## 3 Engineering Applications
### Measurement and Processing of Surface Strain  
To obtain the variation in surface strain on the rock core, a real-time measurement system for the surface strain of surrounding rock was developed. This system includes a self-made drilling rig (Figure 8), a JM3840 multi-channel continuous strain acquisition instrument (from Yangzhou Jingming Technology Co., Ltd., Yangzhou, China) with corresponding software, a laptop, and other components. Compared to a regular water drilling rig, the self-made drilling rig allows the strain gauge line to pass through the middle of the drill bit, enabling real-time measurement of strain data during the drilling process. In the present study, the sidewalls of the \(-930\) strike drift of the upper side and the \(2\#\) across vein roadway were chosen as measurement points to obtain strain data. Four measurement points were arranged in the strike drift of upper side, while five measurement points were arranged in the across vein roadway. The strain data of these measurement points were acquired using the surface stress relief method for subsequent analysis involving in situ stress field inversion. Due to the influences of the geological characteristics of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \(\sigma_{x}\) & \(\sigma_{y}\) & \(\sigma_{z}\) & \(\tau_{xy}\) & \(\tau_{yz}\) & \(\tau_{xz}\) \\ \hline Target value (MPa) & 19.80 & 18.00 & 12.60 & 5.40 & 3.60 & 7.20 \\ Inversion value (MPa) & 18.44 & 16.84 & 11.39 & 4.73 & 4.12 & 7.11 \\ Error & 6.87\% & 6.44\% & 9.60\% & 12.41\% & 14.44\% & 1.25\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Analysis of boundary stress errors (without considering plastic deformation). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Density** & **Compressive** & **Tensile Strength** & **Poisson’s** & **Young’s Modulus** & **Friction Angle** & **Cohesion** \\  & **(kg/m3)** & **Strength** & **(MPa)** & **Ratio** & **(GPa)** & **(c)** & **(MPa)** \\ \hline Average value & 2530 & 101.15 & 8.39 & 0.26 & 72.47 & 45 & 20 \\ Standard deviation & 0.01 & 16.73 & 0.73 & 0.03 & 8.57 & 6.58 & 3.62 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Physico-mechanical parameters of the rock. the surrounding rock and the measurement process, surface strain at six measurement points was measured. The test results from these six measurement points provide sufficient and appropriate data for the inversion of the stress field magnitude. The arrangement of these measurement points is shown in Figure 9. To reduce the error in the results of individual boreholes, two of three boreholes were drilled near each measurement point to obtain an average strain. The spacing between multiple boreholes followed the Saint Venant principle to avoid interference. Before measurement, the rock surfaces needed to be prepared, which involved processes such as polish, attaching strain gauges, waterproofing and wiring, borehole data acquisition, post-drilling waterproofing inspection, and core extraction (Figure 10). Surface strain measurements were used to quantify the in situ stress, and during the coring process, the surface strain gauges undergo corresponding changes due to the elastic recovery of the rock mass. In this process, the JM3840 multi-channel strain acquisition instrument records the data changes in the strain gauges in various directions. Typical strain variations are illustrated in Figure 11. After processing the measurement data using Equation (6), vertical normal strain, horizontal normal strain, and shear strain at the surface measurement points of the drift can be obtained. Considering the results of [PERSON] et al. [21], the results were further corrected using Equation (6). In the table, the \(x\), \(y\), and \(z\)-directions correspond to the established three-dimensional model \(xyz\)-directions, where the \(x\)-direction corresponds to the direction of the 2# seam across vein roadway, and the \(z\)-direction corresponds to the direction of the strike drift of upper side.