# Triaxial Loading and Unloading Tests on Dry and Saturated Sandstone Specimens
## 5 Conclusions  
1. The peak strength difference between dry and saturated sandstones grows with the increasing of initial confining pressure, and the effect of water on rock strength is more remarkable under high initial confining pressures. As a result, when the sandstone specimens are saturated by water, the values of cohesion, internal friction angle and Young's modulus are decreased whereas the Poisson's ratio is increased.
2. In the conventional triaxial compression test, both macroscopic and microscopic failure of the rocks show a combined tension and shear fracture under low initial confining pressure (\(\sigma_{3}^{0}\leq 10\) MPa), but it shows shear fracture under high initial confining pressure (\(\sigma_{3}^{0}>10\) MPa). The microscopic failure of the rock in unloading test mainly shows a combined tension and shear fracture, which is consistent with the macroscopic failure of sandstone under low initial confining pressure (\(\sigma_{3}^{0}\leq 20\) MPa). However, under high initial confining pressure conditions (\(\sigma_{3}^{0}>20\) MPa), it mainly shows shear fracture in macroscopic view. The failure of sandstone in unloading test is more violent than that the failure under triaxial loading test, particularly for dry sandstones.
3. The energy evolution processes confirm that there are some differences for rock specimens under different confining pressures, particularly for the elastic deformation stage. During this stage,  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Group**} & \(\sigma_{0}^{0}\) & \multicolumn{4}{c}{**Unloading Point**} & \multicolumn{4}{c}{**Critical Failure Point**} \\  & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** \\ \hline \multirow{4}{*}{TC-D} & 10 & - & - & - & - & - & 0.85 & 1.01 & 0.41 & 0.43 & \(-\)0.17 \\  
the total energy (\(U\)) and elastic strain energy (\(U_{e}\)) curves are almost overlapped when the rock specimen is under low initial confining pressures. However, when the rock is under high initial confining pressures, \(U\) and \(U_{e}\) begin to separate, and the absorbed energy is basically stored as \(U_{e}\), partially as the dissipative strain energy (\(U_{d}\)). The internal damage has occurred and the internal cracks of the rock develop earlier under the high initial confining pressure conditions.
3. The energy consumption during the failure process mainly contributes to crack initiation and internal damage in loading tests. In unloading test, the energy is mainly consumed for the circumferential deformation and converted into kinetic energy for rock failure. The failure of the saturated rock is relatively moderate because the absorbed energy is mainly used for internal damage and only a part of energy is used for release. Rockburst phenomenon is tended to occur for dry rocks, especially under triaxial unloading conditions. It also indicates that the water injection of the rock may be effective to prevent rockbursts under high in-situ stresses conditions.  
Conceptualization, D.L. and Z.S.; methodology, Z.S.; software, Z.S. and Q.Z.; validation, D.L., Z.S. and K.P.; formal analysis, Z.S.; investigation, Z.S. and Q.Z.; resources, D.L.; data curation, Z.S.; writing--original draft preparation, Z.S.; writing--review and editing, D.L.; visualization, Z.S.; supervision, D.L.; project administration, D.L. and K.P.; funding acquisition, D.L. and K.P.  
This research was funded by the National Natural Science Foundation of China (51474250, 51774058) and Chongqing Basic Research and Frontier Exploration Project (cstc2018 jcyjA3320, cstc2016 jcyjA1861).  
The authors declare no conflict of interest.