Mechanism of Bed Separation Water Inrush during the Mining of Extra-Thick Coal Seam under Super-Thick Sandstone Aquifer

Abstract

The bed separation water inrush disasters in the Jurassic coalfield of Huanglong, China occur frequently, and they seriously threaten the safety of coal mining. This study systematically collected data from multiple instances of bed separation water inrush (BSWI) in the Cuimu coal mine and summarized the characteristics of BSWI. Through the analysis of hydrogeological conditions, hydrochemical characteristics, field detection of a water-conducting fractured zone (WCFZ), and groundwater level monitoring, the water inrush source, water-conducting channel, and the dynamic response of the aquifer water level were studied. The results showed that the water inrush source was mainly Cretaceous groundwater. The height of WCFZ in the extra-thick coal seam mining in the study area was 239.62 m, forming a water-conducting channel of water accumulation in bed separation. There was a strong correlation between water level changes in the Luohe Formation and BSWI. On this basis, we have proposed that four conditions must be met simultaneously for BSWI, and a hydrogeological-mining coupling conceptual model was established to reveal the evolution process and the mechanism of BSWI. The research results are crucial for the prevention of BSWI disasters and for ensuring the safety of coal mine production.

1. Introduction

China is a country with large coal resources (about 5 × 10¹² t), and its raw coal production reached 4.56 billion tons in 2022. Mine water inrush is one of the major disasters in coal mine production, and it brings heavy casualties and economic losses to coal enterprises [1,2,3]. According to statistics, in the past 30 years, more than 300 pairs of mines in China were submerged due to water inrush, and economic losses amounted to more than USD 6 billion. At the same time, it has also caused significant damage to the water resources and environment in the mining area [4,5]. The hydrogeological conditions of many coal fields in China are complex, there are many types of mine water inrush, and the cause mechanism and prevention methods are becoming more and more complicated [6,7,8,9,10].

With the depletion of coal resources in eastern China and the increase in China’s coal demand, the focus of coal mining has begun to shift to ecologically fragile mining areas in western China [11,12]. The Ordos Basin in western China is one of the few giant coal-accumulating basins in the world, with abundant coal reserves. There are four 100-million-ton large coal bases in Shendong, Northern Shaanxi, Ningdong, and Huanglong. At the end of 2016, China issued the national mineral resources plan for 2016–2020, which proposed to focus on the development of the Huanglong coal base in the southwest of Ordos Basin. In recent years, in the process of high-intensity coal mining in the Huanglong Jurassic coalfield, the Dafosi coal mine and Huoshizui coal mine in the Binchang mining area, Yuhua coal mine and Xiashijie coal mine in the Jiaoping mining area, and the Cuimu coal mine and Guojiahe coal mine in the Yonglong mining area have all experienced roof BSWI disasters [13,14]. This not only seriously threatens the mine safety production but also leads to the loss of a large number of valuable groundwater resources.

As the storage medium of groundwater, the bed separation in the overlying strata is a focus of research. Identifying the bed separation position and analyzing the overburden breakage in mining is the primary problem in the prediction of BSWI [15]. Many scholars have predicted the development position of bed separation by means of numerical simulation and mechanical models [16,17]. The existing research results indicate that bed separation is correlated with the physical characteristics of the overburden, including thickness, uniaxial compressive strength of overburdened rock layers, location of rock layer interfaces, and thickness of extracted coal seams [18]. Bed separation fractures mainly appear under each key stratum, and the maximum development height of each one ends at the main key stratum in overlying strata [19]. As a new type of rare and serious water inrush disaster in coal mines, the bed separation water disaster has been paid more and more attention by scholars in recent years. Previous research has mainly focused on the causes of bed separation water, the risk assessment of BSWI, and the prevention and control technology of BSWI. Bed separation water is the source of water inrush, and its formation mainly depends on geological factors and mining factors, including water abundance of aquifer, mechanical properties of rock strata, and mining height [20]. Identifying the roof separation water, including its location and water accumulating volume, is the key work [21]. In order to improve the accuracy of prevention and control of BSWI, mechanical models and fuzzy mathematics were used to evaluate the risk degree of BSWI and divide the risk zoning [22,23,24,25]. In terms of prevention and control of BSWI, various prevention and control technology systems have been proposed, especially the key technology of ground through diversion holes, which effectively avoid the occurrence of BSWI [26,27,28]. However, the mechanism of BSWI is still very clear. On the one hand, due to the concealment of bed separation water, the formation conditions of BSWI disasters need to be further studied due to the complexity of coal mines’ geological conditions and the limitation of underground observation conditions. On the other hand, due to the complex disaster-causing mechanism, the formation conditions and evolution process of BSWI need to be further studied according to the specific geological conditions and mining conditions of coal mines.

In this study, based on a large amount of water inrush data in the Cuimu coal mine, the characteristics of water inrush due to bed separation in the study area are systematically summarized, and the geological conditions of water inrush, water inrush source, and water level change laws are analyzed. On this basis, the conditions for the occurrence of BSWI are clarified, and the evolution process and mechanism of water inrush due to bed separation are revealed through a hydrogeological-mining conceptual model. The research results have important theoretical and practical significance for the prevention and control of BSWI disasters in the Huanglong Jurassic coalfield.

2. Study Area

2.1. General Situation of Mining the Working Face

The Cuimu coal mine is located in Linyou county, Baoji city, Shaanxi province, China (Figure 1), with an area of 34.4 km². The ground wellhead elevation is +1325 m, the mining depth is from +970 m to +626 m, and the approved production capacity of the mine is 4 million tons per year. The Cuimu coal mine is subordinate to the Yonglong mining area of the Huanglong coal base, one of China’s 14 large coal bases. It is located in the eastern section of the Longdong loess plateau and belongs to an arid and water deficient mining area. According to drill holes, the strata from old to new are the Middle Triassic Tongchuan Formation, the Lower Jurassic Fuxian Formation, the Middle Yan’an Formation, the Zhiluo Formation, the Anding Formation, the Lower Cretaceous Yijun Formation, and the Luohe Formation, Neogene and Quaternary. The stratigraphic strike is close to EW, inclined to NW, and with a dip angle of 3°~12°, and the overall distribution is gentle. The coal-bearing stratum is the Yan’an Formation of the Middle Jurassic system. The No. 3 coal seam is located in the middle of the first section of the Yan’an formation. It is the main minable coal seam in the area and belongs to a deep-buried extra-thick coal seam. The thickness of the No. 3 coal seam is 0.35~34.20 m, the average thickness is 16.89 m, the buried depth is 314.42~777.03 m, and the average burial depth is 609.09 m.

The Cuimu coal mine adopts a vertical shaft single-level development mode, the comprehensive mechanized top coal mining technology is adopted for the fully mechanized caving face, and the all-caving method is used to manage the roof. The first mining panel of the Cuimu coal mine is the No. 21 panel of the No. 3 coal seam, with the No. 21301, No. 21302 and No. 21303 working faces arranged as shown in Figure 2. Among them, the No. 21301 working face is the first mining working face. The advancing length of the working face is 968 m, the inclined length is 196 m, and the average thickness of the coal seam is 10 m. The advancing length of the No. 21302 working face is 718 m, and the average mining thickness is 12 m. The advancing length of the No. 21303 working face is 850 m, and the average coal seam thickness is 12 m. According to the coal mine production practice, the Cretaceous groundwater has caused a great threat to the safety of coal mining.

2.2. Hydrogeological and Engineering Geological Conditions

Figure 3 shows the typical stratigraphic histogram in the study area. The Jurassic coal measure strata in the Huanglong Jurassic coalfield are soft at the bottom and hard at the top. The upper part is the huge thick Cretaceous Luohe Formation sandstone, and the lower part is the Jurassic Anding Formation mudstone.

According to the hydrogeological data during the exploration and mine construction in the study area, the changes in aquifer lithology, structure, and water abundance were comprehensively analyzed. The aquifers in the overlying strata of the No. 3 coal seam are divided into seven layers and the aquicludes are divided into two layers. The hydrogeological characteristics of each aquifer and aquiclude are shown in

Table 1. Division of aquifers and aquicludes at the Cuimu coal mine.

Stratum Code	Number of Aquifers (Aquicludes)	Name of Aquifer (Aquiclude)	Aquifer Thickness (m)	Units-Inflow (l/s·m)
Q4	I	Quaternary Holocene alluvial-diluvial pore phreatic aquifer	0~8	-
Q2+3	II	Quaternary Middle-Upper Pleistocene loess pore-fracture phreatic aquifer	5~150	-
N2	III	Neogene clay aquiclude	60	-
N1	IV	Neogene sandy gravel aquifer	3~5	-
K1l	V	Luohe sandstone pore-fracture aquifer	103.1~329.3	0.00899~0.1908
K1y	VI	Conglomerate fractured aquifer of Yijun formation	0~60.9	0.0088
J2a	VII	Mudstone aquifuge of Anding formation	68.56~196.4	0~0.000076l
J2z	VIII	Sandstone fractured aquifer of Zhiluo formation	10~50	0.004578
J2y	IX	Yan’an Formation coal seam and its roof sandstone aquifer	5~60	0.003431

.

2.2.1. Main Water-Filled Aquifers of Coal Roof and Their Characteristics

In the coal-bearing strata, the direct water-filled aquifers of the No. 3 coal seam roof include the Middle Jurassic Yan’an Formation coal seam and its roof sandstone aquifer and the Zhiluo Formation sandstone fracture aquifer. Because of their deep buried depth, poorly developed fractures, poor replenishment conditions, and weak water abundance, water-filling in the shaft and roadway is easy to drain, which, therefore, causes little impact on coal seam mining.

The indirect water-filled aquifer is the lower Cretaceous Yijun-Luohe sandstone pore-fracture aquifer, referred to as the Luohe aquifer formation. The Luohe Formation aquifer is spread throughout the whole area, and its distribution pattern is generally thin in the northeast and thick in the southwest. Its thickness is substantial, with a thickness of 23.2–283.9 m. It is composed of sandstone and glutenite of various grain sizes, with medium to coarse-grained sandstone as the main water-bearing interval. The structure is characterized by loose structure, good water storage, and permeability. The porosity is generally 20–30%, the unit water inflow is 0.00899–0.1908 l/s·m, and the permeability coefficient is 0.0146~0.3763 m/d. It is a weak-medium aquifer with uneven water yield. Due to its substantial thickness, wide distribution, good water abundance, and connection with strong regional aquifers, the Luohe water-bearing rock formation has a great impact on mining safety [29,30]. The hydrogeological types of the Cuimu coal mine are divided into complex types.

2.2.2. Water-Resistant Medium Conditions

The lithology, thickness, distribution laws, and mechanical characteristics of aquicludes and their spatial position within a mineable coal seam determine the risk of water inrush due to roof bed separation. Therefore, the evaluation of the lithology, thickness, and mechanical properties of the water-resistant strata is the basis for the analysis of BSWI.

The overburden thickness between the No. 3 coal and the Cretaceous sandstone aquifer is 125.43–277.80 m, with an average of 185.80 m. It is mainly composed of the Anding Formation, Zhiluo Formation, and Yan’an Formation. The lithologic combination of each program is mainly mudstone–sandy mudstone, silty sand–fine sandstone, and medium–coarse sandstone, as shown in

Table 2. Statistical analysis of the lithologic combination characteristics of the overburden between the No. 3 coal and the Cretaceous strata.

Lithology	Mudstone–Sandy Mudstone	Siltstone–Fine Sandstone	Medium–Coarse Sandstone
Minimum accumulated thickness (m)	5.85	0.00	1.50
Minimum proportion (%)	8.52	0.00	6.47
Maximum accumulated thickness (m)	177.99	150.37	112.67
Maximum proportion (%)	83.92	67.37	74.16
Average accumulated thickness (m)	92.74	33.11	44.63
Average proportion (%)	57.75	16.10	26.14
Minimum total thickness of overburden (m)	125.43
Maximum total thickness of overburden (m)	277.80
Average total thickness of overburden (m)	185.80

.

The total thickness of mudstone and sandy mudstone in the Anding Formation, Zhiluo Formation and Yan’an Formation is about 102.08 m, and their clay minerals have a complex water–rock interaction with water. Their hydraulic characteristics are reflected in disintegration and expansion [31,32]. It can be seen from

Table 3. Expansion and disintegration test data.

Stratum Lithology	Free Expansion Rate δef (%)	Expansive Force pe (MPa)	Expansion Rate under Load at 0.01 Mpa δp (%)	Disintegration Resistance Index Id2 (%)
Sandy mudstone of Anding Formation	27.92	17.8	31.85	4.47
Mudstone of Anding Formation	29.55	36.9	33.2	3.01
Sandy mudstone of Zhiluo Formation	23.61	13.6	25.79	7.65
Sandy mudstone of Yan’an Formation	25.33	16.3	28.15	7.37

that the expansion of mudstone and sandy mudstone in the Anding Formation is larger, while that in the Zhiluo Formation and Yan’an Formation is smaller. The disintegration resistance index is opposite to the law of expansion. To conclude, mudstone and sandy mudstone have the characteristics of easy disintegration and good expansibility in the case of water, which is beneficial to the closure of the water-conducting fractures in the lower part of the separation space. Therefore, the mudstone of the Anding Formation at the bottom of the separation layer has good expansion and disintegration properties, which is an important condition for forming a closed separation space and for forming water accumulation in the bed separation space.

2.2.3. Brief Summary of Geological Conditions for Water Inrush

In general, the engineering geological and hydrogeological conditions of a coal seam roof in this area are as follows: (1) The Cretaceous Luohe Formation sandstone aquifer has the characteristics of very large thickness, high water pressure, and large water volume; (2) The underlying Jurassic strata are weak argillaceous aquicludes with large thickness, which are prone to disintegration and argillization under the action of groundwater seepage. (3) The stratigraphic structure type is of lower soft and upper hard. According to the theory of key strata [33,34,35], the location of water accumulation separation is judged, and it is developed between the contact zone of the Cretaceous sandstone and the underlying Jurassic mudstone. (4) The mined Jurassic coal seam belongs to the thick and extra-thick type, and the water-conducting fractured zone is highly developed, which makes it easy to conduct the bed separation space and form the water conducting channel. To conclude, under the action of these geological conditions, it leads to the occurrence of a BSWI disaster.

3. Methods

3.1. Data Source of Water Inrush

Since the production and operation of the Cuimu coal mine, a total of 26 BSWI accidents occurred in the No. 21 panel. Among them, there were 12 instances at the No. 21301 working face, 7 instances at the No. 21302 working face, and 7 instances at the No. 21303 working face, which seriously affected the safety production of the coal mine. According to preliminary statistics, during the recovery period of the No. 21301 working face, the single water inrush volume was 3072~60,059 m³ and the maximum water inrush intensity was 57~1300 m³/h. The water inrush caused the pressure frame, which resulted in 87 days of stoppage of production, 140 pairs of stents were damaged, and the cumulative drainage time was 48 days. The statistical data of BSWIs in the Cuimu coal mine are shown in

Table 4. Statistics on BSWIs in the No. 21301 working face.

Date	Advancing Distance (m)	Maximum Water Inflow (m3/h)	Cumulative Water Yield (m3)	Water Discharge Time (d)	Downtime (d)	Damage Support (Pair)
13 June 2012	180	64	4320	4	0	0
22 June 2012	210	57	4608	4	0	0
28 June 2012	220	70	3072	2	0	0
17 July 2012	300	110	7920	4	1	0
21 July 2012	325	111	4780	3	1	0
28 July 2012	345	1200	10,800	5	3	0
30 September 2012	495	>1000	5000	6	56	91
3 October 2012	495	>1000	8000	6
27 October 2012	495	500	60,059	10	10	0
18 December 2012	590	150	15,727	4	10	0
2 March 2013	785	1100	8570.8	4	12	18
17 March 2013	841	545	15,508.6	2	6	31

,

Table 5. Statistics on BSWIs in the No. 21302 working face.

Date	Advancing Distance (m)	Maximum Water Inflow (m3/h)	Cumulative Water Yield (m3)	Water Discharge Time (d)	Downtime (d)	Damage Support (Pair)
18 May 2013	219	220	5300	2	4	17
25 May 2013	225	200	3500	1	2	0
31 May 2013	228	120	6700	2	13	0
18 June 2013	232	500	40,000	5	21	0
20 October 2013	422	250	19,391	5	43	95
9 January 2014	578	30	2000	1	12	21
27 January 2014	595	40	3000	1	8	87

and

Table 6. Statistics on BSWIs in the No. 21303 working face.

Date	Advancing Distance (m)	Maximum Water Inflow (m3/h)	Cumulative Water Yield (m3)	Water Discharge Time (h)	Downtime (d)
8 May 2014	313	16	1722	-	0
17 June 2014	567	400	3000	7.5	0
20 June 2014	576	234	6293	23	0
22 June 2014	580	300	5958	40	7
25 June 2014	587.3	420.5	11,332	48	3
14 July 2014	718	257.6	16,771	24	4
26 July 2014	764	260	6247	48	0

.

3.2. Testing of Hydrochemical Features

Accurately identifying the source of mine water inrush can provide a basis for the formulation of mine water disaster prevention and control measures [36,37]. During the period of coal resource exploration, mine recovery, and water inrush in the study area, different types of groundwater were sampled and tested. By analyzing the chemical composition, water quality type, PH value, salinity, and total hardness of water samples, the source of the water inrush due to bed separation was analyzed.

3.3. Field Detection of Water-Conducting Fractured Zone

In this study, the WCFZ height was determined by observing the drilling fluid leakage and borehole TV peep. The observation method of the drilling fluid leakage is a method to comprehensively determine the height of a water-conducting fractured zone by directly measuring the leakage of drilling fluid, drilling water level, drilling speed, sticking, rock cores fragmentation, and other data in the process of ground drilling above the coal mine goaf. The borehole TV peep method is to put a waterproof camera probe with a light source into the underground borehole to detect the development characteristics of rock mass fractures induced by mining. The GD3Q-A/B type full-hole wall imaging system instrument was used in the observation. The G1 borehole was a post-mining drilling fluid leakage detection hole and was also used for post-mining peeping detection. The G1 borehole was located above the No. 20301 working face of the Cuimu coal mine. The mining thickness of the working face was 12 m, and the vertical depth from the coal seam floor to the orifice was 553.22 m.

3.4. Dynamic Monitoring of Groundwater

During the mining period of the Cuimu coal mine, there was a connection between the water level changes and the evolution process of water inrush from the bed separation. The water level changes in the aquifer are regarded as precursor information of BSWI. Through the hydrological long observation holes near the stope, the water dynamic monitoring of the bed separation recharge aquifer was carried out, and the water level, water pressure, and other water regime dynamics of the roof aquifer were observed and recorded in detail.

4. Results and Analysis

4.1. Characteristics of BSWI

(1)

The instantaneous water inrush intensity due to bed separation is large, mainly a medium to large water inrush. A total of 26 water inrush accidents occurred in the No. 21 panel of the Cuimu coal mine. Among them, there were 4 water inrush accidents with a maximum water inrush of less than 60 m³/h, 14 water inrush accidents with a maximum water inrush of 60~300 m³/h, and 8 water inrush accidents with a maximum water inrush of more than 300 m³/h and a maximum water inrush of 1200 m³/h, as shown in

Table 7. Classification statistics of water inrush quantity.

Maximum Water Inflow (m3/h)	Number of Water Inrushes	Percentage
<60	4	15%
60~300	14	54%
>300	8	31%

. The water inrush intensity mainly depends on the amount of water accumulated in the separation layer, which is often controlled by various factors such as the size of the separation layer space, duration, and aquifer recharge conditions [38]. For example, on 30 September 2012, when the No. 21301 working face was advanced to 495 m, the pressure on the working face suddenly increased, and water inrush suddenly appeared behind the support of the No. 80~100 working face, and the water level of the working face rose rapidly, reaching the deepest water level of 2.1 m. According to the water accumulation area and rising speed, the water inflow was estimated. The results showed that the maximum water inflow was more than 100 m³/h, the average water inflow was 350 m³/h, and the cumulative water yield was 5000 m³.

(2)

The amount of water inrush is large at the initial stage, but the amount of water attenuates quickly, and the total amount is limited. This is because the accumulated water in the bed separation space has the characteristic of a closed water body, which is similar to the accumulated water in the old goaf [39]. Through field observations, it was found that the water seepage due to bed separation usually started with the water pouring of working face support, which became a surge in a short period of time, and soon submerged the working face. The instantaneous maximum inrush intensity of the water reached 1200 m³/h. After the occurrence of BSWI, the water inrush decreased rapidly and could be drained in a short time. The duration of BSWI was short, and the amount of water inrush attenuated by 97% after only 3 to 5 h. According to statistics, there were 11 times when the total amount of water inrush was greater than 10,000 m³, 5 times when the maximum cumulative water outflow was 60,059 m³ and 10 times when the total amount of water inrush was 5000–10,000 m³ and the total amount of water inrush was 1000–5000 m³. Generally speaking, the total amount of water inrush quantity due to bed separation was limited.

(3)

The location of a BSWI is closely related to the periodic breaking distance of the overlying strata. Large water inrush accidents mainly occurred at distances of 200–230 m, 300–345 m, 420–500 m, and 570–595 m from the open-off cut, which was basically the same as the initial breaking distance and periodic breaking distance of the hard thick sandstone. When the working face advances to the overburden rupture distance above the bed separation, the overburden fracture and the bed separation closes. Under the action of hydrostatic pressure and the impact force of the overburden rupture, the water in the bed separation discharges to the working face, forming the BSWI.

(4)

A BSWI is accompanied by obvious mine pressure increase, pressing frame, and gas outburst. It is believed that these phenomena are closely related to overlying strata fractures and the roof weight in a square meter. According to statistics, all water inrush accidents with a water yield greater than 400 m³/h were accompanied by roof fall, rib spalling, and other phenomena, indicating that when the water inrush quantity is large, it is often accompanied by a significant increase in underground pressure. That is, the water inrush accident with large water yield is closely related to underground pressure. The instantaneous flow of a BSWI is large, and it is often accompanied by a pressing frame and gas outburst. About 360 pairs of supports were damaged due to water inrush in the mining face of the Cuimu coal mine. The absolute emission of gas in the water inrush working face reached 62 m³/min, causing significant economic losses and seriously threatening the mine’s production safety.

4.2. Water Inrush Source due to Bed Separation

According to the test results of hydrochemical features, the comparison results of water chemical characteristics, such as main chemical composition, water quality type, PH value, salinity and total hardness of water-filled aquifers, working face mine water, and inrush water, are shown in

Table 8. Comparison of chemical characteristics of groundwater, working face mine water and inrush water.

Parameter	Cretaceous Groundwater	Jurassic Groundwater	Mine Water	Inrush Water
K + + Na+ (%)	56.77~57.17	93.22	95.36~97.17	95.95~96.24
Ca2+ (%)	12.54~21.67	4.22	1.76~3.08	1.66~1.81
Mg2+ (%)	21.55~30.14	2.56	1.07~1.54	1.93~2.11
HCO3− (%)	54.65~76.55	5.66	32.71~40.96	44.32~46.74
SO42− (%)	19.76~21.01	0.58	12.60~35.08	19.76~21.01
Cl− (%)	9.10~21.71	93.76	32.22~46.52	32.14~35.86
PH	7.60~8.03	7.40	7.47~7.64	6.78~7.95
Hydrochemical type	HCO3-Na (Na∙Mg)	Cl-Na	Cl∙HCO3-Na	HCO3∙Cl-Na
Salinity (mg/L)	780~1054.72	3674	3099~3136	1252~1355
Total hardness (mg/L)	231.81~296.19	208.63	64.12~69.96	33.74

.

It can be seen from Table 8 that the HCO₃ content in Jurassic groundwater was 5.66%, that in the Cretaceous groundwater was 54.65–76.55%, and that in the working face and goaf was 32.71–40.96%, indicating that some Cretaceous groundwater had infiltrated into the mine water. The percentage content of HCO₃₋ in inrush water due to bed separation was 44.32–46.74%, which indicated that more Cretaceous groundwater was involved. The contents of Cl⁻ and SO₄²⁻ in mine water and inrush water were also between Jurassic and Cretaceous groundwater. From the changes of these anion contents, it can be preliminarily asserted that the water source of bed separation water inrush contained Cretaceous groundwater. In addition, the changes in hydrochemical characteristics, such as salinity and hydrochemical types, also reflected that the hydrochemical characteristics of mine water and inrush water had the characteristics of mixing of the Jurassic groundwater and Cretaceous groundwater. Moreover, the Cretaceous groundwater participated in a higher composition in the inrush water, and the Cretaceous groundwater was one of the main sources of the inrush water due to bed separation.

4.3. Development Height of Water-Conducting Fractured Zone

When the drilling depth was 179.29 m, the flushing fluid completely leaked, and the buried depth of the water level in the borehole decreased from 47.1 m to 79.6 m. When the drilling depth was 302.75 m, the buried depth of the water level suddenly dropped from 86.2 m to 124.5 m, as shown in Figure 4 and Figure 5. In the subsequent drilling, the water level in the borehole continued to drop significantly. Before drilling to the borehole depth of 302.75 m, there was sand sedimentation in the borehole after each lifting. In the subsequent construction, the sand sedimentation gradually disappeared, and the boring crown dropped and stuck many times, resulting in two drill-pipe accidents in the borehole.

Based on the comprehensive analysis of the changes of flushing fluid leakage, water level, and abnormal phenomena during drilling rig construction, the borehole depth of 302.75 m was defined as the top boundary of WCFZ, which was 238.47 m away from the No. 3 coal seam.

A borehole peeping instrument was used to observe the overburden failure in the G1 borehole. For the first time, the depth of the downhole TV probe into the borehole was 347.85 m, reaching the Luohe Formation strata. Due to the collapse in the borehole at the bottom of Luohe Formation, the borehole deformation was serious, and the probe failed to enter the borehole bottom. After the drilling rig took measures to conduct a through-hole operation, the second underground TV detection was carried out, and the detection depth was 345.45 m. In both detections, the TV probe failed to enter the bottom of the borehole due to the broken borehole wall, which indicated that coal mining had caused serious damage to the Luohe Formation sandstone in overlying strata.

A total of 76 groups of fractures were observed during the two times of downhole TV detections. The horizontal and nearly horizontal fractures were developed in the borehole depth of 202.65~288.08 m, which was a bed-separation zone. Vertical fractures were mainly developed in the borehole depth of 301.80 m~347.85 m. Starting from the borehole depth of 301.80 m, due to the increasing degree of mining influence, the number of fractures in the mining rock mass increased gradually with the increase in depth, and the size of the fractures increased continuously. Figure 6 shows part of the mining-induced fractures observed by the borehole TV.

In summary, the borehole depth of 301.80 m is defined as the top of WCFZ. The calculation formula for the height of WCFZ is as follows:

$$H_{\mathrm{f}}=H_{\mathrm{s}}-M-h+W$$

(1)

where H_f is the WCFZ height; H_s is the vertical depth of the No. 3 coal seam floor from the porthole; M is the mining height; h is the vertical depth between the top of WCFZ and porthole; W is the compression value of fractured zone strata during drilling observation (W = 0.2 m here).

By conducting calculations, it has been determined that the height of the WCFZ (Water-Conducting Fractured Zone) measures 239.62 m. This indicates the potential for the development of a water channel formed by the Cretaceous sandstone aquifer, creating a distinct separation within the geological layers.

4.4. Water-Level Response of the Luohe Formation Aquifer

The hydrogeological effects of longwall mining on the overlying aquifer are relatively large, especially on the groundwater level [40]. Taking the water inrush accident in the No. 21301 working face at the Cuimu coal mine on 2 March 2013 as an example, the dynamic evolution process of roof BSWI was analyzed.

According to the underground hydrological observation borehole G1, the change curve of the water level in the Luohe Formation was drawn, as shown in Figure 7. From 14 February to 23 February 2013, the water level continued to rise, indicating that the water level was in the “natural water level restoration” stage affected by the previous water inrush.

From 23 February to 26 February 2013, the water level in the Luohe Formation continued to decline linearly, reflecting the continuous expansion and water-filling of the bed separation. It showed that this period was the ponding process after the formation of bed separation, and the water accumulation in bed separation had not yet entered into the working face. Since 26 February, the working face had obvious water inflow, but the water yield was not large, indicating that the channel for water accumulation in bed separation to enter the working face was not completely connected. The water level in the Luohe Formation continued to drop at this stage, indicating that the Luohe Formation sandstone water continued to fill the bed separation space.

At 1:30, during the night shift, on 2 March 2013, the water yield suddenly increased, and the water accumulation in the bed separation rushed into the mining space along the water-conducting fractured zone. The instantaneous water yield reached a maximum of 1100 m³/h and accompanied the increase of mine pressure and obvious pressure frame phenomenon during the water inrush process. After the roof BSWI in a short period of time ended, as the overburden fractures were compacted by continuous mining, the water inrush channel and bed separation were closed, and the water level in the Luohe Formation rose immediately.

Judging from the evolution process of the water level in the Luohe Formation, which had gone through three stages in sequence in the complete evolution of bed separation water inrush: ① the natural recovery stage of the water level under the influence of the last water inrush; ② the significant decline stage of the water level during the period of water-filling into the separation space; ③ the water level recovery stage after the water inrush channel was closed.

5. Analysis of the Mechanism of BSWI

5.1. Conditions for the Occurrence of BSWI

Under complex geological and hydrogeological conditions in coal mines, separated layer water accumulation is a dynamic water source formed by the bed separation space in the overlying strata receiving aquifer recharge during coal mining. If the water in the bed separation flows into the excavation space under the influence of mining, it will form a BSWI disaster. In general, the occurrence of a BSWI disaster requires the following four basic conditions to be met simultaneously.

(1) Space condition. The accumulated water separation layer must be relatively closed, located in the separation zone above the water-conducting fractured zone. Moreover, there is a certain thickness of a relatively complete impermeable layer (∆H) distributed between the accumulated water separation layer and the water-conducting fractured zone, as shown in Figure 8. Otherwise, the water entering the separation layer space will directly infiltrate into the working face through the water-conducting fracture and cannot form bed separation water accumulation. That is, the position of bed separation space where water can accumulate should satisfy Equation (2).

H > H_f

(2)

where H is the distance between the top boundary of the mining coal seam and the bottom boundary of accumulated water separation and H_f is the maximum height of water-conducting fractured zone.

(2) Water-filling source condition. The surrounding rock of bed separation space must have strong water abundance, that is, after the formation of a relatively closed bed separation, the bed separation space needs to accept the supplement of the surrounding aquifer before the water can accumulate and form the bed separation water. The better the water abundance of the aquifer, the shorter the accumulated water time required. If the surrounding aquifer cannot replenish bed separation, the relatively closed bed separation will form a vacuum structure.

(3) Time condition. For the water accumulation in the overlying strata separation, the greater the amount of separation developed in the overlying strata after coal mining, and the longer the duration, the longer the time for the separation space to accept the surrounding aquifer. The greater the amount of water accumulation in the overlying strata separation, the greater the risk of water inrush from the bed separation.

(4) Water-conducting channel condition. After the groundwater is gathered in the overlying strata separation space, it does not necessarily lead to the BSWI and must have the condition of a water-conducting channel. During the mining process of the coal mining face, the water-conducting fractured zone increases with the increase in the advancing length. When the water-conducting fractured zone directly penetrates the separation water in the overlying strata or the thickness of the water-resistant strata under the separation water is not enough to resist the water pressure, the water accumulation in overlying strata separation flows into the working face through the water-conducting channel, causing the water inrush disaster.

5.2. Mechanism of BSWI

Based on the comprehensive analysis of the basic conditions required for the occurrence of water disaster due to bed separation and the geological conditions of the study area, the hydrogeological-mining conceptual model is constructed to reveal the mechanism of roof BSWI, as shown in Figure 9.

Before coal seam mining, the characteristics of the spatial combination of the coal seam and the groundwater in the study area are that the super-thick Cretaceous aquifer is above and the coal seam is below. The Anding Formation mudstone in the Middle Jurassic system is a stable aquiclude between the coal seam and the overlying Cretaceous system, as shown in Figure 9a.

With the advance of the working face, the coal seam roof subsides, bends, and collapses, and the vertical fractures and bed-separation fractures induced by mining begin to develop. When the working face advances to a certain distance, a cavity-type separation space begins to form between the super-thick sandstone in the Luohe Formation and the Anding Formation mudstone, and the water-conducting fractured zone enters into the lower section of the Luohe Formation sandstone aquifer. The cavity-type separation space receives the recharge of the Luohe Formation aquifer and begins to accumulate water. The groundwater level in the Luohe Formation drops slightly. Because the mudstone below the separation space is thick, and it disintegrates and swells when exposed to water, it will naturally block the fractures induced by mining and form an aquiclude for the second time, making the separation space a relatively stable “closed water storage body”. However, the fracture-type separation layers in the Zhiluo Formation sandstone are pierced by a water-conducting fractured zone, which cannot form a closed water storage space. The water in fracture-type separation layers is the main source of the water yield of the mine, as shown in Figure 9b. As the working face continues to advance, the cavity-type separation at the bottom of the Luohe Formation sandstone expands horizontally and its volume continues to increase. The water-conducting fractured zone develops upward to the maximum height, and its range into the Luohe Formation sandstone aquifer increases. The groundwater of the Cretaceous Luohe formation infiltrates and supplies to the separation space, which is filled with water for a period of time to form a huge water bag, as shown in Figure 9c.

As the working face continues to advance, the overhanging length of the super-thick sandstone above the separation space reaches its limit span; it will rupture, which leads to the failure of the closed water storage space. Under the action of hydrostatic pressure and overburden breaking impact force, the water in the cavity-type separation immediately flows down to the working face, resulting in the mine water inrush disaster, as shown in Figure 9d. The groundwater level in the Luohe Formation drops sharply at this stage. Following this, the working face continues to move forward, the original separation space is gradually closed, and a new separation space between the super-thick sandstone in the Luohe Formation and the Anding Formation mudstone is gradually formed. The groundwater level in the Luohe Formation will rise at this stage. After the expansion and self-sealing of the water-conducting fractures in the Anding Formation mudstone, the Luohe Formation sandstone water continues to supply the separation space. When the super-thick sandstone above the separation space reaches the limit span of its periodic fracture, the water inrush due to bed separation occurs again. With the continuous advancement of the working face, roof BSWI occurs cyclically and dynamically periodically, as shown in Figure 9e.

6. Conclusions

(1)

This paper takes the water inrush accidents in the Cuimu coal mine as the background and analyzes the characteristics of BSWI. The results show that BSWI has the characteristics of large instantaneous water inrush and rapid water depletion. The location of the water inrush is closely related to the periodic fracture distance of the overlying strata and is accompanied by obvious phenomena such as increased pressure frame, gas outburst, and water level change.

(2)

Four basic conditions are required for the occurrence of a BSWI disaster. Only when the bed separation located above the water-conducting fractured zone simultaneously possesses the conditions of water-filling source, water accumulation time, and water-conducting channel can the bed separation water disaster be formed.

(3)

A hydrogeological-mining conceptual model of the BSWI is put forward, in which the whole process of periodic water inrush due to bed separation is described, and the mechanism of the BSWI is revealed. The research results are of great significance for the prediction and prevention of BSWI disasters.