## 4 Engineering Practice
### Stress Comparison within the Plastic Zone  
Given the complex terrain of the power diversion tunnels at the Bulunkou Hydropower Station project and practical challenges like mountain denudation, a comparative test was undertaken between the borehole sleeve core lifting method and the hydraulic fracturing method to validate the reliability of test results. Core release testing was conducted in three borehole test holes, and the measurement outcomes are presented in Tables 4 and 5. To assess the impact of thermal stress on stress within the plastic zone of the surrounding rock, stress analysis was conducted considering the combined influence of ground temperature and ground stress. This was calculated using Equation (3) and compared with monitored stress obtained via the core lifting method (see Figures 11 and 12). Figure 10: Comparison of plastic zone radius when polar angle changes (\(\lambda\) = 2.8). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Modulus of Elasticity** & **Poisson’s Ratio** & **Cohesive Force** & **Angle of Internal Friction** &
\begin{tabular}{c} **Coefficient of Linear Expansion** \\ **of Surrounding Rock** \\ \end{tabular} \\
**E/Gpa** & \(\mu\) & **C/Mpa** & \(\psi/^{c}\) & \(\beta/^{c}\)**C\({}^{-1}\)** \\ \hline
6 & 0.25 & 0.9 & 30 & 0.000005 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mechanical parameters of tunnel envelope. Based on actual measurements of various rock temperatures (T) and burial depths (H), as detailed in Table 3, it is evident that the thermal stress induced in the rock mass due to the ground temperature gradient in the high-temperature tunnel entrance at Bulun is  
\[\sigma^{T}\ =\ \mu\beta EZ\ =\ \beta E(T-T_{0})\ =\ 0.000005\ \times\ 6000\ \times\ (85-20)\ =\ 1.95\ \mathrm{MPa} \tag{5}\]  
Referring to Table 5, the self-weight stress of the tunnel is approximately 15.2 MPa, indicating that the rock thermal stress constitutes 12.83% of the self-weight stress. This means that the in situ ground stress measured by hydraulic fracturing in the tunnel lacks approximately 13% of the rock thermal stress. In Figure 11, at a \(\sigma^{\mathrm{T}}\) of 1.95 MPa, the theoretically calculated radial stress ranges from 14.82 to 18.86 MPa, the circumferential \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Measurement Point** & **Depth of Measurement** & \(\sigma_{\mathrm{X}}\) & \(\sigma_{\mathrm{Y}}\) & \(\sigma_{\mathrm{Z}}\) & \(\sigma_{\mathrm{XY}}\) & \(\sigma_{\mathrm{YZ}}\) & \(\sigma_{\mathrm{ZX}}\) \\ \cline{2-7}
**Number** & **Point/m** & & & & **/MPa** & & \\ \hline SYZK1-1 & 13.3 & 21.9 & 20.9 & 15.6 & \(-5.5\) & 0.5 & 0.1 \\ SYZK1-2 & 14.5 & 22.4 & 22.9 & 14.9 & \(-7.1\) & 0.8 & \(-0.3\) \\ SYZK1-3 & 15.1 & 22.4 & 19.8 & 15.3 & \(-7.3\) & 0.5 & 1.1 \\ SYZK1-4 & 16.3 & 25.0 & 22.0 & 15.3 & \(-8.7\) & 1.1 & \(-1.1\) \\ \hline SYZK2-1 & 13.9 & 20.5 & 16.7 & 15.7 & \(-7.1\) & 1.0 & \(-1.1\) \\ SYZK2-2 & 14.5 & 21.3 & 18.5 & 15.2 & \(-8.8\) & 1.3 & \(-1.5\) \\ SYZK3-1 & 13.0 & 21.0 & 16.5 & 15.4 & \(-12.0\) & 3.9 & \(-2.0\) \\ \hline SYZK3-2 & 13.5 & 17.6 & 21.6 & 14.8 & \(-12.3\) & \(-0.8\) & 1.4 \\ SYZK3-3 & 14.0 & 19.1 & 21.9 & 14.6 & \(-11.9\) & 1.1 & \(-1.4\) \\ \hline \hline \end{tabular} Note: The X-axis is due north, the Y-axis is due west, and the Z-axis is vertically upward in a right-handed system.  
\end{table} 
Table 4: Table of results for stress components. Figure 11: Comparison of theoretical and monitored values of peripheral rock stress under different lateral pressure coefficients. stress ranges from 30.26 to 34.89 MPa, and the tangential stress ranges from \(-4.82\) to \(-1.41\) MPa. Most errors between the theoretical and monitoring values, corresponding to various \(\lambda\), fall within the range of 10.32% to 33.40%, indicating significant discrepancies. The thermal stress within the rock mass significantly contributes to the overall stress.