## 2 Experimental Study
### Experimental Results  
According to the traditional sign convention in geomechanics, the compressive stress is considered here as a positive value. The conventional triaxial compression tests are conducted firstly and the deviatoric stress versus the axial and lateral strains under different confining stresses are shown in Figure 2a. The [PERSON]'s modulus can be determined from the linear part of the axial strain and deviatoric stress curve as \(E^{m}=45\) GPa and the Poisson's ratio can be calculated from the ratio of lateral strain over the axial strain as \(v^{m}=0.15\). Figure 2b presents the curves of the deviatoric stress \(\sigma_{1}-\sigma_{3}\) versus volumetric strain \(\varepsilon_{v}=\varepsilon_{1}+2\varepsilon_{2}\) of Beishan granite under different confining pressures. The transition from compressibility to dilatancy, as well as its dependence on confining pressure are observed on macroscopic scale [37]. The experimental results of single-stage creep tests are shown in Figure 3. A clear trend is observed that creep failure occurs at shorter timeframe when the deviatoric stress is higher. When the deviatoric stress (\(q\)) is 225.7 MPa, the creep failure occurs at around 110 h after the deviatoric stress is applied. When the deviatoric stress is increased to 228 MPa, the creep failure occurs at 64 h; as the deviatoric stress increased to 232.5 MPa, the creep failure occurs at around 1.7 h after the deviatoric stress is applied. It is thus concluded that the creep failure time is very sensitive to the levels of the deviatoric stress. When the deviatoric stress is increased by 3%, the creep failure time is reduced by 98%. If we calculate the axial strain rate and use it as the second y axis in the same plot as shown in Figure 3, clear boundaries of three typical creep stages (primary creep stage, steady creep stage, and accelerated creep stage) can be observed in all tested cases with different levels of deviatoric stresses. As summarized in Table 1, in all three tests, the accelerated creep stage has the highest strain rate in both axial (\(\varepsilon_{A}\)) and lateral (\(\varepsilon_{L}\)) directions, whereas the steady creep stage has the lowest strain rate. It is noted that, while the lower deviatoric stress case is one where \(q\) is equal to 225.7 MPa, the axial strain rates in all three stages  
Figure 1: Servo-controlled rock triaxial testing system. are lower than the axial strain rate in the higher deviatoric stress case where \(q\) is equal to 232.5 MPa. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Deviatoric Stress \(q\)** & **Stage** & \(\varepsilon_{A}\)**(\%)** & \(\varepsilon_{L}\)**(\%)** & \(v_{A}\)**(\%\(\cdot\)\(h^{-1}\))** & \(v_{L}\)**(\%\(\cdot\)\(h^{-1}\))** \\ \hline \multirow{3}{*}{225.7 MPa} & I & \(1.0\times 10^{-3}\) & \(2.1\times 10^{-3}\) & \(4.3\times 10^{-2}\) & \(2.1\times 10^{-2}\) \\  & II & \(6.9\times 10^{-4}\) & \(1.8\times 10^{-3}\) & \(7.1\times 10^{-4}\) & \(2.0\times 10^{-3}\) \\  & III & \(1.3\times 10^{-3}\) & \(4.0\times 10^{-3}\) & \(5.9\times 10^{-2}\) & \(3.1\times 10^{-1}\) \\ \hline \multirow{3}{*}{228 MPa} & I & \(9.6\times 10^{-4}\) & \(1.9\times 10^{-3}\) & \(1.8\times 10^{-1}\) & \(2.2\times 10^{-1}\) \\  & II & \(6.1\times 10^{-4}\) & \(1.5\times 10^{-3}\) & \(1.9\times 10^{-3}\) & \(6.2\times 10^{-3}\) \\  & III & \(1.2\times 10^{-3}\) & \(3.7\times 10^{-3}\) & \(4.3\times 10^{-1}\) & \(2.1\) \\ \hline \multirow{3}{*}{232.5 MPa} & I & \(8.6\times 10^{-4}\) & \(1.1\times 10^{-3}\) & \(1.1\) & \(1.4\) \\  & II & \(3.5\times 10^{-4}\) & \(9.7\times 10^{-4}\) & \(1.0\times 10^{-1}\) & \(2.8\times 10^{-1}\) \\ \cline{1-1}  & III & \(8.0\times 10^{-4}\) & \(2.4\times 10^{-3}\) & \(1.7\) & \(8.2\) \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of single-stage creep test results between Beishan granite. Figure 2: The curves of deviatoric stress versus strain for Beishan granite specimens under triaxial compression with different confining pressures.