# Triaxial Loading and Unloading Tests on Dry and Saturated Sandstone Specimens
## 3 Results
### Fracturing Characteristics  
Figure 5 shows the failure modes of dry and saturated sandstones in conventional triaxial compression tests. It is found from Figure 5,b that the spalling and tensile cracks are visible under  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & **Specimen** & \multicolumn{4}{c}{**Unloading Point**} & \multicolumn{4}{c}{**Critical Failure Point**} \\ \cline{3-10}  & **Number** & \(\sigma_{3}^{0}\) & \(\epsilon_{1}\) & \(\epsilon_{3}\) & \(\epsilon_{\text{p}}\) & \(\sigma_{3}^{f}\) & \(\epsilon_{1}^{{}^{\prime}}\) & \(\epsilon_{3}^{{}^{\prime}}  
uniaxial compression. When the initial confining pressure increases to 10 MPa, axial or radial tensile cracks are often produced along the shear plane, and then tensile cracks develop and form a macroscopic failure surface, which even breaks earlier than the main shear fracture, as shown in Figure 5d. It indicates that the failure mode of rock specimens shows a combined tension and shear failure under low initial confining pressure condition (\(o_{3}^{0}\leq 10\) MPa). However, under high initial confining pressures (\(o_{3}^{0}>10\) MPa), a main shear fracture passes through the entire specimen usually extend to the end of specimen. The specimens are cut into two triangular vertebral bodies by the main fracture, and the main failure surface of sandstone develops from the end of specimen to the side surface as the increase of initial confining pressure. Shear failure occurs under high initial confining pressures (\(o_{3}^{0}\)\(>10\) MPa), and a single shear fracture dominates the failure processes. Besides, all dry sandstone specimens show structural failure while some saturated sandstone specimens fail partly, as shown in Figure 5, implying that the failure characteristics of dry sandstones are more severe than those of saturated sandstones. Figure 5: Failure modes of dry and saturated sandstones in conventional triaxial compression tests: (**a**) TC-D-0; (**b**) TC-S-0; (**c**) TC-D-10; (**d**) TC-S-10; (**e**) TC-D-20; (**f**) TC-S-20; (**g**) TC-D-30; (**h**) TC-S-30; (**i**) TC-D-40; (j) TC-S-40. Figure 6 shows the failure modes of dry and saturated sandstones in unloading confining pressure tests. When the initial confining pressure is 10 MPa, the rock tends to produce longitudinal tensile cracks approximately parallel to the loading direction, as the reduction of circumferential constraints in unloading test. Tensile cracks propagate and coalesce with the shear crack, leading to the final failure of rock specimens. When the initial confining pressure is 20 MPa, there are numerous rock fragments caused by shear friction on the rock failure plane, implying that the rock specimen undergoes shear failure and forms a macroscopic shear surface. Meanwhile, longitudinal (Figure 6c) and radial (Figure 6d) tensile cracks are often produced at the lower part of the triangular vertebral body. Combined shear and tension failure occurs for the rock specimen. Under high initial confining pressures (\(\sigma_{3}^{0}>\) 20 MPa), shear failure dominates the failure process in unloading tests, which is the same as the conventional triaxial compression test. The shear friction of the rock is severe, and a lot of rock fragments are observed on the failure surface. Besides, by comparing failed rock fragments, it can be found that the failure characteristics of dry sandstones are more violent than those of saturated sandstones under unloading test. Figure 6: Failure modes of dry and saturated sandstones in unloading confining pressure tests: (**a**) TU-D-10; (**b**) TU-S-10; (**c**) TU-D-20; (**d**) TU-S-20; (**e**) TU-D-30; (**f**) TU-S-30; (**g**) TU-D-40; (**h**) TU-S-40. The phenomenon of volume expansion occurs significantly in the triaxial tests, particularly for the saturated rock in unloading confining pressure test. The failure characteristics of sandstone in unloading test are more violent than those under triaxial loading test.