*4.2. Results of Numerical Simulation*

Figure 12 shows the distribution of vertical stress and plastic area in the two ribs of the 42202 auxiliary transport roadway after mining at the 42201 working face. The red area and blue area indicate the plastic yielding and elastic states of the roadway surrounding rock, respectively. Parameters σpmax and σscmax indicate the peak vertical stresses in the coal pillar rib and the coal mass rib.

As shown in Figure 12, the plastic area of the roadway surrounding rock and the stress concentration of the two ribs show significant decreasing trends with the increase of coal pillar width. According to Figure 12a, plastic failures, mainly shear failures, occur diagonally across the whole coal pillar when the coal pillar width is 10 m. The peak vertical stress in the coal pillar rib is 57.33 MPa in its middle. The peak vertical stress of the coal mass rib is 43.57 MPa and about 4.0 m from the surface of the roadway. The stress concentration coefficients of the two ribs are 5.97 and 4.54, respectively. Therefore, the roadway is in the stress elevation area formed by the T-shaped remaining coal pillar in the overlying coal seam and the mining at the adjacent working face. As a result, the surrounding rock is subjected to great deviatoric stress, leading to a large number of cracks and a wide range of plastic failures. In the meantime, the coal pillar has a relatively large bearing capacity but is highly unstable after fracture and compaction.

According to Figure 12b, when the coal pillar width is increased to 15 m, the plastic failure does not penetrate the coal pillar, and an elastic core area of about 4.0 m appears in its middle. Compared with those at the coal pillar width of 10 m, the peak vertical stress of the coal pillar rib is reduced by 3.39 MPa, and that of the coal mass rib is reduced by 4.7 MPa. The stress concentration coefficients of the two ribs are 5.62 and 4.05, respectively. The large stress difference between the two ribs causes the asymmetrical distribution of the plastic area, and the surrounding rock failure is severe.

**Figure 12.** Stress and plastic area of surrounding rock under different widths of coal pillars: (**a**) The width of the coal pillar is 10 m; (**b**) The width of the coal pillar is 15 m; (**c**) The width of the coal pillar is 20 m; (**d**) The width of coal pillar is 25 m; (**e**) The width of coal pillar is 30 m.

According to Figure 12c–e, the elastic core area of the coal pillar rib keeps increasing while the range of surrounding rock failure gradually decreases with the increasing coal pillar width. When the coal pillar width is 20–30 m, the plastic area of the coal pillar rib is 3.5 to 4.0 m, and the plastic area of the coal mass rib is about 3.0 m. The vertical stress distribution on the coal pillar evolves from a single-peak shape to an asymmetric double-peak shape, the stress concentration tends to decrease, and the peak stress difference between the two roadway ribs gradually decreases. The reason is that as the coal pillar width increases, the roadway gradually avoids the superimposed stress elevation area, resulting in a decreased load on the surrounding rock and increased roadway stability. Combining the results in Section 4.1, the width of the coal pillar in the 42202 mining roadway section should be 25 m.

#### *4.3. Industrial Testing*
