Mining Height Research Articles

An Enhanced Empirical Model for Predicting Continuous Fracturing in Rock Masses

Abstract A reliable empirical prediction of the height of fracturing (HoF) is significant for mining impact assessment and mine sustainability. However, the prediction accuracy is often limited by the parameter and data coverage involved in empirical models. This paper develops a new predictive model that incorporates mining parameters, as well as the proportion and strength of stiff strata, as predictors for enhanced mining impact assessments. A HoF database has been established by sourcing 80 cases from 50 mines in Australia and China. Statistical interpretation identifies positive correlations of HoF to mining height, panel width, and cover depth. Numerical modelling and sensitivity analysis identify that both the proportion and strength of stiff units negatively correlate to HoF and are more significant than bedding plane frequency. The two critical rock parameters are statistically independent and should be considered for enhanced HoF predictions. A predictive model is established by nonlinear regression over the 80 datasets, with both rock parameters involved by mapping to a dimensionless indicator of strata competency considering dimensional consistency. The model is tested with an $${R}^{2}$$ R 2 of 0.84 and more accurate predictions than several historical models regarding 24 new cases. Prediction bounds with a confidence level of 95% are developed considering the rock parameter variability, providing conservative HoF predictions for cases where significant natural features should be preserved for sustainable mining practices. This paper assesses the influence of rock parameters that have not been considered in historical models on continuous fracturing and provides an enhanced empirical method for mining impact assessment.

Research on Strip Coal Pillar Technology under the Construction Group of Gangue Filling and Recycling

In order to reduce the various impacts of surface movement caused by coal mining on the human living environment, avoid the accumulation of coal gangue on the surface, and recover the strip coal pillar resources left under urban areas, based on the stability of the binary bearing structure composed of backfilled gangue and narrow coal pillars after strip mining and the characteristics of the equivalent mining height model, theoretical analysis and numerical simulation methods were comprehensively used to analyze the factors affecting the stability of coal pillars, the ultimate strength of coal pillars, and the safety factor of coal pillar stress under the condition of binary bearing structure. Through comparative analysis of deformation of surrounding rock after strip mining and coal pillar recovery, it was found that the mining pressure manifestation of strip coal pillar gangue filling is weak, without obvious initial and periodic pressure phenomena, and the top control is relatively easy without the risk of dynamic pressure. The research results provide useful reference and guidance for the first application of fully mechanized mining technology in urban strip coal pillar recovery in coal mines.

Study on the “Two-Zone” Heights in Lower Slice Mining Under Thick Alluvium and Thin Bedrock

The extraction of thin bedrock coal seams with thick alluvium poses a challenging issue in the realm of coal safety production in China. Especially for mining under aquifers, knowing the development height of water-conducting fracture zones above the goaf is crucial for coal mine safety and production. Taking the 11092 working face of lower slice mining in Zhaogu No. 1 Mine as an example, the failure transfer process of the overlying strata is analyzed first. On this basis, the development height of the water-conducting fracture zone is predicted using empirical formulas and the BP neural network. According to the empirical formula, the height of the roof caving zone ranges from 6.93 m to 27.72 m, while the height of the water-conducting fracture zone ranges from 22.17 m to 71.73 m. The BP neural network predicts that the development height of the water-conducting fracture zone in the working face after mining is 56.83 m. CDEM numerical simulation is employed to analyze the development height of two zones of overburden rock. The findings indicate that with a mining height of 2.5 m and a cumulative mining height of 6 m, the maximum caving ratio is 2.61. It is observed that for a cumulative mining thickness of less than 6 m, a bedrock thickness of not less than 30 m, and a clay layer thickness exceeding 5 m, the clay layer effectively obstructs the upward development of the water-conducting fracture zone. Finally, the prediction results of the development height of the two zones of overlying strata in the working face are verified by using the height observation method on the underground water-conducting fracture zone and the borehole peeping method. In conclusion, the height of the overlying strata after mining the lower slice working face in the first panel of the east can be used as a basis for determining the thickness of coal (rock) pillars for waterproofing and sand control safety during the mining of lower slice working faces in mines.

Study on instability mechanisms and control of coal pillar spalling and coal crumbs leakage during working face crossing faults

The stability of coal pillars in fault areas is crucial for ensuring the safe passage of working faces. Based on field observations, frequent coal pillar spalling and substantial tectonic coal crumbs leakage, as well as tilting of hydraulic supports, are observed when working faces transition from primary coal to tectonic coal. To analyze the instability mechanisms behind these phenomena, this paper establishes a mechanical model of coal pillars in fault areas and analyzes the distribution of tectonic stresses and factors affecting the stability of coal pillars. The results indicate that horizontal tectonic stress adheres to an exponential function dependent on the angle factor, where (k0) is a parameter associated with the friction angle of the coal body, the dip angle of the fault, and the friction angle of the fault plane. The stability of coal pillars is influenced by factors such as roof and floor pressures, coal pillar integrity, mining height, and shield support force, with coal pillar integrity being the most critical. To ensure the smooth passage of working faces through faults, this study proposes a combined control technique of “inclined mining” and “grouting,” including reducing mining heights, adjusting the slope of working face advancement, and pre-grouting of coal pillars. Industrial experiments conducted on-site have shown improved integrity of tectonic coal, enabling the working face to pass through faults smoothly and significantly increasing production efficiency.

Coal‐wall spalling prevention mechanism using advance‐grooving pressure relief in the large‐mining‐height working face of a shallow coal seam

AbstractIn mining, the roof structure of a working face with a large mining height in a shallow‐buried coal seam is prone to cutting instability in front of the support, which causes support crushing and coal‐wall spalling. This study analyzes 2201 working faces with large mining heights in the Haiwan No. 3 Mine by investigating the mechanical response law of coal walls under the transient excitation of roof structure instability. A mechanical model of coal walls under dynamic and static load coupling is established, revealing the formation mechanism of coal wall spalling and its main influencing factors. Prevention and control technologies of advanced grooving and pressure relief are proposed, and grooving parameters are determined. The results show that: During the mining process of large mining height working face in shallow buried coal seam, the step change of static response of coal wall is induced by the transient instability of roof structure, and the high abutment pressure is formed on the coal wall. At the same time, the strain energy and gravitational potential energy of roof cutting instability are released to form a dynamic load. Under the action of dynamic and static load, the plastic failure dissipation power of the coal wall is greater than the critical power, and the coal wall spalling occurs. The main influencing factors are mining load, mining height, and internal friction angle of the coal wall. The method of advanced grooving pressure relief can change the roof structure and coal force mechanism, by cutting off the connection between the coal wall and immediate roof, reducing the height of the coal wall force, effectively controlling the position of the roof cut off break line, reducing the static load effect of the roof on the coal, and transferring the position of the dynamic load response to the depth of the coal wall. When the grooving height‐depth ratio k is 0.75, the peak value of the abutment pressure moves 5.57 m to the front of the working face, and the pressure relief effect is the best. The above research results provide an effective active protection method for controlling coal wall spalling in fully mechanised working face with large mining height.

Research on “Playing Football” Type Roof Control in Fully-Mechanized Mining Face with a Super-Large Mining Height under the Background of 5G+ Big Data

With the increase of mining height at the working face, the influence range of roof fractures in the goaf increases, the advanced supporting pressure on the coal wall increases, ground pressure becomes more intense, and roof support becomes more difficult. Based on the analysis of ground pressure behavior in the first mining and caving stage, the normal mining stage, and the final mining breakthrough stage of the fully-mechanized mining face near 12,404, the relationship between conveyor current and coal speed is studied and compared. Based on the intelligent control system of the fully-mechanized mining face with a super-high mining height of 12,404 and the structure of the football team, the “playing football” roof control mode of the fully-mechanized mining face with super-high mining height under the background of 5G+ big data is put forward. The conclusions are as follows: In 12,404, the ground pressure was first mined. During normal mining, when the roof with a buried depth of more than 200 m is broken, the speed of the coal machine is kept within 12 m/min, and the full guard defends and controls the roof, pulling the lead frame through the area with severe ground pressure. When the roof is good, it is necessary to speed up the coal cutting and get rid of the pressure. When it is less than 200 m, it will overcome the local weighting, and show an offensive trend to speed up and increase production. In the final mining breakthrough stage, the speed of the coal machine should be controlled within 8 m/min, with attention to defense, guarding against roof leakage, and reducing waste rock.

Prediction and Analysis of Surface Residual Deformation Considering the Impact of Groundwater in Mines

With economic development and coal resource exploitation, the area of mined-out zones is expanding continuously. The traditional waste disposal methods no longer meet the current demands, making it urgent to evaluate and reuse the surface stability of these mined-out zones. Surface residual deformation is a process where voids and fissures within the mined-out zones are gradually filled and compacted, affecting the overlying rock structure. Additionally, groundwater significantly impacts the strength of the overlying rock, leading to increased subsidence. Therefore, predicting surface residual deformation while considering the effects of groundwater is crucial for forecasting surface deformation and assessing stability in mined-out zones. This study, taking into account the characteristics of subsidence zones and the impact of groundwater on the compaction of fractured rock masses, uses equivalent mining height and probability integral methods to develop a predictive model for surface residual deformation incorporating groundwater effects. Predictions for the study area show that groundwater exacerbates surface residual deformation, with various deformation values ranging from 33.8% to 51.9%. The surface stability categories are divided into stable and essentially stable regions based on the residual deformation’s impact on the working face. This model fully considers the influence of groundwater on residual deformation in mined-out zones, refining existing mining subsidence theories, addressing deformation issues caused by adverse groundwater factors, and providing a theoretical basis for predicting residual deformation and evaluating stability in mined-out zones, promoting the sustainable development of land and environmental resources in mining areas.

Sensitivity analysis of geological mining influencing factors on the pressure relief effect of upper protective layer mining

AbstractTo study the influence of different geological and mining factors on the decompression effect of protective layer mining, a numerical simulation was carried out using FLAC3D numerical software. Taking the Hulusu coal mine as the engineering background, numerical simulation studies were carried out under different mining heights, working face lengths, interlayer lithologies and layer spacing by using FLAC3D numerical software, and the effects of different geological and mining factors on the degree of decompression and the scope of decompression were quantitatively investigated through the control of single‐factor variables. The results show that: the stress at the critical depth of impact ground pressure is 16.44 MPa, and the coefficient of unloading degree C = 0.5 is the indicator of sufficient unloading; the unloading effect of the protected layer decreases with the increase of layer spacing, lithological strength, and length of the working face, and increases with the increase of mining height; the geological and mining parameters of the protected layer show a functional relationship with the critical depth of the unloading, and the order of the influence of pressure relief effect is layer spacing > mining height > interlayer lithology > working face length. The results of the study are very important for the determination of the mining parameters of the protective layer, the estimation of the protective effect, and the design of the management programme of impact pressure.

Subsidence Prediction Method Based on Elastic Foundation Beam and Equivalent Mining Height Theory and Its Application

Grouting technology in overburden separation is recognized as an effective method to prevent surface subsidence and reuse solid waste. This study used mechanical analysis to explore deflection characteristics of key strata and accurately predict and control surface subsidence. Conceptualizing the coal–rock mass beneath the key strata as an elastic foundation, we developed a method to calculate the elastic foundation coefficients for various regions and established an equation for key strata deflection, validated through discrete element numerical simulations. This simulation also examined subsidence behavior under different grout injection–extraction ratios. Additionally, combining the equivalent mining height theory with the probability integral method, we formulated a predictive model for surface subsidence during grouting. Applied to the 8006 working face of the Wuyang Coal Mine, this model was supported by numerical simulations and field data, which showed a maximum surface subsidence of 546 mm at a 33% injection–extraction ratio, closely matching the theoretical value of 557 mm and demonstrating a nominal error of 2%. Post-grouting, the surface tilt was reduced to below 3 mm/m, meeting regulatory standards and eliminating the need for ongoing surface structure maintenance. These results confirm the model’s effectiveness in forecasting and controlling surface subsidence with grouting. The study can provide a basis for determining the grouting injection–extraction ratios and evaluating the effectiveness of surface subsidence control in grouting into overburden separation projects.

Surface damage reduction effect of ultra-high working face in Shangwan coal mine

Green mining is the basic background of high-quality mining, and source reduction is the most important part, among which the optimization of working face height and length is the most active. How to control the working face parameters, reasonably evaluate the surface subsidence characteristics of the super-long working face, and identify the limit working face length and the surface discontinuous deformation threshold is particularly important. In order to solve the problem of irreversible damage to the surface caused by the mining process of the working face, this paper discusses the whole process of surface movement in the 8.8 m super-large mining height working face of Shangwan Coal Mine through field investigation, the maximum subsidence is 6.20 m, with a subsidence coefficient of 0.72. Combined with the inflection point trajectory, advancing degree and subsidence coefficient, the 0.9 coefficient of the working face subsidence basin boundary and the loss reduction strategy far from the limit working face threshold are proposed. The results show that the subsidence basin of 12,401 working face accounts for 34%, the continuous deformation area of surface accounts for 64%, and the influence area of discontinuous deformation area is within 10 times of mining height. With the help of borehole detection verification, the caving zone height within the coal seam overburden to be between 33.2 and 33.25 m and the fracture zone height between 118.08 and 132.83 m. Therefore, the caving ratio is about 3.8, and the fracture ratio is about 13–15, which provides a strong basis for the optimization design of working face and the timing of surface ecological management.

Mechanism of Instantaneous High-Strength Sand–Water Inrush in Steeply Inclined Mining of Soft Coal Seam under Disturbed Rock–Soil Overburden

Abstract Water and sand inrush pose significant threats to underground geotechnical engineering, including shallow buried resource extraction and tunnel construction. To understand the mechanisms behind these phenomena, a mud collapse accident in Chaider Coal Mine was comprehensively investigated through field exploration and laboratory-based testings. Using numerical simulation experiments, we analyzed the failure patterns and seepage characteristics of overlying strata in steeply inclined coal seam mining under various working conditions. We established a structural instability model for water and sand inrush and identified the critical conditions for sand collapse occurrence. Our research indicates that the backfill in the surface mining pit provided a substantial material source for the accident. The overall destabilization of the top coal, due to its insufficient thickness and strength, created a pathway for sand collapse. Furthermore, frequent rainfall during the flood season and the inadequate arrangement of pumping equipment acted as triggers for the sudden water collapse. Preventative measures, such as limiting the mining height, enhancing the shear strength of the top coal, and altering the working face layout, can effectively control the development height of the water-conducting fracture zone. Additionally, timely evacuation and lowering of the aquifer water level, weakening its seepage effect in the top coal area, reducing the moisture content of the bottom soil, and improving its shear strength can mitigate water and sand inrush accidents in the backfill areas of open-pit mines.

Effect of super-high water materials backfilling on stress decrease and energy release during strip coal pillar mining: A case study

Mining strip coal pillars left due to strip mining is important to resource-exhausted coal mines in eastern China. Backfilling mining is an effective means to mine strip coal pillars, which could decrease high stress and high energy release. However, materials with super-high water content were not widely applied in deeply isolated coal pillar mining due to lower strength characteristics and durability. In the paper, taking panel c8301 as engineering background, theoretical analysis, numerical simulation, and field monitoring were used to study the feasibility and application effect of the super-high-water backfilling technology in deep isolated coal pillar mining. The results showed that: (1) Panel c8301 is a strip-filling working face with insufficient mining on both sides, and the mining of the panel will trigger the rebalancing of the overlying rock structure on both sides, which increases the rock burst risk. (2) Simulation results indicate that the super-high-water filling method, in comparison to the traditional caving method, significantly reduces peak stress and elastic energy from 112.3 MPa to 4.6 × 106 J to 79.2 MPa and 2.23 × 106 J, respectively. This represents reductions of 29.6 % and 51.5 %, effectively mitigating the impact of mining activities on overburden movement. (3) On-site measurement data confirmed that the measured equivalent mining height was 1.25 m. The total number of microseisms and the amount of released energy decreased significantly, More specifically, three big energy release instances (>1.8 × 105 J) were recorded during the first roof weighting stage and panel in square meters. Super-high-water filling technology has achieved remarkable results in the mining of strip coal pillars and has significant application prospects.

Research on the Development Height Prediction Model of Water-Conduction Fracture Zones under Conditions of Extremely Thin Coal Seam Mining

Addressing the difficult problem of predicting the height of water-conducting fracture zones in shallow and thin coal seams, a prediction model of water-conduction fracture zones based on a backpropagation (BP) neural network was developed by integrating theoretical analysis, field measurements, and algorithmic advancements. Firstly, through overburden migration analysis and correlation tests, the height index system of the water-conducting fracture zone was determined. This system includes mining height, buried depth, dip angle, working face width, and overburden rock lithology, with five groups of characteristic parameters. Then, 35 pairs of minefield-measured data were collected to establish the measured height data set of the water-conducting fracture zone. Secondly, a BP neural network prediction model and a traditional support vector regression (SVR) prediction model were constructed based on a Pytorch framework, and the models were trained and tested by selecting data sets. Thirdly, the optimal prediction model was determined by comparing the model with the empirical model and multiple regression model of mining regulations for coal pillar maintenance and pressure in buildings, water bodies, railways, and main shafts. Finally, a typical mine was selected for application to verify the suitability of the optimal model. The results show that: (1) the predicted value of the neural network model is consistent with the change trend of the measured value, which accords with the theoretical law; (2) compared with traditional forecasting methods, the error of the BP neural network prediction model is stable and the prediction effect is the best; (3) dropout can effectively mitigate mitigation training overfitting, achieve regularization, and improve prediction accuracy; (4) the field application further verified that the BP neural network model is the best for predicting the height of water-conducting fracture zones of extremely thin coal seams, and the research results can provide technical guidance for similar fragile coal seams.

Floor failure behavior and water disaster prevention system of ultra-wide opposite pulling working face mining on confined aquifer

The floor failure depth of the ultra-wide opposite pulling working face (OPWF) mining on the confined aquifer is larger, and the risk of water inrush is higher. Based on the engineering background of Binhu Coal Mine (BHCM) in North China Coalfield, this paper analyzes the distribution rule of floor support pressure in OPWF mining, carries out numerical simulation before and after roof cutting, and microseismic-electrical joint dynamic monitoring. The results show that the failure depth, vertical stress, maximum principal stress, maximum shear stress, and pore water pressure of floor after roof cutting are decreased by 28.6 %, 15.5 %, 12.1 %, 30.9 %, and 41.7 %, respectively, the stress concentration coefficient is also decreased by 19.5 %. Roof cutting blocks the propagation path of mining stress, reduces the mine pressure concentration and controls the floor failure depth. Using the optimal univariate function between the floor failure depth and mining width, mining depth, mining height, roof cutting height, working face stagger distance, a multi-factor prediction model for the floor failure depth of the OPWF mining is established, and the accuracy of the model is more than 90 %. Combined with the methods of exploration, treatment, and monitoring, the water disaster prevention technical guarantee system in the ultra-wide OPWF mining on the confined aquifer is constructed, which provides a new method for the prevention and control of floor water disasters during mining.

Prediction technology of mine water inflow based on entropy weight method and multiple nonlinear regression theory and its application

Mine water inflow is an important basis for the formulation of mining plans and the utilization of groundwater resources. The mine water inflow is the result of the combined influence of many factors. The weight value of the influencing factors is calculated by the entropy method, and the order of importance of the factors is: precipitation > mining depth > cumulative mined-out area > aquifer thickness > mining area > mining height. The optimal univariate nonlinear regression model of mine water inflow to each influencing factor is obtained by factor scatter analysis and Matlab function programming. On this basis, combined with the weight values of factors, a multivariate nonlinear regression prediction model of mine water inflow based on weighting is innovatively established, which overcomes the defect that the traditional water inflow prediction method that cannot reflect the relative importance differences of various influencing factors. The multivariate weighted nonlinear regression model is used to predict the mine water inflow of typical coal mines, and the prediction results are compared with the linear regression model and the measured value. The results show that the prediction model of mine water inflow based on weighted multivariate nonlinear regression is accurate higher, with higher practical application value.

Analysing the application of a flexible formwork pre-cast wall driving roadway along goaf in a large mining height face

The technology of building a retaining roadway along goaf or a protecting roadway with a small coal pillar has been developed and applied for many years, and a satisfactory supporting effect has been obtained in medium–thick coal seam and thin coal seam mining. However, the gob-side roadway or small coal pillar mining in a thick coal seam is still subjected to technical problems occasioned by factors such as high roadway, high support pressure beside roadway, and waste of coal resources. To solve these problems, the author proposes an innovative technology of coal-free mining: the technology of driving roadway along goaf with a flexible formwork pre-cast wall. The article utilizes the 3503 and 3505 working faces of Wangzhuang Coal Industry Group as the research background, and comprehensively introduces the principle of the technology and the overburden rock movement law. Through theoretical calculations and numerical simulations, the support resistance and support parameters of flexible formwork pre-cast walls have been determined and successfully performed in industrial practice. The results indicate that the combination of the flexible mould pre-cast wall coal pillar-free mining technology and roof cutting process is more conducive to the maintenance of the roadway in the lower working face, and effectively reduces the stress and deformation of the surrounding rock. The roof and floor of the drivage roadway move, and the deformation of the two sides is small; furthermore, the overall roadway retention effect is satisfactory, which meets the requirements of mining in the lower working face. The coal pillar pertaining to the 20 m section of the 5 m high mining height face was recovered for Wangzhuang Coal Mine, and the recovery rate of the coal resources and the driving speed of the roadway were improved. The proposed method can be popularised and applied in this mine and even in the mining of 15# large-height coal seams in the two cities.

Stability prediction of roadway surrounding rock using INGO-RF

In order to more accurately classify the stability of roadway surrounding rock and identify dangerous areas in a timely manner to prevent roadway collapse and other disasters, an INGO-RF roadway surrounding rock stability prediction model is proposed. This model combines the improved Northern Gok algorithm (INGO) and Random Forest (RF) based on the analysis of influencing factors of roadway surrounding rock stability. First, three strategies were employed to enhance the Northern Gob algorithm (NGO): Logistic chaotic mapping, refraction reverse learning, and improved sine and cosine. Subsequently, INGO was utilized to optimize the number of decision trees and the minimum number of leaf nodes for RF species in order to improve the prediction accuracy of RF. Secondly, a data set consisting of 34 groups of roadway surrounding rock data is selected. The input indexes of the model include roof strength, two-wall strength, floor strength, burial depth, roadway pillar width, ratio of direct roof thickness to mining height, and surrounding rock integrity. Meanwhile, surrounding rock stability is considered as the output index. Particle swarm optimization backpropagation neural network (PSO-BPNN), genetic algorithm optimization support vector machine (GA-SVM), Sparrow search algorithm optimization RF (SSA-RF) models were introduced to compare the prediction results with the INGO-RF model, and the results showed that: INGO-RF model has the best performance in the comparison of various performance indicators. Compared with other models, the accuracy rate (Ac) in the test set has increased by 0.12∼0.4, the accuracy rate (Pr) has increased by 0.07∼0.65, and the recall rate (Re) has increased by 0.08∼0.37. The harmonic mean (F1-Score) of the recall rate increased by 0.08∼0.52, the mean absolute error (MAE) decreased by 0.1428∼0.4285, the mean absolute percentage error (MAPE) decreased by 7.15%∼28.57%, and the root mean square error (RMSE) decreased by 0.1565∼0.3779. Finally, the data on surrounding rock conditions of roadways in multiple mining areas in Shanxi Province were collected to test the INGO-RF model. The results indicate that the predicted outcomes closely align with the actual results, demonstrating a certain level of reliability and stability. It shows that it can better meet the practical needs of engineering and avoid the occurrence of mine disasters.

Research on the Movement of Overlying Strata in Shallow Coal Seams with High Mining Heights and Ultralong Working Faces

To study the roof movement and ground pressure evolution characteristics of an ultralong working face in a shallow coal seam with a high mining height, the Shangwan Coal Mine in the Shendong mining area was used as the research background, and the physical and mechanical parameters of the surrounding rock were determined through rock mechanics experiments. A physical simulation model was built considering the 7 m mining height of the 12301 fully mechanized working face of the Shangwan Coal Mine to simulate and study the evolutions of the movement, fracture and collapse of the coal seam, direct roof, and basic roof and overlying strata during the mining process. The mechanical characteristics of the support, mechanism of roof collapse, and changes in the working resistance of the support were analysed and simulated. The research results indicate that when mining at a height of 7 m, the direct roof and basic roof strata collapse in layers; the basic roof strata collapse backwards, the rock block arrangement is more irregular, and the range of the basic roof that can form structural rock layers extends higher. After the basic roof rock fractures, it cannot form a masonry beam structure and can only form a cantilever beam structure. The periodic fracture of the cantilever beam causes periodic pressure on the working face. These research results are of great significance for planning the further mining of shallow coal seams with high mining heights and ultralong working faces in the Shendong mining area, as well as for improving the control of overlying strata.

An Improved Back Propagation Neural Network Based on Differential Evolution and Grey Wolf Optimizer and Its Application in the Height Prediction of Water-Conducting Fracture Zone

Given that the conventional back propagation neural network (BPNN) easily falls into the local optimal solutions, resulting in poor prediction accuracy, an improved BPNN based on the differential evolution and grey wolf optimizer (DEGWO) is proposed, the so-called DEGWO-BPNN. The prediction of the water-conducting fracture zone (WCFZ) height is significant for mine safety operations. A total of 104 sample data are trained and 25 sample data are tested to identify the optimal prediction model. Five evaluation indexes are selected to assess the prediction performance of the models quantitatively. Finally, the DEGWO-BPNN model is applied to a specific engineering case. The main conclusions are as follows: (1) Mining height, mining depth, coal seam dip, panel width, and ratio of hard rock as the main factors affecting the WCFZ height are selected. The topology structure of the model is defined as ‘5-12-1’; (2) the bias between the predicted value and the actual value of the training samples is smaller with an average error of 2.39. Test samples further validate the prediction precision through evaluation indexes. The values of MAE, RMSE, MAPE, and R2 are 2.3952, 3.4674, 5.3148%, and 0.99077, respectively. The prediction accuracy is 94.6852%; (3) ‘Mining Code’, MLR, BPNN, and GWO-BPNN models are treated as the comparison groups. The comparative analysis shows that the prediction performance of ‘Mining Code’ is the worst, while that of DEGWO-BPNN is the best, and it outperforms other algorithms and statistical approaches; (4) the prediction of WCFZ height in the 11601 panel is in line with the actual value. The prediction error of the DEGWO-BPNN model is lower than that of the comparison models. As such, the DEGWO-BPNN model can be well applied to the prediction of WCFZ height and is suitable for coal mines with different regional geological conditions. It can provide a valuable reference for mine safety operations.

The effect of overlying rock fracture and stress path evolution in steeply dipping and large mining height stope

To study the fracture instability characteristics and fracture mechanism of overlying strata in steeply dipping and large mining height stope, through the combination of physical simulation experiment, numerical calculation, theoretical analysis and field measurement, the correlation between the evolution of mining stress field and the fracture of overlying strata in large mining height stope under the effect of dip angle is analyzed, and the stress transfer path and the fracture mechanism of overlying strata are revealed. Finally, the influence mechanism of key strata on the evolution of the mining stress field of overlying strata is explained. The research shows that under the influence of gravity dip angle effect and mining height increase, the mining stress is non-equilibrium transferred and evolved in space, and the principal stress field presents the characteristics of partition evolution. The fracture trajectory of rock strata is multi-step and “八” shaped, and the inclined masonry structure and multi-step key strata form a cooperative bearing structure. The key layer of the ladder is the stress transmission structure between the caving zone and the stress arch, and the high stress in the overhanging area under the main roof is transmitted to the advanced roof. With the increase of dip angle, the height of the caving zone decreases, and the fracture range of ladder and principal stress arch decreases. The cracks in the middle and upper broken rock blocks increase, and the instability of the “high ladder rock layer” is easy to induce the simultaneous fracture of the “inclined masonry structure”, resulting in regional unbalanced instability and impact. The research results can provide some reference and guiding significance for the stability control of surrounding rock in longwall stope with steeply dipping.