## 4 Test Results and Analysis
### Tct  
The TCT was only conducted for mudstone, which is widely distributed in the project area. To avoid the discreteness of the triaxial compressive strength, since the samples are taken from different boreholes, the TCT was conducted on samples whose strength was near the mean value of the UCS. The confining pressure is determined as four gradients of 0 MPa, 4 MPa, 8 MPa and 12 MPa according to the measured in situ stress results and uniaxial compressive strength. The triaxial compressive strength of mudstone conforms to the general law of previous studies on soft rock; that is, with increasing confining pressure, the triaxial compressive strength of the sample also increases. From the stress-strain curve (Figure 8a), it can be seen that the elastic modulus and residual strength of the samples also increase with increasing confining pressure. Figure 8b shows the Mohr-Coulomb strength curve of the mudstone. Figure 8c shows the Mohr's circles obtained according to the mean value of the triaxial test. Through the linear Coulomb envelope, it can be calculated that the cohesion is 2.90 MPa, and the internal friction angle is 40.0\({}^{\circ}\).  
However, in rock masses, due to the existence of joints, fissures and weathering conditions in complex environments, the rock parameters obtained from laboratory tests cannot be directly adopted by the project. It is necessary to provide the mechanical parameters of rock mass in the project area based on the test parameters of intact rock in the laboratory by a reasonable model. The Hoek-Brown criterion is widely used in this process [26; 27; 28], and the expression is:  
\[\sigma_{1}=\sigma_{3}+\sigma_{c}\left(m_{b}\frac{\sigma_{1}}{\sigma_{c}}+s \right)^{\alpha} \tag{1}\]  
\[m_{b}=m_{i}\left(\frac{\text{GSI}-100}{28}\right) \tag{2}\]  
\[s=e^{\frac{\text{CSI}-100}{9}},\alpha=0.5(\text{CSI}>25) \tag{3}\]  
\[s=0,\alpha=0.65-\frac{\text{CSI}}{200}(\text{CSI}<25) \tag{4}\]  
\[\sigma_{cm}=\sqrt{s}\sigma_{c} \tag{5}\]  
\[E_{m}=E_{i}\left(0.02+\frac{1}{1+e^{\frac{60-\text{CSI}}{11}}}\right) \tag{6}\]  
where \(\sigma_{1}\) and \(\sigma_{3}\) are the maximum and minimum principal stresses of the rock, \(\sigma_{c}\) and \(\sigma_{cm}\) are the uniaxial compressive strength of the intact rock and rock mass, respectively, \(E_{i}\) and \(E_{rm}\) are the Young's modulus of the intact rock and rock mass, respectively, \(m_{b}\) and \(m_{i}\) are the Hoek-Brown constants of the rock mass and intact rock, respectively, and GSI is the geological strength index.  
Figure 8: Typical stress–strain curves and failure mode (**a**) of mudstone specimens under triaxial compression tests; (**b**) Mohr-Coulomb strength curve; (**c**) Mohr’s circles with a linear Coulomb envelope.  
\(\sigma_{1}\), \(\sigma_{3}\), \(\sigma_{c}\) and \(m_{i}\) can be obtained from the TCT. The initial survey results in the project area show that there is a strongly weathered layer in the surrounding rock of the project area, and the integrity coefficient \(K_{V}\) is 0.66-0.72. The integrity of the rock mass is relatively good, but with severe weathering conditions. According to the quantified GSI rock mass classification system [29], the GSI value of the rock masses in the project area ranges between 40 and 50. Table 3 shows the mechanical parameters of the rock mass in the project area. With the increase in the GSI value, the UCS of the rock mass increases, but all of them are less than 1 MPa. The engineering mechanical properties of rock masses are very poor.