Taking practical engineering geological environment of 8,102 panel as the background, the panel and the panel on both adjacent sides are selected to establish a model (Figure 3). Specific dimension of the model is 750 m × 300 m × 90 m.

Due to the non-homogeneity of a rock mass within a strata, the physico-mechanical parameters obtained in a laboratory are those of intact rock. The mechanical parameters in the laboratory are modified to apply to the actual conditions of a rock mass. Therefore, a rectangular model with dimensions of 12 m × 12 m × 24 m (length × width × height) is created to simulate the compression process of a rock mass, where the mesh size is consistent with the dimensions of the roadway surrounding rock in this paper. By using a trial-and-error approach, parameter modification is considered complete when the macro-mechanical characteristics in a model are consistent with the field. As the mined space on both sides of N8102 isolated panel is huge, double yielding model is used to simulate the filling effect of the gangue, which can reflect the support action of the gangue caving from the goaf. The model adopts the Moore-Coulomb damage criterion with fixed displacements around all sides and bottom of a model, with vertical loads applied to its top. Buried depth of N8102 panel is 450 m. Bulk density of overlying strata is 2,500 kg/m³. The load applied on the model is 9.922 MPa. Table 1 shows the parameters of each rock stratum when the numerical model is established. Elastic modulus, Poisson’s ratio, cohesion and tensile strength are modified in a certain proportion according to the laboratory test results (Itasca, 2007; Cai et al., 2013).

During the simulation of excavation, Tecplot software was used to extract the distribution data and stress nephogram of the vertical stress in N8102 panel after mining on one side and mining on both sides, as shown in Figure 4. Statistical charts were drawn by Origin platform (Figure 5).

The comparative analysis of Figures 4, 5 shows that.

1) After the mining of N8101 panel is completed and the gob collapse is stable, a bearing pressure concentration area is formed at the side of N8102 panel close to the goaf. Peak stress at the roadway middle line can reach 28 MPa, and the stress concentration factor is 2.55. This is because as N8101 panel is mined, the roof is broken, and the bearing stress is transferred to the panel. Vertical stress of the panel reaches more than 25 MPa within the range of 9–20 m, and the peak stress occurs at 14.5 m from the side wall of the gob. As far as 55 m distance to the coal barrier, the vertical stress in N8102 panel should drop to the in-situ stress. Because N8101 panel has a large mining space, the effect extent of lateral abutment stress is large.

When the numerical calculation software reaches convergence, vertical stress distribution of the yield pillar in N8102 isolated panel is obtained (Figure 6). Tecplot software is used to set the vertical stress measuring line in the middle of coal pillar simulation schemes. Distribution size of vertical stress in the yield pillar extracted by the monitoring line is obtained (Figure 7). Peak stress distribution curve of virgin rib and pillar rib are obtained (Figure 8).

From the above analysis that a lateral abutment pressure concentration area in N8102 panel is located in the middle of the solid coal rib slightly higher than the roadway. The maximum of the stress concentration area is 32.9 MPa. With the increase of CPW, roadway approaches stress concentration area, and peak value of the lateral abutment pressure decreases from 32.9 MPa to 28.9 MPa. With the increase of CPW, surrounding rock stress becomes larger and larger. When CPW is small, the roadway is generally in a safe and stable environment at the side of the adjacent gob. When CPW is greater than 8 m, vertical stress of surrounding rock increases rapidly. When the CPW is less than 8 m, the roadway is less influenced by the high lateral abutment pressure on the solid coal side. When CPW reaches 12 m, lateral abutment stress begins to transfer to the coal pillar side, the stress concentration begins to appear within the yield pillar and peak stress is greater than in-situ stress.

As shown in Figure 7, when CPW increases from 4–22 m, vertical stress of yield pillar presents an obvious unimodal distribution, and presents a pattern of increase followed by decrease from the gob side to the roadway side. Stress concentration occurs in the yield pillar, and stress concentration factor is different, and the factor increases with the increase of CPW. When CPW increases from 4 m–8 m, stress extreme value in the yield pillar is always closer to the gob side. When CPW increases from 8 m–22 m, vertical stress extreme value in the yield pillar is closer to the side of roadway. When CPW is 4 m, maximum of the vertical stress in the narrow yield pillar is lower than the in-situ stress at the depth of coal pillar. It indicates that a large area of crushing area appears in the yield pillar and its support capacity is correspondingly reduced due to breaking rotation of rock and high lateral bearing pressure of the panel. In the process of increasing the CPW from 6 m–22 m, extreme value of vertical stress inside the yield pillar is always greater than the in-situ stress at the depth of pillar, and the area where the stress inside the yield pillar is greater than in-situ stress is closely associated with CPW. After the CPW increases, bearing capacity to the overlying rock is also enhanced.

As shown in Figure 8, when CPW increases from 4–22 m, peak stress on the yield pillars continues to increase. When CPW is 4 m, peak stress of the yield pillar is lower than the initial stress at depth of pillar. It shows that the 4 m yield pillar is greatly influenced by panel excavation, and damage enters a broken state. When CPW is 6 m, peak stress of the yield pillar begins to exceed the initial stress, indicating that the 6 m coal pillar has good bearing capacity.

From Figures 9, 10, the roof subsidence shows an increasing trend as the CPW increases. When the CPW is less than 8 m, the roof subsidence rises slowly. The maximum roof subsidence for the 6 and 8 m CPW is 390 mm and 410 mm respectively, an increase of 24.5% and 33.9% respectively compared to the 4 m pillar width. Meanwhile, as the CPW increases, the position of maximum roof subsidence shifts from the side of pillar towards the centre of roof. When the CPW is greater than 8m, roof subsidence in a roadway begins to rise sharply. The maximum subsidence at 12 m and 20 m CPW increases by 68.1% and 141.6% respectively compared to the 4 m pillar width. Among them, the location of maximum subsidence is distributed in the middle of a roof.

Under the conditions of coal pillar with different widths, the displacement characteristics at pillar ribs and solid coal ribs are that the displacement above the rib is significantly greater than the displacement at the lower end of the rib (Figure 11). It can be seen that the roadway pressure mainly comes from the key block rotary subsidence at the upper part and the high lateral abutment pressure. Because the coal seam of N8102 panel is very thick, the pressure is mainly concentrated in the position inclined to the upper side of a roadway.

As the CPW grows, the solid coal rib is more seriously affected by it, and the horizontal displacement increases (Figure 12). The maximum horizontal displacements on coal pillar ribs and solid coal ribs of 6 m pillar width are 403 mm and 272 mm respectively, an increase of 56.3% and 38% respectively compared to the 4 m pillar. The reason for this is that the 4 m coal pillar is severely fractured, with prominent propagation of plastic characteristics, resulting in a reduction in its load carrying capacity. The 6 m pillar has a good bearing capacity, and the deformation increases with the rise of pressure. As the CPW continues to increase to 12 m, the load bearing capacity of a pillar is rising, resulting in a gradual reduction in the horizontal displacement of pillar rib. When the CPW is greater than 12 m, the horizontal displacement on a pillar rib shows an increasing trend due to the gradual transfer of stresses on solid coal rib into pillar rib.

Figure 13 illustrates a variation curve of the deformation of a roadway and the CPW. The maximum deformation at a roadway with various CPW is obtained in Figure 13.

In order to explain the displacement and advance abutment stress concentration of a roadway under a condition of 6 m width coal pillar, FLAC3D software is employed to analysis of deformation and stress during mining in a panel. Figure 14 illustrates the vertical stress distributions across a panel when a N8102 isolated face is pushed forward by 100 m. The obvious stress concentrating develops in upper corner of a panel, which is 21 m in front of N8102 panel. The peak stress in this region reaches 45 MPa with a stress concentration factor of 4.09. This phenomenon indicates that the excavation of this panel has caused a significant difference in the stress distribution characteristic of a roadway surrounding rock and a more pronounced degree of stress concentration.

Figure 15 illustrates the stress distribution of a roadway for 21 m ahead of the N8102 panel. The peak stress in the 6 m coal pillar increased by 80% from 12.5 MPa to 22.5 MPa under the effect of advance abutment stress, which is greatly affected by the advance abutment stress.

As indicated in Figure 16, under the dual effect of advance abutment pressure and lateral abutment pressure, the roof subsidence of the roadway increases from 390 mm during roadway excavation to 568 mm, an increase of 46%. The maximum displacement of roadway coal pillar rib increases from 403 mm to 642 mm, an increase of 59%. The maximum displacement of the solid coal rib increases by 178 mm. The results display that the displacement of a surrounding rock increases considerably under the influence of advance abutment stress.

Based on the analyses of stresses and displacements in surrounding rocks, 2102 tailgate is greatly affected by the mining of this panel, but the overall stress environment and displacement of surrounding rock can be stabilized within the controllable range through reasonable support. In the process of face mining, it is essential to focus on strengthening the support of the affected section of the advance abutment stress to ensure the roadway stability.

Base on the deformation characteristics of 2102 tailgate in N8102 isolated panel and simulation analysis of different CPW, the final asymmetric joint control technology of asymmetric anchor cable truss, high intensity anchor cable, and coal pillar grouting are employed to support a roadway. A diagrammatic drawing of specific roadway support is displayed in Figure 17.

The solid coal rib is supported by anchor cables with a spacing of 1,000 mm x 900 mm. The anchor cables are installed perpendicular to roadway ribs, except for the anchor cables on the upper and lower sides which are installed at 15° to the vertical direction. The anchoring agent is MSZ2360 resin cartridge, with one anchor rope and one anchor rope. The size of the metal mesh used in the rib is 50 × 50 mm. A diagrammatic drawing of the solid coal rib support is shown in Figure 18.

The coal pillar rib is set up with a row of anchor cables more than the solid coal rib. The anchor cables are Φ21.8–1 × 19–4,300 mm steel strand with row spacing of 1,000×900 mm. Each anchor cable adopts an MSZ2360 resin cartridge. To enhance the control of coal pillar ribs, C12 concrete with a thickness of 100 mm is used for grouting and spraying. A diagrammatic drawing of coal pillar rib support is displayed in Figure 19.

As illustrated in Figure 20, a maximum displacement between the roof and floor and between the two ribs during a roadway excavation are 198 mm and 149 mm respectively, which indicates that a roadway is well controlled using asymmetric combined support scheme.

During the mining of N8102 isolated panel, the maximum displacements between two ribs and between roof and floor are 575 mm and 508 mm respectively. The results suggest that the deformation on surrounding rocks during mining is greater than that during roadway excavation. Therefore, the impact of mining on the N8102 panel is more dramatic. However, the support elements in a roadway are still in a stable condition and a whole displacement of surrounding rock of a roadway is in a controllable range.