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Based on techniques of close upper protective coal-rock layer mining, relieved gas extraction, and underground gangue washing-discharging-backfilling, this paper initiates the concept of mixed fully-mechanized coal mining, which combines a solid backfilling method and a caving method (hereinafter referred to as “backfill and caving mixed mining”). After the principle and key techniques are introduced, a physical simulation experiment and a numerical simulation are used to study the characteristics of the overlying strata’s fracture development, the main roof subsidence, the stress field and its influence area in the transition area with the length ratios of the backfilling section and the caving section, and the advancing distance of the mixed longwall face. Thus, the lengths of the caving section and the backfilling section, the parameters of the support system in the transition section, and the design process of the mixed longwall face are presented. In practice, the mixed longwall face Ji15-31010 in Ping-dingshan No. 12 Colliery proves that the designed lengths of 120 m and 100 m for the backfilling section and the caving section, respectively, are appropriate. The monitoring results of the hydraulic support working resistance show that the supports were working well in general; the maximum growth height of the overlying strata fracture is 18 m; the gas drainage efficiency is up to 80% and the average gas concentration is 0.1 g/m3; a large quantity of gangue generated in the Ji14 seam is disposed underground; coal and gas are extracted simultaneously; and significant environmental and economic benefits are realized.

The mining-induced strata movement and surface subsidence are closely related to the dip angle of coal seam. However, most surface subsidence prediction methods are empirical, and only suitable for nearly flat coal seam mining. In this paper, a new theoretical method is proposed to predict the strata movement boundary and surface subsidence caused by inclined coal seam mining, which considers the influence of key strata, rock quality and coal seam dip angle. The strata movement caused by inclined coal seam mining is generalized and described by three models: analogous hyperbola model (AHM), analogous hyperbola-funnel model (AHFM), and analogous funnel model (AFM). Considering the rock quality of roof and floor strata, the rock mass rating system is adopted to calculate the surface maximum subsidence and its location. Additionally, the distinct element method was used to assess the performance of the theoretical models. The numerical simulation results match well with theoretical predictions of strata movement boundary and surface subsidence. It is discovered that the appearance of surface subsidence troughs is obviously asymmetric. As the dip angle increases, the surface maximum subsidence decreases and its location is laterally displaced. When the dip angle is greater than 50°, the double subsidence troughs can be visualized clearly. Furthermore, the theoretical predictions of surface subsidence are verified by field measurements of two cases. As a result, the theoretical predictions of surface subsidence are greatly improved by comparing with the empirical method.

Violent movement of the roof rock and severe damage to the overlying strata occur in the large mined-out space left by rapidly advancing, high-intensity longwall coal mine extraction, as the goaf forms. Knowing the height of the fractured water-conducting zone (FWCZ) above the goaf is vital in the safety analysis of coal mining, particularly under a water body. The processes of overburden failure transfer (OFT) were analyzed for such high-intensity mining, divided into two stages: transmission development, and transmission termination. Rock failure criteria were used in theoretical calculations of the maximum lengths of ‘suspended’ (i.e., unsupported) rock strata, and of the maximum ‘overhang’ (i.e., cantilever) length of each stratum. Based on this, mechanical models of the unsupported strata and the overhanging strata were established. A new theoretical method of predicting the height of the FWCZ in this form of coal mining is put forward, based on OFT processes. A high-intensity mining panel (the 8100 longwall face at the Tongxin Coal Mine, Datong Coal Mining Group) was taken as an example. The proposed theoretical method, a numerical simulation method and an engineering analogy method were used to predict the height of the FWCZ. Comparison with in situ measurements at the Tongxin mine showed that the theoretical and numerical simulation results were in close agreement with measured data, verifying the rationality of the proposed approach.

Mining deformation of roof strata is the main cause of methane explosion, water inrush, and roof collapse accidents amid underground coal mining. To ensure the safety of coal mining, the distributed optical fiber sensor (DFOS) technology has been applied in the 150,313 working face by Yinying Coal Mine in Shanxi Province, north China to monitor the roof strata movement, so as to grasp the movement law of roof strata and make it serve for production. The optical fibers are laid out in the holes drilled through the overlying strata on the roadway roof and BOTDR technique is utilized to carry out the on-site monitoring. Prior to the on-site test, the coupling test of the fiber strain in the concrete anchorage, the calibration test of the fiber strain coefficient of the 5-mm steel strand (SS) fiber, and the test of the strain transfer performance of the SS fiber were carried out in the laboratory. The approaches for fiber laying-out in the holes and fiber’s spatial positioning underground the coal mine have been optimized in the field. The indoor test results show that the high-strength SS optical fiber has a high strain transfer performance, which can be coupled with the concrete anchor with uniform deformation. This demonstrated the feasibility of SS fiber for monitoring strata movement theoretically and experimentally; and the law of roof strata fracturing and collapse is obtained from the field test results. This paper is a trial to study the whole process of dynamic movement of the deformation of roof strata. Eventually the study results will help Yinying Coal Mine to optimize mining design, prevent coal mine accidents, and provide detailed test basis for DFOS monitoring technique of roof strata movement.

Underground coal excavation has caused a series of geological disasters and environmental problems, especially coal mining subsidence. Backfill-strip mining, which combines the advantages of strip mining and backfill mining, can reduce subsidence and improve the recovery rate of coal. Therefore, predicting the impact of backfill-strip mining on the surface environment and strata structure is essential for the better development of backfill-strip mining technology. Here, a scientific and comprehensive mechanical model is creatively proposed. The mechanical model is divided into two systems at the main key strata (MKS): the lower strata of the MKS are regarded as a rectangular plane mechanical model on the Winkler foundation, comprising spaced filling bodies and coal pillars, and the upper strata of the MKS are regarded as a space-layer mechanical model. First, the subsidence function of the MKS is proposed. Then, this function is transmitted to the space-layer mechanical model through the interface. Finally, the mechanical model is used to predict the subsidence of the strata and the surface. The feasibility of the model is verified by numerical simulation and similar material simulation, and the characteristics of strata movement are analyzed. Using the mechanical model, the influences of geological and mining conditions on strata movement are discussed. This provides theoretical guidance for the study of strata movement and mechanisms in backfill-strip mining.

Aiming at the problem of mining coal resources under the Yuxing Mine Ecological Park in Inner Mongolia, China, we adopted a continuous mining and continuous backfilling (CMCB) cemented-fill mining method; based on the systematic description of the mining and backfilling craft of the CMCB mining process, comprehensive use of theoretical analysis, numerical simulation, and similar simulation test methods to analyze the control mechanism of overlying strata. By establishing the mechanical model of roof movement for the CMCB mining, the roof deflection curve equation is deduced, and the roof instability criterion is put forward based on the maximum tensile stress criterion. Numerical simulation showed that the stress curve at the shallow part of the roof fluctuates up and down in a “w” shape, the stress curve of the upper surrounding rock sinks slowly, and the coal pillars and filling bodies are alternately loaded during the staged mining process to jointly limit the movement of the overlying strata. The similar simulation test results showed that the roof has no structural damage, the roof cracks are not obvious, and the overlying strata control effect is good. On site tests showed that the CMCB mining method can effectively limit the movement of the overlying rock in the stope, control the surface subsidence of the ecological park, and increase the recovery rate of coal resources. It can provide a reference for the development of coal resources and ecological protection in a fragile environment.

In view of the complexity and concealment of goafs, numerical simulations have become an important means for studying the coupling disasters of spontaneous coal combustion and gas. Porosity is an important parameter in the numerical simulation, but this is difficult to detect directly on site, as the current porosity model does not fully reflect the characteristics of \"three horizontal areas\" and \"three vertical zones\" in the goaf. To establish a more accurate porosity model which is based on a theory of overlying strata movement, the Sigmoid function was introduced to reflect the distribution characteristics of the \"three horizontal areas\" in the goaf, and subsidence models of the main roof and overlying strata were established. A model that measures the porosity of the goaf was formulated, which reflects its O-shaped circle characteristics. The accuracy of the theoretical model was verified by obtaining experimental data. The results showed that the main roof subsidence in the natural accumulation area was approximately zero, there was an exponential decrease in the load-affected area, and the maximum subsidence appeared in the compacted area. The descending displacement was very small around the impacted area along the roadway wall. The movement of overlying strata in the fractured zone was controlled by the main roof, and the subsidence of the strata near the roadway wall and the working face was very small. In the vertical direction, the subsidence of the overlying strata decreased as the distance from the mining coal seam increased. The distribution of porosity in the goaf was shaped like a dustpan. In the horizontal direction, the porosity of the natural accumulation area and the impacted area of the roadway wall was the largest. Deep within the goaf, the porosity gradually decreased. The porosity in the fractured zone decreased in the vertical direction in a logarithmic manner. The porosity model proposed in this paper fully reflects the characteristics of the goaf, and provides a more accurate porosity model for the numerical simulation of spontaneous coal combustion and gas coupling disasters in the goaf.

This study employs an integrated methodology combining slip line field theory, FLAC3D numerical simulation, and hydraulic injection testing to systematically investigate the evolutionary characteristics of roof two zones under repeated mining conditions in close range coal seams No.9 and 10 (average interburden thickness is 10.2 m). Theoretical modeling reveals a maximum failure depth of 6.26 m in the No.9 coal seam, with corresponding caving and fracture zone heights measuring 9.6 m and 33.4 m respectively. Numerical simulations demonstrate that subsequent extraction of the No.10 coal seam induces fracture zone expansion to 66 m vertical elevation, representing a 73% increase relative to single-layer mining conditions. Strong congruence was confirmed through MATLAB-based regression analysis. Hydraulic injection tests substantiate the spatial heterogeneity of two zones (caving zone and fracture zone) development, while mechanical analysis proposes a stress arch-dominated fracture propagation mechanism. These findings establish a mechanical framework for safe extraction of close range coal seam clusters, successfully informing optimization of combined bolt reinforcement and grouting curtain configurations in practical applications.

The occurrence of surface strata movement in underground coal mining leads to the generation of numerous ground fissures, which not only damage the ecological environment but also disrupt building facilities, lead to airflow and easily trigger coal spontaneous combustion, induce geological disasters, posing a serious threat to people’s lives, property, and mining production. Therefore, it is particularly important to quickly and accurately obtain the information of ground fissures and then study their distribution patterns and the law of spatial-temporal evolution. The traditional field investigation methods for identifying fissures have low efficiency. The rapid development of UAVs has brought an opportunity to address this issue. However, it also poses new questions, such as how to interpret numerous fissures and the distribution law of fissures with underground mining. Taking a mine in the Shenfu coalfield on the semi-desert aeolian sand surface as the research area, this paper studies the fissure recognition from UAV images by deep learning, fissure development law, as well as the mutual feed of surface condition corresponding to the under-ground mining progress. The results show that the DRs-UNet deep learning method can identify more than 85% of the fissures; however, due to the influence of seasonal vegetation changes and different fissure development stages, the continuity and integrity of fissure recognition methods need to be improved. Four fissure distribution patterns were found. In open-cut areas, arc-shaped fissures are frequently observed, displaying significant dimensions in terms of depth, length, and width. Within subsidence basins, central collapse areas exhibit fissures that form perpendicular to the direction of the working face. Along roadways, parallel or oblique fissures tend to develop at specific angles. In regions characterized by weak roof strata and depressed basins, abnormal reverse-“C”-shaped fissures emerge along the mining direction. The research results comprehensively demonstrate the process of automatically identifying ground fissures from UAV images as well as the spatial distribution patterns of fissures, which can provide technical support for the prediction of ground fissures, monitoring of geological hazards in mining areas, control of land environmental damage, and land ecological restoration. In the future, it is suggested that this method be applied to different mining areas and geotechnical contexts to enhance its applicability and effectiveness.

Disasters such as rock bursts and mine earthquakes became increasingly serious with the increase in mining depth in Erdos Coal Field and became serious problems that restrict high-strength continuous mining of coal mines. In this study, strata movement and energy polling distribution of ultrathick weak-bonding sandstone layers were controlled by the local filling–caving multi-faces coordinated mining technique, which was based on the analysis of subsidence and overlying structural characteristics in the Yingpanhao mining area. Moreover, the influencing factors and the control effect laws were investigated. Surface subsidence and energy polling distribution control effects of different mining modes were compared, which confirmed the superiority of local filling based on the main key stratum. According to the results, the maximum surface subsidence velocity of the first mining face was 1.24 mm/d, which indicates the presence of a logistic functional relationship between the mining degree and subsidence factors. When the mining degree was close to full mining, the practical surface subsidence was smaller than the corresponding logistic functional value. The largest influencing factor for the strata movement control effect of partial filling mining based on the main key stratum was the width of the caving face, followed by the filling ratio, section pillar width, and width of the filling face, successively. With respect to the influencing degree on the energy polling distribution of partial filling mining based on the main key stratum, the order followed as section pillar width > filling ratio > caving working face > width of backfilling working face. Additionally, the comparative analysis from the perspectives of control effect, resource utilization, and cost-effectiveness demonstrated that partial filling mining based on the main key stratum was one of the techniques with high cost-effectiveness in controlling strata movement and relieving rock bursts, mining earthquakes, and subsidence disasters.