###### Abstract
In addition, excavation technology, such as drilling and blasting; mechanical excavation; and the various shapes of tunnel cross sections, also greatly affect the stand-up time of rock. The authors of [6] investigated the interaction of tunnel face dimensions and excavation rates with the stand-up time. As a result, they first concluded that an increase in excavation rate, or a contraction of the tunnel cross-section, leads to an increase in stand-up time. Secondly, they pointed out that tunnelling techniques significantly impact tunnel stability. A mechanical excavation would tend to improve tunnel stability more than blasting techniques. By considering the geometrical parameters of the considered tunnel span and the deformation characteristics of the rock mass around the tunnel, the authors of [3] established the relationship between the unsupported tunnel span (L*) and the stand-up time (*), which is expressed using the formula given below,  
\[L^{*}t^{*(1-\alpha)}=\frac{4E}{3\gamma H}\frac{(1-\alpha)}{\delta}u_{l}^{*} \tag{1}\]  
where: L* is the unsupported tunnel span according to [1]; L* = 2R = B; t* stands for the stand-up time; \(\alpha\) is a dimensionless empirical parameter; \(\delta\) is a dimensional parameter (\(s-1+\alpha\)); E is the modulus of elasticity of the rock mass; \(\gamma\) stands for the unit weight of the rock mass; H is the tunnel depth; and \(u_{l}^{*}\) is the allowable displacement for maintenance of the sustainable condition of sedimentary rocks (for all the parameters used in the equations, please refer to Abbreviations part). To determine the physical and mechanical properties of a rock mass, the modulus of elasticity of rock mass is required to be determined by an empirical method of analysis, and then the formula for stand-up time is established based on the rheological properties of the rock mass. In addition, the parameters of the GSI also present a significant impact on the stability of an unsupported tunnel. As a result, the authors confirm that tunnelling techniques are greatly influenced by the values of stand-up time. However, by precisely assessing rock mass properties (GSI parameters and the modulus of elasticity), engineersare capable of applying adequate supporting structures which are optimal for unsupported tunnel spans. Upon tunnel excavation, the surrounding rock mass may deform gradually, resulting in a potential collapse caused by the creep properties of the rock medium [7; 8; 9; 10]. The research on the influence of time on failure states, and the discussion on creep damage of a rock mass are broadly available in the literature [7; 11; 12; 13]. A great number of investigations of rock creep behaviour indicate that portions of time-varying deformation of rock masses or rock specimens contribute to the development of total displacements [14; 15; 16; 17; 18; 19]. The time dependence of rock mechanical behaviour leads to gradual deformation. Therefore, tunnel structure stability calculation and design demand the precise determination of the rheological properties of surrounding rock mediums, to provide safe conditions during excavation works [14; 15]. Figure 1 presents a typical rheological curve, which comprises four basic stages when a specimen is compressed by a constant axial load [7; 15; 20; 21]. All the stages are represented as follows:  
* The first stage describes the elastic loading; the instantaneous deformation is observed as reversible. * The second stage shows the primary creep, in which the deformation increases nonlinearly. * The third stage denotes the secondary creep process, where the deformation grows steadily and nearly linearly. * The fourth stage is characterised by an explicit curve of irreversible deformation with the acceleration of creep rate. This stage ends up in the entire failure of tested specimens. In rock mechanics, creep is defined as the convergence of shear strain with constant volumetric strain under the maintenance of an unchanging load.