Introduction
Coal serves as the primary energy source in China, fulfilling approximately 60 to 70 percent of the nation’s energy requirements in 2018 [1, 2]. Coal usage is projected to reach 50 percent of China’s total energy resources as early as 2030 [3].
The complex hydrogeological conditions of China’s mines have long been associated with significant casual-ties and economic losses for a long time [4–8]. Currently, four major types of water disasters are observed in China, including high-pressure karst water inrush from the coal seam floor, water inrush in sandstone and the loose layer of coal seam roof, water inrush from abandoned small coal mines and old mine goaf, and water disasters caused by surface water infiltration [9-11]. A comprehensive investi-gation of these water disasters has been conducted in several studies, encompassing mine water inrush mechanisms [12–17], the risk prediction and evaluation of coal mine water disasters [18–21], and mine-water disaster prevention and control technologies [22–24].
Until now, the majority of research has been centered on the first three water disasters. However, literature is scarce regarding the infiltration characteristics of surface water in coal seam mining beneath gullies [6]. The western region of China serves as the primary source of coal, with Inner Mongolia alone producing over 1 billion tons of raw coal in 2019. [25, 26]. The coalfields in this region are characterized by shallow coal seam occurrences, diverse and fluctuating topography, and relatively well-developed loess gullies [12, 27]. During the coal seam mining process, the water-conductive fracture zone reaches the surface in the shallow buried and gully areas, resulting in significant surface damage [28–31]. Surface water is then transported into the goaf along the water-conductive fracture zone, posing a grave threat to the safety of coal mines [32, 33]. Furthermore, the depletion of surface water and groundwater flow has a detrimental impact on the fragile ecological environment of the mines in Western China [34, 35]. Therefore, to facilitate the concurrent development of safe and efficient coal mining practices and ecological conservation in mining areas, it is crucial to investigate the infiltration characteristics of surface water in the coal seam mining beneath gullies.
The present study emphasizes on the typical working face of the Buliangou coal mine in the Zhungeer coalfield of Inner Mongolia intensively examining the infiltration characteristics of surface water in coal seam mining beneath gullies. Moreover, we suggest preventive strategies to curtail surface water infiltration. The outcomes of our research furnish guidelines for the safe and efficient op-eration of the working face, in line with the standards of Nature Journal.
Study Area
The study area is located in the Zhungeer coalfield in China, which belongs to the Ordos Basin. The surface of the study area is covered by loess and aeolian sand, and the bedrock is exposed only in large gully areas. The terrain of this study area is highly complex, with well-developed branched gullies. Some of the important branched gullies within the range of the coalfield are Dabu-liangou, Xiaobuliangou, Xizigou, Fangtangou, Ejiagou, and Qingshuigou. The branch gullies are specially developed, and flash floods occur in the rainy season, with a massive flow rate and strong hydrodynamic force that lead to severe soil erosion. During the mining of the No.6 coal seam in the coal mine, the water-conductive fracture zone develops to the surface in the shallow burial and gully area within the coalfield, and the rainfall in the area is relatively concentrated. With the arrival of the rainy season, seasonal rivers form in the gully, and surface water enters the coal mine along the water-conductive fracture zone, giving rise to disastrous water inrush accidents. In the mining of the typical working face, seasonal rivers in surface gullies pass through this working face (Fig. 1). According to field observations, the water-conductive fracture zone had developed to the surface of the mine of this working face (Fig. 2). The surface river water will later enter the goaf along the fissure, causing water gushing in the coal mine and threatening the safety of production.
Fig. 1.
Location of study area: a Buliangou coal mine in China, b Drainage map of well field and location of typical working face.
Therefore, it is imperative to investigate the infiltration characteristics and develop effective measures for preventing surface water ingress in typical working faces to ensure safe coal mining operations.
The general topography of the study area shows a high in the southwest and a low in the northeast, with an altitude of about +1110 ～ +1346 m. The average annual precipitation is 408 mm, while the maximum monthly rainfall is 247.5 mm. The total yearly evaporation in this region varies between 1824.7 and 2204.6 mm.
We selected the typical working face as our research object. In the coal mine, the 6th coal seam, which has a gentle inclination, is the main minable layer, with a thickness of 5.66–25.51 m. The buried depth varies between 166.80 and 478.00 m. According to the surface drilling exploration data, the strata in the minefield belonged to the periods (from old to new): Cambrian, Carboniferous, Permian, Cretaceous, and Quaternary. The comprehensive pillar of the coal stratum in the typical working face is obtained from the average value of the thickness of each stratum in the data of 7 boreholes (Fig. 3).
The principal aquifers in the mining field include the Quaternary pore aquifer, the Cretaceous pore fissure aquifer, the Permian sandstone fissure aquifer, and the Ordovician karst fissure aquifer. Among them, the Quaternary pore aquifer, the Cretaceous pore fissure aquifer, and the Permian sandstone fissure aquifer are located above the 6th coal seam and have weak water abundance, and before the coal seam mining, the water bodies in the main aquifers of the roof have been drained in advance. During the mining process of the 6th coal seam, minimal infiltration from the roof aquifer was observed within the mine. In most sections of the mining field, there is a well-developed water-conductive fracture zone extending until the Quaternary system. Consequently, surface gully river water and atmospheric precipitation may pose significant hydrological risks to the 6th coal seam in this mining area.
Materials and Methods
Water Sampling and Analysis
Thirteen water samples were collected, with 5 representing surface water, 2 representing cretaceous groundwater, and 6 representing mine water. The sampling locations were recorded using a portable GPS device and visualized in Fig. 1. To ensure representative groundwater samples, groundwater was pumped for 5-10 minutes before collec-tion to remove stagnant water. Sampled water was stored in white plastic bottles, which were rinsed 3–5 times using the water to be sampled. The bottles were then sealed, la-beled, and transported to the laboratory for physicochemical analysis within 24 hours. All groundwater samples were analyzed for pH, total dissolved solids (TDS), and major ions (Na+, K+, Ca2+, Mg2+, HCO3−, SO42−, Cl−). Prior to testing, all groundwater samples were filtered through 0.45 μm membrane filters to remove suspended particu-lates in the laboratory. Major anions (Cl-, SO42-) were analyzed using an anion chromatograph with anion columns with detection accuracies of 1% (Thermo Company, Waltham, USA), while major cations (Ca2+, Mg2+, Na+, K+) were measured using a cation chromatograph with cation columns with detection accuracies of 1%. The accuracy of the analyses for all groundwater samples was verified by calculating the ionic balance error, with a 5% error limit considered the upper limit. Consequently, the calculated ionic balance errors for all groundwater samples were below the established limit.
Physical and Mechanical Property Tests of the Strata
Obtaining physical and mechanical parameters of the strata is the basis of the research to assess the distribution law of mining fissures in the overburden of coal seam mining. In order to elaborately understand the properties of each layer, the strata with different buried depths were sampled through the surface drilling of the typical working face (Fig. 4). Thereafter, the samples were transported to the laboratory for physical and mechanical tests, and the rel-evant experimental data was obtained. The rock samples from different strata were made into 50×100 mm standard samples. The RTR-1500 rapid triaxial rock testing system was used for the mechanical test, while the mercury porosimeter was used for the porosity test. The experimental equipment used in this study is shown in Fig. 5.
A Numerical Model of the Mining Fissures in Overburden in Gully Mining
Distribution characteristics of mining fissures in overburden in gully mining are the basis of studying surface water infiltration [36]. The development characteristics of the mining fissures in the overburden after gully mining were analyzed using the UDEC software based on the engineering geological conditions and the drilling data of the typical working face in the study area [37]. In our study, we established a numerical model that conformed to the actual stratum. The length of the typical working face was 2118.3 m. The presence of a river crossing over the typical working face can be seen from the drainage map of the well field in Fig. 1. When the water-conductive fracture zone develops to the surface after coal mining, the seasonal river in the gully is expected to enter the mine along the fissure. The primary area where surface water enters the mine is in the vicin-ity of the seasonal river in the gully above the working face. Therefore, the area near the river in the gully was selected, and a two-dimensional numerical model with a length of 600 m and a height of 328 m was established according to a comprehensive histogram of the stratum. The thickness of the coal seam was 15.6 m. According to the actual gully position and depth above the working face, we set two gullies in the model; the gully 1 was about 6 m deep, and the gully 2 was about 10 m deep (Fig. 6). The left, right, and lower boundaries of the model were fixed by displacement. Confining pressure was applied to the left and right boundaries to simulate the actual stress of the stope. The parameters used in the numerical model were obtained by physical and mechanical tests (Table 1).
Table 1.
General chemistry of groundwater samples (mg/L).
A Numerical Model of Surface Water Infiltration
Engineering Geological Model of Surface Water Infiltration
Gully mining causes varying degrees of damage to the overburden, which then results in corresponding impacts on the surface water resources. After coal seam mining, the water-conductive fracture zone connects with the surface, leading to a leakage of the surface water. Thereafter, seasonal rivers formed in the gully enter the mine along the water-conductive fracture zone, threatening the safety of the mine [1, 12]. The engineering geological model of surface water infiltration is shown in Fig. 7.
Construction of Surface Water Infiltration Model
(1) Model construction and mesh generation
We aimed to investigate the infiltration characteristics of surface water through fissures following coal mining and quantify the volume of surface water infiltrating into the mine. Additionally, we aim to provide guidelines for preventing mine water disasters based on our findings. Considering the development characteristics of mining fissures in the overburden, we imported these fissures into COMSOL software to construct a numerical model simulating surface water infiltration after coal mining [38]. Herein, we present the numerical model and mesh generation using a scenario where the working face advances by 200 m as an example (Fig. 8).
(2) Calculation of model parameters and boundary conditions
The surface water mainly enters the mine along the fissure formed by mining. The surface water finds it difficult to penetrate the regions of the mine where the rest of the mining fissures are not connected with the surface. Therefore, to simplify the model, the areas other than the mining fissures were regarded as a whole, and the parameters of the whole stratum were used to per-form the corresponding numerical calculations. According to the field water injection test, the permeability coefficient of the Cretaceous, Permian, and Carboniferous aquifer was 0.0194 m/d, 0.021–0.081 m/d, and 0.0056– 0.0577 m/d, respectively. The average of the permeability coefficient of the three aquifers (0.0340 m/d) was used as a representative of the permeability coefficient of the stratum. According to the conversion formula
Using the fracture flow module in COMSOL software, calculate the amount of surface water entering the goaf through mining fissures. The surface water flow in the mining fissures is in general much faster than in the surround-ing porous matrix. The cubic law is a common correlation for modeling fissure flow. It defines the permeability kf in the mining fissures according to
where df (m) is the fissure’s width and ff is the roughness factor. The value of df can be obtained from the numerical model of the mining fissures in overburden in gully mining; The value of ff in numerical calculations is 1. Finally, the permeability of the mining fissures in the numerical model of surface water infiltration can be calculated using the above formula.
Two gullies were set at the upper boundary of the model. With the arrival of the rainy season, seasonal rivers will form in the gullies. The water level in the gullies during the rainy season reaches a height of about 1 m. Therefore, according to the actual situation, the boundary of the model gully was set as 1 m water head. The left, right, and bottom boundaries of the model were selected as the no-flow boundaries. The goaf boundary was designated as the free flow boundary. The surface water in the gully entered the goaf along the fissure.
Results and Discussion
Water Hydrochemistry
To delve into the general hydrochemistry of groundwater, the physicochemical parameters of the samples from surface water, cretaceous groundwater, and mine water are presented in Table 1.
The pH values of water samples within the study area exhibit a range of 7.32 to 9.1, indicating the presence of weakly alkaline water. The average pH values for surface water, chalk aquifer groundwater, and mine water are 8.42, 9.01, and 7.57, respectively. The TDS values of the various water samples range from 457.66 to 1230.37, with some samples exhibiting highly mineralized waters. The average TDS values for surface water, chalk aquifer groundwater, and mine water are 624.29, 452.4, and 905.71, respectively. The pH value and TDS of mine water exhibit a closer resemblance to surface water compared to other samples. The main anions in surface water consist of HCO3− and SO42−, while the primary cations are SO42−. The order of major ions concentration in surface water samples is HCO3−>SO42−> K++Na+>Ca2+. The main anions in Cretaceous groundwater are HCO3− and Cl-, while the primary cations are Na+ and K+. The main anions in mine water are HCO3− and SO42−, with the primary cations being SO42−. Clearly, the main ion composition of mine water exhibits a greater similarity to surface water compared to other samples.
The Piper (1944) trilinear diagram is very helpful in determining connections between different dissolved con-stituents and types of groundwater based on their chemical characteristics. Based on Table 1, the Piper trilinear diagram was drawn (Fig. 9a). The main hydrochemical type of mine water is HCO3-Ca, while the main hydrochemical types of surface water are HCO3-Ca and SO42--Ca. The primary hydrochemical type of groundwater in the Cretaceous system is SO42--Na. The results of the hydrochemical analysis indicate that mine water mainly originates from surface water. The analysis of hydrochemical types indicates that the main source of mine water is surface water. The δD and δ18O values in water are generally less affected by water-rock interactions, making them effective means to determine different water sources and origins. A δD-δ18O diagram (Fig. 9b) was constructed based on the δD and δ18O values of surface water, Cretaceous groundwater, and mine water. The atmospheric precipitation equation was calculated using the δD and δ18O values of atmospheric precipitation: δD=9.38δ18O+7.38. It is evident that the average values of surface water and cretaceous groundwater, as well as their respective δD and δ18O values, fall near the local atmospheric precipitation line, indicating that their main source of recharge is atmospheric precipitation. Furthermore, the proximity between surface water and mine water points demonstrates that mine water primarily originates from surface water.
Physical and Mechanical Properties of the Strata
The experimental data was arranged and summarized to get the physical and mechanical parameters of different rock samples, including compressive strength, tensile strength, modulus of elasticity, Poisson’s ratio, cohesion, internal friction angle, porosity, and bulk density (Table 2). The physical and mechanical properties of the strata obtained from tests presented in Table 2 serve as crucial modeling parameters for establishing a more realistic numerical model of mining fissures in gully mining. Using the parameters in Table 2, we established a numerical model of the mining fissures in overburden in gully mining, and the distribution characteristics of mining fissures obtained by the model were imported into COMSOL software to construct a numerical model simulating surface water infiltration after coal mining.
The rock mass strength and porosity data in Table 2 indicate that the rock mass above the roof of the 6th coal seam exhibits a layered structure characterized by soft and hard interphase. This type of layer structure, when subjected to mining disturbance, promotes extensive development of mining fissures in the overburden [39], as further confirmed by subsequent numerical simulations.
Numerical Simulation Results and Analysis of the Mining Fissures of Overburden in Gully Mining
Our model primarily analyzed the evolution process of the mining fissures in the overburden of the gully mining coal seam. The specific simulation scheme was set to advance from right to left. Considering the influence of the model boundary, the boundary coal pillars were reserved for 100 m, and each advancement was carried out in steps of 50 m, with a total advancement of 300 m. Distribution characteristics of mining fissures in the overburden of coal seam in gully mining have been shown in Fig. 10.
After coal mining, the roof strata moved and underwent deformation to produce open and closed fissures [40, 41]. In Fig. 10, the red part represents the closed fissures, while the blue part represents the open fissures, which comprise abscission and cross fissures. The penetration of the cross fissures into the layered rock and subsequent communica-tion with the abscission layer produces a fluid migration channel or a water-conductive fracture zone [42]. Reference [43] employs PFC numerical simulation software to establish a numerical model of mining-induced fractures in the overlying rock of shallow seam mining through gully mining. It simulates the development characteristics and analyzes the evolution law of these fractures. In this study, we analyzed the development law of mining-induced fractures in the overlying rock during coal seam overlying mining. By comparing our results with those presented in reference [43], we observe consistency in both the simulated development characteristics of mining-induced fractures and the description and analysis of their development process in the overlying rock.
Table 2.
Formation physical and mechanical parameters.
The distribution characteristics of mining fissures in the overburden under different advancing distances of the working face are depicted in Fig. 10. As the working face advanced to a distance of 50 m, abscission fissures emerged along with the upper part of the basic roof. Additionally, a few longitudinal fissures became evident in the middle section of the goaf. As the working face continued to advance, the immediate roof mudstone fell with the mining [44]. When the working face advanced to 100 m, the first collapse of the basic roof occurred; that is, the first pressure occurred. At this stage, the collapsed height was about 38 m away from the coal seam roof, and the height of the water-conductive fracture zone was 98 m. The increase in the advancing distance of the working face led to the periodic collapse of the old roof. Be-sides, the collapse height of overburden and the development height of the water-conductive fracture zone also changed. When the working face advanced to 200 m, the water-conductive fracture zone developed to surface gully 1. Likewise, when the working face advanced to 300 m, the water-conductive fracture zone developed to surface gully 2. Under this scenario, the water-conductive fracture zone directly ran through the surface of the ground. Further, the compaction area was formed in the middle of the goaf due to the action of the mine pressure, and the density of the mining fissures in the compaction area was reduced. Since the stratum was com-posed of soft and hard structures, the development range of the mining fissure was extensive, and the mining fissure was mainly concentrated in the rock mass above the cut hole and the working face [45, 46]. After coal mining, the water-conductive fracture zone developed to the surface, and the seasonal river water in the gully entered the mine along the water-conductive fracture zone, resulting in disastrous water inrush accidents [47]. The numerically calculated maximum height of the water-conductive fracture zone ranged from 230 to 250 m. The development height of the water-guiding fracture zone is determined through the observation method of drilling flushing fluid leakage. This comprehensive approach involves direct measurements of drilling flushing fluid leakage, drilling water level, drilling speed, and observations of drilling anomalies (such as sticking, dropping, and air suction), as well as drilling coring observations and geological descriptions. Currently, it is considered the most reliable method for determining the extent of water-conducting fracture zone development in overlying strata. The final field test confirmed that the development height of the water-conducting fracture zone was 223.86m. These numerical results were found to be consistent with field measurements, indicating the reasonableness of the numerical model used for overburden failure in gully mining. This provides a solid foundation for subsequent numerical calculations related to surface water infiltration.
Numerical Simulation Results and Analysis of Surface Water Infiltration
Based on the above model construction, mesh generation, and boundary conditions, the infiltration characteristics of surface water under different advancing distances of the working face are simulated and analyzed. Fig. 10 shows when the working face advanced by 50 m, the mining fissures did not communicate with the surface. When the working face advanced by 200 m, the mining fissures developed until the surface gully 1. When the working face advanced by 300 m, the mining fissures extended up to the surface gully 2. Therefore, in order to investigate the infiltration characteristics of surface water influenced by mining fissures, two working conditions (surface connected with a surface advancing distance of 200 m and 300 m) were selected for further numerical calculations to determine the volume of surface water. (Fig. 11).
Saturation was used to characterize the infiltration characteristics of surface water in the numerical calculation results. In Fig. 11, the red part represents the saturation of the surface water is 1 in the mining fissure, indicating that the surface water is full of mining fissures. The blue section shows null saturation of the surface water, that is, no surface water enters the mine. Fig. 11 illustrates that when the working face advanced 200 m, the mining fissures connected with the surface, and the fracture zone developed up to the surface gully 2. After one hour, the surface water in gully 2 began to infiltrate the goaf and converged there along the fissure, resulting in a significantly higher saturation of surface water in the goaf and the large mining fissures above it than in other mining fissures. As time progresses, the volume of surface water in mining fissures and goaf gradually increases, accompanied by an increase in saturation. When the working face advances 300 m, the development of mining fissures becomes relatively prominent, extending up to the surface gully 1. The surface water within the gully infiltrates into the goaf through these mining fissures and converges with larger fractures above. Over time, this leads to a gradual increase in the saturation of surface water within both the mining fissures and goaf until it reaches a stable level. This phenomenon occurs due to continuous seepage of surface water into the mining fissures, eventually filling them up and establishing a steady flow towards the goaf.
To further analyze the infiltration characteristics of surface water and quantify the amount of surface water entering the goaf, we performed a real-time integration of the velocity u in the goaf area, i.e. Q = ධධudΩ (integral to get the water volume Q of surface water infiltrating goaf). Only surface water entering through mining-induced fissures was considered while disregarding water from seam roof aquifers that enters through such fissures. This is due to the presence of underground drainage boreholes constructed before coal seams are mined in the research area, effectively pre-draining the main aquifer of working face roofs. Field observations indicate minimal post-mining inflow after coal seams have been extracted; however, when mining fractures intersect with surface rivers or streams in gullies, surface water can infiltrate along these fractures. We report the amount of water infiltrating the goaf under the five conditions with variable advancing distances of the working face (50 m, 100m, 150m, 200 m, 250m, and 300 m) (Fig. 12).
Fig. 12 shows when the working face advanced 50 m, 100 m, and 150 m, the mining fissures were not connected with the surface, and the surface water failed to enter the goaf. Under this scenario, the water volume in the goaf was expectedly 0 m3/h. When the working face advanced 200 m, the mining fissures communicated with the surface, and the surface water infiltrated into the goaf along the fissures. The water volume increased to about 41.9 m3/hour and then reached a stable state. When the working face advanced 250m and 300 m, the water volume of surface water infiltrating the goaf rose to about 56.9 m3/hour and 75.0 m3/hour before reaching a steady state. The development of mining fissures was more thorough when the working face advanced 300 m, and many water diversion channels were formed.
To further validate the numerical calculations, we conducted an analysis of scenarios where the working face advanced by 200 m, 250 m, and 300 m. We compared and analyzed the field measurements with the temporal variation law of water volume in the goaf (Fig. 13).
Fig. 13 illustrates that with the increase of time, the water volume in the goaf calculated by numerical calculation increased rapidly at first and then gradually became stable. When the working face advanced 200 m, 250 m, and 300 m, the calculated water volume after the final stabil-ity was about 41.9 m3/hour, 56.9 m3/hour, and 75.0 m3/hour, respectively. The average measured stable water volume in the goaf was about 40.0 m3/hour, 53.6 m3/hour, and 70.8 m3/hour. The calculated water volume was slightly higher than the field-measured value, with errors of 4.53%, 5.80%, and 5.60%, respectively. These errors are within the allowable range of the project. Therefore, the magnitude of difference between the field measurement and the numerical calculation is small, which confirms the reliability of the latter.
We demonstrate that numerical calculation can provide an accurate estimate of the volume of surface water entering the goaf under different advancing distances. These results can then provide a scientific foundation for the mine water disaster prevention plan and underground drainage capacity design.
Measures to Prevent Surface Water Infiltration
The continuous advancement of the typical working face leads to the gradual development of the mining fissures which communicate with the surface. Once a large through type fissure is formed due to the scouring effect of the surface water, the water will flow into the mine in large quantities, resulting in a significant increase in the water volume of the goaf, causing mine water damage. Therefore, it is crucial to outline corresponding prevention measures for surface water infiltration and ensure the safety of mine production.
Surface Fissure Backfilling
The development of mining-induced fissures in coal mining operations can lead to surface subsidence and the diversion of seasonal river water into the mine. Therefore, it is crucial to backfill these fissures that con-nect with the surface. For larger surface fissures, we recommend a two-step approach: initially backfilling a certain amount of gravel at the bottom, followed by compact clay backfilling in the upper portion. In gully areas, a compact and layered clay backfilling method should be employed first. Subsequently, sealing off any leakage points with clay, gravel, and cement will create an impermeable artificial riverbed to prevent the infiltration of surface water during rainy seasons.
Construction of Soil and Water Conservation Forest
A protective forest should be built on both sides of the gully above the working face, as per the local cli-matic conditions to ensure soil and water conservation. Rigid and drought-resistant trees should be selected to effectively regulate the surface runoff and reduce the soil erosion caused by seasonal rivers in the gully. This will help in achieving soil stabilization and slope stabilization.
Reasonable Arrangement of Mining Times
There are several seasonal surface runoff gullies in the mining area during the monsoon season. Therefore, the majority of mining should be carried out in the dry season to avoid the flood in the rainy season. This can effectively prevent surface water from entering the mine. However, the specific prevention and control measures must be matched with the mining planning of the mine.
Conclusion
(1) With the progression of the typical working face, the water-conductive fracture zone continuously evolved. Upon the advancement of the working face by 200 meters, the water-conductive fracture zone extended to the surface. In the numerical calculations, the maximum height of the water-conductive fracture zone was estimated to be 230-250 meters, while the measured value was 223.86 meters. Thus, the numerical calculations were consistent with the field measurement results.
(2) The infiltration model of surface water in coal seam mining was established using the COMSOL software based on numerical simulation outcomes. When the working face advanced by 50 m, 100 m, and 150 m, there were no connections between the mining fractures and the surface, resulting in negligible water volume in the goaf. However, as the working face advanced by 200 m, 250 m, and 300 m respectively, surface water infiltrated into the goaf through mining fissures at a stable rate of approximately 41.9 m3/hour, 56.9 m3/hour, and 75.0 m3/hour respectively. Subsequently, we compared numerically calculated stable water volumes with those estimated from field measurements for a working face advancement of 200m, 250m, and 300 m. The numerically calculated values were found to be approximately equal to measured average stable water volumes of about 40.0 m3/hour, 53.6 m3/hour, and 70.8 m3/hour, respectively. The minimal discrepancy between these two estimation methods indicates that our numerical calculation model accurately predicts surface water infiltration volume during coal seam mining under gully conditions.
(3) Based on a numerical calculation of the surface water infiltration of the coal seam under the gully mining, we put for-ward corresponding prevention and control measures such as surface fissures backfilling, construction of impervious artificial riverbed or impermeable membrane in the gully, construction of water conservation forest, and reasonable arrangement of mining time. Through the above-mentioned prevention and control measures, the surface water can be effectively blocked from entering the goaf, which minimizes the occurrence of mine water disasters and ensures safe mining.
