# Creep Behavior of Rocks and Its Application to the Long-Term Stability of Deep Rock Tunnels
## 11 Conclusions and Perspectives for Future Research
### Perspectives for Future Research  
One can notice throughout this article that the study of rock creep still represents salient areas of research, despite the enormous efforts already made by many scholars. Although it is very difficult, based on rock creep, improving the accuracy of prediction of the long-term stability of deep tunnels should be continuously investigated. Future research could focus on the following:  
* The effects of potential influencing factors on rock creep need further investigation. In fact, it is noteworthy to study the simultaneous effects of the hydraulic pressure, water content, stress level, temperature, and damage factor on long-term stability of deep tunnels built in heterogeneous rocky environments and subject to high stress.
* Based on different specific case studies, it is extremely important to compare the accuracy of different existing creep constitutive models for stronger conclusions. The influence of individual factors on different creep constitutive models can also be further investigated.
* Based on the heterogeneity and anisotropy of the rocky media, quantify the creep life of deep rock tunnels considering the effects of the potential influencing factors. It is also of high interest to accurately estimate the level of damage that can trigger accelerated creep for various rock types.  
As the tertiary creep needs to be modeled more reliably, it is extremely significant to accurately assess the remaining lifetime of deep tunnel after the onset of tertiary creep conditions in the secondary linings and in the surrounding rocks in realistic conditions.  
Since natural rocks are three-dimensional stress media, it is of utmost importance to establish accurate creep constitutive models based on efficient 3D derivation methods. In addition, by taking into account the characteristics and realistic conditions of rocks, these models will be much more suitable for deep rock tunnels. To accurately predict the long-term life of deep tunnels, it is also of huge importance to establish constitutive creep models of the complex formed with the surrounding rocks and the support systems.  
So far, there are numerous research outcomes regarding the design of creep constitutive models based on loading process of rock mechanical properties, but few on the unloading process. Developing creep models based on the rock unloading process has enormous advantages for the safety and long-term stability of deep underground structures.  
It is significant to monitor the creep behavior of rocks based on the combinations of many traditional methods such as computed tomography, ultra-pulse wave velocity, scanning electron microscope, acoustic emission, etc. Appropriate and robust remote sensing techniques and geographic information systems should be considered for in-depth in situ monitoring.  
Rock creep results will be more accurately exploited and the long-term stability of tunnels will be accurately predicted when all the reported concerns are addressed.  
Conceptualization, W.F.; methodology, W.F.; software, W.F.; validation, H.P.; formal analysis, J.Z.; investigation, W.F.; resources, W.F.; data curation, W.F.; writing--original draft preparation, W.F.; writing--review and editing, W.F.; visualization, J.Z.; supervision, H.P.; project administration, H.P.; funding acquisition, H.P. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the China Scholarship Council, grant number 2019 GBJ008203.  
Not applicable.  
The authors declare no conflict of interest.