# Creep Behavior of Rocks and Its Application to the Long-Term Stability of Deep Rock Tunnels
## 7 On the Applicability of Constitutive Models of Rock Creep to Deep Tunnels  
A constitutive model of rock creep is extremely important in deep rock engineering. It can generally indicate the total creep strain of a given rock type. It is mainly used as a basis for designing appropriate support systems to ensure the long-term stability of host rocks of deep underground structures such as tunnels, mines, and caverns. Therefore, constitutive rock creep models should reflect rock characteristics and conditions as closely as possible. Otherwise, these models will not be effective in engineering applications. According to [PERSON] et al. [166], constitutive models of rock creep must be reliable. In fact, premature tunnel failures can be caused by inaccurate estimation of the rock's constitutive creep models. Consequently, it is important to continually revise the existing creep models in order to adopt the necessary considerations for their suitable applications. As examples, based on the creep constitutive model of deep sandy mudstone, [PERSON] et al. [167] proposed a robust mixed support system to resist the overwhelming deformation of deep soft rock roadway located at the coal mines in Pingdingshan mining area of Henan province, China. Similarly, to solve creep hazards generating considerable instability on a siltstone roadway, [PERSON] et al. [168] proposed a strong support system. Nonetheless, such propositions can be improved if the creep constitutive modeling is improved. In ultra-exaggerated deformations, tunnel support systems must be ultra-efficient and highly adapted to the real situations they face. To achieve such objectives, the constitutive creep models must be as accurate as possible. In fact, the need to improve the degree of precision according to their applicability should be prioritized. Note that significant losses are generally related to rock creep and to their constitutive models that are inappropriately characterized in underground structures [30]. Therefore, an ongoing review of pertinent existing equations that interpret and reflect such models is of tremendous importance. As presented in Tables 4 and 5, researchers have developed more and more sophisticated constitutive models of rock creep based on certain assumptions and conditions, but as pointed out by [7], their suitability is required. It can be noticed that the mathematical expressions are particularly complex for the majority of such models. Their applicability may therefore be difficult. Additionally, due to the complexity of rock behavior, the actual rock properties and conditions are very difficult to be determined and represented with extreme accuracy. Thus, scholars often utilize simplifying assumptions and conditions to study and establish constitutive models of rock creep. However, the exactness of such assumptions is rarely discussed. For convenient applications to deep rock tunnels which are substantial engineering structures and which are mostly expected to have long lifespans, it is worth questioning these assumptions. The following are the most commonly used assumptions or considerations. \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Study (Year)** & **Rock Type** & \begin{tabular}{c} **Testing Type** \\ **and** **Conditions** \\ \end{tabular}  & \begin{tabular}{c} **Duration of** \\ **Testing Days** \\ \end{tabular}  & \begin{tabular}{c} **Creep** \\ **Constitutive** \\ **Model** \\ \end{tabular}  & \begin{tabular}{c} **Creep Equations or Creep** \\ **Constitutive Equations** \\ \end{tabular}  & \begin{tabular}{c} **Equation** \\ **Number** \\ \end{tabular}  & **Comments** \\ \hline \multirow{4}{*}{ \begin{tabular}{c} [PERSON] et al., \\ (2021) \\ (165) \\ \end{tabular}  } & \multirow{4}{*}{ \begin{tabular}{c} Salt rocks \\ (deep caverns) \\ \end{tabular}  } & \multirow{4}{*}{\ \begin{tabular}{c} Long-term \\ uniaxial creep \\ tests. \\ simulation. \\ \end{tabular}