Evolution of Overlying Strata Bed Separation and Water Inrush Hazard Assessment in Fully Mechanized Longwall Top-Coal Caving of an Ultra-Thick Coal Seam

Abstract

Bed-separation water hazards are a common and very harmful mining disaster in the mining areas of western China in recent years, which seriously threatens the safe mining of rich and thick coal seam resources in the West. The Yonglong mining area has become a high-risk area for bed-separation water hazards due to its particularly thick coal seams and strong water-rich overlying strata. In view of this, this paper investigates the development height of a water-flowing fractured zone in the fully mechanized caving mining of an ultra-thick coal seam in the Yonglong mining area, the evolution law of the bed separation of overlying strata, and the process of water inrush from a bed separation. Based on the measured water-flowing fractured zone height data of the Yonglong mining area and several surrounding mines, a water-flowing fractured zone height prediction formula suitable for the geological conditions of the Yonglong mining area was fitted. By using discrete element numerical simulation and laboratory similarity simulation, the evolution law of overlying strata separation under the conditions of fully mechanized caving mining in the study area was analyzed, and the space was summarized into “four zones, three arches, and five zones”. Through the stress-seepage coupling simulation of the water inrush process of the roof separation in the fully mechanized caving mining of an ultra-thick coal seam, the migration, accumulation, and sudden inrush of water in the aquifer in overlying strata under the influence of mining were analyzed, and the variation in the pore water pressure in the process of water inrush during coal seam mining separation was summarized. The pore water pressure in the overlying strata showed a trend of first decreasing, then increasing, and, finally, stabilizing. Combined with the height, water inrush volume, and water-rich zoning characteristics of the water-flowing fractured zone of the 1012007 working face of the Yuanzigou Coal Mine, the danger of water inrush from the overlying strata separation of the working face was evaluated. It is believed that it has the conditions for the formation of water accumulation and separation, and the risk of water inrush is high. Prevention and control measures need to be taken on site to ensure mining safety. The research results have important guiding significance for the assessment and prevention of water inrush hazards in overlying strata during fully mechanized longwall top-coal caving of ultra-thick coal seams with similar geological conditions worldwide.

1. Introduction

In recent years, as coal resources in eastern China have been gradually depleted, the focus of coal resource development has gradually shifted to the northwest. The depth of coal seams has increased, and roof water disasters have emerged, especially abscission water disasters and water inrush accidents in weakly cemented conglomerate aquifers [1,2]. The coal seams in some mining areas are thick and the overlying strata is highly water-rich and has a large water content. During the strong movement and deformation of the overlying rock caused by the fully mechanized caving mining of ultra-thick coal seams, the separation space in the overlying strata will be connected to the lower working face through mining cracks and bed separation. A large amount of water in the space quickly broke into the working surface, causing water inrush disasters in the bed separation (Figure 1) [3,4]. The main reason is that, under the conditions of the fully mechanized caving mining of the ultra-thick coal seam in this area, the overburden disturbance range is large and the degree of damage is severe. The special combined structure of the overlying strata and the hydrogeological conditions make the formation mechanism of roof water hazards very complicated. Under the influence of mining, due to the differences in rock mass structure, lithology, and physical and mechanical properties, the timing, speed, and amplitude of the breaking, rotation, and sinking of adjacent rock layers are different, and the separation space is formed after the uncoordinated deformation occurs [5,6]. When there are aquifers in the rock formations around the delamination space, the delamination space is filled with water to form cavity water accumulation. As the working face continues to advance, the delamination space generated by the adjacent rock formations is penetrated by vertical cracks, forming a water-conducting fracture network. Under the synergistic effect of the sinking of the upper rock formation, the internal water pressure and the lower rock formation that tends to break, the delamination water body quickly flows into the lower goaf or working face along the lower water-conducting fracture network, forming a mining-induced overburden delamination water hazard [7,8]. The characteristics of coal seam mining-induced overburden delamination water hazards are large instantaneous water inflow, short duration, and rapid attenuation. Delamination water hazards can cause losses and hazards of varying degrees. In mild cases, it will increase the water inflow of the working face and the mine drainage burden, affecting the normal production of the coal seam. In severe cases, it will cause the working face to be flooded, resulting in casualties and damage to mechanical equipment, causing major safety and economic losses [9,10].

According to incomplete statistics, a total of 1196 major water inrush accidents occurred in China in the past 20 years (Table 1), resulting in 4775 deaths and disappearances, and direct economic losses of more than tens of billions of yuan [12,13,14]. Therefore, effectively solving this problem is crucial for the safe and efficient realization of fully mechanized caving mining in extra-thick coal seams. Water inrush disasters are relatively common in underground engineering, but the causes of water disasters are not the same. The current methods for studying the mechanism of underground water disasters can be roughly divided into theoretical analysis, numerical simulation, similarity simulation, field research, and a combination of multiple methods [15,16,17,18,19,20,21]. Compared with other water disasters in coal mines, water inrush from coal seams has the characteristics of unclear signs of water inrush, large water volume, and instantaneous flooding, making it difficult to effectively prevent and control [22]. The formation of water in the coal seam overburden layer must meet three basic conditions: the existence of a water-accumulating layer, the existence of a replenishment water source around the layer, and the duration of the layer space is long enough [23]. The characteristics of overburden damage caused by mining are the basis for studying the mechanism of water inrush from delamination. It is generally believed that overburden damage caused by mining is zoned, and the different degrees of deformation, movement, and damage of the overburden caused by coal seam mining are divided into collapse zones, fracture zones, and curved subsidence zones [24,25,26,27,28,29]. Gao et al. [30], based on the “three-zone” model, divided the lower part of the curved subsidence zone into a separate delamination zone and proposed the “four-zone” model, which provided a basis for the delamination grouting construction design [31] and delamination water hazard prevention and control. Considering that the hydrogeological conditions in different mining areas vary greatly, the laws and prevention methods of water inrush during coal seam mining also vary greatly. Identifying the mechanism of water inrush is the key to mine water hazard prevention and control [4]. Relevant scholars at home and abroad have conducted a lot of research on the mechanism and prevention of water inrush in overburden delamination. Fan [2], Qiao [23], Ji [32], Zhang [33], and others studied the mechanism of water inrush and disaster caused by overburden delamination. Some scholars [34,35] studied mining methods, including strip mining and room-and-pillar mining, to control the deformation and damage of the overburden, slow down the surface subsidence, and reduce the risk of water inrush from delamination; however, this method has a low coal recovery rate. In recent years, with the introduction of the concept of green mining in coal mines, delamination grouting and subsidence reduction technology has gradually gained favor. This method effectively slows down and prevents the expansion of delamination by controlling the key layer from breaking, greatly reducing the damage to the aquifer [31,36,37,38]. Many scholars [39,40,41,42] used different methods to predict the development height of water-conducting fracture zones in different regions after coal seam mining. Teng [17], Jiang [18], Li [19], and Feng [20] obtained the factors and laws that affect the development height of water-conducting fracture zones.

Although many studies have been conducted on the formation mechanism of abscission water disaster, the height of the water-conducting fracture zone, the disaster-causing mechanism, and prevention and control technology [43,44,45,46], there are few studies on the evolution law of overburden abscission and the evaluation of water inrush hazards in fully mechanized caving mining of extra-thick coal seams, and the evolution mechanism of abscission water inrush needs to be further studied [47,48,49]. In particular, the movement and deformation of overburden, the evolution process, and the mechanism of abscission in fully mechanized caving mining of extra-thick coal seams are still unclear. In view of this, taking the fully mechanized caving mining of extra-thick coal seams in the Yonglong mining area of Huanglong Coalfield in China as the research background, the height of water-conducting fracture zone, the evolution law of overburden abscission, and the water inrush process of roof abscission in the fully mechanized caving mining of extra-thick coal seams in this area were studied in depth in order to provide theoretical support for the prevention and control of roof abscission water inrush in extra-thick coal seam mining in the Yonglong mining area and ensure the safe production of coal mines.

Table 1. Summary of major water inrush incidents [12,50,51,52,53,54,55].

Coal Mines Name	Sudden Water Time	Maximum Surge Capacity (m3/h)	Hydrogeological Features	Impact
Cuimu Mine	2013	1300	Water in the Yijun Formation sandstone and Luohe Formation sandstone aquifers	Discontinued
Guojiahe Coal Mine	2016	2300	Water in the Yijun Formation sandstone and Luohe Formation sandstone aquifers	Discontinued
Yuanzigou Coal Mine	2019	570	Water in the Yijun Formation sandstone and Luohe Formation sandstone aquifers	Discontinued
Gaojiabao Coal Mine	2015	3000	Low River Formation sandstone aquifer	Discontinued
Zhaojin Coal Mine	2013 2016	-	Rock River Formation Sandstone Fissure Aquifer	11 deaths
Yangliu Coal Mine	2017	-	Water in the sandstone aquifer of the Stone Box Formation	Potential threats
Fangezhuang Coal Mine	2016	60	Hydraulic fracture zones, sandstone fracture bearing aquifers	No impact
Daliu Coal Mine	2013	430	Water-conducting fracture zones, geological formations	Discontinued
Wanglou Coal Mine	2013	1345.7	Jurassic sand and conglomerate fissure aquifer	Discontinued
Lijialou Coal Mine	2016	3158	Water conductive rift zone	Discontinued
Shajihai Coal Mine	2016	-	Water from the Headunhe Formation aquifer, water from the Xishangyao Formation medium-coarse sandstone aquifer	Discontinued
Shilawusu Coal Mine	2018	921.4	Cretaceous Luohe Formation sandstone aquifer	Discontinued

Table 1. Summary of major water inrush incidents [12,50,51,52,53,54,55].

Coal Mines Name	Sudden Water Time	Maximum Surge Capacity (m3/h)	Hydrogeological Features	Impact
Cuimu Mine	2013	1300	Water in the Yijun Formation sandstone and Luohe Formation sandstone aquifers	Discontinued
Guojiahe Coal Mine	2016	2300	Water in the Yijun Formation sandstone and Luohe Formation sandstone aquifers	Discontinued
Yuanzigou Coal Mine	2019	570	Water in the Yijun Formation sandstone and Luohe Formation sandstone aquifers	Discontinued
Gaojiabao Coal Mine	2015	3000	Low River Formation sandstone aquifer	Discontinued
Zhaojin Coal Mine	2013 2016	-	Rock River Formation Sandstone Fissure Aquifer	11 deaths
Yangliu Coal Mine	2017	-	Water in the sandstone aquifer of the Stone Box Formation	Potential threats
Fangezhuang Coal Mine	2016	60	Hydraulic fracture zones, sandstone fracture bearing aquifers	No impact
Daliu Coal Mine	2013	430	Water-conducting fracture zones, geological formations	Discontinued
Wanglou Coal Mine	2013	1345.7	Jurassic sand and conglomerate fissure aquifer	Discontinued
Lijialou Coal Mine	2016	3158	Water conductive rift zone	Discontinued
Shajihai Coal Mine	2016	-	Water from the Headunhe Formation aquifer, water from the Xishangyao Formation medium-coarse sandstone aquifer	Discontinued
Shilawusu Coal Mine	2018	921.4	Cretaceous Luohe Formation sandstone aquifer	Discontinued

2. Statistics and Analysis of Case Studies on the Development Height of Water-Flowing Fractured Zones in Longwall Mining

The geology and hydrogeological conditions (Figure 2) of each wellfield in the Yonglong mining area are similar, but they are unique compared with other coalfields. The main characteristics include the following: The main mining is the Jurassic Yan’an Formation coal seam, and the main aquifer is the Cretaceous Luohe River. The formation sandstone aquifer, the lithology of the strata between the coal seam and the Luohe Formation, is mainly mudstone, sandstone, and conglomerate; the Luohe Formation is extremely thick, with a thickness of more than 100 m and a maximum thickness of 550 m. The water-richness of the Luohe Formation varies greatly; the coal seams are extremely thick, with a thickness of more than 10 m and a maximum thickness of 20~30 m. The coal seams are buried deep and vary greatly, about 300~1000 m. It is precisely this specific geological and hydrogeological condition that determines that the height of the water-flowing fractured zone in the roof of the fully mechanized caving coal mining in the Yonglong mining area, which has a special law different from other mining areas.

Based on the collected measured data of the water-conducting fracture zone in the Yonglong mining area of the Huanglong coalfield, scatter plots of the relationship between the conduction height, mining height, and working face width were drawn, as shown in Figure 3. SPSS software (SPSS Statistics 29.0) was used to fit the measured data of the water-flowing fractured zone development height, working face width, and coal seam mining height, as shown in

Table 2. Measured data of water-flowing fractured zone height in the Yonglong mining area.

Coal Mine Name	Working Face	Working Face Width/m	Buried Depth of Coal Seam/m	Mining Height /m	Measured Data of Water-Flowing Fractured Zone Height
					Water-Flowing Fractured Zone Height/m	Water-Flowing Fractured Zone and Mining Height Ratio
Hujiahe Mine	101	175	608.4	10.1	225.43	22.32
	401	196	553.22	12	239.12	19.93
Huoshizui Mine	8712	200	628.16	10	220	22
Cuimu Coal Mine	21301	200	552.19	12	239.42	19.95
	21303	200	552.19	6.5	190.51	29.31
				9	172.75	19.19
				15.86	368.42	23.23
	21305	150	552.19	10.86	230.97	21.27
				8.6	188.54	21.92
				15.31	325.64	21.27
Guojiahe Mine	1305	235	573.5	14.8	135.78	9.17
				14.8	164	11.08
Tingnan Coal Mine	106	116	483.5	9.1	121.03	13.3
	107	116	650	9.9	165.83	16.75
	204	200	575	6	135.23	22.54
	304	204	529.5	9.1	254.04	27.92
Xiagou Mine	ZF2801	93.4	330	9.9	111.81	11.29
		96.2	332	9.9	125.81	12.71
	ZF2802	96.2	331.98	11	165.61	15.06
	ZF2803	96.2	330	8.7	97.47	11.2
	ZF2804	95	330	8.9	149.48	16.8
Dafosi Mine	40106	180	391.5	9.1	245.52	26.98
	40108	180	391.5	11.22	189.05	16.85
				12.55	191	15.22
				12.12	193.76	15.99
Yuhua Coal Mine	1405	165	450	8	156	19.5
Xiashijie Mine	223	240	620	7	187.4	26.77
Yuanzigou Coal Mine	1012001	200	720	10.7	213.36	19.95

, and an empirical formula for calculating the height of water-flowing fractured zone height was obtained.

$$H_d=0.424B+10.802H+9.016$$

(1)

In the formula: H_d is the height of the water-flowing fractured zone, m; B is the width of the working face, m; H is the mining height of the coal seam, m. After calculation, the complex correlation coefficient of this formula is 0.622, R₂ is 0.786, the adjusted R₂ is 0.939, the significance coefficient F of the mining height is 0.007, and the significance coefficient of the surface width is 0.063, indicating that the mining height has a significant impact on the conduction height. The effect is more significant than that of surface width.

The Yuanzigou Coal Mine is located in the Yonglong mining area (Figure 4). There is a huge thickness of Yijun Formation conglomerate aquifer above the 2# coal seam, and it is very hard. After the coal seam is mined, it is easy to form a certain separation space at the interface with the underlying rock layer. The accumulation of groundwater provides space. The Yijun Group conglomerate aquifer has a certain recharge water source and meets the three conditions of “existence of a separation layer that can accumulate water, existence of a recharge water source around the bed separation, and sufficient duration of the separation space” [21]. The 1012001 fully mechanized top coal caving working face is located in the 101 panel area of the Yuanzigou Coal Mine. It is arranged along the coal seam strike, with a length of 2618 m, and the 2# coal seam is mined. The average burial depth of the coal seam at the working face is about 400 m. The comprehensive columnar view of the working face is shown in Figure 5.

The measured data of the Yuanzigou Coal Mine were used for calculation. The width of the 1012001 fully mechanized caving working face is 200 m, the coal seam mining height is 10.7 m, the roof is mainly a combination of sand and mudstone interlayers, and the roof is managed by the full collapse method. The measured height of the water-flowing fractured zone is 213.36 m, and the fracture-mining ratio is 19.95. Based on the water-flowing fractured zone height prediction formula obtained by fitting the above measured data, the data of the first mining face of the Yuanzigou Coal Mine were substituted into the calculation, and the water-flowing fractured zone height was 209.40 m, and the fracture-mining ratio was 19.50. The predicted value differs from the measured value by 3.96 m, and the relative error is 1.85%. It shows that the formula has good adaptability in this mining area and can provide a reference for the prediction of the height of the water-flowing fractured zone in the newly opened area and working face of coal mines in the study area.

3. Physical Simulation Study on Evolution Law of Overlying Strata Bed Separation in Fully Mechanized Caving Mining of Ultra-Thick Coal Seams

In order to study the evolution law of the overlying strata under the influence of fully mechanized caving mining in ultra-thick coal seams combined with the geological conditions of the 1012007 fully mechanized caving working face of the Yuanzigou Coal Mine in the Yonglong mining area, numerical simulation and physical model test methods were used to study the temporal and spatial evolution law of the overlying strata under the influence of mining. This section introduces the physical simulation test and result analysis.

3.1. Physical Simulation Experiment

3.1.1. Physical Model Building

Conducting similar simulation experiments in the laboratory is a feasible method to better understand the evolution law of overlying strata under mining [56]. According to the comprehensive columnar diagram of rock strata in the Yuanzigou Coal Mine and combined with similarity theory, a physical model with a length × height × width of 130 cm × 80 cm × 20 cm was constructed with a geometric similarity ratio of 1:300. A 10 cm wide (corresponding to 30 m on site) safe coal and rock pillar was left on the left and right sides of the model, and a 10 MPa vertical equivalent load was applied to the top boundary (Figure 6). The detailed experimental model formation and the proportion of its main materials are shown in

Table 3. Similar simulation test model and main material ratio table.

Layer Number	Lithology	Thickness/m	Model Thickness/cm	Model Thickness/cm	Density g/cm3	Ratio	Material Consumption/kg					Layer Thickness/cm	Repeating Layers
							Total Weight	Sand	Calcium Carbonate	Plaster	Water
13	Medium sandy conglomerate interbedded	27	8	80	2530	537	11.84	9.87	0.59	1.38	1.32	3	1
12	Conglomerate rock group	48	12	72	2630	437	49.23	39.39	2.95	6.89	5.47	3 × 4	4
11	Coarse sandy mudstone interbedded	16	4	60	2430	773	15.16	13.27	1.33	0.57	1.68	2 × 2	2
10	Mudstone	37	9	56	2490	773	34.96	30.59	3.06	1.31	3.88	3 × 3	3
9	Coarse sandstone	8	2	47	2430	773	7.58	6.63	0.66	0.28	0.84	2	1
8	Fine sand and mudstone interbedded	20	5	45	2490	773	19.42	16.99	1.70	0.73	2.16	2 × 2 + 1	3
7	Mudstone	50	12	40	2590	773	48.48	42.42	4.24	1.82	5.39	3 × 4	4
6	Fine sand and mudstone interbedded	12	3	28	2430	946	11.37	10.24	0.45	0.68	1.26	3	1
5	Fine sand and mudstone interbedded	20	5	25	2590	773	20.20	17.68	1.77	0.76	2.24	2 × 2 + 1	3
4	Mudstone	27	7	20	2510	773	27.41	23.98	2.40	1.03	3.05	2 × 3 + 1	4
3	Fine sand and mudstone interbedded	15	4	13	2590	773	16.16	14.14	1.41	0.61	1.80	2 × 2	2
2	Coarse sandstone	20	6	9	2430	946	18.95	17.06	0.76	1.14	2.11	2 × 2 + 1	3
1	Coal	10	3	3	1390	773	8.67	7.59	0.76	0.33	0.96	2 × 2	2
Total							289.46	249.85	22.09	17.52	32.16		33

. The mining distance of the simulated coal mining face was 440 m, and the mining step distance was 20 m each time. A horizontal survey line (a total of six lines) was arranged every 10 cm in the rock strata above the coal seam, and a measuring point was arranged every 10 cm on the survey line. The surface of the model was painted white to facilitate the clear observation of the collapse of the overlying strata and the development position of the strata during the mining process. By binarizing the model surface images taken by a high-pixel camera, the number and position of black and white pixels in the overburden at different advancement positions are identified, and then the evolution characteristics of the overburden fractures as the working face advances are quantified [57,58].

3.1.2. Analysis of Physical Simulation Results

As shown in Figure 7, when the working face advanced 40 m, the structure of the overburden did not change significantly. When the working face advanced to 60 m, the lower layer of the roof collapsed under the load and self-weight of the overburden. When the working face advanced to 80 m, the lower layer of the direct roof continued to collapse, and the collapse height was about 16 m. As the working face continued to advance to 160 m, the first periodic roof weighting (periodic roof weighting) appeared, with a step distance of 40 m. The direct roof stratum collapsed in a “mining and bubbling” manner, and the overburden stratum slipped and became unstable again, sinking to the goaf as a whole, and the collapsed overburden and goaf formed a trapezoidal structure. When the working face advanced to 320 m, the water-flowing fractured zone had developed to a height of 129 m above the working face. When the working face advanced to 400 m, the lower stratum in the overburden was compacted, and a new stratum in the upper part was generated and finally developed below the conglomerate layer. At this time, the height of the water-flowing fractured zone was about 195 m.

Figure 8 shows the dynamic evolution of the subsidence of the overlying strata during the advancement of the working face. It can be seen that when the working face was advanced to 160 m, the rock layer where survey line 6 was located did not undergo obvious deformation, which shows that the rock layer was hard and had good integrity. However, when the working face was advanced to 200 m, the roof of the coal seam at a height of 10 cm (survey line 6, corresponding to the roof of 30 m) began to sink (Figure 6), with a maximum sinking of about 1.13 cm (4.52 m); part of the low-level roof below it collapsed, and transverse cracks and tiny bed separation appeared in the direct roof, and the movement and deformation of the rock layers where other survey lines were located above it were not obvious. As the working face continued to advance, the subsidence of the rock layer where survey line 6 was located continued to increase. When it was advanced to 320 m, the subsidence, bed separation, and fracture of the rock layer had spread to survey lines 4, 5, and 6. The subsidence of the rock layer at the height of survey line 6 was the largest, about 3.5 cm (10.5 m). At this time, in the rock layer above the coal seam, the longitudinal cracks rapidly expanded upward along the fault line near the opening eye. At the same time, the rock layer above the coal seam showed obvious stratification. Finally, when the working face continued to advance to 440 m, the rock layer where each survey line was located reached the maximum subsidence value within 60–100 cm from the cut hole, and, at this time, the range was 40 cm away from the working face.

By sketching similar simulation test photos and calibrating the bed-separation cracks, they can be recognized by digital image processing software so that the length and area of the bed-separation cracks in similar simulation morphology photos at different mining stages can be calculated. The calibration results are shown in Figure 9. The area data obtained by the analysis software is used to analyze the evolution law of the overlying strata bed-separation layer under the influence of mining.

During the bed-separation analysis process, four significant separation layers were selected for analysis of separation layer area changes (Figure 10). When the working face was excavated to 200 m, as the overlying rock layer collapsed, separation layer 1 appeared at this time, and the lateral length of bed separation 1 is 32 cm, the maximum depth is 4 cm, and the area of the bed separation is 28 cm². As the working surface continues to advance, the area of bed separation 1 continues to increase, and the lateral length of the bed separation continues to grow. When the excavation reaches 280 m, it reaches the maximum value, the maximum length reaches 52.2 cm, which is equivalent to the actual mining of 208.8 m, and the bed-separation area reaches the maximum 39 cm². At the same time, separation layer 2 begins to appear. Two separation layers appear in the same rock layer. Layer 2 is only 3 cm away from bed separation 1, and the length of newly formed separation layer 2 reaches 48 cm. With the emergence of bed separation 2, bed separation 1 is gradually compacted, and the area becomes smaller. When the working face advances to 350 m, separation layer No. 3 appears. Bed separation 1 and bed separation 2 continue to be compacted, and the bed separation is compacted into a long and narrow crack. When the working face advances to 400 m, bed separation 3 develops to its maximum. The maximum value is 35.6 cm², which is smaller than the maximum values of separation layer 1 and separation layer 2. When the working face was mined to 420 m, the No. 4 detachment layer was generated, and the No. 3 detachment layer was rapidly compacted. At this time, the upper surface of the model began to sink. After the working face advanced to 440 m, the upper surface of the model continued to sink slowly, and the upper surface of the model continued to sink slowly. The rock layers are all compacted to varying degrees, and all separation layers continue to become smaller. The No. 1 separation layer increases rapidly due to the sudden subsidence of the underlying rock layer. This may be due to the overflow of fine particles and debris from the model during the compaction process of the model. The sudden subsidence of the underlying rock strata is caused by the external force.

3.2. Discrete Element Numerical Simulation Based on UDEC

3.2.1. Numerical Model Construction and Parameter Selection

The numerical simulation was established by selecting the same research background and engineering profile as the similar simulation. The top of the model was used as a free boundary, the left, right, and bottom boundaries were used as single constraint boundaries with zero displacement, and a vertical load of 10 MPa was applied to the top boundary. The rock strata follow the Mohr–Coulomb failure law, and the block connection joints in the model adopt an elastic-plastic contact surface model with Coulomb slip failure properties. The single step distance of the mining process was 40 m, and the mining distance of 400 m was completed in 10 rounds. The layout of the displacement measurement line was based on arranging a measurement point every 40 m above the coal seam, with each measurement point 10 m apart, for a total of 40 measurement points. The final model layer thickness and rock mechanic parameters can be found in

Table 4. UDEC numerical simulation rock mechanics parameters.

No.	Lithology	Density (kg/m3)	Bulk Modulus (GPa)	Shear Modulus (GPa)	Friction Angle (deg)	Cohesion (MPa)	Tensile Strength (MPa)	Porosity	Permeability Coefficient (m2/Pa·s)
1	Topsoil layer	1450	3.59.	2.06	40	0.53	0.79	0.3	3.67 × 10−9
2	Medium conglomerate	2530	15	10	55	1.16	1.52	0.1	1.02 × 10−10
3	Conglomerate	2630	19	16	57	1.25	0.69	0.1	1.02 × 10−10
4	Coarse sand mudstone	2490	9.52	7.59	59	0.75	0.62	0.025	2.04 × 10−13
5	Middle sandstone	2430	7.29	5.39	59	0.75	0.62	0.08	1.22 × 10−11
6	Mudstone	2590	5.96	4.48	45	0.95	0.56	0.025	2.04 × 10−13
7	Fine sandstone	2490	5.52	3.59	59	0.75	0.62	0.06	2.04 × 10−12
8	Fine sand mudstone	2590	7.96	5.48	45	0.95	0.56	0.025	2.04 × 10−13
9	Siltstone	2480	6.31	4.61	46	0.25	0.43	0.03	8.16 × 10−13
10	Medium sandstone	2430	5.29	4.19	59	0.75	0.62	0.07	1.02 × 10−11
11	Coal	1390	4.83	2.55	41	0.33	0.13	0.02	6.12 × 10−13
12	Fine siltstone	2400	7.45	4.60	39	0.58	0.56	0.03	8.16 × 10−13

.

3.2.2. Analysis of Numerical Simulation Results

Figure 11 and Figure 12 are the numerical model coal seam mining overburden collapse diagram and fracture development diagram, respectively. It can be seen from the figure that when the model is mined to 80 m, the coal seam direct top collapses, and a narrow stratum is formed at this time, and the stratum is located in the collapse zone. When the working face advances to 120 m, the stratum appears at a high level, the stratum at the low level is compacted, and the water-flowing fractured zone develops into the fracture zone. At this time, the stratum and the fracture are connected. When the working face is mined to about 220 m, the connected water-conducting fractures have developed to the middle and upper part of the Anding Formation, that is, 208 m above the coal seam roof, and the fracture-mining ratio is about 19.54. At this time, the stratum in the middle part of the model is compacted, and the narrow stratum in the coal wall support area on both sides of the model still exists, mainly in the form of longitudinal fractures. As the working face continues to advance, the stratum in the lower part of the Zhiluo Formation in the direction of the goaf gradually moves toward the mining direction, the stratum space begins to close, and the subsidence of the stratum above the goaf continues to intensify, reaching about 7 m. When the working face was advanced to about 340 m, the separation layer and water-flowing fractured zone were basically closed, the subsidence of the overburden rock had reached about 12 m, and the upper part of the goaf had undergone obvious deformation and damage. When the working face was advanced to about 360 m, the deformation and damage of the upper part of the goaf further expanded, the subsidence of the Anding Formation and Yijun Formation strata had reached more than 15 m, and the upper rock strata of the goaf had completely collapsed.

Figure 13 shows the amount of overburden subsidence in coal seam mining in numerical simulation. It can be seen from the figure that the numerical simulation results are basically the same as the results of similar simulation tests. When the working face advances to the first 160 m, the rock layer where survey line 1 is located has not collapsed fully, resulting in a “V”-shaped settlement. When the working face advances to 240 m, the amount of subsidence of survey line 1 increases, and the settlement shape changes from a “V” shape to an irregular “U” shape. When the working face advances to 320 m, the rock layers where survey lines 4, 5, and 6 are located all sink to varying degrees, and the settlement shape of all rock layers changes from a “V” shape to an irregular “U” shape. At this time, the overburden has sunk to create contact with the gangue and enters the compaction stage. When the working face continues to advance to 440 m, the rock layers where each survey line is located reach the maximum subsidence value.

3.3. Comprehensive Analysis of Simulation Research on Evolution Law of Overlying Strata

Comprehensive analysis of numerical simulation and similarity simulation results shows that the dynamic development law of the water-flowing fractured zone can be divided into three stages: the initial development stage, the rapid development stage, and the stable stage. As shown in Figure 14, the height of the water-flowing fractured zone obtained by the similarity simulation test is 207 m, the height of the water-flowing fractured zone obtained by the UDEC numerical simulation is 208 m, and the height obtained by the fitting formula of the water-flowing fractured zone is 209.4 m. The above research results are close to the field measured results (213.36 m). Therefore, the calculation formula for the height of the water-flowing fractured zone obtained by fitting the water-flowing fractured zone data are consistent with the actual situation on site and can be used for prediction. Finally, the height of the water-flowing fractured zone of the Yuanzigou 1012001 fully mechanized caving working face was determined to be 209 m.

Through the comprehensive analysis of the research results of similar simulation and numerical simulation, it is found that the spatial development and evolution process of the overburden stratum under the influence of coal seam mining can be divided into three parts (Figure 15): In the early stage of working face mining, under the influence of the pressure of working face mining, a collapse zone appears above the goaf, but no stratum is formed; with continued mining, the pressure arch expands upward, and a non-sufficiently mined stratum space appears above the collapse zone, and with mining, the stratum continues to expand upward. The pressure arch formed at this time is called a stratum expansion arch; with the continued mining of the working face, the stratum space at higher levels gradually develops. At this time, the stratum expands upward, the development of the pressure arch reaches the limit, and a fully mined stratum space is formed, which is called a stratum limit arch; at the end of the working face mining, the movement direction of the limit arch changes to the direction of the working face advancement, which is called a stratum moving arch. At this time, the lateral cracks of the developed stratum space increase, and the volume of the stratum space also increases significantly. Therefore, in the process of coal seam mining, the morphology and development process of the stratum in the overburden stratum will be affected by the mining speed and mining distance, and there will be a certain hysteresis. In addition, at different periods of mining, the separation space can be divided into two states, namely, fully mined separation space or incompletely mined separation space. The incompletely mined separation space is “bowl-shaped” with a “V”-shaped section, while the fully mined separation space is “disk-shaped” with a “U”-shaped section.

Combined with the characteristics of coal seams in the Yonglong mining area and the results of similar simulation tests and numerical simulation tests, based on the basic idea of the “three-zone” theory, the structural law of overburden after fully mechanized caving of ultra-thick coal seams in the Yonglong mining area is summarized and divided into “four zones”. Since there are widespread water-logged strata in this area, the thickness of the overburden strata and topsoil layer is large, and there is almost no strata bed separation in this section of rock, this paper further subdivides the overall moving zone of the traditional three-zone theory into a curved sinking zone and a bed-separation zone (Figure 16) according to the research situation; that is, four zones from top to bottom are formed: curved sinking zone, water-logged bed-separation zone, fracture zone, and collapse zone. During the continuous advancement of the working face, the fractures and bed separation experienced a cycle of “fracture formation-fracture penetration-fracture closure”, forming a vertical fracture zone, a narrow bed-separation zone, a bed-separation compaction zone, a narrow bed separation zone, and a vertical fracture zone in the horizontal direction, which are symmetrically distributed on both sides. The narrow-sense abscission layer and the overburden abscission layer are defined. The overburden abscission layer is formed in the abscission zone, while the narrow-sense abscission layer is mainly formed in the collapse zone. Numerical simulation and similar simulation tests show that the vertical fracture zone is mostly longitudinal fractures and small transverse fractures, and most of these fractures will be retained during the mining process. The overburden abscission layer is prone to occur in the compacted zone, and the overburden abscission layer will undergo the process of “abscission formation-abscission expansion-abscission closure” during the mining process. The narrow-sense abscission zone is the concentrated area of the starting and ending positions of the abscission expansion and is the main occurrence area of abscission formation to closure, and the narrow-sense abscission layer in the collapse zone often appears in this area.

4. Numerical Simulation of Roof-Water Inrush in Fully Mechanized Caving Mining of Ultra-Thick Coal Seams

Roof bed-separation water inrush is a serious disaster accident, and its suddenness and danger pose a serious threat to the lives and property of coal miners. In the process of bed-separation water inrush, the most critical factor is the law of stress field change, pore water pressure field, and seepage field. Comprehensive analysis of these laws can provide a better understanding of the nature and mechanisms of water inrush and provide a basis for the implementation of prevention and control measures. Based on the previous research results on the dynamic evolution process of overburden bed separation, this section mainly uses the FLAC3D fluid-solid coupling calculation function to study the water migration, accumulation, and sudden surge process of the aquifer in the overburden under the influence of mining. In order to facilitate comprehensive comparative analysis, the geological model is consistent with the previous article. A three-dimensional numerical model is established to simulate the overburden bed-separation water inrush process of ultra-thick coal seam mining, and the law of stress field change, seepage field, and pore water pressure field in the overburden bed-separation water inrush process of ultra-thick coal seam mining is analyzed, providing theoretical support for the prevention and control of bed-separation water inrush disasters.

4.1. Numerical Model Establishment

According to the comprehensive column chart and rock physical and mechanical parameters of the 1012001 first mining working face of the Yuanzigou Coal Mine, a three-dimensional numerical model with a length, width, and height of 800 m × 400 m × 710 m was established. Combined with the actual mining situation of the 1012001 first mining fully mechanized caving working face, the coal seam mining height was simulated to be 10 m, the working face dip width was 200 m, and the strike length was 800 m. The 100 m isolation coal pillars were reserved around the model working face, and the simulated actual mining length was 600 m. Model parameters are shown in Table 4. In this model, each element adopts the Mohr–Coulomb criterion.

4.2. Analysis of Stress Field During Water Inrush Process

As shown in Figure 17, during the process of the working face advancing from 80 m to 640 m, the maximum principal stress in the stress concentration area at both ends of the goaf increased from 19.3 MPa to 22.3 MPa. Among them, the maximum value was reached when the mining reached 320 m. After mining 320 m, the maximum principal stress in the concentrated area at both ends of the goaf began to gradually decrease. When mining reached 640 m, the maximum principal stress dropped to 21.4 MPa, and the area of the compressive stress concentration area also increased accordingly. Before mining to 640 m, the area of the maximum stress concentration area of the coal pillar on the side of the cut was larger than the area of the stress concentration area at the right end of the working face. However, when mining reached 640 m, the area of the maximum stress concentration area on the left was smaller than the area on the right. The law of stress field change during coal seam mining is summarized, and the stress state of the overburden will change significantly. Due to the instability of the roof, the stress distribution state inside the overburden also changed, which led to the change in the stress field. Studies have shown that the law of change in the overburden stress field during the bed-separation water inrush is as follows: the stress is concentrated in the unstable area, and the stress value increases sharply. The impact of the mining working surface on the area close to it (such as the back of the coal pillar) gradually increases, and the stress value in this area also gradually increases. The stress value has a certain relationship with the existing overburden cracks. The more frequent and the wider the overburden cracks are, the smaller the stress value is.

4.3. Analysis of Pore Water Pressure Field During Water Inrush Process

During coal mining, as the working face advances, the peak value of pore water pressure continues to increase (Figure 18). According to the numerical change diagram of pore water pressure in the coal seam roof at different advancement distances, it can be seen that under different advancement distances, peak values will appear at the cut and the rightmost end of the working face, and the peak value data at the cut is slightly higher than the data at the working face end. Only when the advancement is 640 m, the peak value data at the rightmost end is greater than the data at the cut (Figure 19). The main reason is that the protective coal pillar on the rightmost side of the model is too short. The data of each pore water pressure line within its goaf area is wavy, which is because the pore water in the goaf has been fully infiltrated and formed a regular diffusion. As the working face continues to advance, the coal seam and surrounding rock are deformed to varying degrees, resulting in changes in pore water pressure. When the working face is mined, the surrounding rock is squeezed and deformed, and the pore water pressure will first increase, reach a peak value, and then gradually decrease. This is mainly due to the change in the stress distribution of the coal seam during the mining process, which makes it difficult to discharge pore water, and the pressure gradually increases. With the further development of coal seam fissure expansion and rock deformation, the number of pore water infiltration channels increases, the pore water flow rate accelerates, and the pore water pressure gradually decreases. After the coal seam mining is completed, the pore water pressure will basically stabilize. The change law of the pore water pressure field in the process of coal seam mining and bed-separation water inrush is summarized. When the roof is unstable, groundwater flows out from the channels generated by the overburden cracks to form a pore water pressure field. The study found that when the rock formation breaks, the groundwater pressure will increase sharply, forming bed-separation water inrush. The change law of the pore water pressure field in the process of bed-separation water inrush is as follows: As the working face continues to advance, the groundwater pressure will increase rapidly. When the roof sinks, the volume of the rock formation cracks will also increase, followed by a large amount of groundwater outburst. Water pressure is related to the permeability and speed of water in the cracks. When the water pressure increases to a certain extent, the rock formation will break and then form bed-separation water inrush.

4.4. Analysis of Seepage Field During Water Inrush Process

Figure 20 is a cloud diagram of the change in seepage velocity at different mining advancement distances. It can be seen from the figure that when the working face advances to 80 m, the overlying rock strata have not yet moved and the water-conducting channel in the stratum has not been formed. At this time, the seepage velocity in the goaf is relatively small. After mining to 160 m, the infiltration velocity at the working face is significantly accelerated, and the maximum infiltration velocity gradually shifts from the open cut end to the middle of the goaf, and two main seepage channels are formed. One is the seepage channel where the front and rear coal walls converge to the middle, which is shown as the uncrossed green arrow area in the figure; the other is the water-conducting fracture channel formed by the previous mining, which is infiltrating from directly above, and the flow rate of this seepage channel is significantly greater than the previous one.

By drawing the maximum seepage velocity change curves at three different positions (Figure 21), the seepage velocity at the cut is basically the same as that at the working face. With the mining of the working face, the seepage velocity drops rapidly. When the mining reaches about 150 m, the downward trend slows down. When the mining reaches about 450 m, it drops further. The reason for this phenomenon may be that the cracks and separation layers generated in the overburden strata are compacted at this time. The seepage velocity at 200 m before the cut first increases and then decreases. When the working face is mined to about 140 m, the seepage velocity begins to rise, indicating that the mining at this time affects the position 60 m later. When the mining reaches 200 m, the seepage velocity reaches the maximum value, which is slightly lower than the seepage velocity at the cut, which is consistent with the law reflected in Figure 20.

During the bed-separation water inrush process, the seepage velocity will also change with the different stages of the bed separation water inrush event. The change law of the seepage velocity during the bed-separation water inrush process is as follows: before the water inrush, the seepage velocity in the overburden is relatively slow. In the early stage before the water inrush, the seepage velocity is related to the time of the water inrush and the water level. The seepage velocity is slow, and the water level is not high. During the peak of the water inrush, the seepage velocity reaches the highest peak because the cracks in the overburden become larger and the water flow route becomes straighter, resulting in a significant increase in the seepage velocity.

Comprehensive analysis shows that during the bed-separation water inrush process, the stress field, pore water pressure field, and seepage field are coupled with each other. When the coal seam is mined and the roof is unstable, the stress distribution inside the overburden changes significantly, and the stress is concentrated in the unstable area, resulting in a sharp increase in the stress value in the area, and then the overburden bed-separation phenomenon occurs. At the same time, groundwater penetrates into the unstable area through the water-conducting cracks and forms a high-pressure water body, thereby increasing the pore water pressure field. The increase in the opening of the cracks in the rock formation will increase the permeability coefficient of the rock formation, thereby affecting the change in the seepage field. The interaction of these factors causes the instantaneous outburst of high-pressure water, forming abscission water inrush.

5. Assessment of Water Inrush Hazard at 1012007 Working Faces and Suggestions for Water Prevention and Control

5.1. Risk Assessment

Taking the 1012007 working face of the Yuanzigou Coal Mine in the Yonglong mining area as a research case, the water-flowing fractured zone height prediction formula, the overlying strata bed-separation evolution law, and the results of the numerical simulation of water inrush from the roof stratum separation studied in the previous article were used, combined with the on-site hydrological long drilling data of the 1012007 working face, a comprehensive analysis and evaluation of the water inrush hazard of the working face was carried out.

The 1012007 working face is the second fully mechanized caving working face in the 101 panel area of the Yuanzigou Coal Mine. The working face has a strike length of 2618 m, a dip width of 211.19 m, and a designed mining length of 2020 m. The average coal seam thickness of the working face is 11.19 m. The burial depth is 560–750 m. The working face mainly mines the 2# coal seam, which has a stable occurrence and a simple structure. By using the formula for the water-conducting fracture zone fitted in Section 2, the working face length of 200 m and the mining height of 11.19 m are substituted into the fitting formula for the height of the water-flowing fractured zone to obtain a height of 218.1 m. Through a comprehensive analysis of the development position of the water-separated layer in the working area and the water-rich distribution map, it is believed that there is a risk of the water-flowing fractured zone connecting to the Cretaceous aquifer in the 1012007 working face, that is, there is a risk of water inrush.

Through the study and analysis of the location map of the detachment development of the 1012007 working face on site, the bottom boundary of the Luohe Formation, the bottom boundary of the Yijun Formation, the bottom boundary of the Anding Formation, the top boundary of the Zhiluo Formation, and the Yan’an Formation all have the conditions for the formation of detachment. Among them, the detachment developed at the top boundary of the Zhiluo Formation and the Anding Formation poses the greatest threat to the working face, and the two uppermost working faces can form water-accumulated detachment, which has the prerequisite for detachment water inrush. According to the calculation of the water-flowing fractured zone in the previous section, the height is 218.1 m, and the nearest water-accumulated detachment is 243.35 m, which is very close. Under the influence of dynamic pressure, it is easy to induce roof detachment water inrush accidents, and prevention work needs to be performed well.

According to the hydrogeological conditions of the coal mine, the water inflow of the 1012007 working face is estimated. The water inflow of the 1012007 working face is calculated using the “water collection corridor method”: the normal water inflow is 100 m³/h, and the maximum water inflow is 130 m³/h. The large well method is used to prove that when the water-conducting fracture zone conducts the Cretaceous aquifer, a large water inflow may occur, and the predicted water inflow is 659 m³/h. Therefore, the maximum water inflow when the 1012007 working face has a water inrush accident is 789 m³/h (

Table 5. Roof separation water inrush hazard assessment table.

Project	Classification Basis			Dangerousness
	Aquifer	Characteristics and Supply Conditions	Unit Water Inflow q(L/(s·m))
Aquifers and water bodies affected by mining damage	Luohe Formation conglomerate aquifer	Pore and fissure water, medium water richness, the main source of groundwater recharge is regional lateral runoff, followed by atmospheric precipitation. Sufficient recharge conditions, with a certain recharge water source.	0.07~0.19	Medium
Mine water inflow (m3/h)	Normal water inflow	100 m3/h		Medium
	Maximum water inflow	130 m3/h
Predicted water inrush volume (m3/h)		Q3 = 659 m3/h		Relatively dangerous
Height of water layer from coal seam		243.35 m		Relatively dangerous
water-flowing fractured zone height		218.1 m
The extent of mining affected by water hazard		Mines occasionally experience water inrush, and the mining process may be affected by the abscission water disaster, threatening the production safety of the mine. A water inrush accident has occurred at the first mining face of the same coal seam.		Dangerous
Difficulty of water prevention and control work		The workload of water prevention and control is large, and the difficulty is high.		Relatively complicated
Comprehensive assessment results of water inrush hazard of roof separation at working face.				Relatively dangerous

).

According to the estimation results, the maximum height of the water-flowing fractured zone of the 1012007 working face entering the Cretaceous aquifer is mainly distributed in two areas: within 130 m outside the side of the cut-eye return airway, the trapezoidal area between the belt roadway, and the return airway outside the cut-eye. In summary, during the coal mining process, groundwater in the Cretaceous sandstone aquifer may directly enter the tunnel system and indirectly fill the mine with water. Therefore, during the mining process, the mining height should be reasonably controlled according to the actual situation and, when necessary, a drainage system that meets the requirements should be built to minimize the impact of water hazard after the Yijun aquifer at the bottom of the Cretaceous is conducted.

5.2. Prevention and Control Measures

By analyzing geological and hydrogeological data and combining the actual situation of the Yuanzigou Coal Mine, the overall idea of water prevention and control at the 1012007 working face of the Yuanzigou Coal Mine is summarized into five countermeasures: “exploration, prevention, drainage, drainage, and monitoring”:

(1) Exploration: Identify abnormal water-rich areas through geophysical exploration and carry out underground working face drilling work. Drilling verification is carried out on various abnormal water-rich areas that have been delineated to provide a basis for formulating relevant water prevention and control measures.

(2) Prevention: Prevent the occurrence of water leakage during the mining process, especially when the working face enters the predicted water-conducting fracture zone that may develop into the Cretaceous aquifer. Take preventive measures in a timely manner. Drill holes directly through the ground to destroy the closed space of the stratum and slowly drain the accumulated water in the stratum to prevent the stratum water from instantly rushing into the working face, thus ensuring safe and orderly mining at the working face.

(3) Drainage: Drain the sandstone aquifer and water-rich abnormal areas in the coal seam roof and take relevant measures to achieve safe mining.

(4) Drainage: Establish a complete drainage system for the 1012007 mining face and carry out regular inspection and maintenance to ensure normal production.

(5) Monitoring: Establish a hydrological telemetry system to monitor the hydrological changes in the main threatening underground aquifers in real time and accurately, so as to provide preliminary hydrological basic data for the mine and meet the production needs of the mine. The main monitoring indicators include water level and water temperature, and the main monitoring layer is the Cretaceous Luohe Formation aquifer.

Combined with the above-mentioned “five-word” countermeasures for water prevention and control, the overall idea of this design is to actively prevent and control the abscission water by constructing direct-through drainage holes on the ground; actively construct ground hydrological observation holes to find the correlation between the water inflow at the working face and the water level changes in the observation holes; insist on underground drilling verification in the water-rich abnormal area of geophysical exploration, and pre-drain the water in the water-rich abnormal area; strengthen the analysis of abnormal water inflow in the mine, predict possible water inrush, and activate the flood emergency plan as required; and strengthen the inspection and maintenance of the temporary drainage system on the working face, so that the recovery of the working face is not affected by water inflow or water accumulation, or the impact of water inflow and water accumulation is minimized. Through relevant ground and underground projects, supplemented by corresponding technical and institutional guarantee measures, the safety of the recovery of the working face is ultimately ensured.

6. Conclusions

Delamination water hazards are a common and harmful form of water hazard in mining areas in western China in recent years, which seriously threatens the safe production of the rich ultra-thick coal seam resources in the region. The Yonglong mining area has become a high-incidence area of delamination water hazard due to its thick coal seams, deep burial depth, and thick overlying aquifers. In this regard, this paper takes the fully mechanized caving mining of ultra-thick coal seam in the Yonglong mining area as the research background, studies the height of water-flowing fractured zone in the fully mechanized caving mining of ultra-thick coal seams, the evolution law of the bed separation of overlying strata, and the process of water inrush from roof bed separation, and mainly obtains the following conclusions.

(1) By collecting and summarizing the measured water-flowing fractured zone height data of the mines in the Yonglong mining area, the collected data information and main influencing factors were fitted using SPSS software, and a water-flowing fractured zone height prediction formula H_d = 0.424B + 10.802H + 9.016 suitable for the Yonglong mining area was obtained. The accuracy of the fitting formula was verified by field measured data and numerical simulation research results; comprehensive analysis showed that the development law of the water-flowing fractured zone height can be roughly divided into three stages: the initial development stage, the rapid development stage, and the slow and stable stage.

(2) The evolution law of overlying strata under the influence of mining in the study area was obtained by comprehensive analysis of the UDEC numerical simulation and laboratory physical similarity simulation test, which can be summarized as “four zones, three arches, and five areas”. According to the distribution of overburden structure after mining, the evolution law of overburden structure is summarized as “four zones”, which are curved sinking zone, strata separation zone, fracture zone, and collapse zone from top to bottom; according to the spatiotemporal evolution of overburden strata during mining, the evolution law of overlying strata bed separation is summarized as “three arches”, which are divided into strata separation expansion arch, strata separation limit arch, and strata separation movement arch. According to the distribution of strata separation and fractures during the evolution of overburden strata separation, the distribution law of strata separation fractures is summarized as “five zones”, which are vertical fracture zone, narrow strata separation zone, strata separation compaction zone, narrow strata separation zone, and vertical fracture zone from left to right, symmetrically distributed on both sides.

(3) By using FLAC3d to numerically simulate the process of water inrush from the roof of an ultra-thick coal seam, a comprehensive analysis shows that in the process of water inrush from the delamination layer, the stress field, pore water pressure field, and seepage field are mutually coupled. When the coal seam is mined and the roof becomes unstable, the stress distribution inside the overburden changes significantly, and the stress is concentrated in the unstable area, causing the stress value in this area to increase sharply, and then the phenomenon of delamination of the overburden layer occurs. At the same time, groundwater penetrates into the unstable area through the water-conducting fractures and forms a high-pressure water body, thereby increasing the pore water pressure field. The increase in the degree of fracture in the rock layer will increase the permeability coefficient of the rock layer, which in turn affects the change in the seepage field. These factors interact with each other, causing the high-pressure water body to gush out instantly, forming delamination water inrush.

(4) Based on the above research results, the risk of water inrush from roof separation at the 1012007 working face of the Yuanzigou Coal Mine was evaluated. It was found that the mine had the conditions for the formation of water accumulation and separation. The height of t water-flowing fractured zone was calculated to be 218.1 m, and the lowest water accumulation and separation layer was 234.35 m. Under the influence of dynamic pressure, roof separation and water inrush accidents were easily induced. The maximum water inflow during water inrush from the working face was calculated to be 790 m³/h. A comprehensive assessment showed that the risk of water inrush was high, and the mine owner needed to pay attention to it and deploy waterproofing measures in advance to ensure safe production in the mine.

7. Perspectives for Future Work

The multi-scale coupling simulation method and the “four belts, three arches and five zones” overburden damage evolution model proposed in this study provide a theoretical framework and technical tools for the prevention and control of super-thick coal seam delamination water hazards. Based on the current results, future research can be further expanded in the following directions:

(1) Fusion of multi-field coupling and intelligent algorithms: Combining machine learning (such as convolutional neural network CNN, graph neural network GNN) with real-time monitoring data, developing an adaptive prediction model for the dynamic evolution of fracture networks to solve the problem of parameter uncertainty under complex geological conditions. For example, using generative adversarial networks (GAN) to simulate the spatial distribution of heterogeneous fractures, or optimizing grouting reinforcement schemes through reinforcement learning.

(2) Multi-scale cross-process modeling: Establish a multi-scale coupling model from micro-cracks (μm level) to the mining site scale (hundred-meter level) to reveal the cross-scale mechanical mechanism of fracture initiation-extension-penetration. Combining focused ion beam scanning electron microscopy (FIB-SEM) with discrete element simulation (DEM), the contribution rate of micro-cracks to macro-seepage paths can be quantified.

(3) Development of green prevention and control technologies: exploring ways to utilize the resources of the absorptive space, such as combining absorptive grouting with CO₂ storage and geothermal mining, to promote a green mining model of “prevention and control-resource synergy”.

(4) Collaborative research among multiple mining areas: Although this study has made progress in the mechanism and prevention of stratified water disasters in the Yonglong mining area, it is still necessary to recognize the limitations of model simplification and data coverage. In the future, through collaborative research among multiple mining areas, a “risk classification-prevention and control optimization” technical system adapted to different geological conditions will be constructed to promote the development of intelligent early warning tools and provide a reference for safe mining in similar mining areas around the world.