# Triaxial Loading and Unloading Tests on Dry and Saturated Sandstone Specimens
## 3 Results
### Fracturing Characteristics
In order to compare and analyze the microscopic and macroscopic failure characteristics of the rocks under different confining pressures, stress paths and water contents, the microscopic analysis of dry and saturated sandstones with an initial confining pressure of 10 and 40 MPa were carried out using the scanning electron microscopy (SEM) method. Figure 7 shows the SEM photos of sandstone fractures with the initial confining pressure 10 and 40 MPa in conventional triaxial compression test. In Figure 7a,c, the spalling fractures occurs in microscopic view under the initial confining pressure of 10 MPa, which is similar to the macroscopic spalling failure of rock specimen. The microscopic fractures retain sharp and smooth crystal surfaces. Meanwhile, the tearing traces (left arrows) are formed in Figure 7b, which are the typical microscopic characteristics of tensile failure [25; 38]. Besides, there are tiny rock fragments (as shown at two sides of Figure 7a,d) and slip scratches (as shown in Figure 7d) left behind by shear friction [39]. It means that the micro-cracks of sandstone are shown as a combined tension and shear failure under low initial confining pressures (\(\sigma_{3}^{0}=10\) MPa represents low initial confining pressure). It is consistent with the macroscopic failure characteristics of sandstone. However, under high initial confining pressures (\(\sigma_{3}^{0}=40\) MPa), the microscopic fractures of the rock are stepped and serrated as shear effect [40]. In addition, there are lots of rock fragments left in the low-lying area, as illustrated in Figure 7e-h. Furthermore, the slip scratches along the crystal are also existed in Figure 7e. The rock crystals were cut off and angular edges were flattened in Figure 7h. The fractures almost have no traces of tension failure on the microscopic section of sandstone. Under high initial confining pressure conditions, the microscopic failure of sandstone is dominated by shear, which is consistent with the macroscopic failure characteristics of sandstone in the conventional triaxial compression test. Figure 7: _Cont._  
Figure 8 shows the SEM photos of sandstone fractures with the initial confining pressure 10 and 40 MPa in unloading confining pressure tests. It can be seen clearly from Figure 8 that there are visible tensile cracks in dry and saturated sandstones under different confining pressures. In Figure 8a,c,e,g,h, the microscopic section shows sharp angular edges and smooth crystal surfaces. Moreover, there are evident traces of tension and spalling fractures in microscopic view. The microscopic section exhibit a typical tensile failure. In addition, as shown in Figure 8b,d,f, there are a large number of rock fragments left behind on the roughly microscopic section. The scratches caused by shear friction appear on the right part of Figure 8h. The microscopic failure section also shows shear failure characteristics. Thus, the microscopic failure of sandstone mainly shows a combined tension and shear failure in unloading test. It is consistent with macroscopic failure of sandstone under low initial confining pressure, but it is different from macroscopic failure of sandstone under high initial confining pressure, where the specimen only shows shear failure in macroscopic view. Figure 8: _Cont._  
Figure 7: Typical scanning electron microscopy (SEM) images of sandstone fractures with the initial confining pressure 10 and 40 MPa in conventional triaxial compression tests: **(a,b)** TC-D-10; **(c,d)** TC-S-10; **(e,f)** TC-D-40; **(g,h)** TC-S-40 (Note: Opposite arrows and inverse arrows represent slip trace and tensile trace respectively; and the letters F, S and T represent fragments, shear crack and tensile crack, respectively).