# Triaxial Loading and Unloading Tests on Dry and Saturated Sandstone Specimens
## 3 Results  
### Mechanical Properties  
Figure 2 shows the variation of the peak strength with the initial confining pressure of dry and saturated sandstone specimens. It can be seen that the peak strength of sandstone specimen increases as the initial confining pressure increases, but the increase rate slows down with the increase of the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & TU-D-10 & 2380.7 & 3299 & 10 & 0.8\(\sigma_{(10)}\) & 0.1 mm/min-1.5 & 3 \\  & TU-D-20 & 2391.3 & 3292 & 20 & 0.8\(\sigma_{(20)}\) & 0.1 mm/min-1.5 & 3 \\  & TU-D-30 & 2387.8 & 3315 & 30 & 0.8\(\sigma_{(30)}\) & 0.1 mm/min-1.5 & 3 \\  & TU-D-40 & 2391.3 & 3313 & 40 & 0.8\(\sigma_{(40)}\) & 0.1 mm/min-1.5 & 3 \\ \hline  & TU-S-10 & 2454.7 & 3396 & 10 & 0.8\(\sigma_{(10)}\) & 0.1 mm/min-1.5 & 3 \\  & TU-S-20 & 2457.7 & 3347 & 20 & 0.8\(\sigma_{(20)}\) & 0.1 mm/min-1.5 & 3 \\  & TU-S-30 & 2457.8 & 3349 & 30 & 0.8\(\sigma_{(30)}\) & 0.1 mm/min-1.5 & 3 \\  & TU-S-40 & 2457.7 & 3399 & 40 & 0.8\(\sigma_{(40)}\) & 0.1 mm/min-1.5 & 3 \\  & & & & & & & MPa/min \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._initial confining pressure. The difference of the peak strength between dry and saturated sandstones grows with the increasing of initial confining pressure. It means that the effect of water on rock strength is more remarkable under high confining pressure. Figure 3 illustrates the strength parameters of different groups of specimens under triaxial loading and unloading tests. The strength parameters of sandstone specimens are differed in different stress paths. The strength parameters of \(C\) (cohesion) and \(\varphi\) (internal friction angle) under loading conditions are higher than those of unloading tests with the same water condition. Meanwhile, both \(C\) and \(\varphi\) values of saturated sandstones are lower than those of dry sandstones, which indicate that water has a deep impact in decreasing strength parameters (\(C\) and \(\varphi\)) of sandstone samples. When the sandstone samples are saturated by water, their shear strength parameters are reduced, where changes in results of \(\varphi\) are more remarkable. The weakening of strength parameters further affects the ultimate bearing capacity of sandstone. The values of the axial strain (\(\varepsilon_{1}\), \(\varepsilon_{1}^{\prime}\)), circumferential strain (\(\varepsilon_{3}\), \(\varepsilon_{3}^{\prime}\)), and volumetric strain (\(\varepsilon_{\varphi}\), \(\varepsilon_{\varphi}^{\prime}\)) at the unloading point (\(\sigma_{3}^{0}\)) and the critical failure point (\(\sigma_{3}^{f}\)) are listed in Table 3. It can be seen that the absolute value of volumetric strain for rock in unloading test is greater than that of rock samples in conventional triaxial compression test, particularly for saturated samples. It means that the characteristics of volume expansion of rock under unloading conditions are more pronounced than the rock samples in loading tests. In addition, the incremental rates of circumferential strain are several times larger than those of axial strain from unloading point to critical failure point. The rock failure occurs mainly for the circumferential expansion under unloading conditions. Furthermore, except for the initial confining pressure of 20 MPa, the confining pressures of saturated sandstones at failure point (\(\sigma_{3}^{f}\)) are higher than that for dry sandstones which indicate that saturated sandstones fail earlier than dry sandstones. This is due to the fact that the structures of rock samples are weaker when they are saturated by water as compared to structures of dry rock samples. Figure 3: Strength parameters of sandstone specimens. Figure 2: Peak strength of sandstone specimens. The relationship between the deformation parameters (i.e., the Young's modulus, \(E\), and the Poisson's ratio, \(\mu\)) and the initial confining pressure (\(\sigma_{3}^{0}\)) is shown in Figure 4. As shown in Figure 4, \(E\) values of sandstone firstly increase and then decrease as the initial confining pressure increases.