Based on the engineering background of gob side entry retaining in W1319 working face of Gaohe mine of Lu’an Group, this paper adopts the methods of field investigation, theoretical analysis and physical experiment to tackle the key technical problems of surrounding rock stability control during the effect of secondary mining and successfully retain roadway on site. The main research results are as follows: Firstly, the deformation and failure characteristics of surrounding rock of W1319 intake entry are investigated, the main factors affecting the stability of surrounding rock of gob-side entry retaining are determined, the key points to control the stability of surrounding rock of the gob-side entry are obtained, and the stability of top coal, coal wall and shoulder coal is determined as the starting point to control the stability of surrounding rock. Secondly, according to the stress distribution law of W1319 intake entry and the deformation and failure characteristics of surrounding rock, the reinforcement scheme of “high strength grouting + reinforcement anchor cable” combined support before secondary mining is put forward, and the concrete support parameters are obtained by theoretical analysis combined with site conditions. Industrial experiment and field mine pressure monitoring data show that the reinforcement support technology proposed can effectively control the deformation of surrounding rock, and verify the correctness of the reinforcement scheme proposed in this paper. As the first successful case of gob-side entry retaining reuse in non-pillar mining of Gaohe mine, it has a popularization significance.
Keywords: Fully-mechanized caving face; gob-side entry retaining; plastic zone; stress distribution; control of surrounding rock
Based on the theory of BP neural network, the monitored data of the cyclic end resistance of hydraulic support in 02178 working face of Huopu mine is trained. Through analyzing the errors produced by the different nodes of network hidden layer, the periodic weighting prediction model whose network structure is 4-12-1 is built. After field monitoring, the 5th weighting is predicted. And the results showed that the average periodic weighting step is 9.1 m, and the influence range covers 1.53 m, and the average dynamic load coefficient is 1.28. Evidently, the output values of network are basically consistent with the monitored data. Therefore, these predicted data can provide a theoretical basis for supporting design and safety production of roadway with the same conditions.
Keywords: BP neural network; periodic weighting; pressure prediction; roof control
After the upper coal seam mining, stress concentrations under the residual pillar generate an inhomogeneous stress field and the stress on the tunnel increases. This can lead to tunnel destruction if the external forces exceed the strength of the composite rock–bolt bearing structure (CRBBS). The Weibull distribution function was introduced to modify the calculation formula of the CRBBS strength and stress distribution of the CRBBS. The radial stress within the CRBBS under the inhomogeneous stress field was calculated. The tunnel instability coefficient (η) was derived to quantitatively describe the relationship among the tunnel position, tunnel parameters, support parameters, and rock mechanical properties. The coefficient is the ratio of the maximum radial stress within CRBBS to the CRBBS strength; the CRBBS is stable only when η<1. The instability coefficient of the 21178 return tunnel Huopu colliery was calculated to obtain the optimised support parameters.The bolt strength was adjusted from 335MPa to 500MPa and the bolt length was changed from 2.2m to 2.5m; thus, the large deformation of the 21178 return tunnel was controlled. The optimum distance between the 21178 return tunnel and the pillar edge was at least 12.4m. The proposed method was validated by comparison with results by the tunnel stability index method (>20m), abutment pressure influence edge method (14.6m), and rateo fstress change method (>16.2m). Since the resistance of the bolt support is quantified, the tunnel position evaluated by the tunnel instability coefficient is more practical.
Keywords: Gob pillar; CRBBS; tunnel instability coefficient; optimization of support parameters; tunnel position
Based on the special conditions of Changzhi 3θ coal seam in Shanxi province, the stress field and plastic zone of surrounding rock in thick coal seam roadway with large cross section under different side pressure coefficient ( λ ) are studied by using finite element software FLAC3D. The weak position of roadway is analyzed, and the stress direction of roadway support under different side pressure coefficient (λ<1) is put forward. (2)λ When λ = 1, both sides of the roadway and the roof are equally important and should not be neglected; (3) When λ>1, the stress concentration zone gradually shifts to the vicinity of the floor, and the maintenance of the floor should also be considered. According to the measured lateral pressure coefficients and numerical simulation results of the auxiliary roadway in the Changzhi coal mine, the main support direction of the roadway is provided. According to the convergence of the field section, the deformation law of the surrounding rock is basically consistent with the simulation results, which provides a new idea for the design of the roadway support.
Keywords: Numerical simulation, lateral pressure coefficient; stress field; plastic zone
The measured data are taken as training samples, and the surrounding rock deformation of ventilation roadway of the 21175 working face in Huopu coal mine is predicted by building a wavelet neural network time series model whose network structure is 4-18-1. The result shows that there is high coincidence degree between the prediction deformation and the field monitored data. The maximum relative error is less than 1%, and the average relative error is less than 0.5%. Therefore, the prediction values provide the theoretical basis of support design and safe production for roadways with the same conditions.
Keywords: Wavelet neural network; time series; surrounding rock deformation of roadway; prediction on surrounding rock deformation
Based on the measured in-situ stress, the stress, displacement and plastic zone distribution in surrounding rock of the eastern 2# ventilation roadway of a mine in Shanxi are analyzed by using the generalized plane strain model, Mohr-Coulomb strength criterion and FLAC3D software. And it is considered that the stress concentration of surrounding rock is significantly affected by the circumferential stress in the range of 0~3m and the radius of plastic zone is 5m. Both of them have strong uniformity in distribution and it is approximately located in the direction of 135°-315°, showing the asymmetric characteristics. Combining with the influence characteristics of in-situ stress on the coal seam roadway, an asymmetric support scheme for roadway section is proposed. The practice shows that the support is effective, which provides theoretical guidance for the design and optimization of support parameters of roadway with similar engineering conditions.
Keywords: In-situ stress; coal seam roadway; stress of surrounding rock; displacement of surrounding rock; asymmetric plastic zone; support design
Due to overe xploitation in the last years, coal under buildings and other structures has become a significant resource to maintain the sustainability of coal mines in China. However, buildings and other structures above mining area are easily damaged caused by surface subsidence after mining. In order to recover coal under surface structures and ensure the stability of surface structures, the short-strip coal recovery with backfill method is proposed. By using a numerical simulation, this paper simulates the subsidence of overlying strata to select the mining sequence and the strip width. A reasonable mining sequence is determined and the strip width is primarily selected as 5m or 6m. Afterward a physical simulation is conducted to validate the numerical simulation. And the subsidence law of overlying strata during mining and filling is analyzed for the 5m-width and 6m-width strip respectively. Finally, the reasonable strip width is determined as 6m according to the surface subsidence and production efficiency. By employing the above mining scheme, the surface subsidence and deformation of two surface structures above the mining area of Sima coal mine is predicted, which shows the mining scheme can ensure the stability of two surface structures above mining area. The research results provide a reasonable and reliable reference for coal recovery in thick coal seam under buildings and other structures.
Keywords: Coal recovery; thick coal seam; mining scheme; numerical simulation; physical simulation
The key to the selection of coal pillar width lies in the stability of the roadway space and the stability of the coal pillar itself. Based on the theory of coal pillar stress distribution, the reasonable retention width of the coal pillar in the deep buried thick coal seam is discussed. Reasonable coal pillar width and scientific roadway support parameters can maximize the bearing capacity of the surrounding rock, improve the stability of the roadway and reduce the coal mining loss. Reducing the size of existing large coal pillars in a stable stress interval is of great significance for coal mines to increase production and green mining. Through theoretical analysis, numerical simulation and on-site measurement, the stress, strain and displacement of the coal pillar are analyzed, and the width of the section coal pillar is reduced to a safe and scientific scope, supplemented by engineering verification.Therefore, it provides a reference for the narrow coal pillar mining in thick coal seam; learn from the meaning.
Keywords: Thick coal seam; coal pillar width; coal pillar stress; reasonable width
Fully mechanized caving is a common practice in mining thick coal seams in China. The characteristics of parting failures affect the stability and cavability of the top coal. In this paper, based on the geological conditions of and partings in a production site in the Wobei mine, the characteristics of parting failures on the front of a working face are calculated using elasticity theory and the cavability of the coal seam and the parting are obtained. Based on the theory of a cantilever beam under a uniformly distributed load, the cantilever length and the conditions under which a parting at the back of a working face fractures are calculated. These results are combined to obtain the characteristics of parting failures in thick coal seams during fully mechanized caving in the Wobei mine. In addition, this provides basic parameters for improving the top coal caving ratio of a working face.
Keywords: Fully mechanized caving; parting; top coal; failure