# Stability and Control of Surrounding Rock of a Trapezoidal Roadway Retained with Hard Roof Cutting
## 3 Deformation and Control Mechanism of Surrounding Rock for a Top-Cutting Roadway
### Stress and Deformation Rules of Surrounding Rock  
The stress distribution and deformation characteristics of the surrounding rock during the excavation period, mining period and top-cutting conditions of the 21,100 panel are shown from Figures 7-9. As can be seen from Figure 7a,b, before mining the 21,100 panel, the vertical stress and displacement of the surrounding rock were distinctly asymmetrical due to the influence of the trapezoidal roadway's asymmetric structure. Moreover, as opposed to the regular rectangular roadway, there was a stress reduction area at the side of the roadway with a larger height. The surrounding rock in this area changed from an elastic state to a plastic state, resulting in a decrease in the load-bearing capacity of the surrounding rock. As shown in Figure 8a,b, after mining the 21,100 panel, influenced by the subsidence of the rock strata, the vertical stress on one side of the gob was mainly characterized as compressive stress, while on the other side it was primarily tensile stress. Compared with the excavation period, the maximum value of vertical compressive stress increased from 0.47 MPa to 0.51 MPa, and the maximum value of vertical tensile stress increased from 26.7 MPa to 38.6 MPa. Affected by the side abutment pressure of the gob, high stress areas were mainly distributed in the side of the roadway side and the interior of the solid coal. There was a stress concentration area in the roadway side of the solid coal. Rock mass in this area may undergo plastic deformation, damage or instability due to excessive stress. Due to the staggered spatial relationship between the 21,100 panel and the trapezoidal roadway, the vertical displacement distribution of the roof and floor in the gob was asymmetrical. There may be a small-scale floor heave phenomenon in the area of the roadway adjacent to the gob floor. At this time, the maximum subsidence of the roof and gob side were 66 mm and 110 mm, respectively. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Rock Strata** & **Elastic Modulus** & **Cohesion** & **Tensile** & **Poisson’s** & **Friction** & **Density** \\  & **GPa** & **MPa** & **MPa** & **Ratio** & **(\({}^{\circ}\))** & **Kg/m\({}^{-3}\)** \\ \hline Coal & 2.66 & 1.5 & 0.52 & 0.22 & 26.1 & 1387 \\ Limestone & 28.99 & 4.0 & 3.30 & 0.32 & 33.7 & 2664 \\ Sandy Mudstone & 11.13 & 2.2 & 2.02 & 0.25 & 29.8 & 2588 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical and mechanical parameters of coal and rock mass. As shown in Figure 9a, affected by the pressure relief of top-cutting, the vertical stress on the roof and the solid coal was reduced, which meant that the pressure relief effect was obvious and the high stress area was transferred to the inside of the solid coal. The maximum vertical stress on the roof decreased from 6.73 MPa to 2.04 MPa; on the solid coal roadway side, it decreased from 38.60 MPa to 15.27 MPa, and it decreased from 17.85 MPa to 15.63 MPa inside the solid coal. Moreover, influenced by the deflection of the roof strata, the area of stress reduction at the top-cutting seam increased. As can be seen from Figure 9b, after the top-cutting, the roof exhibited slip deformation along the top-cutting line, and showed an asymmetric feature of \"large deformation on the top-cutting side\". The maximum subsidence of the roof decreased from 66 mm to 32 mm, and it increased from 110 mm to 119 mm on the gob side. The above shows that when setting the cutting height at 5\({}^{\circ}\) and the cutting angle at 10\({}^{\circ}\), the cantilever structure of the retained roadway roof can be effectively severed, optimizing the stress state and controlling the displacement of the surrounding rock. Figure 8: Stress distribution and deformation characteristics of surrounding rock during recovery period: (**a**) vertical stress; (**b**) vertical displacement. Figure 7: Stress distribution and deformation characteristics of surrounding rock during excavation period: (**a**) vertical stress; (**b**) vertical displacement.