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Gob-side entry retained with roadside filling (GER-RF) plays a key role in achieving coal mining without pillar and improving the coal resource recovery rate. Since there are few reports on the cyclic filling length of GER-RF, a method based on the stress difference method is proposed to determine the cyclic filling length of GER-RF. Firstly, a stability analysis mechanics model of the immediate roof above the roadside filling area was established, and then, the relationship between the roof stress distribution and the unsupported roof length was obtained by the stress difference method. According to the roof stability above the roadside filling area based on the relationship between the roof stress and its tensile strength, the maximum unsupported roof length and rational cyclic filling length were determined. Combined with the geological conditions of the 1103 thin coal seam working face of Heilong Coal Mine and the 1301 thick coal seam working face of Licun Coal Mine, the suggested method was applied to determine that the rational cyclic filling lengths were 2.4 m and 3.2 m, respectively. Field trial tests show that the suggested method can effectively control the surrounding rock deformation along with rational road-in support and roadside support and improve the filling and construction speed. [ABSTRACT FROM AUTHOR]

In retained gob-side entry (RGE) of coal mining, the immediate roof above roadside backfill area (RBA) plays a key part in transferring the main roof load downward and transferring the roadside backfill body (RBB) resistance upon the main roof upward. Numerical investigation was performed to evaluate the stress distribution, deformation characteristics, and plastic zone distribution of the immediate roof above RBA based on a field case of RGE in a Chinese colliery. The double-yield model for gob, strain-softening Mohr–Coulomb model for RBB and strain-softening Mohr–Coulomb model for the roof above RBA were proposed to improve the reliability of the 3D global numerical model in FLAC3D. The numerical results of the calibrated global numerical model indicate that the vertical stress of the immediate roof above RBA shows a V-shaped distribution when the RBB width is less than 2.4 m. The horizontal stress of the immediate roof above RBA reaches the maximum when the RBB width is 2.0 m. The vertical displacement and horizontal displacement of the immediate roof above RBA positively are associated with the RBB width. In addition, the tensile failure area of the immediate roof above RBA, and the ratio of the tensile failure depth of the immediate roof above RBA to the tensile failure depth of the road-in immediate roof increases with the increase of the RBB width while the tensile failure depth of the immediate roof above RBA decreases. The proposed modeling procedure can be repeated in RGE to evaluate surrounding rock stability with different geological and engineering conditions. [ABSTRACT FROM AUTHOR]

Based on the analysis of the monitoring data of surface displacement and internal multipoint displacement of coal pillars near three inclined strata roadways, which have similar burial depth and different surrounding rock conditions in a coal mine in the east China, the coal pillars' stress change was combined with discontinuous numerical simulation software to simulate the deformation process of one of the roadways. Field monitoring data and numerical simulation results show that when this working face roadway is dug, the coal pillar enters plastic state after being affected by multiple mining influence, but it can still maintain the basic stability of the roadway and control coal pillar deformation under the current widely used supporting method. With the shortening of the distance between the monitoring area and the working face, the vertical and horizontal stress in the coal pillar increase gradually, but the maximum value of the vertical stress decreases by 1.4 MPa, and deformation amount and velocity of the coal pillar show the characteristics of nonlinear rapid increase. The shape of the vertical stress core area in the coal pillar changed from oval to rectangle, and the width of the core area increased, and it moved 1 m to the goaf side. In the process of coal pillar deformation, there is no zero displacement surface within the monitoring range of 6 m. [ABSTRACT FROM AUTHOR]

An innovative gateroad layout in highly gassy longwall top coal caving in a Chinese coal mine based on gob-side entry retaining (GER) was introduced. The numerical model was used to design the rational roadside backfill body (RBB) with high-water quick-setting materials. In order to improve the reliability of the numerical modeling, the double-yield model for gob modeling and the strain-softening model for RBB modeling are validated in detail. The results of the validated model indicate that when the RBB width is 1.5 m, the RBB peak vertical stress is minimal and surrounding rock convergence variation of GER becomes gentle. Consequently, the rational RBB width can be estimated as 1.5 m. Field test and field monitoring indicate that the GER in N2105 panel with a width of 1.5 m roadside backfill body and high-strength cable-rockbolt support could meet the requirement for gas drainage and ventilation during panel N2105 was retreating. It also proved that the new gateroad layout met the requirement for the highly gassy thick seam operation, and provided a new way to the coal mine safety production in similar conditions. [ABSTRACT FROM AUTHOR]

• A verified 3D numerical model based on in-situ observation data was established. • Floor stress distribution under the remnant pillar was obtained. • Stress concentration factor and stress transmission angle was analyzed. • An appropriate location of the lower coal seam entry was determined. Extraction of multiple seam using fully mechanized coal mining technology caused severe deformations in the lower coal seam roadways in Zhaiyadi Coal Mine in Shanxi Province, China. Understanding the characteristics of the lower coal seam entry failure mechanism under the remnant pillar is the first step in determining a reasonable location of the lower coal seam entry. Characteristics of the lower coal seam entry failure mechanism under the remnant pillar are investigated by means of numerical simulations and in-situ observations. To improve the reliability of the numerical simulations, the global model is validated by comparing the surrounding rock deformation of 3905 headgate with the in-situ observation data due to the extraction of Panel 3805. The numerical simulation results of the global model indicate that the original 3905 headgate under the remnant coal pillar is located in the vertical stress concentration zone and horizontal stress concentration zone. Firstly, the peak vertical stress concentration factor in the coal seam 9# (the lower coal seam) is 4.9, located 100 m behind the active Panel 3805 while the peak horizontal stress concentration factor in the coal seam 9# is 2.1, and located 30 m behind the active Panel 3805. When the delay distance to the active Panel 3805 is 80 m, the stress transmission angle reaches a constant value of 30.9°. Secondly, both the ratio of coal pillar rib deformation to solid coal rib and the ratio of roof subsidence to floor heave increase as the delay distance to the active Panel 3805 increases. Finally, numerical results show that the designed 3905 headgate located 25 m to 30 m away from the middle of the remnant coal pillar would be an alternative scheme, located out of the floor horizontal stress concentration and vertical stress concentration in the coal seam 9#. The findings in this study will help to provide a basis to select a reasonable location for lower coal seam roadways under similar mining and geological conditions. [ABSTRACT FROM AUTHOR]

To increase the recovery rate of coal mining operations, the gob-side entry retained (GER) technique has been extensively implemented in China. However, due to the mining-induced macro-cracks in the immediate roof above the roadside backfill area (RBA) under cyclic loading, the roof failure may occur and threaten the safety of GER. Mechanical characterization of rocks is crucial for understanding of the failure mechanism of this roof failure phenomenon. Based on the theoretical analysis and triaxial unloading confining pressure tests with cyclic loading on sandy mudstone, the strain energy evolution and conversion were studied. Results indicate that the total strain energy, elastic strain energy, and plastic-damage strain energy can be divided into various stages during the test. An index was introduced to describe the energy conversation and deformation of damaged rock specimen during the test, which can be expressed as the ratio of the plastic-damage strain energy to the total strain energy. This index increases gradually from the peak point to the post-peak unloading point. Then, it increases sharply from the first axial stress level to the first failure state, and finally decreases from the second axial stress level to the second failure state. Failure patterns of the damaged rock suggest that the macro-shear failure bands are mainly caused by the axial compression, whereas the micro-crack propagation is due to the circumferential expansion. The findings on the strain energy evolution and conversion of the immediate roof under the loading and unloading conditions of GER provides an insightful method to evaluate the roof stability above RBA, which also sheds lights on the GER design in coal mining to prevent the roof failure above RBA. [ABSTRACT FROM AUTHOR]

Taking gob-side entry retaining with large mining height (GER-LMH) of the 4211 panel in the Liujiazhuang coal mine as the engineering background, a numerical simulation was conducted to study the surrounding rock deformation, stress, and plastic zone distribution of GER-LMH with respect to retained entry width. The concept of critical retained entry width of GER-LMH was proposed. In view of the deformation characteristics of surrounding rock, an innovative approach to determine the critical width of GER-LMH based on the cusp catastrophe theory was proposed. The cusp catastrophe functions were set up by approximate roadside backfill body rib convergence and roof subsidence series with respect to different retained entry widths. The critical retained entry width of GER-LMH was 4.0 m according to bifurcation set equations. Surrounding rock stability control principle and technique of GER-LMH was proposed, including "rib strengthening and roof control": road-in support with high pre-stress rockbolts and anchor cables, roadside backfill body construction technology with high-water quick consolidated filling materials and counter-pulled rockbolt, road-in reinforced support technology with hydraulic prop support and roof master. Field test and field monitoring results show that GER-LMH with supercritical retained entry width in the 4211 panel could meet the requirements for ventilation when the 4211 panel was retreating. [ABSTRACT FROM AUTHOR]