Mechanisms behind strong strata behaviour in high longwall mining face-ends under shallow covers

Abstract Safe and efficient mining of shallow coal seams relies on the understanding and effective control of strata behaviour. Field measurements, theoretical analysis and numerical simulations are presented in this study to investigate the mechanism behind abnormal strata behaviour, such as roof collapse and severe roadway deformation, that occurs in high longwall face-ends under shallow cover. We observed that coal pillars with two sides being mined out become unstable when the cover depth exceeds a certain value. The instability of the coal pillar can alter the fracture line of the overlying strata, triggering a reversed rotation of the ‘curved triangle blocks’ that form after the breakage of the overlying main roof. The revolving blocks apply stress on the roof strata directly above the longwall face-end, resulting in roof collapse. The collapse of both the coal pillars and the roof also leads to the advancement and increase of the overlying abutment pressure, which further causes severe roadway deformation in front of the working face. The strong strata behaviour that occurs in high longwall face-ends with shallow cover is presented in this study and countermeasures are proposed, such as widening or strengthening the coal pillar, or implementing destress blasting. The countermeasures we proposed and the results of our analyses may facilitate the safe mining of shallow coal seams with similar problems in the future, and may improve the safety and efficient working of coal mines.

Download Full-text

Automatic Roadway Backfilling of Caving Gangue for Cutting Roofs by Combined Support on Gob-Side Entry Retaining with No-Pillars: A Case Study

Advances in Materials Science and Engineering ◽

10.1155/2019/8736103 ◽

2019 ◽

Vol 2019 ◽

pp. 1-13

Author(s):

Shengrong Xie ◽

Xiaoyu Wu ◽

Dongdong Chen ◽

Yaohui Sun ◽

Junchao Zeng ◽

...

Keyword(s):

Longwall Mining ◽

Control Process ◽

Coal Pillar ◽

Main Roof ◽

Control Effect ◽

Mining Technology ◽

Thick Coal Seam ◽

Immediate Roof ◽

Seam Mining ◽

And Control

Automatic roadways on gob-side entry retaining with no-pillars are used for longwall mining technology. The mining technology with no-pillars can recover coal pillar resources and reduce the amount and cost of roadway excavations. Automatic roadway technology for cutting roofs by combined support on gob-side entry retaining with no-pillars is adopted for the condition of thick immediate roof and medium-thick coal seam mining, cutting off the immediate roof and the main roof on the gob by combined support. The fractured roof forms gangue blocks to fill the gob and loads the overlying strata. The gangue control system is placed on the roadside, which controls the caving gangue to form a gangue rib. In this paper, the viewpoints and key technologies (the roof-cutting technology, the reinforcement and support technology, the gangue rib control technology, and the auxiliary support technology) of automatic roadway technology for cutting roofs by combined support on the gob-side entry retaining with no-pillars are introduced. Furthermore, the formation and control process are explained. The numerical simulation is used to simulate and analyze the roof hanging and the roof cutting structures. In addition, a field engineering test is performed. The field test shows that automatic roadway technology for cutting roofs by combined support on gob-side entry retaining with no-pillars is feasible. This process uses construction techniques and technologies to meet on-site production needs. The combined support has high resistance strength and is shrinkable. In engineering applications, the combined support has a low damage rate. The deformation of the automatic roadway with gob-side entry retaining is small, and the control effect is significant.

Download Full-text

Research of New Filling and Mining Technology Using Waste Rocks in Steep Coal Seams

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.204-208.1395 ◽

2012 ◽

Vol 204-208 ◽

pp. 1395-1400

Author(s):

Chuan Wei Zang ◽

Chuan Le Ma ◽

Xue An Zhuang

Keyword(s):

Coal Mining ◽

Coal Pillar ◽

Recovery Ratio ◽

Mining Method ◽

Coal Seams ◽

Coal Recovery ◽

Waste Rocks ◽

Filling Method ◽

Gob Area ◽

Coal Pillars

During the extraction of steeply inclined coal seams, the coal recovery ratio is low be-cause of the coal pillar loss and the production of waste rock is high due to lots of rock roadways which causes serious environmental pollution. This status is conflicted with the strategy of Clean Coal Mining and Green Coal Mining in China, so it is necessary to develop new coal mining method. In this paper, Downward Stratified Gangue Self-filling Method on the Flexible Shield (DSGSMFS) is put forward first. It means that the coal face is lain horizontally and advances along the dip; the flexible shield is used to separate the gob area; the waste rocks are self-filled downward to the top the shield; the coal is broken by drilling and blasting method under the shield, and the broken coal is transported by the electrical winch and the scraping mucker; the flexible shield moves downward automatically by the weight of itself and waste rocks. Field test shows that the strata displacement is effectively controlled by using DSGSMFS, so some coal pillars are recovered; as a result the problem of large quantity gangue and low coal recovery ratio in steep coal seam is solved. DSGSMFS is proved to be a new hopeful and effective coal green mining method.

Download Full-text

Modeling the Spatial and Temporal Evolution of Stress during Multiworking Face Mining in Close Distance Coal Seams

Advances in Civil Engineering ◽

10.1155/2021/5624972 ◽

2021 ◽

Vol 2021 ◽

pp. 1-11

Author(s):

Yong Zhang ◽

Jinkun Yang ◽

Jiaxuan Zhang ◽

Xiaoming Sun ◽

Chen Chen ◽

...

Keyword(s):

Numerical Simulation ◽

Stress Distribution ◽

Temporal Evolution ◽

Coal Pillar ◽

Coal Seams ◽

Geological Conditions ◽

Coal Pillars ◽

The Stability ◽

Close Distance ◽

Close Distance Coal Seams

Mining in close distance coal seams (CDCSs) is frequently associated with engineering disasters because of the complicated nature of stress distribution within CDCSs. In order to establish a layout of a roadway to minimize the occurrence of disasters associated with mining CDCS, here the spatial and temporal evolution of stress distribution during the multiworking face mining of a CDCS was explored through numerical simulation based on the engineering and geological conditions of the Nantun Coal Mine. The numerical simulation results indicate that, after the extraction of adjacent multiple working faces, the spatial distribution of stress can be characterized with areas of increased, reduced, and intact stress. The superposed stress of inclined seams that are very close to each other propagates through coal pillars in the bottom floor, and this propagation follows neither the line along the axis of the coal pillar nor the line perpendicular to the direction of the floor. It instead propagates along a line angled with the axis of the coal pillar. The roadway can be arranged in the area with reduced stress, to improve its the stability. Based on the computed spatial and temporal evolution of stress, an optimized layout of roadway was proposed. This layout features a reasonable interval between the mining roadway and a minimal proportion of increased stress areas along the mining roadway and is aligned with geological structures.

Download Full-text

Research on Failure Mechanism and Strengthening of Broken Roadway Affected by Upper Coal Pillar

Advances in Civil Engineering ◽

10.1155/2019/8132817 ◽

2019 ◽

Vol 2019 ◽

pp. 1-13 ◽

Cited By ~ 2

Author(s):

Fulian He ◽

Zheng Zheng ◽

Hengzhong Zhu ◽

Bo Yang

Keyword(s):

Stress Distribution ◽

Failure Mechanism ◽

Coal Pillar ◽

Control Area ◽

Surrounding Rock ◽

Horizontal Line ◽

Protective Function ◽

Working Face ◽

Coal Pillars ◽

Roadway Deformation

The principal stress difference is introduced as a new evaluation index in order to better understand the failure mechanism of roadways affected by upper coal pillars and characterize failure of rock mass. Compared with traditional methods, it facilitates quantitative analysis. Moreover, we combine the semiplane theory and we obtain the stress distribution on the coal pillar’s bedrock and the strengthening control area from the “change point” position along a 21 m horizontal line. The influence of multiple stresses induced from mining on a roadway is analyzed. It is found that rock failure is most likely while mining the 051606 working face, followed by mining the 051604 working face, and the stress influence on the upper pillar has the lowest failure probability. In addition, based on the asymmetry of the surrounding rock stress distribution, this study proposes strengthening control technology of surrounding rock on the basis of a highly stressed bolting support and anchor cable, adding to the steel ladder beam, steel mesh, and shed support’s protective function to the roadway’s roof and ribs. Finally, through field observations, it is concluded that the roadway deformation is within the controllable range.

Download Full-text

Accident Analysis in Relation to Main Roof Structure When Longwall Face Advances toward a Roadway: A Case Study

Advances in Civil Engineering ◽

10.1155/2018/3810315 ◽

2018 ◽

Vol 2018 ◽

pp. 1-11 ◽

Cited By ~ 4

Author(s):

Chang Liu ◽

Zengqiang Yang ◽

Peilin Gong ◽

Kai Wang ◽

Xiaoqiang Zhang ◽

...

Keyword(s):

Sharp Increase ◽

Coal Pillar ◽

Main Roof ◽

Mechanical Analysis ◽

Longwall Face ◽

Good Effect ◽

Key Factors ◽

Roof Structure ◽

Similar Simulation Experiment ◽

The Stability

Practices show that hydraulic supports crushing accidents or roadway supports failure often take place when a longwall face advances toward an abandoned roadway or a predriven equipment recovery room. Therefore, a 2D similar simulation experiment is conducted to reveal the loading mechanism. The result shows that when the workface advances close to roadways, the main roof breaks ahead of the workface and leads to instability of higher strata. These two changes induce a sharp increase of the load on supports and lead to an accident. Thus, more attention should be paid to the advanced fracture. Therefore, mechanical analysis is used to explain the advanced fracture. Results show that the failure of coal pillar being excavated induces a sharp increase in the main roof’s hanging length. Once the hanging length reaches the limit, the advanced fracture takes place. Therefore, the stability of the coal pillar and the hanging length of roof strata are two key factors that may induce an accident. To prevent the a similar supports crushing accident, the partial backfilled technology which partly backfills the abandoned roadway in height and length to maintain the stability of the coal pillar is put forward and put into practice. The field test shows a good effect.

Download Full-text

Improvement of the Loading Capacity of Narrow Coal Pillars and Control Roadway Deformation in the Longwall Mining System. A Case Study at Khe Cham Coal Mine (Vietnam)

Inżynieria Mineralna ◽

10.29227/im-2020-02-15 ◽

2020 ◽

Vol 1 (2) ◽

Author(s):

Le QUANG PHUC ◽

V. P. ZUBOV ◽

Phung MANH DAC

Keyword(s):

Longwall Mining ◽

Coal Pillar ◽

Rock Bolt ◽

Mining System ◽

Pillar Stability ◽

Roadway Support ◽

Coal Pillars ◽

The Stability ◽

Safe Condition ◽

Cable Bolt

Currently, the application of coal pillars to protect an adjacent roadway is a common method in Vietnam when exploiting according to the longwall system. Therefore, the width of a coal pillar is an important issue for the stability of a roadway. In order to reduce coal loss in these coal pillars, they tend to be designed in a narrow coal pillar style but still have to ensure that the adjacent roadway can meet safe coal production conditions. The stability of roadways and coal pillars is related to many factors such as technical mechanical characteristics, physical and mechanical properties of coal, stress environment and support methods. The bearing structure of the coal pillar and the around rock a roadway is analyzed and it has been shown that enhancing roadway support and improving the carrying capacity of coal pillars can control the deformation of the surrounding rock. A study related to the stability and safety of roadways and small coal pillars in the longwall mining system has been carried out. Stabilization factors have been considered, especially the state of stress in the coal pillars and the deformation of the roadway. By applying the numerical simulation method, the stress of the coal pillar and the deformation of the adjacent roadway under different supporting solutions were analyzed and evaluated. By using this method, the rock bolt roadway support solution combined with the long cable bolt in the roadway roof and the coal pillar was selected in the safe condition of the mining process. Because cable bolt can improve the flexibility of the coal pillar such as: reducing the size of the plastic area on both sides of the pillar; enhancing coal pillar stability in the core area by providing great drag and tensile for coal pillars; contributing to improving the anchor point fixation of rock bolt. The conclusions obtained may provide a certain reference parameters to improve mining efficiency and labor safety in underground coal mines.

Download Full-text

Dynamic Evolution Law and Width Determination of Section Coal Pillars in Deep Mining Height Working Face

10.21203/rs.3.rs-442910/v1 ◽

2021 ◽

Author(s):

Lei Zhaoyuan ◽

Cui Feng ◽

Liu Jianwei ◽

Lai Xingping ◽

Yi Ruiqiang

Keyword(s):

Dynamic Instability ◽

Coal Pillar ◽

Field Measurements ◽

Deep Mining ◽

Evolution Law ◽

Working Face ◽

Large Mining Height ◽

Coal Pillars ◽

Lower Corner ◽

Mining Height

Abstract The coal column undergoes three types of force evolution from the formation to the end of mining. This paper takes large mining height working face at No.2 Coal Mine as example to study the ways to avoid dynamic instability of the coal column triggered by the deep mining. By means of geological survey, theoretical analysis, numerical calculation and field verification, the load processes under the three stress stage are proposed, and the evolution law of the coal column is analyzed. The study shows that the depth, large mining height working face, coal pillar force and size altogether determine the damage characteristics of the coal pillar. With the combination of Flac3D and 3DEC, it can be analyzed that the plastic failure and displacement characteristics of the 35m coal column under the action of secondary dynamic load coincide. The perturbation stress distribution is stable, which finally determines the reasonable width of the 35m coal column. Field measurements show that the top and gang of the 35m coal column undergo three kinds of displacement characteristics. The lower part is more stable. The top plate of the upper and lower corner completely collapsed in the emptying area, which can play a good supporting role.

Download Full-text

Mechanisms of Floor Heave in Roadways Adjacent to a Goaf Caused by the Fracturing of a Competent Roof and Controlling Technology

Shock and Vibration ◽

10.1155/2020/5632943 ◽

2020 ◽

Vol 2020 ◽

pp. 1-17 ◽

Cited By ~ 1

Author(s):

Shuaigang Liu ◽

Jianbiao Bai ◽

Xiangyu Wang ◽

Bowen Wu ◽

Wenda Wu

Keyword(s):

Longwall Mining ◽

Coal Pillar ◽

Field Measurements ◽

Field Studies ◽

Simulation Methods ◽

Mining Equipment ◽

Floor Heave ◽

Continuous Mining ◽

Numerical Simulation Methods ◽

Three Stages

In traditional sequential single-wing mining practices, one-entry longwall mining systems make it challenging to efficiently and smoothly transfer mining equipment during a continuous mining sequence. In two-entry longwall systems, the headgate of the current panel and the tailgate of the next panel are excavated parallel to one another, effectively creating space for the transfer of mining equipment. The tailgate of the panel, however, is subjected to high-mining-induced stresses, causing severe floor heave, which seriously affects the efficiency of coal production. In this paper, field measurements and numerical simulation methods are used to reveal the mechanism of floor heave induced by the rupture and instability of a competent roof. The results show that the positional relationship between the adjacent tailgate and the longwall face is divided into three stages. Throughout the three stages, the area in which the coal pillar is not horizontally displaced moves from the center of the pillar to the goaf, and the area of peak vertical stress within the coal pillar shifts from the center of the pillar to the side nearest to the tailgate. Field studies suggest that the proposed technologies can effectively control floor heave in the tailgates of two-entry longwall mining systems.

Download Full-text

Geophysical Evaluation of Effectiveness of Blasting for Roof Caving During Longwall Mining of Coal Seam

Pure and Applied Geophysics ◽

10.1007/s00024-019-02321-1 ◽

2019 ◽

Vol 177 (2) ◽

pp. 905-917 ◽

Cited By ~ 4

Author(s):

Łukasz Wojtecki ◽

Petr Konicek ◽

Maciej J. Mendecki ◽

Iwona Gołda ◽

Wacław M. Zuberek

Keyword(s):

Coal Seam ◽

Stress Drop ◽

Longwall Mining ◽

High Energy ◽

Longwall Face ◽

Seismic Moment Tensor ◽

Coal Seams ◽

Rockburst Hazard ◽

Roof Caving ◽

Roof Rocks

Abstract Deep longwall mining of coal seams is made in the Upper Silesian Coal Basin (USCB) under complicated and mostly unfavourable geological and mining conditions. Usually, it is correlated with rockburst hazard mostly at a high level. One of the geological factors affecting the state of rockburst hazard is the presence of competent rocks in the roof of extracted coal seams, so rock falling behind the longwall face does not occur, and hanging-up of roof rocks remains. The long-lasting absence of caving may lead to an occurrence of high-energy tremor in the vicinity of the longwall face. Roof caving behind the longwall face may be forced by blasting. The column of explosives is then located in blastholes drilled in layers of roof rocks, e.g. sandstones behind the longwall face. In this article, a characterization of tremors initiated by blasts for roof caving during underground extraction of coal seam no. 507 in one of the collieries in the USCB has been made using three independent methods. By the basic seismic effect method, the effectiveness of blasting is evaluated according to the seismic energy of incited tremors and mass of explosives used. According to this method, selected blasts gave extremely good or excellent effect. An inversion of the seismic moment tensor enables determining the processes happening in the source of tremors. In the foci of provoked tremors the slip mechanism dominated or was clearly distinguished. The expected explosion had lesser significance or was not present. By the seismic source parameters analysis, among other things, an estimation of the stress drop in the focus or its size may be determined. The stress drop in the foci of provoked tremors was in the order of 105 Pa and the source radius, according to the Brune’s model, varied from 44.3 to 64.5 m. The results of the three mentioned methods were compared with each other and observations in situ. In all cases the roof falling was forced.

Download Full-text

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

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.724.100 ◽

2015 ◽

Vol 724 ◽

pp. 100-110

Author(s):

Shi Guang Ren ◽

Yong Ping Wu ◽

Jian Hui Yin

Keyword(s):

Coal Seam ◽

Longwall Mining ◽

Coal Pillar ◽

Beam Theory ◽

Coal Seams ◽

Working Face ◽

Rock Structure ◽

Longitudinal Bending ◽

Mining Face ◽

The Stability

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.

Download Full-text