Hengzhong Zhu’s research while affiliated with Shandong University of Science and Technology and other places

In accordance with the distinctive characteristics observed in the sandstone-mudstone composite roof within Zhaogu coal mine, Jiaozuo coalfield, this paper delves into the influence of the non-uniform key stratum of Dazhan sandstone, characterized by varying thicknesses, on the overall strength of the composite roof. A mechanical model for the sand-mudstone composite body, considering different sandstone thickness ratios, was established. The study employed ESLS DIC and AE systems to investigate the mechanical behavior. The findings reveal a general decline in compressive strength, with a stepwise reduction in post-peak stress correlating with diminishing sandstone thickness ratios in the combined samples. Specifically, when the sandstone thickness ratio is 25%, the elastic strain energy accumulated in the sample is discharged before the initial peak, resulting in a distinctive "double-peak" shape in the stress–strain curve. Notably, deformation coordination between sandstone and mudstone is evident during loading, wherein mudstone horizontal deformation near the interface is constrained by sandstone. This constraint induces circumferential inward shear stress, gradually diminishing with increasing distance from the sand-mudstone interface. Post-failure, the strain concentration degree and range of the sample undergo significant changes, reflected in fluctuations in the corresponding fractal dimension. Acoustic emission (AE) events predominantly manifest in the upper section of samples as the sandstone thickness ratio decreases. Throughout the loading process, multiple peak periods of counts concentration are observed, accompanied by noticeable fluctuations in the cumulative counts curve.

Taking the return-airway 4204 with roof cutting in Longquan Coal Mine as the engineering background, roof structure, key parameters, and deviatoric stress evolution were studied. Conclusion: The Key Stratum within a 4–8 times mining height is considered as Near Key Stratum. Cutting the roof makes it possible to form a cantilever structure of the Key Stratum on the solid coal side, which is more conducive to the stability of gob-side roadway. During cutting angle of 90–55°, the deviatoric stress increases linearly, and the increase rate is coal pillar > solid coal > roof > floor. While cutting length from 0 to 35 m, the deviatoric stress decreases linearly, and the decreasing range: coal pillar > solid coal > roof > floor. When coal pillar width is from 30 to 4 m, the deviatoric stress of left side and floor presents a “single peak” distribution. The deviatoric stress of coal pillar changes from an asymmetric “double peak” to a bell-shaped distribution, and the deviatoric stress of roof changes from a “single peak” to an asymmetric “double peak” distribution. Under same coal pillar width, the deviatoric stress of left, coal pillar and roof after roof cutting decreases most obviously, followed by the floor. Finally, the coal pillar width is 8 m, the cutting angle is 75°, the cutting length is 20 m, and the hole spacing is 1.0 m. The support scheme is bolt + metal mesh + steel belt + anchor cable combined support. The stable period of roadway is about 10 days.

Taking the double wide-strip mine gangue backfilling work face of 1101 and 1102 in the Tingnan Coal Mine as the background, this study utilizes theoretical analysis, numerical simulation, laboratory experiments, and engineering practices to investigate the complete process of compression-consolidation-creep-diagenesis of the compacted gangue backfill body, the equivalent mining height model, and the surface subsidence characteristics. The results: The maximum control roof distance in the gangue backfilling work face=Tip-to-face distance + Cutting depth + Roof beam length + Post-support gap. The advance subsidence amount of roof is equal to the maximum control roof distance multiplied by the sine value of the roof inclination angle. The height of available backfill space behind the support is equal to the mining height minus the roof advance subsidence, the unfilled height under roof, and the amount of floor heave behind the support. The compacted gangue body undergoes four stages of compression-consolidation-creep- diagenesis within the available filling height, with their respective compressibility rates as follows: skeleton adjustment stage (7.42%), slow compression stage (12.81%), consolidation settlement stage (13.15%), and creep diagenesis stage (25.53%). The equivalent mining heights corresponding to the four stages are 0.94m, 1.10m, 1.11m, and 1.48m, respectively. By comparing the predictions from probability integral method, numerical simulations, and subsidence observations, it is evident that the surface subsidence curves of the 1101 and 1102 working faces, along the strike main section, exhibit a "bowl" shape, with a gentle slope in the first two stages and a steep slope in the latter two stages. h t = L max × sinθ The subsidence curves of the 1101 and 1102 working faces, along the dip main section, show a "bowl" shape in the first two stages, and a "W" shape in the latter two stages.

Mining-induced subsidence is critical for ecological environment reconstruction and damage prevention in coal mining areas. Understanding the characteristics of surface subsidence with multi-seam mining is the first step. Surface subsidence of different mining panel layout configurations was investigated by means of UDEC numerical simulation. Based on the simulation results, it was indicated that mining panel layout configuration had a significant impact on surface subsidence, including ground surface subsidence, horizontal displacement, crack propagation, and ground surface fissure development. The overlapped region of the upper panel and the lower panel is the key region, where existing bedding separations and strata cracks close and activate, the integrity and strength of the interburden layer are reduced, and the subsidence magnitude is enhanced. The subsidence profile of the overlapped region for the stacked configuration, external staggered, the edge of the lower panel internal staggered, two edges of the lower panel internal staggered are steeper and deeper, and the corresponding values of ground surface subsidence and horizontal displacement are greater than other regions. The ground surface fissures with the types of stepped, slided, and graben developed on the ground surface above the edge of the mining panel, and the development location is closely related to the strata movement edge. Because of the support activities of the reserved coal pillar, the ground subsidence of the external staggered (internal staggered) of the upper panel with the coal pillar is slight. The external staggered (internal staggered) and external staggered (internal staggered) of the upper panel with the coal pillar can be selected as the preferred layout configuration. The proposed description of surface subsidence of different mining panel layout configurations can be applied in subsidence prediction.

The mining-induced ground response (MIGR) has a critical impact on safety management, the mining plan, and entry support. A clear understanding of the characteristics is the foundation of the MIGRs scientific control. This study is the result of the MIGRs development of the non-pillar mining panel with gob-side entry by roof cutting (GSERC). Comprehensive research of the in situ measurements, numerical simulation, and theoretical analysis to determine the ground response characteristics, including mining panel and GSERC, were implemented. The results indicate that the MIGR presents the characteristic of asymmetric development and that the ground response near the non-roof cutting side is more significant than that near the roof cutting side. The development stage of the entry convergence of GESRC can be divided into seven stages; the primary rapid development stage should be paid more attention to in the support process. The entry convergence rapidly increases to 275 mm, 380 mm, 410 mm, and 525 mm, respectively, for the roof cutting rib to the virgin coal rib, the roof near the virgin coal side, the roof of the middle section, and the roof near the cutting side. The hydraulic support end cycle resistance at the roof cutting side and the middle section of the mining panel with the value of more than 30.8 MPa is greater than that at the non-roof cutting side with the value of less than 26 MPa, which presents the asymmetric feature. The numerical simulation results regarding vertical stress development, vertical displacement, and horizontal displacement also presents the asymmetric feature. The MIGR division is divided into five divisions. Division II (the middle section of the panel) and division IV (the entry range near the roof cutting side) should be paid more attention to in the panel mining process. The results of this study can provide technical guidance and theoretical reference for similar engineering practices.

The key parameters of pressure relief pre-splitting blasting technology (PRPBT) contribute to the implementation effect of goaf-side entry by roof cutting (GSERC). This study is an attempt to design the key parameters including the roof cutting height, the roof cutting angle, the spacing between blasting boreholes, the charge structure and the sealing length by using theoretical analysis, numerical simulation and field implementation. The basic quality index of the main roof is 666.27, and combined with the peeping observation result, the main roof belongs to the category of a hard main roof. Different from a weakened roof and a compound roof, the unreasonable parameters of the PRPBT for the hard main roof lead to three problems including an insufficient pre-splitting blasting effect, roof suspension in the goaf and serious damage to the rock mass for the roof cutting rib. The PRPBT effect is closely related to key parameters, including the roof cutting parameters and the pre-splitting blasting parameters. Hence, to solve those three problems, a new design strategy was proposed based on the optimized directions of increasing the roof cutting height, decreasing the spacing between the blasting boreholes, changing the charge structure and adjusting the sealing length. According to the results of the theoretical calculation, numerical simulation and in situ measurement, the roof cutting height, the roof cutting angle, the spaced distance between the blasting boreholes, the charge structure and the sealing length were determined as 10 m, 15°, 500 mm, 4 + 4 + 3 + 3 + 2 and 2.4 m, respectively. The fissure rate was proposed to assess the developed effect of different pre-splitting blasting schemes. The proposed design strategy achieved a better effect of pre-splitting blasting for the entry, and eliminated the roof suspension of the hard main roof in the goaf. The design strategy in this study could contribute to other similar coal mines under the condition of a hard main roof.

In this study, we present a comprehensive investigation of the ground stability of a gob‐side entry by roof cutting in relation to the mining stage, based on field measurements, theoretical analysis, numerical simulation, and in situ practice. Differing from the normal mode of gob‐side entry, the mining‐induced stress and strain evolution under gob‐side entry by roof cutting (GSERC) is extremely complex, and unreasonable support can cause serious deformation and roof collapse of the GSERC. Taking Dadougou coal mine as the engineering background, a field investigation of the ground response, a rock mechanics experiment, and field measurement of the surrounding rock mass were first conducted. The roof‐to‐floor convergence presented an asymmetric deformation feature, based on the central axis. Numerical simulation is used to obtain results regarding the evolution of the second invariant of deviator stress and the third invariant of deviator stress (DSTI) in the primary use stage and the secondary reuse stage. It was found that the roof cutting action caused the peak value of the DSTI to transfer to the deep part of the roof. The DSTI value is greater than 0, indicating that tensile failure is dominant at 6–8 m of the roof above tailgate 5202, the upper part of the virgin coal rib, and 1.4 m away from the gangue rib. Finally, a support strategy for the following mining stages, including a support scheme for the primary use stage and the secondary reuse stage, is proposed based on the evolution law of the GSERC. The field‐measured results demonstrate that this support strategy can effectively control the convergence of tailgate 5203, thus leading to a better environment for coal mining. Additionally, the proposed mining stage support strategy can provide a reference for other similar projects.

Traditionally, the study on the bearing structure of overlying strata mainly focuses on bedrock with the effect of the overlying unconsolidated layer. It simply considered as uniformly loading on the top interface of the bedrock, and ignores the bearing structure for unconsolidated layer. Combined with the geological conditions of shallow burial thick unconsolidated layer in Shendong mining area,the mechanical model of the arch structure for the unconsolidated layers was established. The relationship between the arch height and span was analyzed with theoretical formula and the dynamic evolution mechanism of arch structure was studied with numerical simulation. Besides, formula of arch structure ultimate height was derived. The influence mechanism of arch structure on ground fissures was revealed and criterion was obtained, which was verified by field monitoring data. Furthermore, a classification method of arch structure in thick unconsolidated layers was proposed.