# Creep Behavior of Rocks and Its Application to the Long-Term Stability of Deep Rock Tunnels
## 5 Potential Factors Influencing Rock Creep
### Consideration on Tunnel Lining Structures  
Tunnels are commonly supported by an initial lining which may be shotcrete, rock bolts, steel arch, and a secondary lining [122]. Figure 4 illustrates the typical position of tunnel linings. Such supports are necessary and must be put in place within a reasonable time to effectively limit the time-dependent behavior of the EDZ evolving in three phases, the third of which is the accelerated phase, especially in a contiguous rock mass [123]. The linings can be seriously impacted under various factors, including the creep effect of the surrounding rocks. As reported by [PERSON] and [PERSON] [124], rock creep is the primary factor causing lining deformation. For example, due to severe creep deformation which occurred only after 3 years in the secondary lining of Erlangshan tunnel (China), new reinforcement was applied in an attempt to solve the problem [125]. Indeed, there is no fixed time for the appearance of severe creep deformations in the secondary lining of tunnels. This depends on several relevant factors including the strength of the linings. The mechanical properties of the secondary lining can be altered due to their general exposure conditions [127]. Moreover, such lining typically confronts the time-dependent creep behavior of its own materials and the effects of the tunnel construction process [128]. It is of the greatest consideration to install the secondary lining of tunnels as soon as possible [122,129,130]. This implies that the primary lining must previously be installed more rapidly. Indeed, a significant reduction in ground deformation is a great advantage of placing the primary lining very early [131]. Tunnel excavations are usually done in different phases or sections [132], and usually only after a few minutes of excavation, the rocks start their deformation which will evolve continuously [41]. The application time of the secondary lining should be in accordance with the time of the excavation sequence. This can prevent the rapid evolution of rock creep and thus ensure the stability of the rocks near the tunnels. In addition, referring to [PERSON] et al. [133], the earlier the lining is installed, the lower the ultimate support and rheological pressures. More explicitly, for tunnels built in soft rocks where large deformations usually occur, their secondary lining must be put in place within a period not exceeding 40 days after excavation [129]. Such a practice makes it possible to limit the creep rate of the host rocks and to keep their creep behavior in the secondary stage for as long as possible (Figure 5). Additionally, the limitation of the time for placing the secondary lining is necessary to stabilize the pressure of the rock mass around the tunnels. As an example, the rock mass pressure was stabilized after 35 days of secondary lining installation in Lianchengshan highway tunnel, Hanzhong City, Shaanxi Province, China [134]. Stabilizing rock pressure as quickly as possible is essential for the lining stability. In fact, one of the causes of lining rupture is the exorbitant pressure of rocks eng  
Figure 4: Typical positions of secondary and primary linings of tunnels, adapted from [126]. deformations in the tunnels [135]. Under such circumstances, squeezing rock behavior is common. From a constant deviatoric stress, the triggering of such behavior can also be favored by the increase in shear strain [136], which can eventuate during the evolution of creep strain and before failure [8]. As related by [PERSON] et al. [137], squeezing rock can strongly appear in tunnel constructions and its effects are considerable. Such behavior, jeopardizing tunnel stability, is also frequent in weak rocks under high stress, and its extent reflects tunnel deformations [138]. As reported by [PERSON] and [PERSON] [139], squeezing may last for a long-time and is basically related to creep. It is of extreme consideration to stabilize rock pressure and creep by placing the linings as soon as possible.