# Analysis of Key Factors Affecting Water Disaster in Deep Mining and Establishment of a Water Disaster Evaluation Method Suitable for Different Mining Depths

Authors: Jianghui He; Wenping Li; Wei Qiao; Zhi Yang; Qiqing Wang
Journal: Geofluids
 (2021)

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## Abstract

Deep mining is the current and future mining focus in the coalfield of North China. Therefore, it is necessary to establish a water inrush assessment method suitable for deep mining. The conventional water inrush coefficient (T) method is simple to use, but its accuracy is low when the mining depth is very small or large because it neglects the role of the aquifer water yield. The main purpose of this paper is to introduce a simple water inrush risk assessment method that is more applicable in deep mining than the T method. In this study, the hydrogeological characteristics in deep mining were studied and the role of the aquifer water yield in water inrush was analyzed. Afterwards, an improved T method considering the aquifer specific yield (q) was established. In the new method, the critical water inrush coefficient changes with changing q following a negative correlation. The parameter thresholds were determined based on systematic data analyses of 186 mining cases. The results of the statistical analysis show that the accuracy of the new method at different mining depths is higher than that of the T method.


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## Body

## 1. Introduction

Coal will remain the major energy source in China for a long time. From the coal resources in China, 53% are located below a depth of 1 km [1]. Following the depletion of shallow coal resources, the coal mining depth (Dm) has increased. However, the exploitation of deep coal seams in the North China coalfield (Figure 1) faces a threat from the underlying confined aquifers in Ordovician limestone.Figure 1
Distribution of coal resources in China.Floor karst water inrush has been extensively studied in literature. For example, Wu et al. have conducted extensive and detailed research on water inrush from mine floors, proposing some risk-assessment methods, such as the master-control index method and the vulnerability index method and its improved version [2, 3]. These methods were successfully applied to practical mining. Liu and Hu adopted the water-resistance coefficient method to identify floor water inrush risks [4]. Meng et al. presented a geological-assessment method to evaluate the vulnerability of the floor water inrush by considering its lithology, structure, and water inrush coefficient [5]. Wang et al. used a secondary fuzzy comprehensive evaluation system to analyze the risk of mine water inrush [6]. He et al. developed a P−H−q evaluation system for risk assessment of water inrush based on the water-pressure-transfer theory [7]. Hong-fei et al. established a coal floor water inrush model that considered the structural characteristics of floor strata to evaluate the antipermeability strength of an aquiclude [8]. Bai et al. explained the groundwater inrush mechanism in terms of variable mass and nonlinear dynamics [9]. Chen et al. used an engineering analogy method to predict the water inrush risk areas in mining, using Fisher’s discriminant analysis [10]. Physical simulations performed by Zhang et al. revealed that the permeability of floor rocks had regional and temporal characteristics [11]. Gao et al. found that the water inflow increased with an increasing permeability coefficient following a power function [12]. Many other methods are available to evaluate the possibility of mine water inrush and related water-rock coupling problems [13–34]. Some scholars have specially studied floor water inrush through faults [35–39]. In summary, the common methods used to predict the mine floor water inrush can be categorized as (1) empirical formula and GIS technology, (2) mathematical analysis, (3) nonlinear mathematical analysis, and (4) simulation experiments [40]. However, most of these methods require sophisticated software, equipment, and/or operational technology as well as complex analyses and calculations. Therefore, widespread implementation of these methods is difficult.The conventional water inrush coefficient (T) method is the most practical method used to predict water inrush in coal mines under various conditions. T (in MPa/m) is obtained from the following empirical equation based on large water inrush data:
(1)T=PM,where P (in MPa) is the water pressure in a confined aquifer in the coal-seam floor and M (in m) is the thickness of the aquiclude of the coal seam floor.T denotes the water pressure that can be supported by a unit thickness of the coal seam floor aquiclude and can quantitatively indicate the risk of water inrush from an underlying confined aquifer during coal mining. A larger T value indicates more susceptibility to water inrush accidents. The minimum value of T that can initiate water inrush is called the critical water inrush coefficient (Ts), which ranges from 0.06 to 0.1 MPa/m, as provided in the Stipulation on Prevention and Control of Coal Mine Water of China [41]. Floor water inrush incidents are likely to occur when T>Ts. This formula is simple and practical because it incorporates two important parameters that govern the characteristics of the aquifer and aquiclude (P and M, respectively). Therefore, it has been incorporated in relevant standards and coal mine water prevention and control regulations.Practical mining data collected from the North China coalfield, which include 186 water inrush cases and 92 safety mining cases, have proven that the results predicted using theT method are generally consistent with the actual results when 2.5MPa<P<3.5MPa (Figure 2).Figure 2
Change in the accuracy of the prediction results from the water inrush coefficient method (T) under different water pressures (P).In addition, according to the data collected from the North China coalfield [42], P exhibits a linear positive correlation with the aquifer depth (D) (Figure 3).Figure 3
Distribution of the water pressure (P) at different depths (D).The following equation can be obtained from Figure3:
(2)D=101.28P+11.212.BecauseD will change with Dm (Figure 4), it is necessary to analyze the change in the accuracy of the T method with changing Dm.Figure 4
Relationship betweenD and Dm. Di: buried depth of the aquifer directly threatening the safety mining of coal seam i; Dmi: mining depth of the coal seam i; Mi: thickness of the aquiclude directly protecting the safe mining of coal seam i.Figure4 shows that the relationship between D and Dm can be expressed by the following formula:
(3)Dm=D−M.After statistically analyzing the data in Figure2 with the help of Equations (2) and (3), the variation of the prediction accuracy from the T method with changing Dm can be obtained (Figure 5).Figure 5
Change in the prediction accuracy of theT method with changing Dm.TheT method was put forward in 1964 in Jiaozuo’s hydrogeological work. The corresponding Ts was also the empirical value based on the mining conditions at that time. Because the average value of Dm was about 300 m at that time, the empirical value of Ts obtained according to the mining conditions at that time was more suitable when Dm was about 300 m. This conclusion is basically consistent with the law of fitting line in Figure 5, which shows that the accuracy of the T method is the highest when Dm is equal to 311.16 m. Figure 5 shows that when Dm is very shallow or very deep, the prediction accuracy from the T method is very low. Because coal seams buried at great depths are the object of future exploitation in the North China coalfield, evaluation of water hazards during mining of deeply buried coal seams is the focus of the present study. The Dm of deeper coal seams is generally more than 400 m, mostly exceed 700 m, or exceed 1000 m in the 33 mines in the North China coalfield [43]. Hence, the T method is unsuitable for deep-mining risk assessment.In addition to theT method, a common method is the GIS-based multifactor superposition analysis (represented by the vulnerability index method). The GIS-based multifactor superposition analysis method is a good method because many related factors are considered. However, in the discriminant map obtained by this method, we can only obtain the relative magnitude of water inrush risk in different positions, but not the absolute magnitude of the possibility of water inrush in different positions. When applying the GIS-based multifactor superposition analysis method, a specific boundary which can divide the safety zone and water inrush zone is needed, such as Ts in the T method. Therefore, it is necessary to study the hydrogeological conditions in deep mining and to establish a simple water inrush risk assessment method that is more applicable than the T method.


## 2. Hydrogeological Conditions in Deep Mining

The coal seams in some mines are buried at depths of 1,200 m and face the threat of limestone aquifers (Figure6), which cause the P and T values to reach 13 MPa and 0.144–1.256 MPa/m, respectively. In addition, when D is large, aquifers have low porosity (ф) and permeability (K) owing to high ground stress (Figures 7 and 8). Ф determines the size of the water-storage space in aquifers, and K determines the movement efficiency of water in aquifers. Reduced ф and K lead to a reduced water yield (or effluent capacity) of the aquifer.Figure 6
Stratigraphic column of the Permian coal-bearing strata in the Huaibei mining area, China.Figure 7
Changes in ground stress (σ) at different depths (D): σhmax: maximum horizontal principal stress; σhmin: minimum horizontal principal stress; σv: vertical principal stress.Figure 8
Changes in the porosity (ф) and permeability (K) of aquifers with changing depth (D).The water yield of aquifers is often described in terms of the specific yield (q, in L/(s·m)), which indicates the sustainable water yield from a drill hole with a diameter of 91 mm when the water levels drop by 1 m per unit time during a pumping test. Figure 9 shows a steady-flow pumping test in a confined aquifer. In this figure, h0, Q, R0, sw, RW, a, and K denote the initial water head of the confined aquifer, the pump flow, the radius of the cone of depression, the drop of the water level in the well, the radius of the pumping well, the thickness of the aquifer, and its permeability, respectively. Under these conditions, the Dupuit formula of the confined pumping well can be obtained as follows:
(4)Q=2πKaswlnR0−Rw.Figure 9
Pumping test of a steady well flow in a confined aquifer.According to the definition ofq, it can be calculated as follows:
(5)q=Qsw=2πKalnR0−Rw.The value ofq decreases with increasing D because both ф and K decrease (Figure 10). Qiao et al. and Zhao reached the same conclusion [44, 45].Figure 10
Change in the specific yield (q) for different depths (D).


## 3. Analysis of the Role ofq in the Occurrence of Water Inrush

q reflects the effluent capacity of the aquifer. The water inrush process involves draining water from the aquifer to other spaces. Therefore, theoretically, q is closely related to the occurrence of water inrush. In contrast, water scarcity significantly reduces the likelihood of the confined water to break through an aquiclude. Some mining cases have proven that water inrush is related to the q value. When the T method is used to evaluate the risk of water inrush, unrealistic results are produced if q is neglected (Table 1). The list in Table 1 indicates that from the first to the fifth cases, because the q values were all small, no water inrush occurred, although T>Ts. From the sixth to the tenth cases (Table 1), q was large and water inrush occurred during the mining process, although T did not exceed Ts. Therefore, it is confirmed that q significantly influences the occurrence of water inrush. Thus, in order to improve the accuracy of the results, the change in q in deep mining must be considered. Shi et al. reached the same conclusion [46].Table 1
Mining cases in the North China coalfield, wrongly evaluated by theT method.
No.Mining faceP (MPa)q (L/(s·m))T (MPa/m)Ts (MPa/m)Evaluation resultsField verification1III628, Zhuzhuang mine, Huaibei mining area4.220.0160.1400.07DangerSafety2III616, Yangzhuang mine, Huaibei mining area5.500.0520.1100.07DangerSafety31044, Liudian mine, Huaibei mining area6.930.0240.2100.07DangerSafety41037, Liudian mine, Huaibei mining area6.600.0240.3300.07DangerSafety51088, Heishan mine, Zibo mining area6.230.0320.1300.07DangerSafety611022, Jiulishan mine, Jiaozuo mining area1.1924.4500.0520.07SafetyDanger,Q=714m3/h79507, Xiangzhuang mine, Feicheng mining area0.928.3300.0380.07SafetyDanger,Q=300m3/h84221, Jiaoxi mine, Jiaozuo mining area1.0012.6200.0530.07SafetyDanger,Q=201m3/h912031, Jiulishan mine, Jiaozuo mining area0.9211.3300.0410.07SafetyDanger,Q=588m3/h101007, Fucun mine, Zibo mining area1.467.1200.0430.07SafetyDanger,Q=256m3/hNote:Q refers to the water inflow in a water inrush case.This analysis revealed that under deep mining conditions, the aquifer related to water inrush is characterized by largeP and low q, and q is positively correlated with the possibility of water inrush. Therefore, in deep mining, the possibility of water inrush can be reduced using small q, even if T>Ts. Thus, the conventional T method is oversimplified for assessing the water inrush possibility because it only considers P, while neglecting q. The water yield of aquifers that recharge the mine must be considered to accurately assess water inrush risk. Therefore, the objectives of this study are to analyze the role of q in water inrush and to establish a convenient method for water inrush risk assessment that considers q.


### 3.1. Before Formation of Water Inrush

From the point of view of mechanics, the floor water inrush involves breaking the aquiclude under continuous water pressure within a certain period followed by the gushing of water from the aquifer below the aquiclude, along the rupture surface, into the goaf. Because the aquiclude breakage only occurs after a period of deformation, a time limit exists for the occurrence of water inrush. First, the aquiclude takes time to deform to its critical breaking state. After a goaf with lengthL is formed, the aquiclude gradually bends and deforms under the water pressure to reach its critical breaking state. The time required to reach this condition is called required time (tr). Second, the deformation and breaking of the aquiclude must be completed before the goaf is filled and compacted by the caved overburden [47, 48]. The time before the goaf is filled and compacted is called the available time (ta) for water inrush formation. Water inrush can only occur when tr<ta.The water pressure that is continuously exerted on the aquiclude, called the aquiclude bending pressure (Pb), constitutes the external force directly responsible for the eventual breakage of the aquiclude. Pb also refers to the water pressure accumulated in the cavity caused by the deformation of the aquiclude (i.e., ③ in Figure 11). Unless otherwise stated, this is the cavity referred to hereafter. P refers to the water pressure or undisturbed water pressure in the confined aquifer before or after mining, respectively. The water-pressure variation and water runoff in the aquifer after mining are shown in Figure 11. The persistence of Pb is assured by the continuous flow of water into the cavity. The efficiency of the water movement in the aquifer is related to the water yield of the aquifer. Obviously, a lower water yield in the confined aquifer results in lower efficiency of the seepage flow to the cavity, slower deformation in the aquiclude, longer time for the aquiclude to deform to its breaking state, greater possibility for tr to exceed ta, and less possibility of water inrush. The water yield of aquifers is often described in terms of q. In other words, the smaller the q value, the lower the possibility of water inrush.Figure 11
Definition of terms used for risk assessment of water inrush from the coal-seam floor. (a) Numerical model designed by RFPA software. (b) Water pressure distribution after self-stable equilibrium of the model before excavation. (c) Water pressure distribution after excavation. (d) Schematic diagram of water migration characteristics in floor aquifer. ①: areas with undisturbed water pressure; ②: area with disturbed water pressure; ③: cavity caused by the deformation of the aquiclude, which is situated between the aquiclude and the aquifer. The blue arrows indicate the water seepage direction.
(a)(b)(c)(d)For water inrush caused by dangerous geological structures (i.e., faults, collapse columns, etc.), the initiation process involves erosion of the filler in the cracks of these structures owing to water flow, which is also caused by the continuity of seepage. Similar to the water inrush in the cavity described earlier, the smaller theq, the lower is the possibility of water inrush in the dangerous geological structures.


### 3.2. After Formation of Water Inrush

Water inrush data from 186 excavation faces in the North China coalfield (Tables2–4) were collected to analyze the role of q in the water inrush.Table 2
Specific yield (q) and water inrush coefficient (T) values in small water inrush cases.
q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)1.6670.0241.3330.0250.1670.1080.5830.0681.4170.0701.1670.0301.1170.0451.1670.0520.9550.0420.3330.0511.3330.0560.1670.0471.3000.0371.0000.0571.6280.0891.1670.0401.2100.1680.3330.0521.3330.0571.1170.0780.7330.0891.6500.0471.3330.0820.4170.0621.2500.0791.0000.0510.1670.0920.8600.0580.0830.0581.5800.1001.4500.0541.0330.1080.6670.1621.6670.0600.7670.0460.7170.0630.0830.1140.4430.0650.7000.0550.5000.0521.6670.0180.1670.1611.1330.0590.4430.0770.5000.0461.1670.0570.6670.1610.5000.1661.3120.0691.3330.0451.1670.0470.5000.1700.5000.0461.2100.0831.4170.0500.8330.0460.5000.1820.3330.0270.4430.0720.0830.0692.0000.1910.0830.0520.3900.0790.0670.0730.5000.0451.3330.0381.0180.0670.5830.0610.1250.1050.3330.0550.5000.0370.0830.1011.6970.0560.1670.0600.8170.0561.6670.0380.3330.0850.3900.0781.0000.072Table 3
Specific yield (q) and water inrush coefficient (T) values in medium water inrush cases.
q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)8.3330.0288.3330.0383.7500.0512.4500.0525.8820.07112.5000.01810.0000.0460.8670.0574.0000.0366.1470.06511.000.02411.5000.04312.8330.07411.6670.0464.5780.0544.0000.0172.5000.0761.7830.0689.1670.0486.3780.0514.6670.01810.4170.0222.5330.0523.3330.0592.1670.06011.0000.01612.6170.05310.550.0612.6630.0593.8830.06813.0000.0187.1670.0744.1670.0515.0000.0492.7170.04314.1670.0438.8330.03411.3330.0413.3330.0219.1670.0753.3330.0153.0530.0793.4670.05610.0000.0965.0000.0625.1670.0269.6330.05615.5000.06910.0000.0522.5000.0268.3330.0237.6000.0656.6670.0127.1170.0432.6670.07813.3330.0183.3330.0394.5330.17610.7500.05211.7000.0345.0000.0445.5830.0839.7330.0646.6670.0245.4170.0873.3330.0262.5330.0614.0330.0723.3330.0433.6670.0864.6670.0212.1670.0851.8330.0412.5000.0554.3330.0858.3330.0212.7670.0773.7830.0314.3330.0536.1670.09716.6670.0134.1670.0462.2830.0376.2500.0703.0000.09113.3330.0222.000.0455.4170.0253.5430.1679.5000.0918.3330.0244.000.1284.8330.0262.4170.0694.0000.085Table 4
Specific yield (q) and water inrush coefficient (T) values in large water inrush cases.
q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)20.0000.05028.0000.01121.6670.04924.3330.05719.2830.05224.4500.05219.8170.03720.6670.05716.7000.02825.0000.04123.1830.07517.0670.061Usually, according to the volume of water inflow (Q) recorded in each case, water inrush is classified as a small case (0<Q≤60m3/h), medium case (60<Q<600m3/h), or large case (Q≥600m3/h). Among the 186 water inrush cases studied, 79 were small, 95 were medium, and 12 were large.The statistical summary of the 186 water inrush cases (Table5) shows that almost all the small-Q cases were located in areas with 0.06≤q<1.7L/s·m, almost all the medium-Q cases were located in areas with 1.7≤q<16.7L/s·m, and all the large-Q cases were located in areas with q≥16.7L/s·m. No water inrush cases occurred when q<0.06L/s·m.Table 5
Number of water inrush cases for each volume of water inflow (Q) and different magnitudes of specific yield (q).
Magnitude ofq (L/(s·m))Number of water inrush casesSmall-Q water inrush casesMedium-Q water inrush casesLarge-Q water inrush casesq<0.060000.06≤q<1.778101.7≤q<16.71940q≥16.70012After a water inrush occurs, the larger the value ofq, the greater the amount of water poured out because of the greater water yield of the aquifer, the larger the amount of water released from the aquifer in a short period, and the larger the amount of water that flows into the mining area. Therefore, q can determine the magnitude of the water inrush after an event.


## 3.1. Before Formation of Water Inrush

From the point of view of mechanics, the floor water inrush involves breaking the aquiclude under continuous water pressure within a certain period followed by the gushing of water from the aquifer below the aquiclude, along the rupture surface, into the goaf. Because the aquiclude breakage only occurs after a period of deformation, a time limit exists for the occurrence of water inrush. First, the aquiclude takes time to deform to its critical breaking state. After a goaf with lengthL is formed, the aquiclude gradually bends and deforms under the water pressure to reach its critical breaking state. The time required to reach this condition is called required time (tr). Second, the deformation and breaking of the aquiclude must be completed before the goaf is filled and compacted by the caved overburden [47, 48]. The time before the goaf is filled and compacted is called the available time (ta) for water inrush formation. Water inrush can only occur when tr<ta.The water pressure that is continuously exerted on the aquiclude, called the aquiclude bending pressure (Pb), constitutes the external force directly responsible for the eventual breakage of the aquiclude. Pb also refers to the water pressure accumulated in the cavity caused by the deformation of the aquiclude (i.e., ③ in Figure 11). Unless otherwise stated, this is the cavity referred to hereafter. P refers to the water pressure or undisturbed water pressure in the confined aquifer before or after mining, respectively. The water-pressure variation and water runoff in the aquifer after mining are shown in Figure 11. The persistence of Pb is assured by the continuous flow of water into the cavity. The efficiency of the water movement in the aquifer is related to the water yield of the aquifer. Obviously, a lower water yield in the confined aquifer results in lower efficiency of the seepage flow to the cavity, slower deformation in the aquiclude, longer time for the aquiclude to deform to its breaking state, greater possibility for tr to exceed ta, and less possibility of water inrush. The water yield of aquifers is often described in terms of q. In other words, the smaller the q value, the lower the possibility of water inrush.Figure 11
Definition of terms used for risk assessment of water inrush from the coal-seam floor. (a) Numerical model designed by RFPA software. (b) Water pressure distribution after self-stable equilibrium of the model before excavation. (c) Water pressure distribution after excavation. (d) Schematic diagram of water migration characteristics in floor aquifer. ①: areas with undisturbed water pressure; ②: area with disturbed water pressure; ③: cavity caused by the deformation of the aquiclude, which is situated between the aquiclude and the aquifer. The blue arrows indicate the water seepage direction.
(a)(b)(c)(d)For water inrush caused by dangerous geological structures (i.e., faults, collapse columns, etc.), the initiation process involves erosion of the filler in the cracks of these structures owing to water flow, which is also caused by the continuity of seepage. Similar to the water inrush in the cavity described earlier, the smaller theq, the lower is the possibility of water inrush in the dangerous geological structures.


## 3.2. After Formation of Water Inrush

Water inrush data from 186 excavation faces in the North China coalfield (Tables2–4) were collected to analyze the role of q in the water inrush.Table 2
Specific yield (q) and water inrush coefficient (T) values in small water inrush cases.
q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)1.6670.0241.3330.0250.1670.1080.5830.0681.4170.0701.1670.0301.1170.0451.1670.0520.9550.0420.3330.0511.3330.0560.1670.0471.3000.0371.0000.0571.6280.0891.1670.0401.2100.1680.3330.0521.3330.0571.1170.0780.7330.0891.6500.0471.3330.0820.4170.0621.2500.0791.0000.0510.1670.0920.8600.0580.0830.0581.5800.1001.4500.0541.0330.1080.6670.1621.6670.0600.7670.0460.7170.0630.0830.1140.4430.0650.7000.0550.5000.0521.6670.0180.1670.1611.1330.0590.4430.0770.5000.0461.1670.0570.6670.1610.5000.1661.3120.0691.3330.0451.1670.0470.5000.1700.5000.0461.2100.0831.4170.0500.8330.0460.5000.1820.3330.0270.4430.0720.0830.0692.0000.1910.0830.0520.3900.0790.0670.0730.5000.0451.3330.0381.0180.0670.5830.0610.1250.1050.3330.0550.5000.0370.0830.1011.6970.0560.1670.0600.8170.0561.6670.0380.3330.0850.3900.0781.0000.072Table 3
Specific yield (q) and water inrush coefficient (T) values in medium water inrush cases.
q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)8.3330.0288.3330.0383.7500.0512.4500.0525.8820.07112.5000.01810.0000.0460.8670.0574.0000.0366.1470.06511.000.02411.5000.04312.8330.07411.6670.0464.5780.0544.0000.0172.5000.0761.7830.0689.1670.0486.3780.0514.6670.01810.4170.0222.5330.0523.3330.0592.1670.06011.0000.01612.6170.05310.550.0612.6630.0593.8830.06813.0000.0187.1670.0744.1670.0515.0000.0492.7170.04314.1670.0438.8330.03411.3330.0413.3330.0219.1670.0753.3330.0153.0530.0793.4670.05610.0000.0965.0000.0625.1670.0269.6330.05615.5000.06910.0000.0522.5000.0268.3330.0237.6000.0656.6670.0127.1170.0432.6670.07813.3330.0183.3330.0394.5330.17610.7500.05211.7000.0345.0000.0445.5830.0839.7330.0646.6670.0245.4170.0873.3330.0262.5330.0614.0330.0723.3330.0433.6670.0864.6670.0212.1670.0851.8330.0412.5000.0554.3330.0858.3330.0212.7670.0773.7830.0314.3330.0536.1670.09716.6670.0134.1670.0462.2830.0376.2500.0703.0000.09113.3330.0222.000.0455.4170.0253.5430.1679.5000.0918.3330.0244.000.1284.8330.0262.4170.0694.0000.085Table 4
Specific yield (q) and water inrush coefficient (T) values in large water inrush cases.
q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)q (L/(s·m))T (MPa/m)20.0000.05028.0000.01121.6670.04924.3330.05719.2830.05224.4500.05219.8170.03720.6670.05716.7000.02825.0000.04123.1830.07517.0670.061Usually, according to the volume of water inflow (Q) recorded in each case, water inrush is classified as a small case (0<Q≤60m3/h), medium case (60<Q<600m3/h), or large case (Q≥600m3/h). Among the 186 water inrush cases studied, 79 were small, 95 were medium, and 12 were large.The statistical summary of the 186 water inrush cases (Table5) shows that almost all the small-Q cases were located in areas with 0.06≤q<1.7L/s·m, almost all the medium-Q cases were located in areas with 1.7≤q<16.7L/s·m, and all the large-Q cases were located in areas with q≥16.7L/s·m. No water inrush cases occurred when q<0.06L/s·m.Table 5
Number of water inrush cases for each volume of water inflow (Q) and different magnitudes of specific yield (q).
Magnitude ofq (L/(s·m))Number of water inrush casesSmall-Q water inrush casesMedium-Q water inrush casesLarge-Q water inrush casesq<0.060000.06≤q<1.778101.7≤q<16.71940q≥16.70012After a water inrush occurs, the larger the value ofq, the greater the amount of water poured out because of the greater water yield of the aquifer, the larger the amount of water released from the aquifer in a short period, and the larger the amount of water that flows into the mining area. Therefore, q can determine the magnitude of the water inrush after an event.


## 4. Establishment and Application of theT‐q Method

In this section, the development of a convenient method for water inrush risk assessment that considersq is discussed.A confined aquifer is a source of power and the medium for the water inrush. An aquiclude is the medium that can obstruct water inrush. Water inrush is related not only to the potential of the confined water to break through the aquiclude but also to the extent of release of this potential (unless otherwise stated, this is the potential referred to hereafter).P and M parametrize the potential of the confined water to break through the aquiclude, and q is a nonnegligible parameter that determines the extent of this potential release. The contributions of these parameters to the possibility of water inrush are described below.First, whenP is big and M is small, the potential for water inrush is high. Therefore, the ratio of P to M (i.e., T) can be used to indicate the potential for water inrush. The greater the value of T, the higher the potential for water inrush. Second, the greater the value of q, the better the hydrodynamic conditions in the aquifer, the greater the extent of this potential release, and the higher the possibility of water inrush. Therefore, the possibility of water inrush is positively correlated with both T and q values.


### 4.1. Division of Water Inrush and Safety Zones in theT‐q Method

From the previous analysis, the possibility of water inrush was determined in terms ofT and q. When the potential for water inrush is greater, the energy that can be directly applied to break through the aquiclude or the water inrush channels is larger and the possibility of water inrush is greater. In other words, for constant q, the greater the value of T, the higher the possibility of water inrush. Figure 12 shows the 186 water inrush cases used to obtain the Ts-q function curve. Because Ts is the minimum T that can initiate occurrence of water inrush, the Ts points should be in the area where T is relatively small for a certain q value of the aquifer. The points marked by the symbol “×” in Figure 12 were selected as the critical points, and the Ts-q function curve was obtained by fitting these points. The points in the area above the Ts-q curve were identified as the water inrush points, and those in the area below the Ts-q curve were the safety points. Figure 12 shows that except for the fitting points; all the other water inrush points are above the Ts-q curve. The water inrush points between the Ts-q and the stipulated Ts curves were wrongly evaluated as safety points by the T method, because at these points, T was less than the stipulated Ts. Therefore, the new method is more practical than the T method in determining the water inrush and safety zones.Figure 12
Discrimination diagram using theT-q method.TheTs-q curve also shows that Ts is not fixed but changes with changing q. There is a negative correlation between Ts and q. The relationship between them is expressed as follows:
(6)Ts=0.0246q−0.315,q≥0.06L/s·m.The areas below and above theTs-q curve were defined as the safety and water inrush zones, respectively.The list in Table5 indicates that no water inrush accidents occurred when q<0.06L/s·m. Therefore, the line at q=0.06L/s·m can also be used as the division between the water inrush and safety zones, considering that the area with q<0.06L/s·m is the safety zone. In this interval, Ts was not affected by q. A larger but not definitive value could be considered as the value of Ts, which is represented by the symbol ∞.
(7)Ts=∞,q<0.06L/s·m.


### 4.2. Risk Level Classification in Water Inrush Zone in theT-q Method

For the area above theTs-q curve, the risk level was determined by the value of q in the aquifer.Table5 and Figure 12 show that the boundary between the small and medium water inrush cases was q=1.7L/s·m. The area where the small water inrush cases were distributed was defined as a relatively safe zone because Q in these cases was sufficiently small to be easily drained and could not affect mine production. The boundary between the medium and large water inrush cases was q=16.7L/s·m. The area where the medium water inrush cases were distributed was defined as a danger zone because Q was relatively large and would have a negative influence on mine production. Applying the same logic, the area where the large water inrush cases were distributed was called a high-danger zone because Q was too large for rapid draining and would jeopardize safe mine production.


### 4.3. Novelty Analysis of theT-q Method

The abovementioned analysis enabled the development of theT-q method.Figure12 shows the discrimination diagram with T on the y-axis and q on the x-axis. The Ts-q curve separates the safety zone from the water inrush zone. The water inrush zone consists of relatively safe, danger, and high-danger zones, which are indicated by different colors. The risk level of the water inrush from the coal-seam floor can be easily and quickly determined by introducing the q and T values of an excavation face onto Figure 12. Thus, the T-q method provides a quick and handy risk assessment in mining. Smaller q indicates lower possibility of water inrush, whereas larger q correlates with higher magnitude of water inrush after the water inrush initiation.The novelty of this study is reflected in the following two aspects.


#### 4.3.1. TheT-q Method Is More Accurate and Practical

The occurrence of water inrush actually includes two stages: (1) the formation stage of a water inrush channel and (2) the water-discharge stage from the aquifer into the excavation face. In the formation stage of the water inrush channel, the instability and breakage of the intact aquiclude or dangerous geological structure occur under the action of continuous water pressure. The continuity of water pressure is guaranteed by the water movement in the aquifer. Therefore, when the possibility of water inrush is evaluated, the efficiency of water movement in the aquifer must be considered to obtain more accurate results. In the water-discharge stage from the aquifer to the excavation face, the amount of water inflow into the excavation face is also determined by the efficiency of the water movement in the aquifer. The efficiency of water movement in the aquifer is related toq. Therefore, the q of the aquifers that recharge the mine must be considered to accurately assess the water inrush risk. Compared with the traditional T method, the T-q method proposed herein provides a more realistic risk assessment in deep mining because q is considered.As it can be seen from Figure13, the method proposed in this paper has high accuracy for different Dm. The accuracy is generally higher than 80%. Given that the results of this paper were obtained through the statistical analysis of the existing sample data, the accuracy of the method will inevitably be affected by the number of samples and the distribution range of the Dm of the samples. If we get a large number of sample data with different Dm, the accuracy of the method will increase. Although the accuracy of the method is affected by the number of samples, the conclusion stating that the accuracy of the method is higher than that of the T method is convincing because Ts is treated as a q-related variable. Especially when the mining depth is very shallow or deep, the accuracy of the method is higher than that of the T method.Figure 13
Change in the accuracy of the prediction with different mining depths (Dm) using the T-q method.


#### 4.3.2. TheT-q Method Is Simple and Can Be Quickly Used

Many factors related to water inrush exist, including the size of the excavation face,Dm, and complexity of the geological structure. Therefore, as more factors are considered in the evaluation of water inrush risk, more accurate are the evaluation results. However, considering more factors involves a more complex analysis and calculation, which can make difficult its application. Therefore, the selection of the most decisive factors needs to consider both the accuracy of the results and the simplicity of the discrimination process. Several decades of mining practice have proven that the results of the water inrush risk assessment based solely on the size of P, M, and q can fully provide a guide to the prevention and control of mine floods. Compared with the existing multifactor superposition and analytical methods with complex operating process, the proposed method provides a quicker risk assessment in deep mining because only one T-q discrimination diagram is needed to complete the discrimination.


### 4.4. Application and Verification of theT-q Method

Water inrush cases and safety cases are used to verify the accuracy of the proposed method (Table6). According to Equations (6) and (7), the Ts values of each sampling point in Table 6 can be obtained.Table 6
Application cases in the North China coalfield evaluated by theT-q method.
No.Mining faceDm (m)P (MPa)q (L/(s·m))T (MPa/m)Ts (MPa/m)Evaluation resultsField verificationAIII628, Zhuzhuang mine, Huaibei mining area408.474.220.0160.140∞SafetySafetyBIII616, Yangzhuang mine, Huaibei mining area518.255.500.0520.110∞SafetySafetyC1044, Liudian mine, Huaibei mining area680.086.930.0240.210∞SafetySafetyD1037, Liudian mine, Huaibei mining area659.666.600.0240.330∞SafetySafetyE1088, Heishan mine, Zibo mining area594.266.230.0320.130∞SafetySafetyF11022, Jiulishan mine, Jiaozuo mining area108.851.1924.4500.0520.0090Water inrush withQ≥600m3/h (high danger)Water inrush withQ=714m3/hG9507, Xiangzhuang mine, Feicheng mining area80.180.928.3300.0380.0126Water inrush with60<Q<600m3/h (danger)Water inrush withQ=300m3/hH4221, Jiaoxi mine, Jiaozuo mining area93.621.0012.6200.0530.0111Water inrush with60<Q<600m3/h (danger)Water inrush withQ=201m3/hI12031, Jiulishan mine, Jiaozuo mining area81.950.9211.3300.0410.0115Water inrush with60<Q<600m3/h (danger)Water inrush withQ=588m3/hJ1007, Fucun mine, Zibo mining area125.131.467.1200.0430.0133Water inrush with60<Q<600m3/h (danger)Water inrush withQ=256m3/hK4231, Jiaoxi mine, Jiaozuo mining area109.881.173.330.0590.0168Water inrush with60<Q<600m3/h (danger)Water inrush withQ=91m3/hL9304, Shida mine, Zibo mining area182.121.965.880.0710.0141Water inrush with60<Q<600m3/h (danger)Water inrush withQ=210m3/hM10401, Taoyang mine, Feicheng mining area43.400.541.670.0240.0209Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=60m3/hN42013, Jiaoxi mine, Jiaozuo mining area72.240.81.170.040.0234Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=42m3/hO11051, Jiulishan mine, Jiaozuo mining area139.321.50.720.0630.0273Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=26m3/hP9301, Shida mine, Zibo mining area225.282.420.390.0780.0331Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=14m3/hQ9302, Shida mine, Zibo mining area176.571.910.580.0680.0292Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=21m3/hR9307, Shida mine, Zibo mining area148.541.6410.0570.0246Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=36m3/hS9019, Shida mine, Zibo mining area140.631.540.080.0580.0545Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=3m3/hT1095, Xiazhuang mine, Zibo mining area190.362.0510.0720.0246Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=36m3/hNote: ∞ indicates a larger, but not definitive, value ofTs; the distribution of the cases can be seen in Figure 14.It can be seen from Table6 that since the q values of points A–E are very small, the Ts of points A–E are all ∞, which is greater than their actual T values, indicating that water inrush will not occur at these points. The Ts values of points F–T are smaller than their actual T values, indicating that water inrush will occur at these points. Furthermore, the water quantity range of these water inrush points can be predicted according to the magnitudes of q. The actual results show that the risk level of water inrush at all points and the water quantity range of each water inrush point are accurately predicted (Figure 14, Table 6). On the contrary, it can be seen from Table 1 that if q is not considered, the water inrush risk of these points will not be predicted accurately. Compared with Tables 1 and 6, we can conclude that the T-q method is reliable, and q is a key factor that cannot be ignored in water inrush assessment.Figure 14
Distribution of application cases recorded in Table6 in the T-q discrimination diagram.


## 4.1. Division of Water Inrush and Safety Zones in theT‐q Method

From the previous analysis, the possibility of water inrush was determined in terms ofT and q. When the potential for water inrush is greater, the energy that can be directly applied to break through the aquiclude or the water inrush channels is larger and the possibility of water inrush is greater. In other words, for constant q, the greater the value of T, the higher the possibility of water inrush. Figure 12 shows the 186 water inrush cases used to obtain the Ts-q function curve. Because Ts is the minimum T that can initiate occurrence of water inrush, the Ts points should be in the area where T is relatively small for a certain q value of the aquifer. The points marked by the symbol “×” in Figure 12 were selected as the critical points, and the Ts-q function curve was obtained by fitting these points. The points in the area above the Ts-q curve were identified as the water inrush points, and those in the area below the Ts-q curve were the safety points. Figure 12 shows that except for the fitting points; all the other water inrush points are above the Ts-q curve. The water inrush points between the Ts-q and the stipulated Ts curves were wrongly evaluated as safety points by the T method, because at these points, T was less than the stipulated Ts. Therefore, the new method is more practical than the T method in determining the water inrush and safety zones.Figure 12
Discrimination diagram using theT-q method.TheTs-q curve also shows that Ts is not fixed but changes with changing q. There is a negative correlation between Ts and q. The relationship between them is expressed as follows:
(6)Ts=0.0246q−0.315,q≥0.06L/s·m.The areas below and above theTs-q curve were defined as the safety and water inrush zones, respectively.The list in Table5 indicates that no water inrush accidents occurred when q<0.06L/s·m. Therefore, the line at q=0.06L/s·m can also be used as the division between the water inrush and safety zones, considering that the area with q<0.06L/s·m is the safety zone. In this interval, Ts was not affected by q. A larger but not definitive value could be considered as the value of Ts, which is represented by the symbol ∞.
(7)Ts=∞,q<0.06L/s·m.


## 4.2. Risk Level Classification in Water Inrush Zone in theT-q Method

For the area above theTs-q curve, the risk level was determined by the value of q in the aquifer.Table5 and Figure 12 show that the boundary between the small and medium water inrush cases was q=1.7L/s·m. The area where the small water inrush cases were distributed was defined as a relatively safe zone because Q in these cases was sufficiently small to be easily drained and could not affect mine production. The boundary between the medium and large water inrush cases was q=16.7L/s·m. The area where the medium water inrush cases were distributed was defined as a danger zone because Q was relatively large and would have a negative influence on mine production. Applying the same logic, the area where the large water inrush cases were distributed was called a high-danger zone because Q was too large for rapid draining and would jeopardize safe mine production.


## 4.3. Novelty Analysis of theT-q Method

The abovementioned analysis enabled the development of theT-q method.Figure12 shows the discrimination diagram with T on the y-axis and q on the x-axis. The Ts-q curve separates the safety zone from the water inrush zone. The water inrush zone consists of relatively safe, danger, and high-danger zones, which are indicated by different colors. The risk level of the water inrush from the coal-seam floor can be easily and quickly determined by introducing the q and T values of an excavation face onto Figure 12. Thus, the T-q method provides a quick and handy risk assessment in mining. Smaller q indicates lower possibility of water inrush, whereas larger q correlates with higher magnitude of water inrush after the water inrush initiation.The novelty of this study is reflected in the following two aspects.


### 4.3.1. TheT-q Method Is More Accurate and Practical

The occurrence of water inrush actually includes two stages: (1) the formation stage of a water inrush channel and (2) the water-discharge stage from the aquifer into the excavation face. In the formation stage of the water inrush channel, the instability and breakage of the intact aquiclude or dangerous geological structure occur under the action of continuous water pressure. The continuity of water pressure is guaranteed by the water movement in the aquifer. Therefore, when the possibility of water inrush is evaluated, the efficiency of water movement in the aquifer must be considered to obtain more accurate results. In the water-discharge stage from the aquifer to the excavation face, the amount of water inflow into the excavation face is also determined by the efficiency of the water movement in the aquifer. The efficiency of water movement in the aquifer is related toq. Therefore, the q of the aquifers that recharge the mine must be considered to accurately assess the water inrush risk. Compared with the traditional T method, the T-q method proposed herein provides a more realistic risk assessment in deep mining because q is considered.As it can be seen from Figure13, the method proposed in this paper has high accuracy for different Dm. The accuracy is generally higher than 80%. Given that the results of this paper were obtained through the statistical analysis of the existing sample data, the accuracy of the method will inevitably be affected by the number of samples and the distribution range of the Dm of the samples. If we get a large number of sample data with different Dm, the accuracy of the method will increase. Although the accuracy of the method is affected by the number of samples, the conclusion stating that the accuracy of the method is higher than that of the T method is convincing because Ts is treated as a q-related variable. Especially when the mining depth is very shallow or deep, the accuracy of the method is higher than that of the T method.Figure 13
Change in the accuracy of the prediction with different mining depths (Dm) using the T-q method.


### 4.3.2. TheT-q Method Is Simple and Can Be Quickly Used

Many factors related to water inrush exist, including the size of the excavation face,Dm, and complexity of the geological structure. Therefore, as more factors are considered in the evaluation of water inrush risk, more accurate are the evaluation results. However, considering more factors involves a more complex analysis and calculation, which can make difficult its application. Therefore, the selection of the most decisive factors needs to consider both the accuracy of the results and the simplicity of the discrimination process. Several decades of mining practice have proven that the results of the water inrush risk assessment based solely on the size of P, M, and q can fully provide a guide to the prevention and control of mine floods. Compared with the existing multifactor superposition and analytical methods with complex operating process, the proposed method provides a quicker risk assessment in deep mining because only one T-q discrimination diagram is needed to complete the discrimination.


## 4.3.1. TheT-q Method Is More Accurate and Practical

The occurrence of water inrush actually includes two stages: (1) the formation stage of a water inrush channel and (2) the water-discharge stage from the aquifer into the excavation face. In the formation stage of the water inrush channel, the instability and breakage of the intact aquiclude or dangerous geological structure occur under the action of continuous water pressure. The continuity of water pressure is guaranteed by the water movement in the aquifer. Therefore, when the possibility of water inrush is evaluated, the efficiency of water movement in the aquifer must be considered to obtain more accurate results. In the water-discharge stage from the aquifer to the excavation face, the amount of water inflow into the excavation face is also determined by the efficiency of the water movement in the aquifer. The efficiency of water movement in the aquifer is related toq. Therefore, the q of the aquifers that recharge the mine must be considered to accurately assess the water inrush risk. Compared with the traditional T method, the T-q method proposed herein provides a more realistic risk assessment in deep mining because q is considered.As it can be seen from Figure13, the method proposed in this paper has high accuracy for different Dm. The accuracy is generally higher than 80%. Given that the results of this paper were obtained through the statistical analysis of the existing sample data, the accuracy of the method will inevitably be affected by the number of samples and the distribution range of the Dm of the samples. If we get a large number of sample data with different Dm, the accuracy of the method will increase. Although the accuracy of the method is affected by the number of samples, the conclusion stating that the accuracy of the method is higher than that of the T method is convincing because Ts is treated as a q-related variable. Especially when the mining depth is very shallow or deep, the accuracy of the method is higher than that of the T method.Figure 13
Change in the accuracy of the prediction with different mining depths (Dm) using the T-q method.


## 4.3.2. TheT-q Method Is Simple and Can Be Quickly Used

Many factors related to water inrush exist, including the size of the excavation face,Dm, and complexity of the geological structure. Therefore, as more factors are considered in the evaluation of water inrush risk, more accurate are the evaluation results. However, considering more factors involves a more complex analysis and calculation, which can make difficult its application. Therefore, the selection of the most decisive factors needs to consider both the accuracy of the results and the simplicity of the discrimination process. Several decades of mining practice have proven that the results of the water inrush risk assessment based solely on the size of P, M, and q can fully provide a guide to the prevention and control of mine floods. Compared with the existing multifactor superposition and analytical methods with complex operating process, the proposed method provides a quicker risk assessment in deep mining because only one T-q discrimination diagram is needed to complete the discrimination.


## 4.4. Application and Verification of theT-q Method

Water inrush cases and safety cases are used to verify the accuracy of the proposed method (Table6). According to Equations (6) and (7), the Ts values of each sampling point in Table 6 can be obtained.Table 6
Application cases in the North China coalfield evaluated by theT-q method.
No.Mining faceDm (m)P (MPa)q (L/(s·m))T (MPa/m)Ts (MPa/m)Evaluation resultsField verificationAIII628, Zhuzhuang mine, Huaibei mining area408.474.220.0160.140∞SafetySafetyBIII616, Yangzhuang mine, Huaibei mining area518.255.500.0520.110∞SafetySafetyC1044, Liudian mine, Huaibei mining area680.086.930.0240.210∞SafetySafetyD1037, Liudian mine, Huaibei mining area659.666.600.0240.330∞SafetySafetyE1088, Heishan mine, Zibo mining area594.266.230.0320.130∞SafetySafetyF11022, Jiulishan mine, Jiaozuo mining area108.851.1924.4500.0520.0090Water inrush withQ≥600m3/h (high danger)Water inrush withQ=714m3/hG9507, Xiangzhuang mine, Feicheng mining area80.180.928.3300.0380.0126Water inrush with60<Q<600m3/h (danger)Water inrush withQ=300m3/hH4221, Jiaoxi mine, Jiaozuo mining area93.621.0012.6200.0530.0111Water inrush with60<Q<600m3/h (danger)Water inrush withQ=201m3/hI12031, Jiulishan mine, Jiaozuo mining area81.950.9211.3300.0410.0115Water inrush with60<Q<600m3/h (danger)Water inrush withQ=588m3/hJ1007, Fucun mine, Zibo mining area125.131.467.1200.0430.0133Water inrush with60<Q<600m3/h (danger)Water inrush withQ=256m3/hK4231, Jiaoxi mine, Jiaozuo mining area109.881.173.330.0590.0168Water inrush with60<Q<600m3/h (danger)Water inrush withQ=91m3/hL9304, Shida mine, Zibo mining area182.121.965.880.0710.0141Water inrush with60<Q<600m3/h (danger)Water inrush withQ=210m3/hM10401, Taoyang mine, Feicheng mining area43.400.541.670.0240.0209Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=60m3/hN42013, Jiaoxi mine, Jiaozuo mining area72.240.81.170.040.0234Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=42m3/hO11051, Jiulishan mine, Jiaozuo mining area139.321.50.720.0630.0273Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=26m3/hP9301, Shida mine, Zibo mining area225.282.420.390.0780.0331Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=14m3/hQ9302, Shida mine, Zibo mining area176.571.910.580.0680.0292Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=21m3/hR9307, Shida mine, Zibo mining area148.541.6410.0570.0246Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=36m3/hS9019, Shida mine, Zibo mining area140.631.540.080.0580.0545Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=3m3/hT1095, Xiazhuang mine, Zibo mining area190.362.0510.0720.0246Water inrush with0<Q≤60m3/h (relatively safe)Water inrush withQ=36m3/hNote: ∞ indicates a larger, but not definitive, value ofTs; the distribution of the cases can be seen in Figure 14.It can be seen from Table6 that since the q values of points A–E are very small, the Ts of points A–E are all ∞, which is greater than their actual T values, indicating that water inrush will not occur at these points. The Ts values of points F–T are smaller than their actual T values, indicating that water inrush will occur at these points. Furthermore, the water quantity range of these water inrush points can be predicted according to the magnitudes of q. The actual results show that the risk level of water inrush at all points and the water quantity range of each water inrush point are accurately predicted (Figure 14, Table 6). On the contrary, it can be seen from Table 1 that if q is not considered, the water inrush risk of these points will not be predicted accurately. Compared with Tables 1 and 6, we can conclude that the T-q method is reliable, and q is a key factor that cannot be ignored in water inrush assessment.Figure 14
Distribution of application cases recorded in Table6 in the T-q discrimination diagram.


## 5. Conclusions

The main contribution of this paper is to improve theT method by obtaining a simple and easy to use method with a higher accuracy. Through this study, the following findings are obtained.The conventionalT method is mostly applicable when Dm is about 300 m. The corresponding Ts is also extracted based on the prevention and control experience of water disaster in those mines whose Dm is about 300 m. The accuracy of the T method is low when Dm is very shallow or very deep because in the T method, the Ts of different Dm is set as a fixed value. Thus, the method neglects the fact that Ts is a variable related to q, affected by Dm.The possibility of water inrush is positively correlated with bothT and q. T indicates the potential of the confined water to break through the aquiclude, and q is the key parameter that determines the extent of this potential release. The value of the critical Ts is not fixed but changes with changing q and has a negative correlation with q. The T-q method proposed in this paper has high accuracy for different Dm, generally being higher than 80%. The risk level of water inrush from the coal seam floor can be easily and quickly determined by introducing the q and T values of an excavation face onto the T-q discrimination diagram. The evaluation results of 20 application cases show that the T-q method has high accuracy at different Dm.Compared with the traditionalT method and the multifactor superposition analysis method, the method established in this paper has the characteristics of convenient field application and high accuracy. The establishment process of the T-q method can provide reference for the establishment of relevant methods in other coal fields.



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*Source: 9948763-2021-07-06.xml*