Longwall Mining Face Research Articles

A Geometry Surveying Model and Instrument of a Scraper Conveyor in Unmanned Longwall Mining Faces

To maintain the straightness of an unmanned longwall mining face, a track geometry surveying model of the scraper conveyor was constructed based on the position of the shearer. A surveying instrument was developed employing an inertial measurement unit and axial encoder. Surveying tests were conducted using a longwall mining face mock-up, and the 3-D accuracy of the surveying instrument based on the mean radial spherical error and the spherical error probable (SEP) radius was determined to be 20.78 mm and 16.57 mm, respectively. The accuracy of the surveying system satisfied the requirements of a longwall mining face.

Investigating the shape and size distribution of dust in mechanized longwall method in Tabas coal mine

ABSTRACTWhen the average size of coal dust particles is reduced, risk of explosion as well as probability of emergence of numerous diseases is increased. To find the rate of negative effects in mechanized longwall mining face, the amount of spread coal dust and the scattering manner of particles in each part of the face and tail gate are investigated. Accordingly, 19 samples were taken from the face and tail gate of Tabas mechanized coal mine. Then, using the Zetasizer device and TEM (Transmission electron microscopy), the samples were analyzed and their shapes were determined, respectively. Results showed that shape of the particles was almost spherical and their size followed Rosin–Rammler distribution in the tail gate and along the longwall. In the face, since there was no opportunity for the deposition of coarse particles, some disturbances were observed in their distribution. At the beginning of the tail gate and close to the face, as the height increased, coarser particles were observed, which deposited after passing through the tunnel and getting away from the face. Considering these results, the aggregation and sequestration regions of dust could be found and the ventilation system can be modified. Also, more suitable solutions can be adopted for dust control or more advanced equipment can be proposed for miners in terms of confronting these particles in their working environment.

Studying Hygienic Status of Mechanized Long Wall Face from the Perspective of the Existing Dust Containing Silica in Tabas Coal Mine

In order to consider the quantity of the existing negative effect in mechanized longwall mining face in Tabas coal mine, the concentration of the available coal dust and amount of its toxicity in every part of the mining face and tail gate were investigated and compared with the permissible amount. After sampling and preparing dust, its concentration was determined by gravimetric method. Then, using Zetasizer, the particle size distributions were determined and also the available amount of silica in the samples was found using SEM and XRF device. The results of the elemental analysis of dust in the mine showed that the weight percentage of silica in dust was between 27.9 and 26.7 %. The correction of the permissible dust concentration from the perspective of silica was done and the result was reported as 0.36 mg/m3. Computational fluid dynamics can be used to determine dust accumulation zones and the zones with high dust concentration. Finally, D50 of the samples showed that the existing toxicity in dust can be transferred to dozens of meters far from the source (shearer).

Stress Changes and Deformation Monitoring of Longwall Coal Pillars Located in Weak Ground

Coal pillar stability is strongly influenced by the site-specific geological and geotechnical conditions. Many geological structures such as faults, joints, or rock intrusions can be detrimental to mining operations. In order to evaluate the performance of coal pillars under weak roof degraded by igneous rock intrusion, stress and deformation monitoring was conducted in the affected tailgate areas of Nos. 8208 and 8210 longwalls in Tashan coal mine, Shanxi Province, China. The measurements in the 8208 longwall tailgate showed that the mining-induced stresses in 38-m-wide coal chain pillars under the overburden depth of 300–500 m started to increase at about 100 m ahead of the 8208 longwall working face and reached its peak level at approximately 50 m ahead of the longwall face. The peak stress of 9.16 MPa occurred at the depth of 8–9 m into the pillar from the tailgate side wall. In comparison, disturbance of the headgate block pillar area was negligible, indicating the difference of abutment pressure distribution between the tailgate and headgate sites where the adjacent unmined longwall block carried most of the overburden load. However, when the longwall face passed the headgate monitoring site by 360–379 m, the pillar stress increased to a peak value of 21.4 MPa at the pillar depth of 13 m from the gob side mainly due to stress redistribution in the chain pillar. In contrast to the headgate, at the tailgate side, the adjacent goaf was the dominant triggering factor for high stress concentrations in the chain pillar. Convergence measurements in the tailgate during longwall mining further indicated the evolution characteristics of coal pillar deformation, clearly showing that the gateroad deformation is mainly induced by the longwall extraction it serves. When predicting the future pillar loads from the monitored data, two stress peaks appeared across the 38-m-wide tailgate coal pillar, which are separated by the lower stress area within the pillar center. This 10-m-wide elastic pillar core area indicates that the coal pillar may be narrowed to 30 m to improve coal recovery. The measurements further indicate that, if the headgate of the next panel can be developed after the adjacent gob becomes stable, the coal pillar width may be further reduced to 20 m. This study is applicable for the chain pillar design, the gateroad secondary support design ahead of the longwall mining face, and the gateroad preparation of the next longwall panel under similar geological and geotechnical conditions.

Bearing characteristics of coal pillars based on modified limit equilibrium theory

There are two states for the coal-mass on the goaf-side which is in stress equilibrium: the state of limit equilibrium (the bearing stress in the coal-mass equals its ultimate bearing stress) and the state of non-ultimate equilibrium (the bearing stress in the coal-mass is less than its ultimate bearing stress). To analyze the bearing characteristics of a coal pillar in the state of limit equilibrium and guide the design of pillar width, we established a mechanical analytical model of the non-ultimate equilibrium zone in the coal-mass on the goaf-side combined with the limit equilibrium theory as well as adopting the methods of theory analysis and mechanical analysis based on the assumption of a state of non-ultimate equilibrium. The width correction coefficient of the limit equilibrium zone has been given. The influence of mining depth, stress concentration coefficient of the surrounding rock, the non-limit strength of the coal-mass and stability of the coal rock interface has been studied. On this basis, we have confirmed that when the width of a longwall mining face roadway protection coal pillar is between 11.6m and 13.16m in No. 4 coal seam of Xinrui coal mine in Lvliang in Shanxi province the elastic core region in the coal pillar can be assured and the roadway will be located in the area of lower stress which is outside the peak stress. So the revised width of the limit equilibrium zone is more practical.

Research on Stability of Overburden Structure around Longwall Mining Face in Steeply Dipping Seam Group

The steeply dipping seam group is defined by the two or more coal seams ,a pitch between 35°~55°. Using masonry beam theory, longitudinal bending theory and “R-S-F” dynamics control theory built a lower area overburden structure mode. Analysed the stability of low position coal seam. The balance requirement and the strength of the structure which is affected by the caving rock and lower coal roof were given. It easily generates two lower position steps rock structure in inclination along working face. Regular breaking of the second structure is the main reason leads to the imbalance of the structure between upper coal pillar and upper coal mining face.The interaction among multiple coal seam panels and overburden structures is the main reason that causes the rock disaster, the unbalance of the lower area may lead to pushing accident, the imbalance of the upper area can generate shock pressure.

Mechanical Analysis of Shell Structure in Longwall Mining Face of Steeply Dipping Seam

Taking the longwall mining face area in steeply dipping seam as the research object, the stability of asymmetrical rock mass structure is analyzed using shell theory and masonry beam theory. The results show that asymmetrical shell structure is formed at the top strata of mining stope and its magnitude is mainly determined by the mechanical characteristic of overburden strata of working face, mining space size and so on. Localized instability of the shell structure is the main factor that leads to the overall structural instability in the stope. The instability modes includes: the shell roof instability dominated by tensile failure, shell shoulder instability initialized by compression-shear failure, and shell bases composite instability occurs in tensile and compression-shear modes. The shell structure stability dedicates rational strata control measures to choose, and the caving height of overburden strata is the key for shell structure instability.

Research of Large-Area Hard Roof Caving Prevention and Control System in Shortwall Mining Face

In the mining process, if the roof is very hard, it will hang in a large area in the gob. The hanging roof may cave suddenly, which will crash the mining face and generate hurricanes so as to cause casualties and equipment damage. The hard roof control technology has mainly been used in the longwall mining face in coal mines for a long time, and made little use in shortwall mining face. The factors of roof caving is analyzed in shortwall mining area, which mainly consists of the coal pillar size and layout, the length of coal face, roof thickness and strength, the overburden load, the supporting conditions of roof and so on and the relationship of the factor and the limit caving area is studied. And then a new integrated control technology system is put forward, which includes: support and isolation by coal pillars, roof caving pressure relief, monitoring and early warning of roof fall, gob sealing by the stopping walls, and the principle of different technology and the parameter selection method is claimed. This technology is applied in Shangwan Mine of Shendong Group in China and the results shows that the hard roof caves naturally in small area, the hidden danger of large-area roof caving is eliminated completely, so the coal production is guaranteed to be safe.

F-structure model of overlying strata for dynamic disaster prevention in coal mine

The rupture and movement scope of overlying strata upon the longwall mining face increased sharply as the exploitation scale and degree growing recently, and the spatial structure formed by fractured strata became much more complex. The overlying strata above the working face and adjacent gobs would affect each other and move cooperatively because small pillar can hardly separate the connection of overlying strata between two workfaces, which leads to mining seismicity in the gob and induces rockburst disaster that named spatial structure instability rockburst in this paper. Based on the key stratum theory, the F-structure model was established to describe the overlying strata characteristic and rockburst mechanism of workface with one side of gob and the other side un-mined solid coal seam. The results show that F-structure in the gob will re-active and loss stability under the influence of neighboring mining, and fracture and shear slipping in the process of instability is the mechanism of the seismicity in the gob. The F-structure was divided into two categories that short-arm F and long-arm F structure based on the state of strata above the gob. We studied the underground pressure rules of different F-structure and instability mechanism, thus provide the guide for prevention and control of the F-structure spatial instability rockburst. The micro-seismic system is used for on-site monitoring and researching the distribution rules of seismic events, the results confirmed the existence and correct of F-spatial structure. At last specialized methods for prevention seismicity and rockburst induced by F-structure instability are proposed and applied in Huating Coal Mine.

Parameter Design and Application of Hydraulic Support with Hanging Chain and Splicing Beam in Shallow Seams

In view of actual situation of serious resources waste and less than 30% of face recovery ratio to shallow seams with existing features of harder roof at the process of room and pillar mining in northern Shaanxi province in China, reasonable support strength has been studied in both cases of normal mining and force caving for improved longwall mechanized mining face, and obtained reasonable working support resistance at the condition of force caving which was better than normal mining. Based on this condition, Shandong Xin Mei machinery Co, Ltd. has developed hydraulic support with hanging chain and splicing beam which was suitable to existing features of shallow seams. The support had advantages with flexible connection between supports to ensure that all pillars reached rated setting load, sub-size top beam to resolve "protection" and "cutting" dilemma for the roof, adjusting and locking support, etc. The company also has given concrete support parameters. By the field industrial test verification, the developed new support fully adapted strata behavior at the process of force caving with existing features of shallow depth and harder roof. The research has provided a useful reference for other coal mines with similar geological features.

A Method to Relate the Affecting Parameters and Estimate Dilution in Coal Mines

This study provides an overview of the various issues influencing Out-of-Seam Dilution (OSD) in longwall mining method. The collected data has been statistically analyzed to examine the effect of the some factors causing OSD in front of the longwall mining face. Multiple parameter regression analysis was conducted on affecting parameters and the OSD. The SPSS (Statistics Package for Social Sciences) for Windows software package was used for the statistics analysis. Finally, a relationship between affecting parameters and the OSD is established by using the multiple parameter regression results. Results of this study have revealed that depth of seam, dip of seam, roof quality and variation in seam thickness are the most important influence factors for OSD. The proposed method may be utilized for the estimation of OSD for similar mines since, it was based on actual collected data from the coal mines.

Fluid permeability of sedimentary rocks in a complete stress–strain process

The permeability of sedimentary rocks during triaxial compression tests was investigated to relate it to the complete strain–stress process. It was found that the permeability was not constant, but varied with the stress and strain states in the rocks. Prior to the peak strength, the permeability decreases with increasing load. A dramatic increase in permeability occurs during the strain softening period. In the present study, in situ measurements of fluid flow and pressure in floor strata was carried out in a double longwall mining face in the Yangzhuang colliery. These measurements show that both the strata pressure and the position with respect to the mining face influence the hydrogeologic properties. The permeability increased in the floor strata behind the mining face because those mining induced fractures opened as the strata pressure decreased. To better understand this change in hydraulic behavior around the mining faces, 3-D numerical modeling was carried out. The model provides the general picture of the stress distribution and failure zone both in the floor and roof strata. The field and model results demonstrate the importance of changes in the stress and strain states on the hydrogeology of a site.

Review and interpretation of primary floor failure mechanism at a longwall coal mining face based on numerical analysis

This paper describes the conceptual model of primary rock failure mechanisms in the floor ahead of the longwall coal mining face based on underground observations and numerical modelling. During longwall mining, the stress fields and displacements of strata are constantly changing. High stress concentrations ahead of the longwall face can initiate fractures in the rock floor that can, under unfavourable stress conditions, develop into secondary floor failure, where large displacements of strata can occur. Severely broken floor strata or floor heave can seriously interfere with mining operations. Underground observations indicate that the sub-vertical shear fractures and the shear failure along the bedding planes in stratified rock are the most common fracture types observed in the rock floor below the longwall face. To explain how these fractures occur, basic principles of rock failure mechanism are applied to an overstressed rock subject to stress concentrations typically found at the longwall face. The computational simulation of the progressive longwall mining operations was undertaken in stratified rock to model complex stress distributions ahead of the longwall face and to simulate the rock failure in the mining floor. The model indicates that the shear fractures and the bedding plane failure dominate the floor failure ahead of the moving longwall face, as was observed underground.

Floor failure analysis at a longwall mining face based on the multiple sliding block model

This paper presents an analytical model of a floor failure at a longwall coal mining face based on the multiple sliding block model. During longwall mining, stresses and displacements of strata are constantly changing. High stress concentrations at the coal face can exceed the rock strength and initiate fractures in the strata that can, under unfavourable conditions, lead to large floor displacements and disruption of mining operations. Underground observations of the rock floor and the computational modelling of the longwall face indicate that two types of fracture dominate floor failure. Extensive lateral fracturing often develops along the numerous weak bedding planes that are typically present in the sedimentary strata while sub-vertical fractures form in response to changing stress abutments ahead of the longwall coal face. The fractures that initially develop ahead of the longwall face are subject to a “secondary” movement when exposed ahead of the longwall supports. In response to the stress relief as the coal is mined from above, strata move towards the opening and bending of the floor occurs. If fractures in the floor exist, the floor blocks will displace in response to the floor movement and interact at the fractured surfaces. This analysis attempts to explain how the stress distribution develops within the broken floor during an active movement of floor strata leading to high stress concentrations at the floor level. The analytical formulation in this paper is supplemented by numerical modelling, and results are presented to verify that the analytical solutions are in accordance with the numerical predictions.