Study on the bolt-mesh-anchor support technology for mining roadway in complex coal seam

The stability of mining roadway is affected significantly by the condition of the surrounding rock and stress regime, which is the key factor for determining the roadway support program. Located in syncline axis, the No.15 seam is a steep seam at - 230 m level in Changgouyu coal mine, the original flexible shield cannot meet the safety and production requirement. The No.15 seam is classified as unstable and relatively complex according to the analytic hierarchy process (AHP). The distribution characteristics of stress in the seam around the synclinal axis before and after mining are analysed by use of FLAC2D. According to the complexity of the coal seam and the stress distribution characteristics, the supporting parameters using bolt-mesh-anchor were selected and implemented successfully in an underground roadway, which can be referred to as in the design of roadway support in similar complicated coal seams

Experimental study on permeability evolution of slender coal pillar of entry driven along goaf

Under the condition of roadway driving along goaf, slender coal pillar is affected by multiple mining-induced disturbances, and the permeability of coal and rock mass affected by mining will change due to the development and compaction of mining fractures and primary fractures. Determining the evolution of slender coal pillar permeability at different mining stages is the theoretical basis for the prevention and control of gas water disasters in adjacent goaf at the same layer. Taking the mining with slender coal gate pillar of the Carboniferous extra thick coal seam in Datong Mining Area as the engineering background, the distribution characteristics of the stress field for the slender coal gate pillar of the coal seam in different mining stages are comprehensively determined by the methods of geostress testing and numerical simulation, which provides a basis for the determination of the stress path for experimental research. The DJG - Ⅱ triaxial loading coal rock seepage testing equipment was used to conduct experimental research on the evolution of coal pillar permeability in different mining stages. The research results are as follows: The quantitative influence relationship between permeability and stress of slender coal gate pillar in different mining stages is established. The overall performance is that the permeability decreases with the increase of axial stress, and the permeability increases with the decrease of axial pressure in unloading stage; It reveals the evolution of stress strain permeability of the coal pillar in different mining stages. When loading and unloading in the first and second stages, the deformation of coal sample is still in the elastic deformation stage, and the change amplitude and rate of permeability are relatively gentle. In the third mining-impacted stage, the irreversible plastic failure of the specimen made the permeability increase sharply, and the rate of increase was also significantly greater than the first two mining stages. The permeability of slender coal pillar increased by 324.389 times compared with the initial permeability. In this stage, the slender coal pillar was damaged and lost its gas water barrier performance. It was clear that the 6 m small coal pillar was not damaged in the first two mining stages of the super thick coal seam gob side entry project. The research results can provide reference or theoretical support for the study of permeability evolution characteristics of slender coal pillar in different mining stages, and the prevention and control of gas water disasters in adjacent goaf under the condition of gob side entry mining in hard roof extra thick coal seams

Anchorage mechanical characteristics of newly designed rebar bolt and optimization of its support scheme

In Shendong coalfield, large cross-section underground gateways are often excavated for large machine transportation. It causes large deformation of the surrounding rock and the phenomenon of rib spalling. In this study, taking Xiaobaodang coal mine as the engineering background, a newly designed rebar bolt for large deformational roadway support was introduced to control large deformation. Results that the peak anchoring force of the new bolt increased by 20-30%, the total energy absorption of the system is increased by 35-50%, the post-peak displacement increases by 4.5mm comparing with traditional left-spiral non-longitudinal rebar bolt. The support schemes were analyzed by using FLAC3D, the newly bolting effect was better than the original scheme when the new rebar bolt was used in full-length anchorage and the increased bolting interval. This study provides a referencr for the similar roadway support

Study on the Analysis Method of Swelling Deformation of Protected Seam During Protective Seam Mining

In view of the study on swelling deformation analysis method of protected seam during the mining process of protective seam, the analysis method of “four invariant points around area” is put forward for the first time. The method determines the swelling deformation of protected seam and analyzes it from the perspective of plane by analyzing the variability of “four invariant points around area” of protected seam before and after the mining of protective seam. Monitoring scheme and area analysis and calculation method are respectively designed applied in coal mine and laboratory; the monitor of “four invariant points around area” has been realized in the mining practice by arranging two measuring lines in the roof and floor of protected seam. The study scheme is designed to analyze the swelling deformation of the protected seam by the application of “four invariant points around area” in the engineering practice; the theoretical calculation method of irregular “four invariant points around area” after swelling deformation of the protected seam is put forward under laboratory conditions based on the Freeman boundary encode vector and measuring the length of quadrilateral side directly with the vernier caliper.; the reasonable scale of the four invariant points around area is discussed, it is suggested that different “four invariant points around area” should be established with different scale of 1 times, 1/2 times, 1/4 times and 1/8 times thickness of coal seam. The study shows that the method of “four invariant points around area” of swelling deformation is more accurate than the analysis method of “two fixed-point”; the more cells are divided at 1 times of the thickness of coal seam, the higher the accuracy of calculation is

Deformation and failure mechanism of full seam chamber with extra-large section and its control technology

In order to effectively predict and control the rib spalling and roof leakage, it is necessary to reveal the deformation and failure mechanism of the chamber and propose the corresponding surrounding rock control technology. Based on uniaxial compression experiments and numerical simulations, it is concluded that coal body damage is dominated by shear failure during uniaxial compression, which indicates to some extent the main form of damage of the surrounding rock in the chamber. Then the combined finite and discrete element method is used to establish a numerical model to reveal the evolution law of fracture in the surrounding rock. The simulation results show that after the excavation of the chamber, a large amount of shear failure occurred in the ribs and the roof. Then those cracks further developed, expanded, penetrated, and finally connected with the surface of the chamber. Under the effect of the mine pressure, the coal body is separated from the surface of the chamber, leading to the occurrence of rib spalling and roof leakage. So it was given that support method by controlling crack development. The grouting and high-strength anchor bolt and anchor cable are proposed to improve the shear strength of the surrounding rock, which helps to reduce the occurrence of cracks, and inhibit the cracks from interpenetrating. An industrial test was carried out in the chamber of Tashan Coal Mine, which showed good control effect of the surrounding rock in the chamber

Permeability Enhancement of Coal Seam by Lower Protective Layer Mining for Gas Outburst Prevention

Risk prediction of dynamic disasters such as rock burst, gas outburst, and water inrush is closely related to the permeability evolution of coal seam. According to the characteristics of the lower protective layer mining, the basic assumption of gas-solid coupling model of the coal was proposed in this paper. The permeability enhancement coefficient of equivalent layer spacing was first put forward. Based on the three-zone-shaped dynamic evolution of the permeability of the overlying protective layer during the lower protective layer mining, the theories of seepage mechanics and damage mechanics were applied to introduce the permeability enhancement coefficient of equivalent layer spacing. A mathematical model of permeability evolution of the protected coal seam in the lower protective layer mining was established. Based on the engineering background of the lower protective layer mining in Changping Coal Mine, the numerical simulation using the proposed mathematical model was performed. The results showed that the stress and permeability of the protected layer in #3 coal seam evolved dynamically with the advancement of the working face of the protective layer in #8 coal seam. When the working face of the protective layer in #8 coal seam advanced to 80 m, the stress reduction rate in the relief area tended to be stable, and the stress in the stress reduction area was about 50% of the original rock stress. When advanced to 80 m, the permeability of the protected layer of the #3 coal seam increased sharply, and the permeability increased by 873 times. With the continuous advancement, the permeability of the protected layer in #3 coal seam tended to increase steadily, and the permeability increased by 1100–1200 times. The calculated magnitude of permeability increment is consistent with that in the engineering practice, indicating that the permeability evolution model is basically reasonable. The research provides a theoretical guidance for the gas drainage field application in the lower protective layer mining for prevention of coal and gas outburst

A Field Study Implementing New Monitoring Technology for Roof Caving and Systematic Monitoring for Gob-Side Entry Retaining via Roof Cutting in Underground Coal Mining

The longwall mining method with gob-side entry retaining via roof cutting is a new underground coal mining method which has the characteristics of a high resource recovery ratio and environmental friendliness. Due to the complexity of this method, the research method of case-based dynamic on-site monitoring, analysis, adjustment, and optimization is usually adopted. Based on a roadway retaining via roof cutting project, in addition to the traditional indirect monitoring method of hydraulic support pressure, this study innovatively establishes a direct monitoring method for roof caving by monitoring the gangue pressure in the goaf, which provides data for the roof cutting effect and offers a new method for studying the overlying strata movement. In the project, a comprehensive monitoring and analysis system was established, including gangue pressure, cable bolt stress, bracket pressure, roadway deformation, and roof separation, which was used to dynamically analyze the effect of roof cutting and optimize the support design. The results show that the pressure of the hydraulic support close to the roof cutting is low, indicating that roof cutting is favorable in the roadway retaining mining method. The roadway deformation in the advanced abutment pressure area of the working face is small. The mining-induced stress caused by the collapse and compaction of the overlying strata in the goaf is the dominant factor affecting the effect of roadway retaining, especially in the 50–100 m range behind the working face, where the dynamic load causes high bearing capacity of the support elements, large roadway convergence, and roof separation. Temporary support and supplementary reinforcement should be added when necessary. The monitoring system presented in this study is highly comprehensive, simple, reliable, and low in cost, providing a reference for roof cutting roadway retaining projects and roof caving-related studies