# Creep Behavior of Rocks and Its Application to the Long-Term Stability of Deep Rock Tunnels
## 10 Discussions
The safety conditions of the secondary lining can signal the creep evolution of the rocks surrounding the tunnels. In fact, the visible growth of cracks and damage in the secondary lining can be taken as an indication that the deformations of the rocks near the tunnels are significant and therefore the creep strain rate could be high. As a result, the phases of rock creep may last less than expected when the host rocks are initially cracked and damaged by the excavations. Then, tunnels could then fail prematurely due to the shortening of the time-to-failure of rocks. With suitable and reliable monitoring systems, adequate measures can be taken in time and serious accidents can be prevented. For instance, after a short operating period of three years, the Xuecheng Tunnel of China was taken out of service due to the development of numerous cracks in its lining and it took two years to restore [122]. Although countless research efforts have already been conducted, the ideal constitutive models of rock creep are not yet precisely determined. Therefore, predicting long-term stability of tunnels remains arduous in spite of many advances in the field [189]. This is mainly due to the complexity associated with deep rocks. Consequently, rock creep is strongly influenced by many factors which are very difficult to account for accurately. On top of that, in many existing creep constitutive models, all these factors are not even taken into account and simplifying assumptions are mostly employed. Another important issue to address is the modeling of tertiary creep. As most of the existing constitutive models do not take into account the time-to-failure which reflects the lifetime of the creep and which is a potential internal influencing factor, the modeling of the tertiary creep needs to be studied much more. Despite most of the existing models having been designed by combining nonlinear models intending to better describe the nonlinear behavior of the accelerated creep, they are limited by not properly considering the three-dimensional characteristics of the rocky media. Such situations should therefore be further investigated. Consequently, the applicability of such models cannot be rigorous under real conditions. Furthermore, it is also observed that the existing creep models mainly concern the rock masses. Only few of them take into account the rocks surrounding the tunnels. Moreover, it remains difficult to assess the actual rheological properties of the surrounding rocks in complex geological conditions [182; 190]. In both cases (rock mass and surrounding rocks), all the factors that strongly influence the creep behavior of the rocks are not taken into account. In addition to this, as the linings or components of tunnels integrate the surroundings rocks and deform with them, they should also be considered in the development of creep models of rocks surrounding the tunnels. For example, the combination of rock bolts and rocks is always expected to behave as a new material with upgraded properties [191]. The control of the creep deformation of the complex generated by the surrounding rocks and the support systems is necessary. Importantly, for instance, depending on the type of surrounding rocks, the tunnel may be deformed even though it is reinforced [192; 193; 194; 195; 196; 197; 198]. However, rock bolt and cable bolt properties are rarely incorporated into the creep models of the rocks surrounding the tunnels. Convinced of such a shortcoming, [PERSON] et al.