Longwall Research Articles

A novel numerical modeling method for studying the failure mechanism of the main roof with different thicknesses in longwall coal seam mining

A novel numerical modeling method for studying the failure mechanism of the main roof with different thicknesses in longwall coal seam mining

Analytical Estimation of Strain Energy Accumulation in Retreating Longwall Mining and Sensitivity Analysis Using the Orthogonal Testing Method

AbstractExcessive strain energy accumulation in the coal seam is one of the essential conditions for rockbursts to occur. It is imperative to understand the parameters that affect the strain energy accumulation in retreating longwall mining to optimise the design and minimise rockbursts occurrences. In this paper, new analytical models were developed to calculate the strain energy accumulation considering the current state-of-the-art longwall machinery being used in the industry. Seven parameters, i.e., mining depth, length of the cantilever roof, coal seam thickness, thickness of the immediate roof, Young’s modulus of coal and the roof, and Poisson’s ratio of the coal seam, were identified as the parameters affecting the strain energy accumulation. A detailed statistical analysis of the parameters was conducted using the orthogonal testing method, which revealed that mining depth, Young's modulus of coal and the coal seam thickness significantly influence the strain energy accumulation within a 99% confidence interval.

Straightness Measurement Based on FOG-IMU and Shearer Motion Constraints for Longwall Coal Mining

Straightness measurements of fully mechanized coal faces are a core requirement for intelligent, comprehensive coal mining. In coal mining, using an inertial measurement unit (IMU) assisted by an odometer for straightness measurement is vital for automating longwall mining. IMUs are less vulnerable to dim and harsh underground mining conditions. However, IMU divergence cannot be controlled solely by the forward velocity obtained from the odometer. To address this problem without external sensors, this paper proposes a straightness measurement method based on a fiber optic gyroscope-IMU/odometer and shearer motion constraints. The motion of the shearer is constrained to correct the pitch and roll attitude angle errors in strapdown inertial navigation solution (SINS) attitude solutions. An extended Kalman filter was established to combine the SINS and dead reckoning using the output from the odometer. Finally, the position estimation obtained by Kalman filter fusion was converted into straightness. An experiment based on a custom-built test rail demonstrated that the proposed algorithm effectively improved the accuracy of straightness measurements in multiple cutting cycles. The error of the results in the horizontal plane was reduced by 45% compared to the traditional IMU/odometer method. This study is important in promoting the unmanned and intelligent development of China’s coal industry.

Numerical Investigation for Estimation of Behaviour of Barrier Pillars, Gateroads and Face of a Deep Longwall Mine: a Case Study

Numerical Investigation for Estimation of Behaviour of Barrier Pillars, Gateroads and Face of a Deep Longwall Mine: a Case Study

An exploration of improving the stability of mining roadways constructed in soft rock by roof cutting and stress transfer: A case study

In the confinement of traditional supports, reinforced supports are often used to control the deformation of roadways constructed in soft rock. Due to the failure to weaken the factors that induce deformation at the source, the roadway remains damaged after the restoration period. To this end, taking inspiration from roof cutting and pressure relief concepts, the protection mechanism of stress transfer in mining roadways constructed in soft rock of longwall remaining coal pillars was studied in the context of a typical coal mine. Unlike the technical scheme of gob-side entry retaining, no components are added in this study. The roof cutting and coal mining progress are consistent to ensure the integrity of the roadway in front of the longwall coal mining face (LCMF). The effect of the roof-cutting parameter on the roadway is studied via numerical simulations, and the optimal parameters are subsequently applied to similar material simulation experiments. It was found that roof cutting relieved the mechanical connection between the gob roof and the roadway roof, and the high-stress area was transferred from the roadway roof to the front of the LCMF. Roof cutting diminished the abutment stress in the surrounding rock in the area of intense dynamic pressure in front of the LCMF (average reduction of 12.57 %), which effectively improved the stability of the roadway. The roof cutting impaired the traction effect of the roadway roof deformation behind the LCMF on the strata ahead and effectively reduced the roadway deformation (27.3 % reduction) and coal pillar deformation (15.7 % reduction). On-site practice has demonstrated the effectiveness of this research, and the results of this research provide a new idea for preventing the destruction of roadways constructed in soft rock.

Stochastic distribution characteristics of void fraction in overburden disturbed by longwall mining in shallow seam

Stochastic distribution characteristics of void fraction in overburden disturbed by longwall mining in shallow seam

A method for reconstructing the pose of hydraulic support group based on point cloud and digital twin

The pose information of the hydraulic support group in underground mining is crucial for controlling the straightness of the longwall mining faces and ensuring safe production operations. However, currently, the acquisition of hydraulic support group pose information is not complete, particularly the lack of position information for the hydraulic support. In this paper, a method for reconstructing the hydraulic support group pose based on point cloud and digital twin is proposed. This method employs virtual and physical point clouds reconstructs the pose of the shearer and the hydraulic support group sequentially. The reconstructed pose of the hydraulic support group is then assessed for risk level. Firstly, an analysis was conducted on the inherent defects of the physical point cloud and the objective conditions for reconstructing the pose of hydraulic support groups. As a result, the idea of using virtual point clouds to assist in pose reconstruction was proposed. Subsequently, based on the working principle of LiDAR, a digital twin of the LiDAR was constructed, and methods for acquiring point clouds in both physical and virtual spaces were put forward. Then, based on the analysis of the mining process, the pose reconstruction of shearer is clarified as a prerequisite for hydraulic support pose reconstruction. Subsequently, the programs of their pose reconstruction were elaborated and a method for assessing the risk level of the hydraulic support group pose based on the reconstructed pose information is proposed, with the assistance of a case study. Finally, experimental verification was conducted in the laboratory environment for the three key parts: shearer pose reconstruction, hydraulic support group pose reconstruction, and assessment of the risk level of hydraulic support group pose. The experimental results confirmed that the method based on point cloud and digital twin are feasible for the reconstruction of the hydraulic support group pose. This method offers critical information for obtaining the hydraulic support group pose and provides a reference for determining whether manual intervention is necessary to adjust the hydraulic support group.

О ВЛИЯНИИ ОЧИСТНЫХ РАБОТ НА ХАРАКТЕР ДЕФОРМИРОВАНИЯ ДЕМОНТАЖНЫХ ВЫРАБОТОК

The paper presents the results of numerical modeling of the geomechanical behavior of the system “lava — operational excavation — enclosing potash rock mass” during mining operations. Numerical modeling includes a forecast of the mechanical behavior of the system at all stages of the system existence: from the natural stress-strain state (SSS) of the rock mass to excavation and safety measures installation, as well as the longwall mining operations. The results of numerical experiments are verified using the data from field studies of convergence of the roof and floor of the operational excavation and the side walls of this excavation. As a result of the studies, it is established that longwall mining has a significant impact on the operational excavation at a distance of about 40 m or less. In addition, the paper describes an algorithm for numerical simulation of the geomechanical behavior of the considered geotechnical system.

Numerical modelling approach for estimation of a yield zone in the face of a deep longwall panel

In the Longwall mining method, the coal face is being supported with two-legged or four-legged shield supports which improve better roof control between the face and canopy tip. The development of load on the face increases with the face retreating rate or the increase of the overhung length behind the shield support and as a result, the coal face tends to yield/fail. The yield/failure zone in the face extends depending on intensity of developed load due to the depth of mining, panel size, height of excavation and geometry of the overhung. The broken coal from the yielded face may fall on the shearer, armoured flexible conveyor (AFC), shield supports or may hit the deployed manpower in the face and causes the face stoppage which in turn results in to the loss of the project. It is therefore, a study is conducted to understand the face yield/failure behaviour considering various geo-mining conditions based on input parameters such as main roof thickness, overhung length, and material type and setting load. This paper develops a unique statistical model to predict the yield/failure zone in the longwall face. For this purpose, a total of fifty-four (54) three-dimensional finite element models in ANSYS software are developed and analyzed considering Drucker-Prager failure criterion.

A Carbon Nanocomposite Material Used in the Physical Modelling of the Overburden Subsidence Process.

Carbon nanomaterial is widely used in structural health monitoring due to the advantage of sensitivity and good mechanical properties. This study presents a novel approach employing carbon nanocomposite materials (CNMs) to characterize deformation and damage evolution in physical modelling. As the primary measurement method, the CNM is used to investigate the deformation characteristics of a 200-400 m thick sandstone bed at a 1 km deep longwall mine. The sandstone unit is identified as an ultra-thick key stratum (UTKS), with its thicknesses varying across different mining panels of the UTKS. The results of CNM monitoring show that the UTKS remains stable even after a consecutive excavation of 900 m in width. This stability impedes the upward propagation of overlying strata failure, leading to minimal surface subsidence. The study demonstrates the huge potential of CNM in the mining area, which can be useful for investigating material damage in physical modelling studies. The findings suggest that the cumulative extraction width in individual mining areas of the mine should be controlled to avoid a sudden collapse of the UTKS, and that special attention should be paid to where the UTKS's thickness changes substantially. The substantial variation in UTKS thickness significantly impacts the pattern of overburden subsidence.

Longwall Mining Automation—The Shearer Positioning Methods between the Longwall Automation Steering Committee and China University of Mining and Technology

The shearer positioning method is of great significance to the automation of longwall mining. The research teams in the Longwall Automation Steering Committee (LASC) of Australia and China University of Mining and Technology (CUMT) have focused on shearer positioning and identified the shearer inertial navigation system, the measurement of longwall retreat and creep displacement, and the backward calibration of the shearer trajectory as three key technologies to obtain accurate shearer positioning information. In underground environments without GPS, due to the characteristics of inertial navigation system (INS) autonomous full-parameter navigation, shearer positioning based on INS is adopted by the LASC and CUMT, and error reduction algorithms are proposed to inhibit the rapid error accumulation of INS. In order to obtain the periodic calibration information when the shearer reaches the end of the longwall face, it is necessary to measure the retreat and creep displacements in order to back-correct the shearer trajectory. Finding a suitable measurement method for this task is challenging, especially in the presence of dust and moisture. The LASC used a scanning laser and FMR 250 microwave radar to measure these two displacements, while CUMT adopted an ultra-wideband (UWB) radar. In terms of the backward calibration method, minimum-variance fixed-interval smoothing (MFS) proposed by LASC and the global optimization model (GOM) for the shearer trajectory from CUMT are described in detail. The experiment demonstrates that the GOM outperforms MFS in terms of accuracy but requires more computational resources. Therefore, our next research objective is to develop an efficient and accurate algorithm for performing backward calibration on the shearer trajectory.

Floor heave mechanism for gob-side entry retaining with concrete blocks and control method: A case study

The issue of significant floor heave deformation in gob-side entry retaining has long been a challenging problem in the context of longwall mining. This paper studies the floor heave failure mechanism and control method for gob-side entry retaining with concrete blocks in Guizhou Faer Coal Mine in China. Based on Rankine’s earth pressure theory, the effective shear stress equation for the plastic slip of roadway floor is established. The deformation mechanism of floor heave in a retaining roadway with a block wall is revealed in this study. The new comprehensive control method is proposed, encompassing roof pre-splitting blasting for pressure relief, reinforcing cables for roof control, double directions control bolts for concrete block, and pliability cushion yielding pressure. FLAC3D numerical calculation model is established, which shows that the new method can effectively reduce the average vertical stress peak value of the entity coal floor by 34.6% and significantly reduce the pressure source causing the roadway floor heave. Besides, a multi-parameter real-time online monitoring system for mine pressure was designed, and field tests were carried out. The results show that the maximum value of roadway floor heave under the new method is 163 mm, reduced by 66.9%, and the roadway floor heave is effectively controlled. These research findings offer a fresh perspective and new ideas for controlling floor heave in mining operations.

Solid waste filling and roadway retaining for longwall mining by numerical investigation

Solid waste filling and roadway retaining can effectively control surface subsidence and alleviate solid waste accumulation pollution. In order to effectively evaluate the advantages of solid waste filling in deformation control of overlying strata and surrounding rock of retained roadways, this study used theoretical analysis and numerical simulation methods to analyze the factors affecting surface subsidence, as well as the deformation characteristics of surrounding rocks and retaining tunnels during backfill mining. By calculating the influence of factors such as the foundation coefficient and the filling rate on the subsidence of the roof, it is concluded that the filling rate is the main controlling factor affecting the subsidence of the roof. Through simulation and comprehensive analysis of the impact of different filling rates on overlying rock migration, it was found that when the filling rates are 70 % and 80 %, it can effectively control the subsidence of overlying rock in the mining area. By simulating the effects of these two filling rate conditions on the deformation of surrounding rock within the retained roadway zone, the results show that the optimal filling rate that can effectively control the subsidence of the overlying rock and improve the stability of the retained roadway is 80 %.

Mechanism and control of deformation in gob-side entry with thick and hard roof strata

Deformation of gob-side entries has always been a critical concern for ensuring stability in longwall coal mines. This paper addresses the significant deformation and support challenges that arise in thick and hard roof longwall faces (THRLF) due to dynamic pressure. The study aims to elucidate the characteristics and mechanisms of deformation during the retreat of the longwall face. The research findings indicated that the primary cause of deformation was the combination of advanced abutment stress resulting from longwall face mining and the movement of the lateral roof over the chain pillar. To mitigate this issue, we propose a deformation control method known as cutting off the lateral roof (COLR) over the chain pillar. Simulation results demonstrate a significant reduction in roof stress and deformation of the gob-side entry after implementing the lateral roof-cutting technique. These findings provide valuable guidance for effectively managing deformation in gob-side entries, particularly when dealing with thick and hard roof strata.

Investigating Effects of Reverse Osmosis-Treated Water on the Corrosion Rate of Chains in Armoured Face Conveyor for Longwall Mining

Premature failure of armoured face conveyor (AFC) chains due to corrosion is a significant proportion of the unplanned downtime experienced on longwall equipment. The AFC chains are constantly in contact with water and wet coal. The premature failure issue of AFC chains has become more prominent since the introduction of longwall top-coal caving with its additional AFC. Reverse osmosis (RO) is a popular water treatment method for reducing salinity and dissolved solids, but its impact on the corrosion of the AFC chains is unclear. This study has been commissioned to investigate the direct effects of RO water on the corrosion of AFC chains. An immersion test was carried out using AFC chain steel submerged in two water samples: untreated dam water and treated water from an RO treatment plant. Elemental analysis was conducted for both water samples, and four corrosion indices were measured for both water samples. The RO water more vigorously dissolves calcium carbonate scales leading to increased corrosion of the AFC chains although the dam water has much higher levels of calcium, chloride, sulphate, sodium, and magnesium. Bicarbonate ions are the main alkaline factor of water that provides the buffering capacity to acids. Decreased alkalinity without balancing other ions in water causes high corrosivity and decreased scaling tendencies.

An attempt to use machine learning algorithms to predict strong tremors during longwall mining of a coal seam

Strong tremors during longwall mining are usually associated with the fracturing of thick sandstone layers. The prediction of such tremors is important and can be treated as a form of rockburst prevention. The article presents the possibility of predicting the occurrence of strong tremors based on the random forest algorithm in the 48-h forecast horizon. The study concerns a longwall panel in coal seam No. 506 in one of the hard coal mines in the Upper Silesian Coal Basin, Poland. The extraction of this longwall panel was carried out at a great depth, surrounded by thick layers of sandstone and with the occurrence of edges in the previously mined seams. Therefore, seismic activity was high and strong tremors occurred. Based on the seismic catalogue and data related to the longwall face advance, 12 parameters were selected that were most significantly positively or negatively correlated with the occurrence of strong tremors. Based on 10 randomly selected datasets, models of the random forest algorithm were trained and then tested. Due to the much smaller number of cases of strong tremors compared to days when there were no strong tremors, the set of training data was balanced using the ADASYN algorithm. The obtained models had an average recall value of 0.79, and average accuracy equalled 0.912. The trained models showed high efficiency in predicting minority cases of days with strong tremors for up to 48 h and a relatively low percentage of misclassifications.

Distributed strain monitoring of overburden delamination propagation at a deep longwall mine

Clear understanding of overburden failure propagation during underground mining is critical in managing safety and environmental issues such as rock burst hazards, groundwater inrush and longwall convergency. This paper presents an in-situ monitoring study of overburden strain dynamics at a kilometre-deep longwall mine using distributed fibre optic sensing technology (DFOS). A deep borehole was drilled from the ground surface to near the mining seam and installed with an optical fibre cable to detect rock deformation in a contiguous fashion. In addition, a Global Navigation Satellite System (GNSS) device was installed at the borehole collar to monitor surface subsidence. The results suggested that strata delamination contiguously extended to 347 m, about 34 times the mining thickness. The remaining 600 m thick strata above this height behaved as the intact zone. Such an overburden deformation pattern resulted in minor surface subsidence of only 0.6 m or 6% of the mining thickness. A step-wise, upward breakage of the optical fibre cable (due to excessive strain) was observed and found to correlate with other ground behaviours, including an increase in the total microseismic energy and periodic weighting on the longwall supports. These observations corroborate that the dynamic deformation of overburden is primarily governed by competent stratum units, i.e., the key strata. The monitoring results have served to develop a reliable mine-scale 3DEC model for predicting stress redistributions in the subsequent longwall panels for preventing dynamic hazards. The study also demonstrates that DFOS-based strain monitoring can be a valuable tool to investigate overburden responses to mining and has significant potential for applications in various mining methods, such as block caving, sub-level caving and open stopping.

Longwall Face Automation: Coal Seam Floor Cutting Path Planning Based on Multiple Hierarchical Clustering

Space adaptability between mining equipment and coal-rock mass, to ensure the machines cut in a coal seam, is an importance technique in longwall mining automation. In order to guide the mining equipment cutting in the coal seam, a cutting path planning method based on multiple hierarchical clustering was proposed. Morphology similarity and the coplanarity measurement method were defined to evaluate the similarity of clusters. The coal seam floor series in the face-advancing direction were clustered according to the morphology similarity and coplanarity, respectively. Taking the morphology-based and coplanarity-based cluster centers as generating lines and stretching angle, respectively, the coal seam floor was reconstructed. The reconstructed floor can be regarded as the cutting path. The coal seam geological model of the 18,201 longwall face was analyzed with the proposed cutting path planning method. Comparing the reconstructed floor and original floor, the amounts of coal left and cut gangue were 1999 m3 and 1856 m3, respectively, for the segmental floor. For the case of whole floor, the amounts of coal left and cut gangue were 5642 m3 and 5463 m3, respectively. The coal loss rates only were 0.57% and 0.87% for the segmental and whole coal seam, respectively.

Evaluation of Seismic Potential in a Longwall Mine with Massive Sandstone Roof Under Deep Overburden: an Update

Evaluation of Seismic Potential in a Longwall Mine with Massive Sandstone Roof Under Deep Overburden: an Update

The Process of Developing Model Research for the Technology of Obtaining Energy Resources

The current problems associated with the maintenance of hard coal longwall mining depend on the application or use of extraction technologies. In order to make the best use of these technologies, a new approach based on simulation studies is necessary. This paper aims to develop a mathematical model for the powered roof support’s operation. The three groups of professionals involved in the testing of the roof support were involved in the work on changing the hydraulic system of the powered roof support stand. These professionals were powered roof support’s designers, researchers and users. The research subject was the development of a mathematical model as a starting point for conducting simulations. The model is based on d’Alembert’s principle and the equation of the balance of flow rates. Based on the developed model, it is possible to determine the pressure in the space under the piston of the hydraulic prop. The results obtained in the simulations are the basic assumptions for the development of a prototype that would solve the current problems in the hydraulic systems of powered roof supports. The adopted research methodology assumed the development of a mathematical model, simulation in the MATLAB environment and verification of the model on a test stand. The obtained results of simulation tests based on the developed mathematical model were confirmed in bench tests. Simulation and bench tests determined the correctness of the assumptions made for the development of the prototype model. Based on the analysis of the results, the nature of the work of the future prototype has been predetermined. The next stage will be the testing of the prototype, which is to be included in the hydraulic system of the prop of powered roof support in the future. The model mentioned before is the baseline model, and it will be modified depending on the application of the future design in real conditions. Simulation studies of powered roof support will allow the structure that is used currently to be optimised, so as to adapt it to increasingly difficult working conditions.