*4.2. Stress Variation Characteristics for the Whole Coal Cavern Group*

In order to analyze the distribution law of stress and plastic zone in the cavern groups with different heights, three sections were selected along the *z*-axis. The sections are located at h = 3 m (the middle position of the large roadway), h=5m (the roof position of the large roadway), and h = 8 m (the roof position of high- and low-negative pressure caverns) from the coal seam floor. The simulation results are shown in Figures 6–8.

**Figure 6.** Surrounding rock variation characteristics for h = 3 m. (**a**) Maximum principal stress distribution; (**b**) vertical stress distribution; and (**c**) plastic differential distribution.

**Figure 7.** Surrounding rock variation characteristics for h = 5 m. (**a**) Maximum principal stress distribution; (**b**) vertical stress distribution; and (**c**) plastic differential distribution.

**Figure 8.** Surrounding rock variation characteristics for h = 8 m. (**a**) Maximum principal stress distribution; (**b**) vertical stress distribution; and (**c**) plastic differential distribution.

(1) The simulation result ath=3m

The height of the roadway near the underground pumping station is 3.5 m, and the surrounding rock variation in the position 3 m away from the coal seam floor is shown in Figure 6.

The distribution range of tensile stress generated by the excavation of the cavern is larger than that of the roadway (see Figure 6a). The maximum principal stress is larger in the area between the cavern groups, and the highest value is 8.09 MPa. The distribution law of vertical stress is roughly similar, but the maximum vertical stress value is 22 MPa (see Figure 6b). The larger the section of the cavern, the larger the scope of the plastic zone. The failure mode of the surrounding rock is mainly shear failure (see Figure 6c);

(2) The simulation result ath=5m

The surrounding rock variation in a position 5 m away from the coal seam floor is shown in Figure 7.

The peak values of the maximum principal stress and vertical stress decrease to 7.04 MPa and 19.3 MPa, respectively (see Figure 7a,b). The roof position of the roadway is obviously in the tensile stress zone, and the stress concentration area of the vertical stress begins to increase. The larger the section of the cavern, the greater the range of stress concentration. The larger the section of the cavern is, the larger the scope of the plastic zone is. The tensile failure occurs on the two sides of the cavern, the shear failure occurs in the rest of the scope, and the overall scope of the plastic zone increases gradually;

(3) The simulation result ath=8m

The surrounding rock variation in a position 8 m away from the coal seam floor, is shown in Figure 8.

The peak values of the maximum principal stress and vertical stress decrease to 6.45 MPa and 15.6 MPa, respectively (see Figure 8a,b). In this section, the stress near the cavern is not affected by the main roadway. The larger the section of the cavern, the greater the stress on the surrounding rock, and obvious stress asymmetry occurs. The plastic zone of the cavern continues to expand to the two sides as it moves away from the floor. The larger the section of the cavern, the larger the scope of the plastic zone. The range and distribution of the plastic zone obtained by numerical simulation are consistent with the theoretical results. These results can provide theoretical support for determining the supporting parameters, such as the length of the bolts and the cables;

(4) Stress distribution characteristics of cavern groups at different heights

Three sections were selected along the *z*-axis: the bottom of the cavern, the middle of the cavern, and the roof of the cavern. The stress curves for different heights were obtained, as shown in Figure 9.

The maximum principal stress of the roof attenuates sharply near the cavern and shows obvious stress concentration on the two sides of the cavern (see Figure 9). The variation trend of vertical stress is consistent with that of maximum principal stress. At the boundary of the model, the maximum principal stress and vertical stress increase and decrease sharply. In addition, the larger the section of the cavern, the greater the stress peak. In the middle of the cavern, the stress peak of the surrounding rock increases as a whole, and the distribution of the maximum principal stress is relatively stable among the cavern groups while the vertical stress increases gradually from high to low. Compared with the middle part of the cavern, the stress peak of surrounding rock near the bottom of the cavern gradually decreases, and the maximum and minimum principal stresses between the cavern groups show uniform distribution, but the influence range gradually decreases.

#### *4.3. Displacement Characteristics of the Whole Coal Cavern Group*

The displacement cloud chart of the cavern group obtained by the numerical simulation is shown in Figure 10.

**Figure 10.** A cavern displacement cloud chart. (**a**) The vertical displacement cloud chart; and (**b**) the horizontal displacement cloud chart.

The larger the section of the cavern, the greater the displacement of the roof caused by the excavation of the cavern. The maximum displacements of low- and high-negative pressure caverns are 103.2 mm and 92.4 mm, respectively (see Figure 11). The maximum two-sided displacement is 160 mm and 125 mm, respectively. The roof displacement curve of the low-negative pressure cavern is asymmetrical. The roof displacement of the highnegative pressure cavern has a saddle-shaped distribution and reached a maximum in the cavern roof left and right, respectively. The two sides of the cavern have an asymmetrical arch distribution. It indicates that a two-sided displacement in the middle and upper parts of the cavern is larger.
