# Creep Behavior of Rocks and Its Application to the Long-Term Stability of Deep Rock Tunnels
## 1 Introduction
For this, they fitted the gradual creep curves using the exponential empirical law and employing the method of polynomial regression analysis. Their study can be applied to engineering structures built on a steep rocky slope where the limestone is in low interlayer. For their part, [PERSON] et al. [20] developed improved creep damage equations for soft rock based on the classical Nishihara model and superposition principle. In this model, the nonlinear least squares method is used to obtain the fitted expression. By reflecting three stages of soft rocks, this model can be applied in the stability study of soft rock tunnels where mica-quartz schist is predominant. By studying salt rocks and using segmentation simulation where segments are fitted using variable-order fractional calculus, [PERSON] et al. [21] proposed an adapted creep model. In stress and time thresholds, continuous damage is considered in such a model describing the creep character of rock salt. Using the basis of factional derivatives and experimental data, [PERSON] et al. [22] conceived a creep model for salt rocks. Their results can be used to judge the long-term stability of deep saline rocks serving as energy storage structures. Recently, to describe the overall rock creep behavior, [PERSON] and [PERSON] [23] conceived a new variable fractional order rheological model. Although their model could be applied to various rock types, not all factors influencing rock creep are taken into account. [PERSON] and [PERSON] [24] analyzed the shear creep of sandstone, marble, and granite under fatigue loading provoked by seismic perturbations, and they suggest that their model can serve as a basis for studying the support of anchored rock mass subjected to dynamic disturbances. More recently, [PERSON] et al. [25] carried out creep tests on granite at different confining pressures to study the deformation of an underground hydropower station in the Western Sichuan province of China, and then established an improved creep constitutive model based on the generalized Kelvin model, taking into account the ratio of deviatoric stress and the maximum strength of the rocks. It is therefore observed that, despite substantial research efforts, comprehensive reviews are lacking in the advancements of this salient field. In fact, the need to accurately select and estimate the creep characteristics of rocks are of utmost importance in order to properly predict the long-term stability of deep rock tunnels. This article summarizes relevant information on creep behavior of rocks. Moreover, it discusses the implication of such rock behavior in predicting the long-term stability of deep tunnels. Since the need to build such structures is remarkably increasing around the world, the accurate prediction of their long-term stability is a major concern. It is important to mention that this study has two main limitations. First, it is difficult to study the influence of individual factors on the obtained results without considering specific cases of deep tunnels. Second, as specific study cases are not presented, it is difficult to quantify the accuracy of the various creep constitutive models, and mechanical models are mainly discussed. These limitations could be addressed in future investigations.