# Triaxial Loading and Unloading Tests on Dry and Saturated Sandstone Specimens
## 4 Discussion  
When a rock specimen approaches the failure point, \(U_{e}\) can be quickly released and it may be transformed into \(U_{d}\). In the conventional triaxial compression test, the internal cracks develop completely within the rock specimen. However, the unloading failure usually occurs abruptly, and the rock specimens fail violently with loud fracturing noise, which has the characteristics of rockburst as observed in deep rock excavation engineering.  
Table 4 lists the average strain energy values of sandstone specimens at the unloading point and the critical failure point in present triaxial tests. In the conventional triaxial compression test, \(U_{d}\) is larger than \(U_{3}\) at critical failure point of the rock, implying that the energy consumption is mainly used for the micro-crack initiation and internal damage of the rock. In contrast, under unloading tests, the energy consumption during the failure process of rock specimens mainly contributes to the circumferential strain (\(\varepsilon_{3}\)) as the reduction of confining pressure, and thus \(U_{3}\) is larger than \(U_{d}\) as shown in Table 4. Moreover, the following energy incremental value can be calculated based on the data in Table 4.  
\[\Delta U_{e}=U_{e}^{\prime}-U_{e},\Delta U_{d}=U_{d}^{\prime}-U_{d},\Delta U_{ 1}=U_{1}^{\prime}-U_{1},\Delta U_{3}=U_{3}^{\prime}-U_{3} \tag{1}\]  
It can be inferred that \(\Delta U_{e}<0\), \(\Delta U_{d}>0\) in unloading tests, and \(U_{e}\) converts into \(U_{d}\) for the development and propagation of internal cracks from the unloading point to the failure point. Meanwhile, the present study discovered that \(\Delta U_{d}>\Delta U_{e}\). The dissipative energy includes not only  
Figure 11: Variation of the \(U_{d}\)/\(U\) values with the initial confining pressure for dry and saturated sandstone specimens at the critical failure point.  
the energy converted from \(U_{e}\), but also the newly absorbed energy from the test machine. In unloading test, there is the following relationship between the dry and saturated rock:  
\[\Delta U_{e}^{TU-D}\geq\Delta U_{e}^{TU-S},\Delta U_{d}^{TU-D}\geq\Delta U_{d}^ {TU-S} \tag{2}\]  
It implies that the failure of dry specimens is more severe than the saturated specimen. Furthermore, \(\Delta U_{1}+|\Delta U_{e}|-|\Delta U_{3}|-\Delta U_{d}>0\) at the critical failure point also exists in unloading test. The energy absorbed by the rock from test machine is mainly consumed for the development of internal fractures, axial deformation, and circumferential deformation, while partly remained. The residual energy (\(\Delta U_{1}+|\Delta U_{e}|-|\Delta U_{3}|-\Delta U_{d}\)) may account for the kinetic energy due to rockbursts, which may occur in the unloading process such as rock excavation with high in-situ stresses in hard rock. Besides, it can be obtained that:  
\[0<\Delta U_{1}^{TU-S}+\left|\Delta U_{e}^{TU-S}\right|-\left|\Delta U_{3}^{ TU-S}\right|-\Delta U_{d}^{TU-S}<\Delta U_{1}^{TU-D}+\left|\Delta U_{e}^{TU-D} \right|-\left|\Delta U_{3}^{TU-D}\right|-\Delta U_{d}^{TU-D} \tag{3}\]  
There is more residual energy for dry rock than saturated rock. It implies that the rock burst is more likely to occur in dry rock, and water infusion is effective to reduce the possibility of rockbursts in the excavation of deep underground rock engineering.