## 4 Results
### Triaxial Compression Test  
Constant normal stresses of 5 MPa, 10 MPa, and 20 MPa are applied to represent typical normal stresses encountered in underground engineering construction. The test results are shown in Table 7.  
Figure 8 shows the typical triaxial compression stress-strain curves of limestone with different \(\sigma_{3}\). It can be seen from Figure 8 that:  
1. The stress-strain curves can be divided into three stages: 1--the initial compression stage, 2--the line elasticity growth stage, and 3--the yield failure stage. The greater  
\begin{table}
\begin{tabular}{c c c c c} \hline
**No** & \(\sigma_{3}\)**/MPa** & \(\sigma_{1}\)**/MPa** & \(\sigma\) & \(\sigma\) \\ \hline \multirow{4}{*}{0.043} & 5 & 154.573 & 27.615 & 25.4 \\  & 10 & 171.748 & 32.956 & 35.2 \\  & 20 & 214.685 & 40.074 & 37.3 \\  & 5 & 136.618 & 26.880 & 24.3 \\
0.357 & 10 & 156.134 & 29.512 & 34.0 \\  & 20 & 195.168 & 38.466 & 40.1 \\  & 5 & 122.956 & 22.576 & 16.6 \\
0.625 & 10 & 136.618 & 26.375 & 25.5 \\  & 20 & 150.279 & 28.721 & 26.4 \\  & 5 & 98.365 & 19.157 & 16.3 \\
0.754 & 10 & 122.956 & 22.675 & 16.8 \\  & 20 & 136.618 & 26.421 & 25.6 \\ \hline \end{tabular}
\end{table}
Table 7: Triaxial test data of different initial damages and confining pressures.  
Figure 7: Plot of axial stress versus axial strain for SHPB test from \(\overline{x}=1\), 10, and 100 s\({}^{-1}\) levels of strain rate. (**a**) \(D=0.043\); (**b**) \(D=0.357\); (**c**) \(D=0.625\); (**d**) \(D=0.754\).  
the surrounding pressure, the greater the peak stress and the strain corresponding to the peak stress.
2. The principal stress increases with increasing confining pressure. For \(\sigma_{3}\) = 20 Mpa, the maximum principal stress is 214.685 Mpa of \(D\) = 0.043, which increases by 10.1%, 42.9%, and 57.1% for \(D\) = 0.357, 0.625, and 0.754, respectively. Moreover, for \(D\) = 0.043, the maximum principal stress is 154.573 Mpa of \(\sigma_{3}\) = 5 Mpa, which decreases by 10.0%, and 28.0% for \(\sigma_{3}\) = 10 Mpa and 20 Mpa, respectively. The confinement pressure inhibits the expansion of the microcracks in the limestone; thus, this affects the growth rate of the damage, which can delay the damage and strain softening of the rock. The side reinforcement behaviors of the confinement pressure are similar to the results reported by [PERSON] [52].
3. The slope of the line elastic phase of \(\sigma_{3}\) = 5 Mpa is greater than that of \(\sigma_{3}\) = 10 Mpa, 20 Mpa and \(D\) = 0.043. The slopes of the line elastic phase for \(\sigma_{3}\) = 5 Mpa and \(\sigma_{3}\) = 10 Mpa are approximate. Furthermore, the differences in the slopes of the line elastic phase are due to the initial damage and are diminished with increases in the confinement pressure. In addition, for \(D\) = 0.625 and 0.754, the line elasticity growth stages of three confinement pressures are approximated, further indicating that the characteristics of the confinement pressure restrained microcrack expansion into the macroscopic crack. A significant initial damage means that the large defects of the internal cracks in the limestone lead to the decreased strength and damage generation within the triaxial compression test process.  
Figure 8: Plot of axial stress versus axial strain for triaxial test from 5, 10 and 20 Mpa levels of confining pressure: (**a**) \(D\) = 0.043, (**b**) \(D\) = 0.357, (**c**) \(D\) = 0.625, (**d**) \(D\) = 0.754.