The coal-forming period is mainly Permian and Carboniferous in the North China coalfield, which is one of the main coal accumulating areas in China. It is characterized by high coal rank, abundant reserves, and varieties. However, water outburst accidents originating from karst aquifers under the coal seam floor have become a terrible disaster in accompany with the deep coal exploited progressively. Water inrush events of the deep limestone have often occurred during excavation in mines. To decrease the risk of high confined water from the coal seam floor and ensure the mining under the safe water pressure of limestone aquifers, the comprehensive exploration and regional treatment are all implemented, such as drainage depressurization, curtain grouting, and grouting transformation of aquifers. Through the comprehensive treatment of the ground and underground, the water channel will be effectively filled with slurry to prevent limestone water bursting into the roadway, and the value of water-inrush coefficient is decreased below the critical value. In the study, utilizing COMSOL Multiphysics based on the finite element method to verify and determine the real layout of grouting parameters, the result shows the design plans satisfy the engineering requirements. 13321 working face located in South No.1 mining area has analyzed the effect of water hazard prevention and control. On the basis of the analysis of geophysical prospecting and validation boreholes, it is concluded that the fracture is filled with grouting slurry to block water-conducting channel effectively. In turn, the rational design parameters of grouting are confirmed as well. Finally, the water-inrush coefficient of Taiyuan formation limestone and Ordovician limestone water is calculated, respectively. The result shows that water-inrush coefficient is less than the critical value after treatment, the safety of excavating coal seam can be further assured.
Although the energy consumption rate of coal resources is decreasing over time, it is still the main energy in domestic economic development. Coal consumption accounted for 57.7% of the total energy consumption in 2019. Besides, China ranks the first in coal production and consumption across the world. Mine water hazard has become one of the main factors restricting coal mine safety production. According to government statistics and the relevant literatures from 2009 to 2019, a total of 238 accidents occurred in coal mines in China, resulting in around 1,000 deaths [
It is precisely because of the great casualties and national economic losses caused by water hazards that contemporary scholars and researchers have studied water disaster-causing factors and mechanisms for decades. The complexity degree of geological structure studied by the intensity index, density of fault intersections, and endpoints has been put forward [
Currently, research on water hazard control technology is gradually mature and complete as the technology and equipment remain to update and progress in practical engineering. According to the different hydrogeological conditions of coal mines, some appropriate preventive measures are taken to achieve the safety production. Gobs are usually threatened by separation layer water. By grouting into the overburden bedding separation, the displacement and deformation of overlying strata are obviously weakened; thus, the separation layer water could not burst into the rear of the mining area [
The main purpose of the paper is to introduce comprehensive treatment technology and evaluate the water damage of limestone. Taking the Gubei coal mine as an example, South No.1 mining area is surveyed by surface geophysical exploration before treatment firstly; the mine implemented multibranch directional hole and mine drainage to low pressure and level of limestone aquifers. Analyzing the validation of grouting hole layout, the grouting effect of the coal seam floor will be appraised by the data of underground geophysical exploration, observation holes, and verification drillings. The water-inrush coefficient is obtained to quantify the analysis of safety and efficient mining under the confined aquifer after treatment. The study summarizes a complete set of “exploration-treatment-evaluation” on the limestone disaster, which can provide the successful experience for other coal mines.
Gubei mine is located in the middle east of Huainan coalfield situated in the southeastern margin of the North China Plate which is adjacent to Bengbu uplift in the north, Hefei basin in the south, Tanlu Fault in the east, and Shangqiu and Macheng Fault in the west, as shown in Figure
Location of the study area in China including (a), Huainan coalfield within Anhui province (b), schematic diagram of stope distribution (c), and 13321 working face located in South No.1 mining area (d).
Gubei mine has lied in the connecting belt between the east wing of Chenqiao anticline and the west wing of Panji anticline. The overall structural form of the mine is a monoclinal structure with a north-south strike and an eastward dip. The strata slope gently, with the dips ranging from 5° to 15°, and there are secondary wide and gentle folds and faults with uneven development. The sedimentary period of coal measure strata in the coal mine is Carboniferous and Permian. The research area is a concealed coalfield. Drilling data reveal that the compound strata (from down to up) are consisted of the following: (1) Cambrian strata, which consist of dolomitic limestone, oolite limestone, limestone alternated with dolomitic limestone, and argillaceous limestone; (2) Ordovician strata, dolomitic limestone, breccia, locally developed with purplish-red and grayish-green argillaceous strips; (3) Carboniferous strata, consisting of bioclastic limestone alternated with dark gray silty mudstone, interspersed with thin layers of carbonaceous mudstone and coal with no industrial use value; (4) Permian strata, with the main coal-bearing strata, consisting of more than 30 coal-bearing layers. The minable coal seams comprise of 1 coal seam in the Permian Shanxi Group, 8 coal seam and 6 coal seam in the Permian Lower-Shihezi Group, and 11-2 coal seam and 13-1 coal seam in the Permian Upper-Shihezi Group; (5) Cenozoic strata, 371 m ~512.6 m in depth and directly covering the Permian coal measure strata, demonstrating thinner in the southeast and thicker in the northwest of the study area.
According to early geological exploration and coal mining production data, aquifers threatening mine safety production include Cenozoic loose aquifers, sandstone fracture water in the Permian strata, karst fissure water in the Carboniferous limestone aquifers and Ordovician aquifers at present.
The sedimentary thickness of the Cenozoic loose layer ranges between 371 m and 520.76 m, with an average thickness of 456.51 m, and the average elevation of the bedrock is -419.77 m. Based on the stratigraphic correlation and lithologic combination characteristics, it is divided into three aquifers, three water-resisting layers, and a red stratum at the bottom.
Permian sandstone fracture aquifers are distributed between coal seam and mudstone; the lithology and thickness vary greatly. There are thick water-resistance layers consisting of mudstone and sandy mudstone between aquifers, which block the hydraulic connection between the sandstone fractured aquifers. The aquifers are mainly static reserves, with uneven water richness. The water inflow is generally not large; the supplement of condition and source is poor and limited. The water-resistance layer located between the 1 coal seam floor and the limestone aquifer roof is 10.07 m to 29.2 m, with an average of 18.27 m in thickness.
The Carboniferous limestone aquifers are divided into three groups (C3I, C3II, and C3III groups) which consist of 10~12 layers; C3I limestone aquifer is the direct source of water inrush from 1 coal seam floor. The maximum limestone water pressure of the coal seam floor is 3.5 MPa measured at 1 coal belt conveyor roadway. Among them, six aquifers (C31, C33up, C33down, C34, C35, and C311) are distributed stably, with thicker limestone than others. The unit water inflow of Taiyuan formation limestone ranges from 0.000597 L/m·s to 0.299 L/m·s, the permeability coefficient varies greatly from 0.002 m/d to 1.96 m/d, and the degree of water abundance is between weak and medium.
About sixteen boreholes pass through Ordovician limestone strata in the mining area, with the thickness of 48.7 m to 92.5 m. According to the drilling data, the lithology is dense without dissolution and no water leakage has rarely occurred during the borehole implementation. On the basis of the pumping test data, the water level elevation of Ordovician limestone is from -5.85 m to 24.59 m and the unit water inflow is 0.000458 L/m·s to 0.763 L/m·s, the degree of water abundance is weak to medium.
In the study, the 13321working face of the South No.1 mining area in Gubei mine is analyzed to prevent and control limestone water inrush hazard under the 1 coal seam. From the detection of abundance zone using ground three-dimensional seismic and underground comprehensive geophysical prospecting to the implementation of draining depressurization and ground grouting, the working face is made reasonable conclusions by evaluating the effect of water disaster control and treatment engineering finally. The procedure of research implementation is shown in Figure
Flow chart of water inrush hazard prevention and control.
According to the priority investigation achievements of three-dimensional seismic and high power transient electron-magnetic exploration on the whole South No.1 mining area, although no collapse column is found, twenty-two low-resistance abnormal zones have existence in the 1 coal seam roof, fifteen low-resistance abnormal zones in the 1 coal seam floor, eleven low-resistance abnormal zones in the section of C3I group limestone, and eleven low resistance abnormal zones in C312 limestone of Taiyuan formation. Among them, there are four abnormal zones with low resistance in the floor and margin of 13321 working face, including two water-abundance areas of 1 coal seam floor sandstone and two water-abundance zones of Taiyuan formation limestone under the 1 coal seam floor, as shown in Figure
Water-abundance zone distribution of the South No.1 mining area, 13321 working face exists in two abnormal water-abundance zones of C312 limestone and two sandstone fissure water-abundance zones.
On the basis of the detailed exploration on the limestone aquifers under the 1 coal seam floor, water-resistance layers should also have been considered into the study. The lithology of sedimentary strata related to aquiclude has been obtained from drillholes described by drawing a series of drillhole columnar sections to explain the occurrence status of water-resistance layers in the coal seam floor during early coalfield exploration. As shown in Figure
The comprehensive columnar of the mining area.
The scale of the South No.1 mining area is designed with near-horizontal multibranch drillings including five main grouting drillholes named as S1~S5; 13321 working face is covered by one main drillhole (S1) and its branch holes, and branch holes of other main drilling holes. The design principle of drilling is
Schematic diagram of “three casings” drilling profile structure.
To verify whether the rational layout of borehole spacing or not, the numerical simulation method is employed to analyze the influence radius of grouting diffusion at different situations, such as grouting pressure, water-cement ratio (R), and dynamic viscosity. COMSOL Multiphysics numerical simulation software based on finite element theory is utilized to complete the study of grouting diffusion [
Two-phase Darcy’s law is applied to compute the grouting diffusion in two dimensions. The hypothesis equations of two-phase Darcy flow are expressed as follows:
The single borehole is analyzed in the geometry model with the size of
Through the simulation of slurry diffusion, the result shows that the scale of slurry diffusion is positively correlated with the grouting pressure and inversely proportional to the dynamic viscosity and water-cement ration at the same time [
The distribution plot of slurry diffusion at different situation with time.
In practical engineering, the performance parameters of slurry are selected as follows: (1) the minimum grouting pressure in all boreholes is given to 9.2 MPa, which is more than 1.5 times of Ordovician limestone water pressure; (2) dynamic viscosity and water-cement ratio are set to 0.006 Pa·s and 2.0, respectively. In terms of grouting simulation before-mentioned, the current ground grouting treatment projects can fully meet the requirement of drilling spacing, which play a vital role in reinforcement of the waterproof layer and block the invasion of deep high confined limestone water.
Carrying out regional treatment on the ground and underground draining depressurization is performed to assure the safety of working face mining at 13121 bed plate tunnel, 13141 bed plate tunnel, and 1 coal floor drainage roadway. The eighteen boreholes, including fourteen directional horizontal holes and four conventional holes, fully cover the whole 13321 working face. By using the artificial drainage measure to lower the limestone water level and pressure [
After comprehensive treatment of limestone water hazard of 1 coal seam floor, the effect of treatment engineering has been confirmed to employ multimethods, such as drilling and geophysical exploration. During the excavation of the belt conveyor bed plate tunnel and return airway bed plate tunnel of the 13321working face, the comprehensive geophysical prospecting (transient electromagnetic and current electric method) combined with regular drillings is used to predict the occurrence of water body in roadway front. It is proven that no low-resistance abnormal area and the phenomenon of borehole water inflow is rarely found during excavation.
While the construction of 13321 working face is thoroughly completed, a three-dimension electrical method and audio-frequency electrical perspective technology are used to further ascertain the water-abundance situation of the limestone aquifers of 1 coal seam floor. Through the comprehensive analysis of geophysical prospecting, six abnormal water-abundance zones have existence within 60 m of the 13321 working face floor, which are designated D-DZ1, D-DZ2, D-DZ3, D-DZ4, D-DZ5, and D-DZ6; the detailed location is shown in Figure
Comprehensive geophysical abnormal interpretation of 13321 working face floor and holes for regional control verification layout.
Based on the verification concept of “geophysical exploration first, drilling supplement,” the mine designs eleven long verification boreholes in the drainage roadway of 1 coal seam floor and the drainage crossheading of 13421 belt conveyor bed plate tunnel, which verify the water-abundance of six low-resistance abnormal zones in the working face and confirm regional treatment effect of the 13321 working face floor in the early stage as well. At least two boreholes are drilled through the abnormal zones during the implementation project, which reduces the uncertainty of interpretation. It is ascertained that there has no water existence in D-DZ4; other low-resistance abnormal zones are verified to have a small amount of water inflow from drilling holes, with the water volume of 0.1 m3/h~1.0 m3/h and the water temperature of 30°C~35°C which is lower than the average temperature of Ordovician limestone by 39°C. Therefore, the degree of water abundance in the low-resistance abnormal zones is weak, the water recharge channel has been effectively sealed by grouting, and Taiyuan formation limestone aquifers have no supplement of deep Ordovician limestone water.
In terms of water inflow in the borehole, water pressure, and water temperature, the control and treatment effect of limestone water hazard obviously fulfills the prior expectation goal. As shown in Table
Regional exploration and treatment effect verification of outlet drilling.
Bore no. | Hole depth (m) | Water exit formation | Water inflow (m3/h) | Water temperature (°C) | Water pressure (MPa) | Water quality type |
---|---|---|---|---|---|---|
Y3-2 | 471 | C33down limestone | 0.30 | 30 | 0 | Cl·HCO3-Na+K |
Y3-6 | 351 | C33down limestone | 0.42 | 32 | 0.10 | Cl·HCO3-Na+K |
Y3-8 | 636 | C35 limestone | 0.10 | 35 | 0 | Cl·HCO3-Na+K |
Y3-9 | 597 | C35 limestone | 1.00 | 34 | 0 | Cl·HCO3-Na+K |
Y3-10 | 615 | C35 limestone | 0.30 | 34 | 0.70 | Cl·HCO3-Na+K |
Comparative analysis of water chemistry characteristics. Taiyuan formation limestone water (C3I, C3II, and C3III groups), Ordovician limestone water, and Cambrian limestone water act as the background value to analyze to discriminate the verification hole outflow water sources.
After ground regional treatment and the validation of the grouting effect, a reasonable and qualitative evaluation can be made on the prevention and treatment effect of limestone water. Water-conducting fissure is fully filled with slurry, consolidating the waterproof floor of 1 coal seam and increasing the effective thickness of the waterproof layer. Meanwhile, the water pressure and level of limestone aquifers have been significantly decreased by mine drainage and grouting on the analysis data of the hydrogeological observation wells located in the South No.1 mining area.
In the South No.1 mining area and nearby, there are seven observation wells for monitoring water level change of limestone aquifers, which contain four Carboniferous observation wells (that is, nine C3I, ten north Kz1, nine C3II, and nine C3III), two Ordovician observation wells (nine O1+2-I and ten-ten south O1+2), and one Cambrian observation well (XLZJ1).
As shown in Figure
Hydrogeological observation well dynamic curves of Carboniferous limestone aquifers (C3I, C3II, and C3III groups) in South No.1 mining area after water damage prevention and treatment.
Because of the short distance between Carboniferous strata and limestone aquifers in the Ordovician and Cambrian. Fracture-conducting has the existence between strata. Thus, while water in the Taiyuan formation has been draining, Ordovician and Cambrian limestone aquifers are indirectly dewatered as well. After water hazard prevention and treatment, the water level drops by more than 10 m, as shown in Figure
Hydrological observation well dynamic curve of Ordovician and Cambrian limestone aquifers in the South No.1 mining area before and after water damage prevention and treatment.
According to the safety evaluation methods of limestone water disaster in North China coalfield by previous scholars and experts, the water-inrush coefficient method is applied to assess the feasibility and safety after treatment in the study. The evaluation method is employed to predict the safety of coal mines in North China during production, which is first incorporated into “the Regulations for Coal Mine Water Prevention and Control, China” (Ministry of Coal Industry 2009) and is partially modified in 2018 [
After implementing limestone water hazard regional treatment and control projects, the direct limestone aquifer (namely, C3I group aquifer) has rarely threatened to 1 coal seam. By analyzing the data of limestone drilling exposed in 13321 working face, the lithology between 1 coal seam floor and C3I group limestone roof is mainly sandy mudstone, siltstone, fine sandstone, and mudstone, which can be regarded as the water-resistance layer with a thickness of 16.5 m to 21 m and the average of 17.9 m; the value of
Considering the water inrush accident of Ordovician limestone aquifers in some coal mines recently, it should be treated equally as the Carboniferous limestone aquifer, although it is defined as the indirect recharge water source. Combined with the actual geological condition of 13321 working face and water damage control projects, the strata from C39 limestone floor to the 1 coal seam floor are treated as the aquiclude layer of the Ordovician limestone aquifer, with the average thickness of 79.07 m. Based on the data of ground area exploration and treatment, the data of S4-2-10 and S5-3 show that the lowest elevation of C39 limestone floor is -655.02 m and-672.01 m, respectively. Meanwhile, collecting the latest data of observation wells in Ordovician limestone aquifers, the water level ranges from -6.18 m to -13.57 m. The water pressure in C39 limestone floor can be reckoned as follow:
After calculation, the Ordovician water pressure sustaining of C39 limestone floor in S4-2-10 and S5-3 is 6.59 MPa and 6.66 MPa, respectively. As shown in Figure
Cross-sectional drawing of S5-3 and S4-2-10, which are located in the C39 limestone aquifer.
To guarantee the safety mining of the 13321 working face, Gubei mine employs multimethods to detect the water-abundance zone distribution of the 1 coal seam roof and floor before and after regional treatment. On the basis of geophysical prospecting, underground drillings are launched to further confirm the results of the previous exploration; abnormal water-abundance zone threat to the working face is ascertained finally. The degree of water abundance in six low-resistance zones is weak, having no threat to excavate 1 coal seam, the detailed and reasonable conclusions are made in the study as follows:
After the comprehensive exploration and analysis of the study area, the distance between the limestone aquifers in Carboniferous and 1 coal seam floor is only less than 20 m. Exploiting 1 coal seam is likely to be affected by the deep limestone water if water hazard control and prevention projects are not carried out. Ground region treatment project is implemented to prevent limestone water damage, utilizing numerical analysis tool to guide and verify the ground grouting drilling layout. When the grouting pressure is 9 MPa, the dynamic viscosity is 0.008 Pa·s, and the water-cement ratio is 1.9, the scale of slurry diffusion is up to about 30 m. The value of project grouting parameters is not less than the standard value actually; the grouting parameters are verified to meet engineering requirements. While the regional treatment projects are thoroughly completed, the comprehensive geophysical prospecting and verification drillings implemented in roadways, combining with the data of ground observation wells, have confirmed the grouting effect to reach the control expectation. On the basis of analyzing exploration and drainage and geological drillings, the water-inrush coefficient of C3I limestone and Ordovician limestone in the working face floor is 0.0333 MPa and 0.0832 MPa, which is less than the critical water-inrush coefficient. The effect of limestone water hazard control engineering meets the requirements of safety mining.
The data used to support the findings of this study are included within the article.
The authors declare no conflict of interest.
Jingzhong Zhu designed and wrote the paper, Seng Yang and Jiajun Fan collected the data for the paper, Ling Li and Yu Cui sorted and analyzed the material, and Yu Liu and Qimeng Liu reviewed and supervised the paper writing.
This research was supported by the Natural Science Foundation of Anhui Province (grant nos. 2008085QD191 and 1908085ME145), the Independent Research Fund of the State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines (Anhui University of Science and Technology) (grant no. SKLMRDPC19ZZ06), and Anhui University of Science and Technology Graduate Innovation Fund Project (grant no. 2020CX2004). The authors gratefully appreciate the Gubei coal mine for providing the research data.