Mining-induced Stress Evolution Research Articles

Mining-induced stress evolution mechanism and control technology of working face in deep coal seam bifurcation and merging area: a case study

Mining-induced stress evolution mechanism and control technology of working face in deep coal seam bifurcation and merging area: a case study

Coal burst and mining-induced stress evolution in a deep isolated main entry area – A case study

Main entries in some underground coal mines are tunneled into coal seams. During coal mining, main entries usually have a one-side gob or two-side gobs. Serious coal bursts have occurred in the isolated main entry area (IMEA). In this study, coal bursts in the IMEA are investigated. A large-scale numerical model is built to depict the static-dynamic stress evolution in the IMEA. Likewise, the study discusses the coal burst mechanism in the IMEA and proposes the measures to decrease the coal burst risk. During tunnelling of the main entry, coal bursts in the IMEA occur in the intersection of roadways. During coal mining, coal bursts mainly arise in the area between the gobs on both sides. Coal bursts in the IMEA are induced by high static and low dynamic stress. When coal is mined, the abutment stress around the gobs on both sides transfers to deeper regions and intersects in the IMEA, leading to higher static stress in coal mass. The seismic-based dynamic stress negatively correlates to the distance between the seismic sources and the IMEA. During coal mining, dynamic stress fluctuates, then the amplitude decreases and finally remains at a low value and exerts slight dynamic disturbance on the IMEA. When static stress is low, only higher dynamic stress can lead to coal bursts. However, when static stress is high, low dynamic stress can induce coal bursts. Coal bursts in the IMEA are caused by the superposition of high static stress in coal seam and low seismic-based dynamic stress disturbance from longwall panels. Roof blasting in the IMEA and roof pre-splitting blasting in longwall panels are proposed and implemented on site. Field monitoring results indicate that the de-stressing measures significantly decrease the coal burst risk in the IMEA.

Breaking and mining-induced stress evolution of overlying strata in the working face of a steeply dipping coal seam

The breaking features and stress distribution of overlying strata in a steeply dipping coal seam (SDCS) differ significantly from those in a near-horizontal one. In this study, the laws governing the evolution of vertical stress release and shear stress concentration in the overlying strata of coal seams with different dip angles are derived via numerical simulation, rock mechanics tests, acoustic emissions, and field measurements. Thus, the stress-driven dynamic evolution of the overlying strata structure, in which a shear stress arch forms, is determined. Upon breaking the lower part of the overlying strata, the shear stress transfers rapidly to the upper part of the working face. The damaged zone of the overlying strata migrates upward along the dip direction of the working face. The gangue in the lower part of the working face is compacted, leading to an increase in vertical stress. As the dip angle of the coal seam increases, the overlying strata fail suddenly under the action of shear stresses. Finally, the behavioral response of the overlying strata driven by shear stresses in the longwall working face of an SDCS is identified and analyzed in detail. The present research findings reveal the laws governing the behavior of mine pressure in the working face of an SDCS, which in turn can be used to establish the respective on-site guidance.

Mechanical Behavior and Permeability Evolution of Coal under Different Mining-Induced Stress Conditions and Gas Pressures

The gas permeability and mechanical properties of coal, which are seriously influenced by mining-induced stress evolution and gas pressure conditions, are key issues in coal mining and enhanced coalbed methane recovery. To obtain a comprehensive understanding of the effects of mining-induced stress conditions and gas pressures on the mechanical behavior and permeability evolution of coal, a series of mining-induced stress unloading experiments at different gas pressures were conducted. The test results are compared with the results of conventional triaxial compression tests also conducted at different gas pressures, and the different mechanisms between these two methods were theoretically analyzed. The test results show that under the same mining-induced stress conditions, the strength of the coal mass decreases with increasing gas pressure, while the absolute deformation of the coal mass increases. Under real mining-induced stress conditions, the volumetric strain of the coal mass remains negative, which means that the volume of the coal mass continues to increase. The volumetric strain corresponding to the peak stress of the coal mass increases with gas pressure in the same mining layout simulation. However, in conventional triaxial compression tests, the coal mass volume continues to decrease and in a compressional state, and there is no obvious deformation stage that occurs during the mining-induced stress unloading tests. The theoretical and experimental analyses show that mining-induced stress unloading and gas pressure changes greatly impact the deformation, failure mechanism and permeability enhancement of coal.

The Mechanism of Mining-Induced Stress Evolution and Ground Pressure Control at Irregular Working Faces in Inclined Seams

Surrounding rock control is problematic at irregular fully-mechanized working faces of inclined seams. This study investigated the evolution of mining-induced stress at a variable-length working face under different coal seam inclination angles by using FLAC3D software to establish an experimental model based on the occurrence conditions of the inclined seam and the layout of fully-mechanized working face in the western district of Pan’er mine in China. Analysis of the simulation data was used to determine the stress evolution of working face in the strike and dip directions under different dip angle conditions and reveal the relationship between mining stress evolution and coal seam inclination. Comparative analysis of the formation and evolution of mining-induced stress before and after the roadway connection was used to determine the mechanisms behind mining-induced stress evolution in the short face to long face connection process and to reveal asymmetrical differences in the mining-induced stress evolution of the upper, middle and lower parts of the coal seam along the dip direction. Based on the relationship between supports and surrounding rock, the structure of hydraulic powered supports, the entry arrangement and supporting characteristics, control methods to ensure the stability of surrounding rock are proposed such as a metal mesh above hydraulic powered supports and reinforcement at the junction. These measures are effective for preventing partial roof fall and rib spalling, loss of support stability and stress concentration at connecting entry and for achieving safe and efficient mining of irregular working faces in thick inclined seams.

Numerical analysis of the dynamic evolution of mining-induced stresses and fractures in multilayered rock strata using continuum-based discrete element methods

In this study, the continuum-based discrete element method (CDEM) was adopted to simulate the evolution of mining-induced stress and fracturing during roadway tunnelling and mining in multilayered heterogeneous rock strata. The CDEM integrates the finite element method (FEM) and the discrete element method (DEM) to characterize the mining-induced stress evolution, the discontinuous fractures, separations, and caving that occur in the interfaces between multilayered rock strata. The maximum tensile-stress criterion and the Mohr–Coulomb strength criterion were used to evaluate the tensile and shear failure of the material elements. The CDEM model for rock strata was constructed by employing image processing and reconstruction approaches, using the geometrical and physical parameters that were measured from a real coal mining site. The stress evolution and compression deformation of the roof and floor strata were computed to evaluate the criticality of mining-induced disasters. The constructed model was employed to simulate and analyse the immediate roof collapse, immediate floor bulges, and the compaction of the collapse blocks, as well as the large deformation, separation, and collapse between the immediate roof and the main roof during coal seam mining. It was shown that the proposed method could predict ranges for the caving zone and fracture zone in the rock roofs that were in good agreement with the observation results from the real coal mining site.

An anisotropic coal permeability model that considers mining-induced stress evolution, microfracture propagation and gas sorption-desorption effects

Anisotropic coal permeability plays an important role in the simultaneous extraction of coal and gas from deep underground. In this study, first, the representative elementary volume of a coal mass is determined based on the natural microfracture distribution in coal. Then, an anisotropic coal permeability model that considers the complex stress evolution process, microfracture propagation and gas sorption-desorption effects was established to comprehensively study the coupled process of the actual mining-induced anisotropic mechanical behaviors and the gas seepage phenomena in the coal. The main characteristic of this model is coupled the stress-fracture-seepage analyses on the mechanical behavior of coal in terms of complex stress evolution. The presented model is verified using two different experimental observations, and a precise mathematical expression of the stress evolution processes induced by different mining layouts is presented to perform the coupled analyses using the presented model. The simulation results exhibit good agreement with the experimental data and reflect the overall trends of the test results fairly well, and the simulation results can appropriately represent the actual mining-influenced coal mechanical behavior. Thus, the model is highly adaptable for simulating various stress boundary conditions and engineering disturbance processes and is applicable for conducting numerical studies on coal fracture propagation, gas sorption-desorption effects and complex mining-induced mechanical behavior. Therefore, this model provides a theoretical basis for further analyses.

Mining-Induced Coal Permeability Change Under Different Mining Layouts

To comprehensively understand the mining-induced coal permeability change, a series of laboratory unloading experiments are conducted based on a simplifying assumption of the actual mining-induced stress evolution processes of three typical longwall mining layouts in China, i.e., non-pillar mining (NM), top-coal caving mining (TCM) and protective coal-seam mining (PCM). A theoretical expression of the mining-induced permeability change ratio (MPCR) is derived and validated by laboratory experiments and in situ observations. The mining-induced coal permeability variation under the three typical mining layouts is quantitatively analyzed using the MPCR based on the test results. The experimental results show that the mining-induced stress evolution processes of different mining layouts do have an influence on the mechanical behavior and evolution of MPCR of coal. The coal mass in the PCM simulation has the lowest stress concentration but the highest peak MPCR (approximately 4000 %), whereas the opposite trends are observed for the coal mass under NM. The results of the coal mass under TCM fall between those for PCM and NM. The evolution of the MPCR of coal under different layouts can be divided into three sections, i.e., stable increasing section, accelerated increasing section and reducing section, but the evolution processes are slightly different for the different mining layouts. A coal bed gas intensive extraction region is recommended based on the MPCR distribution of coal seams obtained by simplifying assumptions and the laboratory testing results. The presented results are also compared with existing conventional triaxial compression test results to fully comprehend the effect of actual mining-induced stress evolution on coal property tests.

Mechanisms influencing the lateral roof roadway deformation by mining-induced fault population activation: a case study

Mining-induced fault population activation often leads to dynamic disasters. It is of considerable significance to understand the strata movement and mining-induced stress evolution induced by fault population activation. This paper studied the movement and rotation of the roof strata which are influenced by a complex interaction between two faults in its strike direction of panel 31100 of no. 1 mine in Pingdingshan, China. Protective coal pillars for the lateral roof roadway were designed to be wide enough in case of eliminating fault disturbance. Severe roadway deformation was also observed throughout advancing. Based on the deformation observation of roadways, a structural instability model of considering the fault population was established. Moreover, mining-induced stress distribution and lateral roof roadway deformation mechanisms during fault population activation were investigated by both theoretical analysis and 3D numerical simulations. According to the mining-induced stress distribution, the sliding instability of roof strata above the working face aggravated the structural rotation of the lateral strata. Intense stress concentration induced by lateral roof rotation resulted in lateral roof roadway deformation. As concerning the effect of fault population activation, results obtained are of great importance for reasonable allocation of working face and roadways, and also for the prevention of dynamic disasters. [Received: February 5, 2015; Accepted: July 20, 2015]

Numerical approach to the top coal caving process under different coal seam thicknesses

Numerical study of mining-induced stress evolution of coal during the top coal caving process under different coal seam thicknesses is carried out, and the numerical prediction agrees well with the field test data. Main characters on stress distribution and dangerous area are elucidated. For the same coal quality, coal layers under 7 m thick fail earlier than thicker coal layers; correspondingly, the internal fracture networks of thin layers are more easily developed. During the mining of a coal layer less than 7 m thick, stress monitoring of the ?dangerous area? in the middle of the top coal should be emphasized, whereas during the mining of coal layers less than 11 m thick, stress monitoring of the ?dangerous area? at the bottom of the top coal should be highlighted. The research is to optimize caving technique and extraction process.