# Creep Behavior of Rocks and Its Application to the Long-Term Stability of Deep Rock Tunnels
## 10 Discussions  
In this article, relevant information about rock creep is highlighted. It can be seen that scholars and researchers have devoted substantial research to rock creep. The properties of the latter are widely applied in different types of underground structures. However, because of its predominant feature of governing the long-term stability of rock engineering [113], rock creep should be much more studied and discussed as it still represents a key research area. In fact, rock creep impacts are increasingly significant at great depth. Under such circumstances, all rock types generally experience time-dependent deformations. There are no exceptions even for high strength hard and brittle rocks [115]. For long deep tunnels that pass through rock media with varying properties, rock creep control can be much more  
Figure 10: Illustration of tunnel convergence. difficult. Therefore, precise information on the types of rocks traversed by these tunnels is necessary for a better assessment of their long-term stability. It is indispensable to ensure the stability of deep tunnels [87], regardless of their function (water conveyance, sanitary drainage, transport systems, hydro-power stations, irrigation, mining, energy storage). The long-term stability of such structures is mainly governed by the secondary creep phase [185; 186]. As stated by [PERSON] et al. [37], at the smallest creep strain rate, secondary creep is considered a long duration transient creep event. It is essential to take adequate measures to keep it as durable as possible. Particular attention should be given to deep soft rocks which can quickly reach their tertiary creep under high stress [187]. In such stress ranges, the existence time of the secondary creep can be shortened, which is very detrimental to the stability and durability of tunnels built in the aforesaid rock types. It is important to emphasize that, whatever the nature of the rocks, high stress coupled to extreme geological conditions are very harmful to tunnel stability. Such conditions have great effects on rock creep life. Information on the extent of loads or stresses that trigger the steady creep stage and the period of reaching this stage is crucial to ensure the stability of tunnels [188]. Although laboratory experiments can reveal the magnitude of stress that causes secondary creep to end and tertiary creep to begin, extensive on-site monitoring is strongly necessary to better control and interpret the creep behavior of rocks. This is very important as the duration of real engineering projects is not exactly reflected in experimental study of rocks [39; 102; 185; 129]. To accurately predict the long-term deformations of rocks, the duration of laboratory experiments should be reasonable. It can be observed that there is no universally accepted duration for laboratory creep experiments. As reported by [PERSON] et al. [142], such tests can last a few hours, days, or months, and rarely a few years. It remains a difficult task to determine the duration of these experiments for the long-term prediction of rock deformation [77]. Tunnel instability can be predicted as soon as it is known that the second creep stage is about to end, and the third stage begins very soon. Consequently, maintaining the creep behavior of the surrounding rocks in its secondary phase is the optimal option. It is certain that the rocks surrounding the tunnels alone cannot maintain this phase for long. It is why that it is strongly recommended that the tunnel linings be installed at soon as possible according to the excavation sequences. Even after the installation of the secondary lining, adequate monitoring is required to control the appearance of cracks and deformations which will evolve slowly [179], and to ensure that the rock deformations still remain stable, avoiding any unexpected failure. In fact, given that failure usually results from reaching maximum crack propagation, careful monitoring is of paramount importance, especially at great depth where large rock deformations typically occur.