# Creep Behavior of Rocks and Its Application to the Long-Term Stability of Deep Rock Tunnels
## 11 Conclusions and Perspectives for Future Research  
### Conclusions  
Relevant information on rock creep and its potential influencing factors has been described throughout this article. Moreover, the article presented the latest advances in constitutive models of rock creep and discussed their applicability to the long-term stability of deep underground structures such as tunnels, mines, and caverns. The main conclusions drawn from this paper are as follows:  
1. Regardless of the rock type, creep character is very common and is influenced by several potential factors which are mainly stress level, hydraulic pressure, water content, temperature, damage, and time-to-failure. The creep deformation mechanisms of rocks are mainly brittle, ductile, and semi-brittle. Under the long-term action of stresses, brittle failure is mostly common in hard rocks, while soft rocks mostly fail in a ductile manner. Semi-brittle failure is predominant in many hard rock types in deep rocky environments when there are combined influences of high stresses, high pressures, and high temperatures. 2. Rock creep properties are usually determined through different laboratory experiments accompanied by monitoring methods. The collected data are adequately processed and used as the basis for decision-making in the early stages of deep rock engineering projects as well as for studying the long-term stability of different types of deep underground structures such as tunnels, mines and caverns. However, one of the key concerns with evaluating the creep properties of rocks is the duration considered in laboratory experiments which does not truly reflect the full exposure duration of rocks in realistic situations. To this end, compared to the exposure times in the field, the observation times in laboratory are too short to detect with great precision the characteristics and conditions of rock creep. To explore more broadly and more concretely the creep properties of rocks, it is suggested to combine their microscopic and macroscopic characteristics. Furthermore, as each monitoring method has its limitations, to achieve more accurate results, it is suggested that these methods be combined appropriately. Importantly, long-term monitoring is necessary to effectively ensure long-term performance of host rocks and therefore long-term stability of the underground structures. 3. Many constitutive models of rock creep have been devised. They are habitually used to design support systems that must keep stable the surrounding rocks of the underground structures over a long period of time. However, much effort is still needed for the rigorous applicability of such models to deep tunnels. In fact, most of such models are established on the basis of simplifying assumptions that do not fully express the characteristics and conditions of actual deep rocks. For instance, since the coupling of many internal factors is unclear, most scholars extend the one-dimensional models to three-dimensional creep models, despite the fact that rocks are naturally three-dimensional stress media. Then, for the external factors, despite their significant individual influences on rock creep, additional investigations on their coupling effects are necessary. Nonetheless, precise and appropriate correction factors  
Figure 11: Diagram showing the importance of accurate constitutive rock creep models to ensure the long-term life of deep tunnels, including deep caverns and mines. can be applied to these models to enhance their suitability to deep tunnels. Therefore, tunnel support systems generally designed to cope with total creep deformation can be more responsive and more efficient. Note that only accurate creep constitutive models can properly represent the overall contribution of rock creep to the time-dependent behavior of rocks. 4. It is also noticed that, until now, numerous constitutive models of rock creep have been designed on the basis of parameters estimated from loading process of rock mechanical properties.