Seam Extraction Research Articles

A review of cavability evaluation in longwall top coal caving

Longwall Top Coal Caving has been considered as one of the most effective technologies for the extraction of underground thick coal seams. A large number of studies on the applicability of Longwall Top Coal Caving into new mine sites have linked the success of its application to the geomechanical understanding of the cavability of the top coal. The paper aims to improve the knowledge of the top coal cavability evaluation. A range of parameters that affect the top coal cavability were first identified. Afterward, a number of cavability assessment methods and classifications were reviewed. The result is important in that it assists researchers in developing an advanced and reliable tool for the top coal cavability evaluation.

A Novel Method for Selecting Protective Seam against Coal and Gas Outburst: A Case Study of Wangjiazhai Coal Mine in China

Protective seam mining is a major and critical regional measure to prevent coal and gas outbursts (CGO) in coal mines. In this study, a novel method for selecting protective seam against coal and gas outburst was studied on the basis of gas geology and rock strata control theories and principles for protective seam mining and relevant regulations, which is that theories of gas geology were used to assess the outburst risk inherent in different seams of this mine, and then make preliminary selection of protective seams, and the technical feasibility of the proposed selection method was then analyzed using the theories and principles for protective seam mining and relevant regulations. The case application study results show that the extraction of the upper protective seam (UPS) caused significant decreases in the predicative indicators of outburst risk in the outburst-prone seam and thereby prevented CGO, and the novel method can provide a theoretical basis for selecting protective seam against CGO.

Multivariate statistical approach for assessment of subsidence in Jharia coalfields, India

Indian coalfields, one of the major coal producers, are facing serious problem of subsidence now-a-days. This paper attempts to investigate various factors and their influence on magnitude and extent of subsidence. The study was conducted in the Jharia coalfields, India where extraction of thick seams at shallow depths has damaged the ground surface in the form of subsidence. For precise pre-estimation of subsidence, it is therefore necessary to know the contribution of each factor to the occurrence of subsidence. In order to achieve the objectives of this study, several multivariate statistical techniques such as factor analysis (FA), principal component analysis (PCA) and cluster analysis (CA) have been used. Two factors were extracted using FA. Factor 1 and factor 2 account for 42.327% and 24.661% of the variability respectively. Factor 1 represents “natural factor” whereas factor 2 represents “subsidence coefficient”. Spatial variations in regarding susceptibility to the subsidence were determined from hierarchical CA using the linkage distance. Further, the findings of this study would be helpful for prediction of magnitude of subsidence empirically.

Investigation of Sloped Surface Subsidence during Inclined Seam Extraction in a Jointed Rock Mass Using Discontinuous Deformation Analysis

Investigation of Sloped Surface Subsidence during Inclined Seam Extraction in a Jointed Rock Mass Using Discontinuous Deformation Analysis

The Influence of In-Situ Rock Mass Stress Conditions on Deformation and Load of Gateroad Supports in Hard Coal Mine

The stress-strain state of the rock mass in the neighbourhood of the mining galleries is absolutely critical considering their behaviour particularly load and deformation. The stress-strain state can be affected by numerous geological and mining factors i.e. depth, mechanical parameters of the rock mass, the presence of faults, previous mining operations in case of multiple seam extraction and by the influence of the current longwall mining process.Considering this fact the advanced measurement plan was developed. The measurements were conducted in one longwall maingate of the Czech coal mines. The holistic investigation program consisted identification of initial rock mass stress tensor and monitoring of its changing during the approach of the longwall face. Two CCBO (modified overcoring) probes were installed to obtain pre-mining full stress tensor and afterwards three CCBM probes were installed to continuous monitoring stress state in rock mass ahead of advancing longwall. The triple height telltales and endoscopic methods were applied to monitor the behaviour of rockmass in close proximity of the gateroad. Hydraulic dynamometers and strain gauged rockbolts were used to determine the support load. Also the gateroad deformation observations were performed. The results of aforementioned measurements allowed to determine the potential relation between stress-strain rock mass state and behaviour of the investigated gateroad in coarse of longwall face approaching.

Green coal mining technique integrating mining-dressing-gas draining-backfilling-mining

Aiming to address the following major engineering issues faced by the Pingdingshan No. 12 mine: (1) difficulty in implementing auxiliary lifting because of its depth (i.e., beyond 1000m); (2) highly gassy main coal seam with low permeability; (3) unstable overlying coal seam without suitable conditions for implementing conventional mining techniques for protective coal seam; and (4) predominant reliance on “under three” coal resources to ensure production output. This study proposes an integrated, closed-cycle mining-dressing-gas draining-backfilling-mining (MDGBM) technique. The proposed approach involves the mining of protective coal seam, underground dressing of coal and gangue (UDCG), pressure relief and gas drainage before extraction, and backfilling and mining of the protected coal seam. A system for draining gas and mining the protective seam in the rock stratum is designed and implemented based on the geological conditions. This system helps in realizing pressure relief and gas drainage from the protective seam before extraction. Accordingly, another system, which is connected to the existing production system, is established for the UDCG based on the dense medium-shallow trough process. The mixed mining workface is designed to accommodate both solid backfill and conventional fully mechanized coal mining, thereby facilitating coal mining, USCG, and backfilling. The results show that: The mixed mining workface length for the Ji15-31010 protected seam was 220m with coal production capacity 1.2million tons per year, while the backfill capacity of gangue was 0.5million tons per year. The gas pressure decreased from 1.78 to 0.35MPa, and the total amount of safely mined coal was 1.34million tons. The process of simultaneously exploiting coal and draining gas was found to be safe, efficient, and green. This process also yielded significant economic benefits.

Quantitative evaluation of stress-relief and permeability-increasing effects of overlying coal seams for coal mine methane drainage in Wulan coal mine

To eliminate the gas outburst hazards at Wulan Coal Mine, China, a methane and coal exploitation strategy is proposed for highly outburst-prone coal seams, in which the exploitation of the lower protective coal seams and pressure-relief methane drainage using surface boreholes are combined. This study investigates the stress-relief and permeability-increasing effects of overlying coal seams to test the effectiveness of this coal and methane exploitation strategy at Wulan mine. According to a numerical simulation of the protected seam extraction process in the Wulan coal mine, the stress-relief effect of the overlying coal seams was analyzed from two aspects: evolution regularities of the relief angle and stress evolution regularities. Due to the exploitation of the lower protective seams, the stress of the overlying coal seams can be fully relieved, which is beneficial for gas drainage.A permeability evolution model of a damaged coal seam was proposed in this study, in which the effect of damage on the permeability was considered, and the reliability of this model was demonstrated through a comparison with the permeability model of undamaged coal and an experimental data matching. Then, based on this permeability model of an undamaged coal seam, along with the stress evolution characteristics of the protected coal seam, the permeability distribution characteristics of the overlying coal seams were obtained. After mining coal seam 7# and coal seam 8#, the permeability maximum of coal seam 3# is 2054 times the value of the initial permeability, and the permeability maximum of coal seam 2# is 1781 times the value of the initial permeability.A field test at Wulan mine showed that this methane and coal exploitation strategy effectively eliminates the outburst hazards of the overlying coal seams and thereby creates suitable conditions for coal mine methane drainage. Over more than 2 years of degasification, the methane content of coal seam 3# decreased from 9.81 m3/t to approximately 3.14 m3/t, and that of coal seam 2# decreased from 10.6 m3/t to approximately 3.63 m3/t. The outburst hazards of coal seams 2# and 3# are eliminated completely, and a safe mining condition could be achieved by executing this strategy at Wulan mine.

Formation and Height of the Interconnected Fractures Zone after Extraction of Thick Coal Seams with Weak Overburden in Western China

Formation of a zone of interconnected fractures during coal mining is a key factor in mine flooding. Coal mines in western China are characterized by thick coal seams with mechanically weak overburden. In situ studies including drill core analysis, drilling fluid loss measurement, and borehole video monitoring were used at the working face 101 in Shaanxi Jinjitan coal mine to explore the maximum height of the interconnected fractures zone (IFZ). Also, tests on a scaled physical model and numerical simulation based on the drilling data were used to study the formation of the fractured zone. By considering data from other mines with similar mining conditions, a logarithmic relationship was found between the maximum height of the IFZ and the thickness of coal excavation. The maximum height of the IFZ was found to be 27 times the thickness of the excavated coal seam, which is far more than in coal mining areas in eastern China. Also, the IFZ in overlying strata of the study area was arch-shaped, not saddle-shaped, as had been observed in previous studies.

Application of artificial neural network technology to predicting small faults and folds in coal seams, China

Small geologic structures pose a great threat to production safety of the coal mines in China. Many water hazards and rock collapses are related to these small geologic structures. Accurate prediction of these structures relies on multiple lines of evidence including coal seam dip, thickness change, amount of gas accumulated, water flow changes, temperature, fracture type and degree of fragmentation of coal seams. Through the use of artificial neural network technology, this article presents a working method for forecasting small geologic structures in coal mines. The methods are applied to Zhangcun Coal Mine, China. A nonlinear model consisting of coal seam dip and thickness is constructed to predict the small structures in the front of working faces. The predictions are verified by field data. The distribution characteristics of the small structures can be accurately predicted in the coal seam extraction process as long as data of the controlling factors are accurately collected.

Geomechanics of subsidence above single and multi-seam coal mining

Accurate prediction of surface subsidence due to the extraction of underground coal seams is a significant challenge in geotechnical engineering. This task is further compounded by the growing trend for coal to be extracted from seams either above or below previously extracted coal seams, a practice known as multi-seam mining. In order to accurately predict the subsidence above single and multi-seam longwall panels using numerical methods, constitutive laws need to appropriately represent the mechanical behaviour of coal measure strata. The choice of the most appropriate model is not always straightforward. This paper compares predictions of surface subsidence obtained using the finite element method, considering a range of well-known constitutive models. The results show that more sophisticated and numerically taxing constitutive laws do not necessarily lead to more accurate predictions of subsidence when compared to field measurements. The advantages and limitations of using each particular constitutive law are discussed. A comparison of the numerical predictions and field measurements of surface subsidence is also provided.

Applying the ground reaction curve concept to the assessment of shield support performance in longwall faces

The longwall method is used in many countries around the world in the underground extraction of coal seams. This method enables significantly improved production results to be achieved when compared to the bord and pillar mining system. However, this mining method requires higher capital investment compared to bord and pillar mining. One of the essential elements required to achieve the anticipated level of production from longwall panels is good shield–strata interaction. This means that the shields used in the longwall faces should have an adequate capacity to ensure the maintenance of roof stability in the longwall working. The issue of determining shield capacity has been the goal of research in many countries resulting in a number of different methods for calculating the required capacity of shields. In recent years, numerical modeling and ground reaction curves (GRCs) have been used to determine adequate shield capacity. An important factor to be considered in analyses using the concept of GRC for shield support selection for ground and mining conditions is roof convergence. This paper presents an analysis of shield–roof strata interaction in two longwall panels with natural roof caving in the gob using the concept of GRC. The GRCs for the specific mining conditions in the two longwall faces were determined by means of numerical modeling using Phase2 software. Performance characteristics of two-leg shields were obtained from underground measurements conducted continuously during the retreat of the longwall panels. In a specially prepared measuring shield, the changes in the leg pressures were measured. In addition, the changes in shield geometry were assessed by means of inclinometers. For the two longwall panels studied, the selected variations of leg pressures and changes of shield height in time are presented for a single shield’s cycle during the longwall operations for shield advance, setting, loading, and lowering. An analysis of the interaction between the shield and the roof strata rock mass was performed based on a comparison of the GRC and the operating characteristics of the shield. The values of the roof convergence, which occurred in the longwall faces during the single shield’s cycle, are presented. It is strongly recommended that a system enabling the characterization and mining conditions appropriate for shield capacity determination and selection be developed.

Parameter Self-Optimizing Clustering for Autonomous Extraction of the Weld Seam Based on Orientation Saliency in Robotic MAG Welding

This paper presents an effective method which needs free parameters as little as possible to autonomously extract the weld seam profile and edges from the molten background in two kinds of weld images within robotic MAG welding. First, orientation saliency detection produced by Gabor filtering nicely highlights the weld seam profile and edges from the molten background. Then, an unsupervised clustering algorithm combing a cluster validity index via an optimization rule, referred to as parameter self-optimizing clustering, is applied to discern the weld seam profile and edges from interference data after the orientation saliency detection result is given threshold segmentation. The validity index is better than the classical ones in two kinds of data sets through considerable tests. Last, two common applications of weld seam identification demonstrate the effectiveness of the proposed method.

The method of coal losses reducing at mining by shovels

Nowadays the main part of coal in Kuzbass is extracted by backhoes. But the part of coal seams, especially on the open-pit mines where the work is coming to its final stage, is extracted by shovels. Considering characteristics of shovels design it is difficult to provide a coal seam extraction on the full height of the slope. At the same time the organisational and economic parameters of mining operations are significantly reduced when the layers of coal seam are worked out by shovels. Consequently, it is necessary first to establish the optimum parameters of the coal face for different types of shovels for complete scooping of the coal seam. Second, it is important to set the limit of seam dip angle at which the excavator operating parameters will be used maximally. For this purpose the graph-analytic model of calculating the coal losses when extracting seams by shovels is given in this paper.

The method of coal losses reducing at mining by shovels

Nowadays the main part of coal in Kuzbass is extracted by backhoes. But the part of coal seams, especially on the open-pit mines where the work is coming to its final stage, is extracted by shovels. Considering characteristics of shovels design it is difficult to provide a coal seam extraction on the full height of the slope. At the same time the organisational and economic parameters of mining operations are significantly reduced when the layers of coal seam are worked out by shovels. Consequently, it is necessary first to establish the optimum parameters of the coal face for different types of shovels for complete scooping of the coal seam. Second, it is important to set the limit of seam dip angle at which the excavator operating parameters will be used maximally. For this purpose the graph-analytic model of calculating the coal losses when extracting seams by shovels is given in this paper.

Underground mining of thick coal seams

This paper reviews underground mining methods for total thickness of a thick coal seam in single lift (TTTCSSL). Review shows the required engineering for extraction of thick seams needs to be fitted with thickness of the seam, behavior of rock-mass and surrounding stress conditions for efficient mining. Variants of TTTCSSL are able to extract a maximum 10–12m thickness only. An improvement in bending moment of the overlying coal band in longwall top coal caving (LTCC) provides better under-winning opportunity for the roof coal band. An acceptable limit of 25MPa compressive strength of coal for the success of LTCC may be increased under favorable geo-technical conditions. Bord and pillar in India adopted induced caving of roof coal band for single lift depillaring of total thickness (SLDTT) of a competent thick coal seam developed along floor. Case studies are given to arrest the adverse effects of extraction height on pillars.

Effect of Water Invasion on Outburst Predictive Index of Low Rank Coals in Dalong Mine.

To improve the coal permeability and outburst prevention, coal seam water injection and a series of outburst prevention measures were tested in outburst coal mines. These methods have become important technologies used for coal and gas outburst prevention and control by increasing the external moisture of coal or decreasing the stress of coal seam and changing the coal pore structure and gas desorption speed. In addition, techniques have had a significant impact on the gas extraction and outburst prevention indicators of coal seams. Globally, low rank coals reservoirs account for nearly half of hidden coal reserves and the most obvious feature of low rank coal is the high natural moisture content. Moisture will restrain the gas desorption and will affect the gas extraction and accuracy of the outburst prediction of coals. To study the influence of injected water on methane desorption dynamic characteristics and the outburst predictive index of coal, coal samples were collected from the Dalong Mine. The methane adsorption/desorption test was conducted on coal samples under conditions of different injected water contents. Selective analysis assessed the variations of the gas desorption quantities and the outburst prediction index (coal cutting desorption index). Adsorption tests indicated that the Langmuir volume of the Dalong coal sample is ~40.26 m3/t, indicating a strong gas adsorption ability. With the increase of injected water content, the gas desorption amount of the coal samples decreased under the same pressure and temperature. Higher moisture content lowered the accumulation desorption quantity after 120 minutes. The gas desorption volumes and moisture content conformed to a logarithmic relationship. After moisture correction, we obtained the long-flame coal outburst prediction (cutting desorption) index critical value. This value can provide a theoretical basis for outburst prediction and prevention of low rank coal mines and similar occurrence conditions of coal seams.

The decoupling visual feature extraction of dynamic three-dimensional V-type seam for gantry welding robot

During the welding manufacturing of a circular pressure vessel, the V-type seam’s position relative to welding torch varies dramatically due to the seam curvature change or axial asymmetry movement. Thus, a decoupling visual seam detection method for a 3-axes gantry welding robotis developed to implement automatic welding. Firstly, a visual system with laser structured light is developed to measure the deviation between the torch and welding seam in horizontal and vertical directions. Secondly, a multiple peak detection algorithm for laser profile is proposed to overcome the interference in high strength reflection area. Then, a decoupling visual detection algorithm is presented to gain approximately uncoupled relationship between the image feature and the movement of adjustment mechanism. Finally, in a gantry robot system, some experiments that verify the proposed methods are well illustrated with satisfactory performance.

Characteristics and applications of gas desorption with excavation disturbances in coal mining

According to the deficiency of experiment system for gas adsorption and desorption in coal mass, a large scale experiment system is developed independently by researchers. This experiment system is composed of primary and auxiliary boxes, power transmission system, mining system, loading system, gas charging system, data monitoring and intelligent acquisition system. The maximum experiment coal consumption is 1200 kg, the mining system is developed to conduct experiment for gas desorption under excavating disturbance, and the plane-charging cribriform ventilation device is developed to realize uniform ventilation for experiment coal sample, which is accord with the actual gas source situation of coal bed. The desorption characteristics of gas in coal are experimentally studied under the conditions of nature and mining using the experiment system. The results show that, compare with nature condition, the permeability of coal and the velocity of gas desorption could significantly increase under the influence of coal pressure relief and destruction caused by mining, and the degree of gas desorption could somewhat increase too. Finally, pressure relief gas extraction of current seam and adjacent seams after mining in a certain coal mine of Yangquan mining area are introduced, and the gas desorption experiment results is verified by analyzing the effect of gas extraction.

Evaluation of induced vertical stress during depillaring in blasting gallery panel workings

Selection of a suitable mining method for extraction of a thick coal seam with optimal recovery and safety is an extremely delicate process. In fact, single lift working of full thickness of a thick coal seam has always a verge over the multi-slice working due to favorable economics and high production and productivity. Blasting Gallery (BG) method is suitable for extraction of virgin thick seams as well as developed pillars in thick seams in single lift. The method was very successful resulting in 85% of extraction with high productivity. But, this method experienced strata control problems during final extraction. The presence of strong and massive overlying roof strata caused high values of the mining induced stress over the pillars facing the goaf line during the depillaring. High values of mining-induced stresses in and around a depillaring face of thick coal seam generally create a threat of pillar overriding. This paper describes the assessment of mining induced vertical stress in a BG panel during depillaring in specific geomining conditions, together with the results of the laboratory and field investigations. The attempt was based on field monitoring data of depillaring faces in BG panel of Godavari Khani (GDK) No 10 Incline, Singareni Collieries Company Ltd. (SCCL). Based on the results of laboratory investigations on simulated (FEM) models using ANSYS software, the numerical simulation (FEM) results are validated with field investigation results. The safety factor of the pillar is evaluated using Hoek and Brown failure criterion at different advances of goaf edge. The research revealed that the width of abutment zone is estimated to be about 35 m to 40 m from the diagonal line of extraction in the BG panels and the induced vertical stress and roof deformation decreases as one goes away from the goaf edge. It was observed that the safety factor of the pillar decreases when line of extraction moves towards center of the pillar. Results of finite element analysis using ANSYS are validated with the strata monitoring instrumentation data in the BG 2B panel of GDK No. 10 Incline, SCCL.

Physical modelling of subsidence from sequential extraction of partially overlapping longwall panels and study of substrata movement characteristics

Field examples of subsidence from sequential extraction of overlapping coal longwall panels have shown that complex and unexpected subsidence effects can occur. Physical models, comprising sand and plaster obeying relevant similarity rules, were used to investigate the subsidence mechanisms. Displacements within the models were measured using a combination of optical transducers, photogrammetry and laser scanning. The observed results were compared with finite element models. The models demonstrate that the incremental subsidence and substrata movement profiles after lower seam extraction can be separated into three different zones. These zones undergo different horizontal and vertical movement characteristics. The zone of overlap of the two longwall panels undergoes greater total subsidence compared to a single seam of equivalent thickness. The limit angle of the subsidence also differs at the two ends of the lower extraction as a result of the effects of the previously mined-out area above.