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AbstractThe traditional index to determine the width of the stopping coal pillar (WSCP) is generally only the front abutment pressure, but the direction of the principal stress will deflect due to the mining action, which brings significant challenges to controlling the surrounding rock. Based on theoretical calculation and numerical simulation, we studied the deflection law of principal stress under mining action, quantified the bearing capacity of roadway surrounding rock, and determined WSCP. The results show that: (1) With the deflection of the principal stress, the distribution of the surrounding rock plastic zone also rotates; (2) [Formula: see text] and repeated mining dominate and aggravate the deflection angle and range of the principal stress in front of the working face, respectively. When [Formula: see text] is close to 1, the deflection of principal stress should be paid attention to in determining WSCP; (3) The anchorage composite bearing structure (ACBS) provides a scientific basis for quantifying the bearing capacity of surrounding rock. WSCP of the lower coal seam is determined to be 60–75 m, and the corresponding principal stress deflection is about 135°. The control of the surrounding rock in the diagonal direction where the maximum principal stress is concentrated should be paid attention to.

Abstract To analyze the plastic failure process and mechanism of gob floor in the process of close‐distance coal seams mining, a simplified mechanical model of the lateral abutment pressure of the gob was proposed to derive the stress of surrounding rock. The distribution characteristics of gob floor stress deviator were obtained, and the effect of geological parameters (coal depth H and horizontal lateral pressure coefficient λ) were studied. By combining Mohr–Coulomb theory, the influence of surrounding rock cohesion c, internal friction angle φ, different coal seam depths H and different horizontal lateral pressure coefficients λ on the distribution of plastic zone of gob floor was obtained. The results showed that the distribution of the plastic zone under different geological factors is roughly the same as that of the stress deviator. This also proves that the excavation of the rock layer causes the redistribution of stress, which leads to the plastic failure of the surrounding rock. To verify the rationality of the proposed model, industrial experiments and simulation experiments were carried out. The results of on‐site investigation showed that the stability and integrity of surrounding rock in the concentration area of stress deviator is far less than that in the low concentration area. In addition, the acoustic wave test and the displacement field monitoring result of surrounding rock in the simulation experiments of a similar model are basically consistent with the distribution of the stress deviator field. These results agree well with the distribution of stress deviator obtained by theoretical calculation.

Abstract Close-distance coal seams are widely distributed over China, and the coal pillars left by the overlying coal seams affect the retracement channel of the underlying coal seam in the stopping stage. Based on the engineering background of close-distance seam mining in a coal mine, the reasonable position of the underlying coal seam's stopping line and the support method of the large section roadway during stopping are investigated using field measurements, similar simulation experiments, and numerical simulations. There are three types of location relationships between the stopping line of the underlying coal seam and the stopping line of the overlying coal seam: "externally staggered with the upper stopping line" (ESUL, stops mining under the overlying goaf), "overlapped with upper stopping line" (OUL), and "internally staggered with the upper stopping line" (ISUL, ISUL-SD for shorter internal staggered distances, ISUL-LD for longer ones). There are different stress arch structures in the overlying strata of the above three positions, and the stress arch evolution process exists in the process of ESUL → OUL → ISUL-SD → ISUL-LD: a front and rear double stress arch structure → the front arch gradually decreases → the front arch dies out, and the double arch synthesizes the single arch → the single-arch range expands → the nested double arch. The relationship between the stress arch structure and the position of the stopping line is evaluated as follows: (1) ESUL: the stress concentration in the roof plate of the retracement channel of the underlying coal seam is the highest, because the overburden block of the extensive collapse zone acts directly on the roof plate of the retracement channel, resulting in relative difficulties in roof support. (2) OUL: although the retracement channel roof pressure is minimal, the overlying rock structure has the potential for rotation or slippage instability. (3) ISUL-SD: the pressure on the roof of the retracement channel is small and the overburden structure is stable, which is conducive to the safe retraction of the support and not limited by the width of the end-mining coal pillar. (4) ISUL-LD: it is basically the same as the condition of stopping under the non-goaf; however, it has a limitation on the width of the end-mining coal pillar. The location of the stopping line is selected as ISUL-SD, and the retraction process of the self-excavating retraction channel was adopted. A partition asymmetric support scheme which is proven by field practice is proposed, through a comprehensive analysis of the pre-stress field simulation of the support scheme, based on the different control requirements of the roof above the support and the roof of the retracement channel in the stopping area. This method realizes safe and smooth withdrawal of the support.

Abstract Different from single coal seam mining, the stress evolution in the end mining stage of close‐distance coal seams is extremely complex. The unreasonable position and support of the lower retracement channel will cause serious deformation and damage to the surrounding rock. Taking the Yanzishan coal mine as the engineering background, the deformation and failure characteristics of the retracement channel under different overlay environments and the key influencing factors of position design were discussed by numerical simulation, theoretical analysis, and field investigation. The results show that the coupling superposition of upper coal pillar high stress and mining dynamic pressure will form a dangerous area with severe ground pressure behavior. The retracement channel should be preferentially designed in zone A (overlying mining roadways), followed by zone B (overlying end‐mining coal pillar), and finally, zone C (overlying section coal pillar). In addition, the rational horizontal distance between the lower retracement channel and the upper end‐mining coal pillar should make the channel in a good stress environment. The safety distance between the retracement channel and the nearest main roadway (end‐mining coal pillar width) should be greater than the severe range of advance abutment pressure. Finally, the design principle and control strategy for the lower retracement channel is proposed. The feasibility and rationality of the study are verified by industrial applications.

Under the condition of fully mechanized top coal caving in close-distance coal seams, the surrounding rock of the stopping space easily loses stability during the withdrawal of mining equipment in the working face because the lower coal seam working face is located under the goaf and the overburden rock has a large range of complex interaction. Field investigation, theoretical analysis, laboratory experiment, similar simulation experiment, numerical simulation, and field industrial tests are used to carry out the research on the stability and control of the surrounding rock in the large section stopping space under the goaf in this paper. The research conclusions are as follows. (1) It is determined that the lower coal seam working face can only stop mining under the goaf, and the reasonable stopping position under the goaf should ensure that the key block fracture line of the main roof is behind the support. (2) The interaction law between the main roof's key blocks of the upper and lower coal seams is analyzed, and the catastrophic conditions for sliding instability and rotary instability of the main roof's key blocks of the upper and lower coal seams are obtained. (3) "Anchorage with push and pull equipment-Embedded anchorages and trays" integral anchoring technology is developed. The dimensions of the push and pull equipment are determined. (4) Through numerical simulation of the distribution characteristics of the anchor cable pre-stress field, the asymmetric control scheme of "Partition long and short anchor cables + Integral polyurethane mesh + Embedded anchorages and trays for roof protection" is determined. The rock pressure observation shows that the withdrawal of the working face equipment is implemented safely. [ABSTRACT FROM AUTHOR]

In order to determine the reasonable width of a stopping coal pillar in close-distance coal seams, the evolution law of front abutment pressure of the working face with repeated mining was studied. Based on the actual engineering project, we conducted field measurement, theoretical analyses, numerical simulations and a physical similarity simulation test to study. The results show that: (1) according to field measurement, the influence range of front abutment pressure increases from 60 m to 75 m with repeated mining; (2) according to theoretical analysis, the arch height and span are negatively and positively correlated with the influence range of front abutment pressure, respectively; (3) with repeated mining, the arch height increased to 165 m, the arch span to 235 m and the influence range to 83.5 m by 14.5 m relative to that before repeated mining; (4) if it is necessary to ensure that the main roadway is less affected by the mining stress, the width of the stopping coal pillar in 2214 working face should be greater than 80 m. The influence range of front abutment pressure increases obviously with repeated mining in close-distance coal seams. The study provides a reference for similar engineering projects. [ABSTRACT FROM AUTHOR]

To analyze the plastic failure process and mechanism of gob floor in the process of close‐distance coal seams mining, a simplified mechanical model of the lateral abutment pressure of the gob was proposed to derive the stress of surrounding rock. The distribution characteristics of gob floor stress deviator were obtained, and the effect of geological parameters (coal depth H and horizontal lateral pressure coefficient λ) were studied. By combining Mohr–Coulomb theory, the influence of surrounding rock cohesion c, internal friction angle φ, different coal seam depths H and different horizontal lateral pressure coefficients λ on the distribution of plastic zone of gob floor was obtained. The results showed that the distribution of the plastic zone under different geological factors is roughly the same as that of the stress deviator. This also proves that the excavation of the rock layer causes the redistribution of stress, which leads to the plastic failure of the surrounding rock. To verify the rationality of the proposed model, industrial experiments and simulation experiments were carried out. The results of on‐site investigation showed that the stability and integrity of surrounding rock in the concentration area of stress deviator is far less than that in the low concentration area. In addition, the acoustic wave test and the displacement field monitoring result of surrounding rock in the simulation experiments of a similar model are basically consistent with the distribution of the stress deviator field. These results agree well with the distribution of stress deviator obtained by theoretical calculation. [ABSTRACT FROM AUTHOR]

Different from single coal seam mining, the stress evolution in the end mining stage of close‐distance coal seams is extremely complex. The unreasonable position and support of the lower retracement channel will cause serious deformation and damage to the surrounding rock. Taking the Yanzishan coal mine as the engineering background, the deformation and failure characteristics of the retracement channel under different overlay environments and the key influencing factors of position design were discussed by numerical simulation, theoretical analysis, and field investigation. The results show that the coupling superposition of upper coal pillar high stress and mining dynamic pressure will form a dangerous area with severe ground pressure behavior. The retracement channel should be preferentially designed in zone A (overlying mining roadways), followed by zone B (overlying end‐mining coal pillar), and finally, zone C (overlying section coal pillar). In addition, the rational horizontal distance between the lower retracement channel and the upper end‐mining coal pillar should make the channel in a good stress environment. The safety distance between the retracement channel and the nearest main roadway (end‐mining coal pillar width) should be greater than the severe range of advance abutment pressure. Finally, the design principle and control strategy for the lower retracement channel is proposed. The feasibility and rationality of the study are verified by industrial applications. [ABSTRACT FROM AUTHOR]

In order to solve the application problem of gob-side entry retaining formed by roof cutting (GERRC) technology of working face under goaf in close-distance coal seams, the 9101 working face and its return air roadway of Xiashanmao Coal Mine were taken as engineering background. The key parameters of GERRC were studied by means of theoretical analysis, numerical simulation and field test. The results show that: The roof cutting height (RCH) should be not less than the height of stope caving, meanwhile, when the rock strata collapsed, broke and expanded within RCH, it should meet the support effect on basic roof. And the RCH should be less than the distance between coal seams. The roof cutting angle (RCA) should meet the mechanical conditions of rock mass failure along the structural plane, and it should meet the geometric condition of not squeezing the roadway roof during the rotation and subsidence of the stope basic roof. When the RCH was 7.5m, the RCA was 15 degrees and the blasting charge structure was 4+3+3+2, the stress distribution and roof subsidence were most optimal. Three rows of constant resistance with large deformation (CRLD) anchor cables were used to strengthen roadway roof. The unit support, hydraulic prop and π-shaped beam were carried out as temporary support in roadway. The surrounding rock of roadway tended to be stable when lagged behind 100m, and the effect of GERRC was good after the temporary support measures were withdrawn. [ABSTRACT FROM AUTHOR]

Under the condition of fully mechanized top coal caving in close-distance coal seams, the surrounding rock of the stopping space easily loses stability during the withdrawal of mining equipment in the working face because the lower coal seam working face is located under the goaf and the overburden rock has a large range of complex interaction. Field investigation, theoretical analysis, laboratory experiment, similar simulation experiment, numerical simulation, and field industrial tests are used to carry out the research on the stability and control of the surrounding rock in the large section stopping space under the goaf in this paper. The research conclusions are as follows. (1) It is determined that the lower coal seam working face can only stop mining under the goaf, and the reasonable stopping position under the goaf should ensure that the key block fracture line of the main roof is behind the support. (2) The interaction law between the main roof’s key blocks of the upper and lower coal seams is analyzed, and the catastrophic conditions for sliding instability and rotary instability of the main roof’s key blocks of the upper and lower coal seams are obtained. (3) “Anchorage with push and pull equipment-Embedded anchorages and trays” integral anchoring technology is developed. The dimensions of the push and pull equipment are determined. (4) Through numerical simulation of the distribution characteristics of the anchor cable pre-stress field, the asymmetric control scheme of “Partition long and short anchor cables + Integral polyurethane mesh + Embedded anchorages and trays for roof protection” is determined. The rock pressure observation shows that the withdrawal of the working face equipment is implemented safely.

The backfill mining of coal-based solid waste in goaf poses a potential risk of heavy metal pollution to the groundwater environment, and the migration behavior of heavy metals differs significantly under the disturbance of backfill mining in close-distance multi-layer coal seams and single-layer coal seams. In this study, a migration model of heavy metals after solid backfilling in the goaf of shallow-buried close-distance thick coal seams was established, and the impact of the overburden damage and the layered distribution of the filling body on the long-term migration behavior of heavy metals were analyzed. The results show that the migration of heavy metals after close-distance coal seam backfill mining exhibits a higher risk of heavy metal pollution. The peak permeability of overburden after close-distance coal seam backfill mining is about 600 × 10−19 m2 higher than that after single-layer coal seam backfill mining. The migration distance of heavy metals in the floor after backfill mining of close-distance coal seams is 7.41 m farther than that of single-layer coal seam backfill mining, and its migration time of heavy metals to the surface is 27 a earlier than that of single-layer coal seam. This research provides theoretical and empirical support for the ecological risk assessment and heavy metal pollution control in close-distance coal seam backfill mining. The main filling material of close-distance coal seams backfill mining is coal gangue. Heavy metal elements such as Mn and Cr will be released in the underground environment for a long time, and the migration behavior of heavy metal elements will have an impact on the groundwater environment for more than 1000 years. This research provides theoretical and empirical support for the ecological risk assessment of close-distance coal seam backfill mining and the mitigation of heavy metal pollution. [Display omitted] • The heavy metal migration model for close-distance coal seam backfill mining were established. • The evolution characteristics of overburden permeability in close-distance coal seam backfill mining were analyzed. • The migration behavior of heavy metals after solid backfilling in the single-layer coal seam were analyzed. • The migration behavior of heavy metals after close-distance coal seam backfill mining were analyzed. [ABSTRACT FROM AUTHOR]

Abstract The Gaojialiang Coal Mine in Inner Mongolia in China was used as an example to investigate the process of chain pillar and gateroad failure in the lower coal seam under the effect of an irregular residual coal pillar in the upper close‐distance coal seam. The results indicated that after the longwall face in the lower coal seam retreated through the area under the large irregular residual coal pillar in the upper coal seam, the bearing area of the residual coal pillar decreased and the roof pressure continuously concentrated in the elastic core area of the residual coal pillar. Isolated triangular slab rock was formed above the triangular residual coal pillar, and the weight of the overlying strata was transferred downward through the triangular slab rock. When the chain pillar under the residual coal pillar was destroyed due to loading that exceeded its carrying capacity, the upper triangular slab rock rotated and sank, which resulted in the intensification of the concentrated stress in the solid rib of the gateroad in the lower coal seam. Based on the analysis, it is suggested that in the design of a longwall system, a gateroad under an isolated residual coal pillar in the upper coal seam should be avoided when mining in close‐distance coal seams.

The Gaojialiang Coal Mine in Inner Mongolia in China was used as an example to investigate the process of chain pillar and gateroad failure in the lower coal seam under the effect of an irregular residual coal pillar in the upper close‐distance coal seam. The results indicated that after the longwall face in the lower coal seam retreated through the area under the large irregular residual coal pillar in the upper coal seam, the bearing area of the residual coal pillar decreased and the roof pressure continuously concentrated in the elastic core area of the residual coal pillar. Isolated triangular slab rock was formed above the triangular residual coal pillar, and the weight of the overlying strata was transferred downward through the triangular slab rock. When the chain pillar under the residual coal pillar was destroyed due to loading that exceeded its carrying capacity, the upper triangular slab rock rotated and sank, which resulted in the intensification of the concentrated stress in the solid rib of the gateroad in the lower coal seam. Based on the analysis, it is suggested that in the design of a longwall system, a gateroad under an isolated residual coal pillar in the upper coal seam should be avoided when mining in close‐distance coal seams. [ABSTRACT FROM AUTHOR]