## 3 Application to the Shuangjiangkou Hydropower Station
### Measured Results of In Situ Stress in Underground Cavern Zones
#### 3.1.2 Analysis of the Field Test  
In situ stress measurements are a direct method for identifying the magnitude, direction, and distribution of in situ stress in the engineering field. Figure 4 displays the layout of in situ stress measurement points around the underground caverns on the left bank. The five measurement points are situated parallel to the main powerhouse's axis and located inside the SPD9# structure at an elevation of 2268 m. The measured data is obtained in the geodetic coordinate system, and thus it is necessary to transform them into the model coordinate system using Equation (1). The transformed results are listed in Table 3.  
As shown in Table 3, the measured stress in the x-axis direction in the cavern area is 10.21-19.26 MPa, which is approximately 1.1 to 1.4 times greater than that in the vertical direction. Similarly, the stress in the horizontal y-axis direction ranges from 15.95 to 23.50 MPa, which is around 1.4 to 1.8 times greater than the vertical stress. The measured stress values are ranked as \(\sigma_{\mathrm{y}}\), \(\sigma_{\mathrm{x}}\), and \(\sigma_{\mathrm{z}}\) in descending order, with horizontal stress \(\sigma_{\mathrm{y}}\) and \(\sigma_{\mathrm{x}}\) being higher than the vertical stress \(\sigma_{\mathrm{z}}\). This suggests that the tectonic movements have a significant impact on the stress distribution in the Shuangjiangkou underground cavern area. The shear stress values in the cavern area are generally low, with an average of 2.3 MPa for \(\tau_{\mathrm{x}\mathrm{z}}\), \(-3.7\) MPa for \(\tau_{\mathrm{y}\mathrm{z}}\), and 2.8 MPa for \(\tau_{\mathrm{x}\mathrm{y}}\) in the three directions.  
The monitoring data used in the back analysis may directly affect the accuracy of the inversed in situ stress distribution, as it is the main input for the back analysis algorithm. Therefore, filtering the illogical measured points is necessary for the inversion process. As shown in Table 3, the stress values at the 3# measured point are smaller than the others, and considering that the 3# point is located near the F1 fault, the stress decreases in this region may be reasonable. Notably, despite the fact that the 5# point is located at the lowest burial  
Figure 4: Distribution of in situ stress measured points.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Test No.**} & \multirow{2}{*}{**Test Point**} & **Burial** & \multirow{2}{*}{\(\sigma_{\mathrm{y}}\)**/MPa**} & \multirow{2}{*}{\(\sigma_{\mathrm{y}}\)**/MPa**} & \multirow{2}{*}{\(\sigma_{\mathrm{x}}\)**/MPa**} & \multirow{2}{*}{\(\tau_{\mathrm{x}}\)**/MPa**} & \multirow{2}{*}{\(\tau_{\mathrm{y}}\)**/MPa**} & \multirow{2}{*}{\(\tau_{\mathrm{y}}\)**/MPa**} & \multirow{2}{*}{\(\tau_{\mathrm{x}}\)**/MPa**} \\  & & **Depth** & & & & & & \\ \hline
1\# & SZK2 & 549 m & 16.92 & 23.50 & 15.04 & 4.44 & \(-3.19\) & 1.54 \\
2\# & SPD9-5 & 470 m & 19.26 & 22.74 & 16.66 & 0.33 & \(-7.11\) & 4.71 \\
3\# & SZK1 & 431 m & 10.21 & 15.95 & 9.10 & 0.86 & \(-2.60\) & \(-0.52\) \\
4\# & SPD9-6 & 357 m & 18.64 & 21.86 & 13.56 & 4.29 & \(-2.47\) & 6.81 \\
5\# & SPD9-1 & 308 m & 15.84 & 22.18 & 24.01 & 1.64 & \(-3.61\) & 1.37 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Stress components from in situ stress measurements around the underground caverns.  
depth of just 308 m compared to the highest 1# point of 549 m, the \(\sigma_{\rm z}\) value at this point is about 24 MPa, ranking first among these five measurement points (the average value being 15.6 MPa). This means that there probably exist stress fluctuations near the 5# point area, and the measured data at this point may not properly reflect the overall in situ stress distribution in the engineering area. However, to consider the comprehensiveness and diversity of the input sample, all five measurement points were included in the inversion calculation.