## 3 Rock Strength and Creep Constitutive Model Based on the Hoek-Brown Failure Criterion
Afterwards, [PERSON] and [PERSON] further derived the estimations of the relevant mechanical parameters of the rock mass [35], based on Equation (1):  
\[\sigma_{\rm cr}=\sqrt{s}\sigma_{\rm c} \tag{6}\]  
\[\sigma_{\rm tr}=\frac{1}{2}\sigma_{\rm c}\left(m_{\rm b}-\sqrt{m_{\rm b}^{2}+ 4s}\right) \tag{7}\]\[E_{\text{rm}}=10^{5}\frac{1-D/2}{1+\exp[(75+25D-GSI)/11]} \tag{8}\]  
Equation (1) shows that the _GSI_ value is the key to determining the mechanical parameters of the Hoek-Brown failure criterion for the rock mass. However, [PERSON] proposed only a general range for the texture type and weathering conditions of the rock mass and offered no quantitative calculation method. Rock quality designation (_RQD_) is the most commonly used index to describe the structural characteristics of rock mass, which is defined as the percentage of the sum of the lengths of intact core pieces longer than 0.1 m relative to the chosen length of an evaluated drill core (\(RQD=\) (\(\frac{\text{the lengths of intact core pieces }\geq 10\text{ cm per footage}}{\text{ the length of an evaluated drill core}}\) (%)); however, the definition of _RQD_ requires that the quality of core pieces must be \"hard and sound\", meaning it can only calculate the percentage of the core with a length beyond 10 cm in the total cored length, which is not suitable for integrity evaluations when characterizing the multi-joint rock mass. This paper introduces the rock mass block index (_RBI_) and the absolute weathering index (_AWI_) to quantify the _GSI_. The _RBI_, proposed by [PERSON] et al. [36], can thoroughly characterize the block dimension, texture type, and structural packing of blocks for the rock mass. The _RBI_ defines the percentages of drilling cores with the measured lengths of 3-10 cm, 10-30 cm, 30-50 cm, 50-100 cm, and \(>\)100 cm as the weights, and the value of the _RBI_ is the sum of the weights multiplied by the corresponding coefficients. Although the rock mass has the same _RQD_ value, the _RBI_ values can be different. For example, the rock mass of _RQD_ = 90% can be 10-30 cm with a mosaic structure, or 30-50 cm with a block structure, or 50-100 cm with a block structure, or even more than 100 cm with intact structure. Under the same _RQD_, the larger the _RBI_ is (from a mosaic structure to an intact structure), the more intact the rock mass is, and the _RBI_ can be regarded as an extension of _RQD_. The calculation formula of _RBI_ is shown below:  
\[RBI=3C_{\text{r}3}+10C_{\text{r}10}+30C_{\text{r}30}+50C_{\text{r}50}+100C_{ \text{r}100} \tag{9}\]  
where \(C_{\text{r}3}\), \(C_{\text{r}10}\), \(C_{\text{r}30}\), \(C_{\text{r}50}\), and \(C_{\text{r}100}\) are the acquisition rates (weights in percentages) of the cores with the lengths of 3-10 cm, 10-30 cm, 30-50 cm, 50-100 cm, and \(>\)100 cm, respectively. The details of the rock mass texture representation by _RBI_, associated with the field description, are shown in Table 2.  
The mechanical properties of rock are also related to the weathering level. Chemical weathering can alter the mineral composition of rock and thus alter its physical and mechanical performance.