal seam. The initial model for numerical simulation is shown in Figure 5.

Considering the impact of boundary effects on the calculation, protective coal pillars of 20 m are left on both sides of the model. As the model does not include surface values, the strata and loose layers not considered are applied as a load on the upper boundary. The model’s upper boundary is buried to a depth of 349 m, with an equivalent load of 8.55 MPa applied. The lower boundary and the left and right sides are treated as zero-displacement constraint boundaries. During the mining simulation, a single excavation step distance of 20 m is used.

5.1. Analysis of the Development Process of Overlying Rock Strata

When excavating the first step, that is, advancing the working face by 20 m, there was no collapse of the roof overlying rock. When excavating the second step, that is, advancing the working face to 40 m, as shown in Figure 6a, the roof rock layer is exposed. Under the influence of gravity, the bent rock beam hangs to a certain span. After the bending settlement reaches the critical strength, a separation space appears between the first and second layers, namely between the Yan’an group of carbonaceous mudstone and the Yan’an group of coarse sandstone, with a separation space length of about 5 m. Some immediate roof collapses, with a collapse height of about 9.8 m, and the collapsed rock layer covers the entire goaf.

In Figure 6b, when excavating in the 5th step, that is, when the mining length reaches 100 m, the pressure on the overlying strata becomes more intense. Breakage occurs at both ends and the middle part at a distance of about 45 m above the top of the coal seam, exhibiting a combined movement and forming a separated space between the overlying strata above. The maximum height of the space is about 0.6 m. At this time, the fissure water in the 4th layer of overlying strata will flow into the goaf; however, since the water-conducting fissures will develop through the separated space in a short period of time, the amount of accumulated water in the space is not large. The water inflow situation does not have the characteristics of rapidity, large instantaneous water volume, difficult to control, etc.

When excavating in the 6th step, as shown in Figure 6c, with an excavation length of 120 m, the separation space rapidly develops upwards. As the separation space between the 3rd and 4th layers gradually closes, the separation space between the 6th and 7th layers begins to appear and gradually increases. In Figure 6d, when excavating to the 8th step, i.e., with a mining length of 160 m, the separation space between the 6th and 7th layers, that is, the separation space between the Anding Formation sandy mudstone and the mudstone of the Anding Formation reaches its maximum The maximum width of the space is about 15 m, with a height of approximately 1 m, located 130 m from the top of the coal seam. The collapsed rock mass and the goaf exhibit a trapezoidal structure, with a collapse angle of 66°.

In Figure 6e, the excavation reaches 200 m, and, for the first time a separation space appears between the 8th impermeable rock layer and the 9th aquifer layer, which may become a water storage space. By 220 m of excavation, as shown in Figure 6f, the separation space between the 8th and 9th layers reaches its maximum development. At this point, the maximum width of the space is about 22 m, and the height is approximately 0.9 m.

When the excavation length reaches 240 m, a second separation space is formed between the 8th and 9th layers in the direction of the advancing working face, and continues to develop with ongoing mining. The first separation space gradually compacts, significantly reducing its width and height. When the excavation length reaches 280 m, as shown in Figure 6g, the second separation space between the 8th and 9th layers reaches its maximum development. At this point, the maximum width of the second separation space is about 19 m, and the height is about 1.2 m, with its water storage capacity reaching its maximum.

5.2. Analysis of the Development Process of Water-Conducting Fissure Zones

As the working face advances to 40 m, the central part of the goaf mainly experiences tensile stress, while compressive stress appears at both ends of the goaf. Subsequently, rock layers fracture and delaminate, with the height of the water-conducting fracture reaching 11 m. When the working face advances to 100 m, there is a significant change in the stress distribution of the surrounding rock, but no obvious change in direction. At this stage, the elevation of the water-conducting fracture zone increases to 39 m. By the time the working face reaches 160 m, the central part of the goaf mainly exhibits horizontal compressive stress, while tensile stress occurs at the sides of the goaf. The water-conducting fracture zone at the top of the goaf develops to 132 m. Subsequently, the height of the water-conducting fracture zone increases slowly.

As the working face advances to 220 m, the squeezing and stretching stress on the overlying strata of the goaf gradually increases. At this point, the height of the water-conducting fracture zone has increased to 175 m. As the working face advances to 240 m, the height of the water-conducting fracture zone increases to 202 m. After that, with the extension of the mining distance, the height of the water-conducting fracture zone no longer increases.

Based on the above analysis, before the working face advances to 100 m, and though the extent of overlying rock damage gradually increases, the height of the fracture zone remains relatively low. During the advancement of the working face from 100 m to 160 m, the height of the fracture zone rapidly rises, jumping from 39 m to 132 m, marking the first abrupt development process of the water-conducting fracture zone height. Subsequently, the height of the water-conducting fracture zone continues to increase slowly. A cantilever beam structure is formed above the goaf, suppressing the upward expansion of the fractures. When the mining face advances from 220 m to 240 m, the height of the mining face fracture zone rapidly increases, jumping from 175 m to 202 m, marking the second abrupt development process of the water-conducting fracture zone height, after which the height of the water-conducting fracture zone no longer increases.

Comparing the results of numerical modeling with the fracture zone range obtained from physical model testing, there is good consistency between the physical simulation and numerical modeling results regarding the range and evolution patterns of the fracture zone. The physical simulation visually demonstrated the formation process and spatial extent of the fracture zone, while the numerical modeling verified the physical simulation results by simulating the dynamic changes in the rock layers at different advancement distances.

6. Conclusions

(1)

According to the key stratum theory, the determination of the separation development position is as follows: between the first layer, Yan’an Formation carbonaceous mudstone, and the second layer, Yan’an Formation coarse sandstone; between the third layer, Zhiluo Formation siltstone, and the fourth layer Zhiluo Formation medium-grain sandstone; between the sixth layer, Anding Formation sandy mudstone, and the seventh layer Anding Formation mudstone; and between the eighth layer, Anding Formation sandy mudstone, and the ninth layer Yijun Formation conglomerate. Based on the estimated height of the developed aquifer fracture zone, we can observe that the main concentration of overlying strata water separation development in coal mining is between the Yijun Formation conglomerate layer and the Anding Formation sandy mudstone. The main aquifer in the Yijun and Luohe groups at the 9th level can serve as a replenishment water source for the aquifer, so, with the disturbance of mining, there is a high possibility of aquifer water inrush between the 8th and 9th levels

(2)

Through a similar experimental method, the evolution process of the overlying strata under mining action was studied, revealing the gradual development of horizontal stratified space from bottom to top along the mining process in the stope. As the working face advances, the horizontally stratified space formed in the early stage and the vertical fissures will gradually compact and form closed zones as the overlying strata collapse. On the side closer to the working face, a new fissure zone will form, while the asynchrony of overlying strata movement promotes the development of vertical fissures, thereby creating conditions for sudden water inrush disasters induced by water storage space. When the working face advances to 240 m, the stratified space develops to about 248 m above the coal seam roof, between the 8th and 9th layers, with a space height of about 0.8 m and a length of about 22 m. At this point, the water-conducting fissure zone has not yet developed into the stratified space and remains relatively stable with subsequent mining. The water-storing stratified space, as surrounding water continuously converges into it, generates a certain static water pressure, exerting pore pressure and load on the underlying rock layers. The pore pressure disrupts the rock structure at the bottom of the stratified space, reducing the effective thickness of the aquiclude. The water load on the strata causes an increase in the height of the fissure zone at the bottom of the strata, potentially leading to water breakthrough in the aquiclude due to the formation of water-conducting fissures.

(3)

Using numerical simulation techniques, we studied the development of the off-layer position and water-conducting fracture height during the mining of the 1304 working face of Guojiahe coal mine. When excavating to 220 m, the first off-layer appears between the 8th sandy mudstone layer and the 9th conglomerate layer, with a width of about 22 m and a height of about 0.9 m. The water-conducting fracture zone jumps to 175 m. When excavating to 240 m, the second off-layer appears, and the first off-layer gradually compacts, with the water-conducting fracture zone height increasing to 202 m. After that, the height of the water-conducting fracture zone does not continue to increase. When excavating to 280 m, the space of the second off-layer reaches its maximum, with a width of about 19 m and a height of about 1.2 m. This off-layer is a water-storing off-layer, and at this point, the water storage capacity of the off-layer has reached its maximum.