3.2. Distribution of Plastic Zone in Roadway after Mining
The plastic deformation zone of the roof of the panel 81305 No.2 return airway in the Baode coal mine was analyzed using numerical simulations and field measurements. Firstly, in order to discuss the changes of the two principal stresses (σ1
) at a fixed point of the roadway on both sides of the coal face after mining, a larger-scale numerical model is established to simulate the stress environment around the panel. As shown in Figure 5
a, the model’s height is 300 m and the length is 550 m. At the same time, the grid at the location of the mining roadway is carefully divided. The grid’s width of this area is 1.0 m. Table 1
shows the rock mechanics parameters of the coal seam and the roof and floor used by numerical simulation. Applying a vertical stress on the upper boundary of the model, which is equal to the weight of the rock above it, about 12 MPa, and the lateral pressure coefficient is 1.2. The initial stress state is: the Z direction is loaded with 12.0 MPa, and the X and Y directions are loaded with 14.4 MPa. The initial stress is assumed to be uniform because the actual depth of the coal seam changes little. In the horizontal direction of the model, the displacement and initial velocity in the x direction are limited. And the displacement and initial velocity in the directions of x, y, and z are limited at the bottom of the model .The Mohr-Coulomb criterion based on the elastic-plastic theory is used for the simulation.
After the excavation of the coal face, the stress around the coal face is redistributed. Figure 5
b shows the distribution of the principal stress in the side of the goaf at the position of 300 m behind the panel. It shows the stress concentration occurs within the range of 5–18 m on the side of the goaf. The maximum principal stress reaches 42–45 MPa, which is about four times of the primary rock stress. It has exceeded the uniaxial compressive strength of the surrounding rock of the roadway. Figure 5
c shows the numerical simulation results of the ratio of the maximum principal stress to the minimum principal stress on the side of the goaf. It can be seen that the farther away it is from the goaf, the more obvious is the gradient of the principal stress ratio decreases. In the range of 18–50 m from the goaf, the principal stress ratio is 2–3, while in the range of 18 m from the goaf, the principal stress ratio is 3–5. Besides, as the distance from the edge of the goaf is different, the direction of principal stress is also changing. The closer it is from the edge of the goaf, the larger the deflection angle of the principal stress direction is. And the maximum principal stress is about 50° to the side of the goaf compared to the primary rock stress at the position about 20 m from the edge of the goaf. The panel 81305 No. 2 return airway shown in Figure 3
is in the high stress difference region of the deflection of the principal stress direction. The shape of the plastic zone in the turbulent superimposed stress field environment will exhibit the maximum depth toward the roof.
The absolute value of the maximum principal stress at the center of the panel 81305 No. 2 return airway is about 35 MPa, and the ratio of the principal stress is about 3.0–3.5. The angle between the maximum principal stress and the vertical direction is about 46°. Under the stress condition, the angle between the normal direction of the rock and the vertical direction is 46° to facilitate the loading of the principal stress, as shown in Figure 6
Combined with the roof rock structure shown in Figure 2
is modeled by ANSYS software, then imported into the FLAC3D simulation software. In the horizontal direction of the model, the displacement and initial velocity in the x direction are limited. And the displacement and initial velocity in the directions of x, y, and z are limited at the bottom of the model. The simulation adopts the Mohr-Coulomb criterion based on the elastic-plastic theory, the behaviour of the strata is perfectly plastic after calculation. The calculation is carried out through the process of loading, the assignment of mechanical parameters of each rockstrata, and the excavation of the roadway.
b shows the numerical simulation results of plastic zone distribution of the panel 81305 No.2 return airway in the Baode coal mine. It can be seen that the plastic zone distribution of this roadway presents the morphological characteristics of maximum failure depth toward the roof, and its plastic zone is obviously distributed in the inner part of the coal seam and the mudstone layer in the upper part of the coal seam, showing penetrability. Moreover, the plastic failure depth of roof at different locations is different, and the plastic failure depth of roof at coal pillar side is obviously greater than that at cutting coal side, which is highly consistent with the distribution characteristics of roof subsidence in field roadway. Figure 6
c is the actual photograph of roof deformation about 300 m behind the coal face of the panel 81305 No.2 return airway in Baode coal mine. The deformation of roof on the coal pillar side reaches 450 mm, and the degree of deformation is obviously greater than that on the cutting coal side. The deformation magnitude of roof corresponds to the depth of roof failure.
Since the plastic failure is essentially compression-shear failure, the rock surface under compression-shear is usually rough, and the rock surface under tension is smooth. Based on this, the distribution of plastic zone in the panel 81305 No.2 return airway in the Baode coal mine is detected by using roof dense boreholes. The detection location is 400 m behind the panel 81305, and the roof deformation at this location is close to the maximum, as shown in Figure 3
. The peep depth is 8.0 m, and five groups of detection holes are evenly arranged in the same section. Because of the serious deformation and damage of roadway surrounding rock, the borehole peep should be carried out in time after the hole is formed, and the shallow hole protection should be carried out to prevent the phenomenon of hole collapse.
The detection results also show that the plastic zone of surrounding rock will penetrate through the strata without plastic failure and redistribute in the upper strata with lower strength. Thus the shallow plastic zone and penetrating plastic zone appear. The shallow plastic zone shows in the shallow part of coal seam. From the roof on the coal pillar side of the roadway to the roof on the cutting coal side of roadway, the failure depth of the shallow plastic zone is successively 2418, 2518, 2076, 1530 and 1168 mm, and the shallow plastic zone has non-uniformity, which has a maximum depth toward the roof, as shown in Figure 7
a. The plastic zone penetration starts from the thin mudstone layer above the coal seam, and all the thin mudstone layer is destroyed within the scope of peeping. At the same time, local damage occurred on the sandy mudstone layer, and the maximum damage depth occurred on the coal pillar side, which was 735 mm from the upper boundary of the coal seam. The rock formation without plastic failure between the high penetrating plastic zone and the shallow plastic zone has a few cracks. And the damage surface is relatively smooth. The on-site plastic zone detective results are almost consistent with the on-site deformation observation and numerical simulations, as shown in Figure 7