Key Strata Theory Research Articles

Using of Key Stratum Theory to Study the Structural Development of Roof Aquifer

Using of Key Stratum Theory to Study the Structural Development of Roof Aquifer

Coupled disaster-causing mechanisms of strata pressure behavior and abnormal gas emissions in underground coal extraction

The generalization and application of fully mechanized caving with super great mining height (SGMH) face a series of new issues, of which strata pressure behavior and abnormal gas emissions are the most prominent disasters and display a close relationship. The Tashan coalmine was chosen as the experiment base because of its SGMH and the overlying hard and thick main roof (HTMR). This coalmine had experienced many damaging hydraulic support and abnormal gas emission accidents induced by strata pressure behavior. The “Key Strata Theory” and the fracture failure process analysis model established by 3DEC were used to analyze the structural characteristics and fracture failure process of the HTMR. The geostress reduction and permeability enhancement redistribution of the adjacent coal measure strata were studied by the geostress and permeability distribution model established by FLAC 3D. The HTMR controlled the failure extent and the movement evolution of the entire overburden stratum. The HTMR acted as a large double-sided embedded rock beam during primary fracturing and a cantilever-articulated rock beam during periodic fracturing, and it generated a huge hanging space above the gob. The fully mechanized caving with SGMH caused large-scale disturbances and permeability enhancements in the adjacent rock and coal strata. Then, the substantial amounts of gas stored in the remaining coal, surrounding rock strata and adjacent coal seam rushed out and aggregated in the caved and fissured zones of the gob, forming a huge gas warehouse. The strata pressure behavior and abnormal gas emission disasters primarily occurred after the primary and periodic fracturing of the HTMR. These disasters are predominantly caused by the escaping of the abundant gas that existed in the gob during mine pressure behavior processes. When the ultimate broken span was reached, as the HTMR rotated and collapsed, the substantial amounts of gas that accumulated in the gob escaped into the working face, which easily caused abnormal gas emissions. Hydraulic presplitting and gas drainage technologies are proposed to solve these disasters.

Fracture failure analysis of hard–thick sandstone roof and its controlling effect on gas emission in underground ultra-thick coal extraction

In underground coal extraction of fully mechanized caving, the overlying hard–thick sandstone main roof could control the failure extent and the movement evolution of the entire overburden strata. The instantaneous failure of the hard–thick sandstone main roof possibly causes strata pressure behaviors, rock-bursts and abnormal gas emissions, which may result in equipment damages and casualties. Tashan coalmine was chosen as a field case study base because of its super great mining height (SGMH) and the overlying hard–thick sandstone roof (HTSR). This mine has experienced a great deal of damaging hydraulic support and abnormal gas emission accidents caused by strata pressure behavior. The fracture failure analysis was analyzed based on “Key Strata Theory” and numerical simulation results. The hard–thick sandstone main roof could perform as a very large double-sided embedded rock beam in the primary fracture and as a cantilever-articulated rock beam in periodic fracture, simultaneously generates a huge hanging space in the gob. The fracture failure of the hard–thick sandstone main roof causes a permeability enhancement in the adjacent rock-coal strata and near face coal seam. The substantial amounts of gas stored in the remaining coal, surrounding rock strata and adjacent coal seams rush out and aggregate in the caved and fissure zone of the gob, thereby forming a huge gas cloud. The disasters due to coupled strata pressure behavior and abnormal gas emissions, which primarily occurred after primary and periodic fracture failure, are predominantly caused by the instantly fracture of main roof. When the main roof reached the ultimate broken span and underwent, rotation and collapse, substantial amounts of gas accumulating in the gob escaped to the working face under the extrusion and impaction of the caving rock strata, which easily produced abnormal gas emissions, some of which exceeded the statutory limit. Shortening the length of the HTSR failure span using hydraulic presplitting and decreasing the gas content of the coal seam using gas drainage technology are recognized as two effective approaches to solve this issue.

Mechanisms of Mining Seismicity under Large Scale Exploitation with Multikey Strata

The dynamic disasters are aggravating with the increase of exploitation scale and intensity in Chinese coal mines, to further understand this problem, we studied the mechanisms of mining tremors induced by key strata movement and instability under large scale exploitation. First the mechanisms were categorized into two groups that is main key strata fracture and movement as well as subkey strata instability again under adjacent mining activities. Based on the key strata theory in ground control we revealed three basic mechanisms of key strata destabilization that are rotary and sliding of low subkey strata, shear sliding of the high subkey strata, and the main key strata rupture and cave at limit span, respectively. The microseismic observing systems were applied to monitor the mining tremor events and verify the theoretical analysis in different coal mines. The characteristics of time-space evolution of tremors show that low inferior key strata causing the most, followed by the high inferior key strata and the main key strata least, however the released energy was just opposite.

Research on the Simulation of the Roof Fracture about the Gob-Side Entry Retaining

The geological conditions of haulage roadway of the working face 1528 in GeQuan Coal Mine, which belongs to the JiZhong Energy Group CO., Ltd., and the research findings of Key Stratum Theory was taken into consideration, based on which established a mechanical model could reflect the basic features of rock stratum and made a profound investigation on the form and conditions of the fractures on main roofs. A systematic study on the stress, deformation, and damage characters of fractures on main roofs beyond a retained entry has been made by the laboratory test. The experiment shows that the deeper the fracture is in the rib, the greater the differences between dimension of compression and deformation will likely be, and the greater the influences on the stability of surrounding rock will it has. At the same time, smaller is the overhanging beam in mined-out area, more beneficial to roadway maintenance. On a certain deformation of main roof, it creates subsidence that is great difference in different position of a roadway. The subsidence near those mined-out areas is much more than those near the ribs. The application of a strong and effective measure could help cut the roof out and assure its stability.

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.

Analysis Floor Failure Characteristics of 7122 Working Face in the Qinan Coal Mine Based on the Plasticity Theory

In order to study the floor damage depth of 7122 working face in the Qinan Coal Mine, we used Key strata theory and elastic-plastic theory to analyze the activities mechanisms and characteristics of the surrounding rock. By theoretical calculation obtained floor strata failure depth 8.79m of 7122 Face Mining; The horizontal distance of from the greatest floor damage depth to the coal wall is about 5.71m; The goaf floor failure distance is about 25.44m. Combined with the results of plasticity theory, we had drawn floor damage feature maps of 7122 working face in Qinan Coal Mine.

Analysis of Numerical Simulation on Breaking Law of the Composite Key Stratum Affected by Coal Mining

Using FLCA3Dnumerical simulation software, analyzed the different models of different extraction from the composite key layer of breaking the law, respectively to the Composite Key Stratum and non-Composite Key Stratum .The results show that Composite Key Stratum breaking interval was significantly greater than The non-Composite Key Stratum, showing a more obvious composite effect, provides a more adequate proof to the Composite Key Stratum theory.

Strata movement controlling effect of waste and fly ash backfillings in fully mechanized coal mining with backfilling face

A fully mechanized coal mining with backfilling (FMCMB) provides advantages of safety and efficiency for coal mining under buildings, railways, and water bodies. According to the field geological conditions, we analyzed the controlling effect of strata movement by the waste and fly ash backfilling in FMCMB face. Based on the key strata theory, we established the equivalent mining thickness model, and analyzed the action of the bulk factor of backfilling body to the equivalent mining thickness. In addition, we numerically simulated the controlling function of the strata movement by backfilling bodies with different strength. And the numerical simulation result show that the deformation of stratum and the subsidence of surface can be controlled by FMCMB. The result provides references to the effective execution of fully mechanized coal mining with solid waste backfilling in goaf.

Research on the Height of Caving Zone and Roof Classification of Mining Whole Height at One Times in Thick Coal Seam

Research on the division of the overlying rock roof of stope has great significance. The existing classification method is based on the determination of existing mining height and loose coefficient , and such studies have been proved exist some limitations in the applications of thick coal seam full coal mining. Theoretical analysis and similarity simulation experiments show that during the mining all height at one times in thick coal seam, as the recovery room increase ,the thickness of immediate roof of the overlying strata which fall with the mining increased signifi-cantly, and the structure of the overlying critical layer to stabilize that is the layer of hypogyny basic roof gradually increased. Through theoretical analysis and summarizing similar simulation experiment phenomenon, based on the definition and characteristics of the immediate roof and main roof, with elastic thin mechanics and the key strata theory as the research foundation, doing scientific classification of mining face's roof in all height at one times in thick coal seam, and combined impact of all factors, which influence the breakage and caving of Basic Roof, to estab-lish a scientific judgment in the length of work face and the pressure of basic roof for practical production relations, provide certain theoretical basis.

The Evolution Rules of Overlying Strata Porosity Influenced by Mining Activation

Rock is a type of natural material containing microcracks. The evolution of mining-induced fracture field is one of the main problems of gas comprehensive control and rock mass seepage. The distribution of fissures and porosity in overlying strata under the mining influence were studied in this paper. The stratum subsidence was represented quantitatively using elastic thin plate theory and key strata theory. Based on the stratum subsidence, a program calculating porosity in overlying strata was developed based on VB. The results demonstrate that the porosity increase with face advances. The porosity becomes small with the decrease of strata depth, and the bigger separation is located in the strata below the key strata. The amount of the fissures in the middle part of overlying strata is more than that in two ends, and the porosity curves present symmetric arch shape in the initial stage. With face advances, the curves of porosity in the strata located the upper position change symmetric arch shape into saddle shape.

Study on Characteristic of Overburden Movement in Unsymmetrical Isolated Longwall Mining Using Microseismic Technique

Based on the key stratum theory, overlying strata structures above a typical unsymmetrical isolated working face (LW10302) was analyzed, and a microseismic monitoring was also applied to characterize the fracturing propagations associated with overburden movement in mining progress. The results show that the overlying strata above LW10302 can be divided into key strata of different grades, and the formed “O-X” fracturing structure have the main and inferior “O-X” ones. The spatial evolution of seismic events demonstrated that seismic activities fits very well with the overburden fracturing patterns and stress manifestation around the longwall face. In the mining process, most of the events located within the surrounding strata of LW10301 and 10302 while low energy events distributed mainly in multiple roof and floor strata, and the strong tremors occurred almost within the super-thick primary key strata and appeared to be related to shear fracturing of large-scale overburden movement. Additionally, seismic signals corresponding to different failure mechanisms show different characteristics in waveform features. The study in this paper indicates that microseismic monitoring can provide invaluable information to characterize the mining-induced seismicity and reveal the failure patterns within strata associated with mining, which will greatly benefit the alleviation and prevention of rock burst hazards in mines

Discrimination conditions and process of water-resistant key strata

Water-preservation mining is one of the most important parts of the ‘Green Mining’ technology system, which can realize the effective regulation of groundwater resources by controlling strata movement, changing passive prevention and governance of water disasters to active conservation and utilization of groundwater resources and thus obtaining coal and water simultaneously in mining. The concept of water-resistant key strata further enriches the content of the key stratum theory and provides a theoretical basis for water-preservation mining. In order to realize the idea of water-resistant key strata as a guideline in the design of water-preservation mining and engineering applications, the conditions for discrimination in the process of water-resistant key strata, we have presented a mechanical model, as well as its corresponding computer program, based on a large number of theoretical analyses and field measurements, as well as on a comprehensive consideration of the position, structural stability and seepage stability of key strata. Practical engineering applications indicate that this discrimination method and its corresponding computer program on water-resistant key strata are accurate and reliable and can satisfy the actual design needs of water-preservation mining and thus have instructional importance for water-preservation mining in mining areas lacking water.