Mining Depth Research Articles

Effect of mining and geology on mining-induced seismicity – A case study

Mining-induced seismicity is a commonly occurring phenomenon in underground mines. This poses a greater challenge to the safety of the mining operation. This paper presents a case study of the Young-Davidson mine in northern Ontario, Canada, where seismic events of magnitude Mn 2.0+ have been observed at mining depths of 600 to 800 m below the surface. The occurrence of large seismic events at such shallow depths is the key issue of this study. A comprehensive study of the microseismic database has been conducted to discern the root causes for the unusually strong seismic activities recorded at shallow depths. The effects of mining activities in the vicinity of two dykes intersecting the orebody on the seismic response are investigated. Variation of the b-value derived from the magnitude-frequency distribution is examined, and moment tensor inversion for three large seismic events is carried out to determine the source mechanisms. It is shown from this investigation that the influence of the sill pillar is more critical, leading to high mining-induced stress and the occurrence of large events. While the findings from this research are specific to this case study, they could be used to shed light on the causes of induced seismicity at other mines with similar conditions.

Effects of Loading Stress and Velocity on Compression and Particle Breakage Behaviour of Waste Rocks in Backfill Coal Mining

Coal mine waste rocks, mainly broken gangue, can be used as filling materials to backfill into goafs. Under the overburden load, the backfill body is vulnerable to compressive deformation and particle breakage. With the increase in mining depth, the overlying strata will impose different loads on waste rock filling materials at different loading velocities, which further affect the material compressive deformation and particle breakage. In this paper, an experimental scheme and a loading device are designed to study the influence of loading stress and velocity on the compressive deformation and particle size distributions of the backfill materials before and after compression. The results show that the axial strain of the gangue filling materials increases rapidly with the axial stress and then gradually stabilizes, showing a logarithmic functional relationship. Increasing the loading velocity will destroy the contact structures among the gangue particles and cause a larger deformation to the filling materials. When the loading stress is relatively low (5 MPa), the gangue particles with a size larger than 20 mm have a stronger bearing capacity compared with particles of 16–20 mm, which are the first particles to be crushed under these conditions. Further increasing the loading velocity will increase the breakage degree of the filling materials. The breakage ratio (BM) has a logarithmic functional relationship with the loading stress and the loading velocity. When the ground stress is lower than 5 MPa, the content of coarse particles should be increased to enhance the bearing capacity of the gangue materials; when the ground stress is higher than 10 MPa, the content of fine particles should be increased to reduce the porosity ratio and the particle breakage ratio.

Experimental analysis on dynamic performance of refrigeration systems in a deep gold mine

With the increment of depth of gold mining, the heat damage has become more and more serious and thus the thermal management systems of refrigeration and ventilation becomes essential. Dynamic performance is of great significance for the stability of system control and energy conservation. This paper proposes a thermal management system for the deep gold mine and builds an experiment platform, which fully utilizes the existing cycle air system. The system includes chilled water loop, intermediate loop and cold water loop. The dynamic response of the system is studied experimentally. The results show that rising and falling steps of the flow rate have similar effects on the system but the response time on the rising step is shorter. The response time of chilled water loop is 750 s for increasing by the step change of frequency and 1120 s for decreasing step change of frequency respectively. The total heat exchange of the air coolers increases from 6.93 kW to 7.31 kW during the rising step, and drops from 6.91 kW to 6.51 kW during the falling step. The response time of increasing and decreasing of pump of the intermediate cooling loop is 320 s and 403 s respectively. The total heat transfer rate of the system has no obvious change. Similarly, the response time of the cooling tower loop is 363 s and 1133 s. The total heat transfer rate of the system has no significant change. The research gives the response characteristic of step changes, which improves the stability of systematic control and energy conservation.

Quantitative evaluation of the effect of gas adsorption and desorption hysteresis on gas flow in coal

• A calculation method for inversing adsorption parameters is proposed by using the modified hysteresis index DHI. • The new gas flow coupling model considering the adsorption–desorption hysteresis effect is constructed in this paper. • The reliability and superiority of the new model is proven by laboratory experiments. • The effect of different degrees of hysteresis on gas extraction is quantitatively evaluated using the new model. Coal mining methane (CMM) is a major hazard source and a typical greenhouse gas in the coal industry. Gas extraction is an effective solution to eliminate coal seam gas disasters and reduce greenhouse gas emissions. Multi-field coupling model of gas flow in coal (MCMGF) is an important theoretical basis for guiding gas extraction, and the adsorption model is an important component of MCMGF. Numerous studies have shown that there is a hysteresis effect in the desorption of gas in coal compared to the adsorption process. Since gas extraction is a depressurized desorption process, HE should be taken into account when building the adsorption model. However, most of the current MCMGFs ignore the hysteresis effect, resulting in an overestimation of the amount of gas involved in the desorption process. Therefore, in this paper, the calculation method of the degree of hysteresis index (DHI) is revised and the adsorption model is modified based on systematically sorting out the adsorption–desorption hysteresis phenomenon. On this basis, a coupled gas flow model considering the hysteresis effect of adsorption and desorption was constructed, and the reliability of the model was verified by isothermal adsorption and desorption experiments on coal pillars. Eventually, the influence of different hysteresis degrees (DHI = 0, 0.25 and 0.59) on gas extraction was quantitatively evaluated by using the new model. The results show that hysteresis effect has a significant influence on gas extraction, and the larger DHI value is, the smaller the daily extraction is. Ignoring the influence of adsorption–desorption hysteresis will make the gas extraction capacity of the borehole overestimated. After 160 d of extraction, the difference between the residual gas content when adsorption–desorption hysteresis is considered and when it is ignored is up to 2.05 m 3 /t. This is because the adsorption–desorption hysteresis limits the ultimate amount of gas that can participate in the flow. In our previous studies, it has been found that the additional resistance to gas desorption is intrinsic to the hysteresis. With the increase of the mining depth, the in-situ stress increases rapidly, therefore the influence of coal adsorption–desorption hysteresis on gas extraction under high stress is one of the problems that need to be considered emphatically.

Identification of mixing water source and response mechanism of radium and radon under mining in limestone of coal seam floor

With the gradual increase of the coal mining depth, the mixing of multiple water sources intensifies and the activity of radium and radon in groundwater increases. Identifying the source of mine water inrush by using radium and radon isotopes is a new choice. In this paper, the mathematical statistics method, radioactive isotope decay theory, the mass conservation principle, and the numerical simulation method are used to analyze the influence of total dissolved solids (TDS), pH, and the hydrochemical ion content in groundwater on the isotope activity of radium, radon, uranium, thorium, and lead. The activity of thorium and lead is lower than the detection limit of the instrument, and the influence of coal mining activities on it is small. The simulation of the radium–radon mass balance in groundwater shows that the greater the adsorption coefficient (k) of solid particles in groundwater is, the more obvious the adsorption effect and the greater the influence on the radium–radon activity balance are. The radium–radon dating method is used to calculate the groundwater age. Results show that the groundwater age in the closed pit coal mining area is generally older than that in the mining coal mining area. Combined with the 222Rn, 226Ra, and 234U radioactive isotopes and temperature, a mixing water source identification model of limestone in the coal seam floor is constructed. The model shows that the radium activity and temperature of the groundwater are inversely proportional to the mixing ratio of the Permian sandstone water. From the closed pit coal mining area to the mining coal mining area, the radium radon activity of the groundwater increases gradually, the groundwater age decreases significantly, the water cycle is accelerated, the mixing ratio of the Permian sandstone water decreases gradually, the mixing ratio of the Ordovician limestone water increases gradually, and the risk of coal mine water inrush increases. The research results prove the feasibility of the new method for accurately discriminating the mixing water sources in coal mine areas, which is of great significance to the improvement of the theory of coal mine water disaster prevention and control.

The role, importance and impact of the methane hazard on the safety and efficiency of mining production

Abstract Underground mining production is an extremely important process for the economy and carried out in very difficult and complex environmental conditions. The disturbance of the balance of this environment makes it also a very dangerous process. Due to the importance of coal, mainly as an energy raw material, the process of its exploitation is carried out all over the world. The specificity of its production is mainly determined by mining and geological conditions, which determine the method of operation and the selection of machines and devices for this process. One of the most dangerous natural hazards associated with this process are ventilation hazards, including methane hazard. The reason for this threat is methane, an odorless and colorless gas, which becomes a flammable and explosive gas under certain criteria. These features make this gas a huge threat to mining operations. Its huge amounts, contained in coal seams, are released into the mine atmosphere during the exploitation process, causing a very high threat to work safety. Events related to the occurrence of methane are most often the cause of mining disasters, in which people die and the technical and mining infrastructure is destroyed. The reason for the growing methane hazard is the increasingly difficult mining conditions, and mainly the increasing depth of mining, and thus also the increase in methane-bearing capacity of the seams. Taking into account the huge impact of methane hazard on the mining process, the article discusses its impact on the safety and efficiency of this process. The results of the literature review with regard to this risk are presented and the accident statistics are presented. On the basis of actual data, an analysis of interruptions in the exploitation process related to exceeding the permissible methane concentrations was carried out in one of the mines. The problem of limiting the production process due to these exceedances is an important factor reducing the efficiency of this process. The obtained results clearly indicate that the losses resulting from these breaks deteriorate the profitability of the entire process and affect the economic efficiency of the industry. In order to effectively counteract the dangerous phenomena related to the methane hazard and to improve the efficiency of the mining production process, solutions were proposed to improve this state and the directions for further research were proposed.

Safety and Protection Measures of Underground Non-Coal Mines with Mining Depth over 800 m: A Case Study in Shandong, China

With the increase in mining depth, the risk of ground pressure disasters in yellow gold mines is becoming more and more serious. This paper carries out a borehole test for the pressure behavior in a non-coal mining area with a mining depth of more than 800 m in the Jiaodong area. The test results show that under a depth of 1050 m, the increase in the vertical principal stress is the same as the increase in the minimum horizontal principal stress, which is about 3 MPa per 100 m. When the depth increases to 1350 m, the vertical principal stress increases by about 3% per 100 m, and the self-weight stress and the maximum horizontal principal stress maintain a steady growth rate of about 3 MPa per 100 m. In addition, based on the test results, the operation of the ground pressure monitoring system in each mine is investigated. The investigation results show that in some of the roadway and stope mines with depths of more than 800 m, varying degrees of rock mass instability have occurred, and a few mines have had sporadic slight rockbursts, accounting for about 5%. There was a stress concentration area in the lower part of the goaf formed in the early stage of mining, and slight rockburst phenomena such as rock mass ejection have occurred; meanwhile, the area stability for normal production and construction was good, and there was no obvious ground pressure. This paper compares the researched mines horizontally as well as to international high-level mines and puts forward some suggestions, including: carrying out ground pressure investigations and improving the level of intelligence, which would provide countermeasures to balance the safety risks of deep mining, reducing all kinds of safety production accidents and providing a solid basis for risk prevention and supervision.

Experimental and numerical study on heat emission characteristics of ventilated air annular in tunneling roadway

With the increase in mining depth, thermal hazard becomes a prominent problem. Reducing heat dissipation of the surrounding rocks on the basis of artificial refrigeration can be an important thermal hazard-control technique. There are several researches on spray and surrounding rock-filling insulations, but only a few researches were conducted on air jacket heat insulation performance. To enrich the research content of air jacket heat insulation, a similar experiment platform was designed. Based on the analysis of the test data, the friction factor for the turbulent flow in the interlayer of air annular is obtained and a modified equation for calculating the friction factor is proposed based on the Blasius equation model. The average Nusselt number of the inner and outer wall surfaces in the annular air of roadway is calculated under turbulent state, and the Gnielinski equation is modified based on the experimental results. Based on the finite volume method, the unsteady mathematical model of the heat dissipation structure of ventilated air annular with surrounding rock was established. A computer program was developed, and the accuracy of the mathematical model was verified using experimental results. The numerical calculation results were used to analyze the variations of heat absorption of airflow in the main roadway and heat exhaust of the ventilated air annular at different wind speeds and annular diameters. The results show that the heat insulation effect of interlayer of ventilated air is the best when the air flow in the annular is at the turning point of laminar flow turbulence. The heat insulation effect of ventilated air interlayer is the best in the early ventilation stage, and the heat insulation rate will decrease with the increase of ventilation time, but the heat insulation rate can reach more than 69.7% in 10 years.

Mechanical Properties and Failure Mechanism of the Weakly Cemented Overburden in Deep Mining

With increases in the mining depth and area in the Ordos coal field, the failure law of the super thick sandstone in the Zhidan group leads to frequent disasters, such as rock bursts and mine earthquakes, which have become a significant issue, restricting large-scale continuous mining. To adequately understand the movement mechanism of the super-thick and weakly cemented overburden, and to promote the large-scale mining of the coal resources under it, this study analyzes the physical and mechanical properties, along with the microstructural characteristics, of the weakly cemented overburden of the Yingpanhao Coal Mine through mechanics tests, scanning electron microscope tests (SEM) and hydrolysis experiments. A two-dimensional discrete element model of the survey region is then built to explore the temporal and spatial evolution laws of the overburden failure. The results show that, even though poorly cemented strata such as the Cretaceous Zhidan group sandstone and the Zhiluo group sandstone are weak in lithology, their unique mineral composition and microstructural characteristics give them a greater rigidity when their thickness reaches a certain value. The surface subsidence exhibits a sudden increase, and the dynamic disaster range of the overlying strata is wide when deep multi-face mining was carried out under the super-thick and weakly cemented overburden. The temporal and spatial evolution laws of the strata subsidence and influence boundary are closely related to their depth, and their relationships evolve into the Boltzmann function and Boltzmann–parabolic function, respectively. The failure mode of the super-thick and weakly cemented overburden is ‘beam–arch shell–half arch shell’, and the failure boundary exhibits arch fractures.

Study on Mechanical Characteristics of Deformation and the Failure of Gas-Containing Coal in the Wuhai Mining Area of China under Different Gas Pressure Conditions

The mechanical properties of gas-containing coal and rock mass play important roles in controlling the occurrence and development of coal and gas outbursts. The gradual increase in mining depth will change the failure mechanism of gas-containing coal and rock mass. In order to further study the failure mechanism of gas-containing coal and rock mass, samples were taken from the gas-containing coal seam in the Wuhushan Coal Mine of the Wuhai Mining area of China. The mechanical parameters of coal samples during the failure process under different gas pressure conditions were measured and analyzed with the SAW-2000 rock mechanics testing machine, the gas-containing coal uniaxial compression device and inflation system. Meanwhile, the failure process and mechanical parameters of coal samples under different gas pressure were simulated by RFPA2D gas plate numerical simulation software. The results show that with increasing gas pressure in the coal there is decrease in Compressive strength, Elastic modulus, Strain, Peak strength and Bearing capacity and increase in Poisson’s ratio. When the failure state appears in the coal, the cracks are longer and wider, more random cracks are genareted, and the damage degree of the coal is greater. The numerical analysis’ results are in good agreement with experimental results. The research results are applicable to the gas bearing coal with the same or similar gas geological conditions. The tests can be carried out repeatedly and reasonable results can be obtained according to the physical and mechanical parameters of the actual coal seam and the occurrence of gas. On this basis, physical experiments and numerical simulations of triaxial compression can also be carried out to further study the mechanical characteristics of deformation and the failure of gas-containing coal under gas pressure and provide technical support for revealing the mechanism of coal and gas outbursts.

Scientific and practical bases for establishing and introducing integrated geological monitoring in Kupol Mine

The location of the Kupol gold–silver deposit governs complexity of its geological features within the Okhotsk–Chukotka Volcanic Belt in the zone of high tectonic activity. The present zones with different thermodynamic conditions in permafrost 250–300 m thick at the mining depth greater than 500 m, the cryopags in frozen rocks and in mineralized pressure water below the permafrost sole, the excessive tectonic stresses in thawed rocks, the aggressive physicochemical, chemical and biochemical conditions of the underground hydrosphere dictate the need to develop and introduce the integrated geological monitoring. The monitoring concept and contents are formulated based on the representation of the underground space (US) of a mine as a multicomponent system: fractured rock mass–underground water–microorganisms of underground water–gases of various genesis–load-bearing structures (supports). For each of the components, the necessity and directions of monitoring and observations are determined in the structure of the integrated monitoring system. The interrelation of all US components is shown, and special attention is paid to the operability and long-term stability of rock bolting systems as the main supporting structures that determine mine operation safety. It is emphasized that the mine support systems experience the multifactorial influence of four components: rocks, groundwater, microorganisms and gases. All types of corrosion of mine support systems, including biocorrosion, are identified and systematized, their nature is disclosed and the methodological aspects of recommended control and observations in the specific monitoring system are formulated, which ensures improvement of mining safety and elimination of emergencies through undertaking preventive measures.

Experimental Study on the Mechanical Behavior of Coal under Triaxial Dynamic Compression

The frequency and intensity of coal-rock dynamic incidents in underground coal mining, such as coal bumps and outbursts of coal and gas, tend to increase with mining depth. These dynamic incidents are closely related to the dynamic mechanical behavior of coal. In this experimental study, the dynamic mechanical behavior of coal was investigated with an active triaxial split Hopkinson pressure bar (SHPB) test system. In the test, the in-situ stress field for coal with an overburden depth of 100 m to 600 m was simulated and the dynamic loading tests of coal were undertaken under low, medium, and high loading rates. The results of the study show that the dynamic compression strength of coal increases with loading rate and axial and confining stress, and the effect of confining stress is more profound than that of axial stress. The results also reveal that the energy consumption and energy density per unit volume of coal are positively correlated with the depth and loading rate. This study may help gain insights into the occurrence mechanism of coal-rock dynamic incidents in underground coal mining.

A Study on the Influence of Mining Depth on the Stress Distribution Characteristics of Stope Surrounding Rock

The occurrence of severe mine pressure in stope is of high risk, and the mining depth is the main control factor which affecting the occurrence of mine pressure in stope. In order to study the influence law of mining depth on stope strata behavior, taking 11318E fully mechanized working face in Xieqiao Coal Mine of Huainan Mining Group as the engineering background, the stress distribution and evolution characteristics of stope surrounding rock under mining depth were studied by similar material simulation. And the evolution laws of mechanical characteristics of stope surrounding rock under different mining depths of 600 m and 1200 m were simulated. It is concluded that with the increase of mining depth, the damage range of surrounding rock on the mining field increases significantly. The initial collapse of the immediate roof above the working face is not affected by the mining depth. The periodic weighting caving step decreases with the increase of mining depth, and the height of two zones above the working face increases with the increase of mining depth. The stress transfer and evolution characteristics of the strata above the stope were revealed. The increase of front caving angle indicates that the pressure on the support of the working face is greater, indicating that the support of the working face should be strengthened with the increase of mining depth. The research results have a guiding role in the control of surrounding rock and disaster prevention of deep mine stope.

Influence of Section Shape and Buried Depth on Rock Loosening Zone around Underground Roadway in Coal Mine.

Owing to differences in deformation characteristics of roadways with different section shapes and depths, it is difficult to determine the support form and grouting depth of a roadway, which can cause serious deformation to the roadway. To address the challenges in determining the shape and grouting depth of a roadway section when the mine depth is known, the loose zone range of the roadway was tested using the acoustic method, and the loose zone evolution law under different conditions was performed by numerical simulations. The research results revealed that when the ratio of the maximum principal stress to the minimum principal stress is η > 3, the distribution of the rock loosening zone under different cross-sectional shapes was roughly “butterfly-shaped”, and the “smoother” the cross section in the design of the roadway, the smaller the range of the rock loosening zone. With the increase of burial depth, the rock loosening zone and sealing depth also increase; the rock loosening zone and burial depth have a power function relationship the rock loosening zone range = a·burial depthb; the sealing depth and burial depth have a linear relationship R = aH + b.

Assembled design and compressive performance simulation of mine waterproof wall based on concrete 3D printing

With the increase of mining depth, the deep mine waterproof wall project faces the test of worse construction environment and higher risk of water damage. With the development of intelligent construction technology, it is necessary to introduce concrete 3D printing technology for the construction of coal mine waterproof walls, so as to adapt to more complex construction environment and engineering needs. Through uniaxial compression tests, the compressive properties of 3D printed concrete were tested under different printing methods and force directions. The results showed that the compressive strength of the 3D printed concrete was higher under the conditions of printing path B and loading along the direction parallel to the bonding layer. According to test results, the mine waterproof wall structure based on concrete 3D printing was designed. Then, ABAQUS was used to simulate the compressive strength and deformation of the 3D printed waterproof wall. The results showed that the ultimate load of the 3D printing module was 6.697e4kN and the deformation range was controllable, which meet the engineering requirements. The advantages of 3D printing mine waterproof walls are more flexible in design, faster in manufacturing, and more intelligent in operation. This work provides new ideas for the design and construction of waterproof walls in deep mines.

Study on Permeability Characteristics of Gas Bearing Coal under Cyclic Load

With the depletion of shallow coal mine resources, the mining depth of coal mines is deepening year by year, therefore, gas explosion and other disasters occur from time to time. Gas drainage is the main measure to prevent gas disasters, and the permeability of coal is one of the main factors affecting gas drainage. In order to explore the seepage characteristics of coal under different confining pressure loading and unloading conditions and different gas pressure, the seepage test of raw coal samples was carried out by using the coal rock triaxial loading seepage test device. The results show that the permeability of coal samples decreases with the increase of confining pressure in the loading stage, and increases with the unloading of confining pressure in the unloading stage; Through calculation, it is found that the permeability loss of coal body in the loading stage decreases with the increase of loading times, and during unloading, this value also decreases with the increase of unloading times, and reaches the maximum value at the first loading and unloading. With the increase of loading and unloading cycles, the permeability loss of coal samples decreases, and the first loading and unloading is the main stage of permeability loss of coal samples. At the same time, it is found that when the confining pressure increases, the permeability loss of coal samples decreases, and the initial permeability of coal samples maintains a good linear growth relationship with the increase of gas pressure.

A Case Study on Ground Subsidence and Backfill Deformation Induced by Multi-Stage Filling Mining in a Steeply Inclined Ore Body

The backfill mining method transports treated tailings to the mined-out area, which not only improves the surrounding environment of the mine but also enables the mined-out area to continue mining and production under the support of the filling body. However, with the growth in the depth and scale of mining, ground subsidence, and backfill deformation are becoming increasingly serious problems. As an example, in the Jinchuan mine, a typical multi-stage filling mining mine in China, the deformation law of surface rock mass and backfill are studied through a method combining field monitoring and numerical simulation. The major findings are as follows: (a) A settlement funnel is formed on the ground, and its radius gradually expands with continuous mining and filling. The location of the settlement center moves toward the surface above the footwall of the ore body, and the maximum subsidence reaches 739 mm in 14.5 years. (b) Three-section mining significantly affects the surface deformation, and the single subsidence center on the upper wall develops into the double subsidence center with the mining and filling. When the three-section mining is finished, the maximum value of the surface subsidence reaches about 1.35 m and the mining area is still in a relatively stable state. (c) The whole filling body presents obvious subsidence, with the development of the multi-stage mining and filling. Bed separation phenomena are found between the filling layers, and the closer to the interior, the more obvious it becomes. The backfill’s subsidence characteristics are similar to the surface’s; that is, both the subsidence amount and speed are higher on the hanging wall than on the footwall. (d) The backfill mainly shrinks inward in the horizontal direction, and the deformation is mainly manifested as an internal uplift and an external subsidence in the vertical direction. The mass instability of the backfill is difficult because of the insufficient deformation space, and the influence of large-scale deformation on the mining and overlying strata needs to be considered, as well as the local deformation near the rock contact zone surrounding the backfill. The results provide technical support for filling mining in the Jinchuan mine and provide a reference for other projects with similar engineering conditions.

Thermal properties and microstructural evolution of coal spontaneous combustion

With the increase of coal energy mining depth in China, coal spontaneous combustion phenomenon becomes more frequent, which makes the safe mining of coal energy in China under serious threat. In order to prevent coal spontaneous combustion, it is significant to investigate the process of coal oxidation and spontaneous combustion by combining thermal and microstructural properties, which is a necessary study to figure out the mechanism of coal spontaneous combustion. A series of coal oxidation experiments were conducted with the aid of high precision Calorimeter and X-ray diffractometer. The results show that the thermal properties of coal spontaneous combustion have obvious segmentation characteristics which is not affected by the degree of metamorphism and heating rates, and the initial exothermic temperature is positively correlated with heating rates. The effect of temperature on microscopic characteristics is not uniform, but the degree of metamorphism has a regular change for microscopic characteristics. Although temperature does not play a decisive role, the spatial structure of coal oxidation process has some reversibility with the change of temperature. Besides, thermal dynamic balance is an important cause of microstructure changes. It is expected that this work will contribute to the refinement of the theory of coal spontaneous combustion.

Main Controlling Factors and Formation Mode of Geothermal Anomaly in Eastern Chenghe Mining Area of Weibei Coalfield

Geothermal anomaly has gradually become a prominent issue affecting the efficient mining of coal with the depth of coal mining increasing in eastern Chenghe mining area of Weibei coalfield. Here, we comprehensively investigated the distribution characteristics of the present geothermal field, analyzed the main controlling factors, and constructed forming mode of geothermal anomaly by the temperature measurement in surface borehole and underground water, combined with coal-rock thermal conductivity test. The results show that the geothermal gradient ranged from 25.7°C/km to 54.3°C/km. The areas with geothermal gradient greater than 30°C/km accounted for 88.31%, and there was the highest gradient value in the southeast F1 fault zone. The heat flow was between 66.81 mW/m2 to 128.49 mW/m2, which belonged to the obvious high heat flow area. Under the action of the main controlling factors such as fault, fold, coal-rock thermal conductivity, and groundwater activity in the region, the geothermal gradient and geothermal heat flow values showed an increasing trend from northwest to southeast. According to the distribution characteristics of heat flow and the action mechanism of main controlling factors, the geothermal anomaly in the study area was finally divided into two forming modes, i.e., fault-deep circulating hot water uplifting type and coal seam heat resistance-fold type. The research provides guidance for the geothermal hazard prevention of coal mine and the rational development and utilization of geothermal resources.

Study of the microstructure and oxidation characteristics of residual coal in deep mines

Residual coal is prone to spontaneous combustion under the influence of high ground temperature, and the risk increases with mining depth. This results in gradual increases in smoke and dust emissions, which seriously affect the ecological and geological environment of the mining area. To avoid spontaneous combustion of the residual coal, protect the resources and ecology of mining areas, and investigate the influence of the deep thermal environment on the microstructure and oxidation properties of coal, the pore evolution characteristics and functional group migration of coal in a thermal environment were studied by N2 adsorption and infrared spectroscopy. A temperature-programmed test system was used to simulate the low-temperature oxidation process occurring in deep coal seams, and the heat release characteristics and oxidation kinetic parameters of the index gas were calculated. The experimental results revealed that the pore oxygen storage capacities and coal oxygen contact areas of residual coal in deep mines are larger than those of coal in shallow seams. As the preheating temperature is increased, the rates of CO and CO2 production and the amount of oxygen consumed in oxidation of heat-treated coal increase. The crossing point temperature (CPT) of heat-treated coal decreases. The deep thermal environment accelerates the rate of condensation decomposition of the aliphatic hydrocarbon groups in coal and fracture of side chain hydrogen bonds connecting strong electron-absorbing groups, which increases the exothermic intensity of the oxidation process. The reaction trigger temperatures for aliphatic hydrocarbon groups, peroxyl radicals, associative hydrogen bonds and other active groups in coal are different. With increasing ambient temperature, the types of active groups in coal increase, and the contents accumulate. With an increase in the deep geothermal temperature, the oxidation activity of coal is enhanced, and the coal is more prone to spontaneous combustion. These research results provide a theoretical reference for deep coal clean mining and prevention of spontaneous combustion disasters and have a potential guiding role in environmental protection and ecological improvement of deep mining areas.