# Triaxial Loading and Unloading Tests on Dry and Saturated Sandstone Specimens
## 2 Materials and Methods  
### Specim Preparation  
Red sandstone samples which were obtained from a quarry in Yunnan Province, were used to test in the present study. A thin section analysis was carried out to examine the microstructure of the sandstone. It was identified as fine-grained sandstone by petrographic microscopy (Leica DM2700P, Leica Microsystems Inc., Wetzlar, German). The microstructure of the sandstone under plane polarization light (PPL) and cross polarization light (CPL) are illustrated in Figure 1. The mineral composition and grain size distribution of the sandstone specimen are listed in Table 1.  
Both oven-dried and water-saturated specimens were prepared to study the influence of water on triaxial failure of the sandstone samples. In drying process, the sandstone specimens were placed in an oven at 105 \({}^{\circ}\)C for 24 h, and then they were removed to a desiccator and weighed after cooling to room temperature. In saturation process, the specimens were placed in water and allowed to absorb water for 48 h under atmospheric pressure, and then specimens were taken out and weighed after removing surface moisture of the rock. These specimens were considered as saturated samples in this study. The natural water content of specimen was about 2.5% and the saturated water content was about 3.3%. The diameter (\(D\)) of the cylindrical specimens was 50 mm and the specimen height (\(H\)) was 100 mm. The density and the P-wave velocity of each testing specimen are provided in Table 2.  
\begin{table}
\begin{tabular}{c c c} \hline
**Rock Composition** & **Grain Size (mm)** & **Mineral Content (\%)** \\ \hline Quartz & 0.03 \(\sim\) 0.2 & 77 \\ Potassium feldspar & 0.05 \(\sim\) 0.2 & 10 \\ Calcite & 0.01 \(\sim\) 0.08 & 7 \\ Sericite & 0.01 \(\sim\) 0.02 & 4 \\ Others & 0.01 \(\sim\) 0.1 & 2 \\ \hline \end{tabular}
\end{table}
Table 1: Mineral composition and grain size distribution of sandstone specimen.  
Figure 1: Polarized light micrographs of sandstone specimen: (**a**) plane polarization light (PPL); (**b**) cross polarization light (CPL). (Note: The letters Qz, Kfs and Cc represent quartz, potassium feldspar and calcite, respectively).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & **Specimen** & **Density** & **P-Wave** & **Initial** & **Unloading** & **Unloading** \\  & **No.** & **(kg/m3)** & **Velocity** & **Confining** & **Point** & **Loading Rate** & **Rate** \\  & & **(m/s)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** \\ \hline \multirow{4}{*}{TC-D} & TC-D-0 & 2387.3 & 2951 & 0 & 0.1 mm/min \\  & TC-D-10 & 2380.4 & 2979 & 10 & 0.1 mm/min \\ \cline{1-1}  & TC-D-20 & 2387.5 & 2916 & 20 & 0.1 mm/min \\ \cline{1-1}  & TC-D-30 & 2398.3 & 3063 & 30 & 0.1 mm/min \\ \cline{1-1}  & TC-D-40 & 2382.1 & 3077 & 40 & 0.1 mm/min \\ \hline \multirow{4}{*}{TC-S} & TC-S-0 & 2456.8 & 3381 & 0 & 0.1 mm/min \\ \cline{1-1}  & TC-S-10 & 2459.8 & 3363 & 10 & 0.1 mm/min \\ \cline{1-1} \cline{2-2} \cline{4-4} \cline{6-6} \cline{1-1}  & TC-S-20 & 2464.1 & 3362 & 20 & 0.1 mm/min \\ \cline{1-1} \cline{2-2} \cline{4-4} \cline{6-6} \cline{1-1}  & TC-S-30 & 2459.7 & 3393 & 30 & 0.1 mm/min \\ \cline{1-1} \cline{2-2} \cline{4-4} \cline{6-6} \cline{1-1}  & TC-S-40 & 2450.0 & 3392 & 40 & 0.1 mm/min \\ \hline \hline \end{tabular}
\end{table}
Table 2: Test scheme and physical parameters of sandstone specimens.