## 4 Test Results and Discussion
### Correlation Analysis  
Correlation analysis was performed to determine the relationship between the chemical weathering indices and the friction angles estimated using limit pressures. As described in Section 4.1, the weathering intensity and site-specific characteristics affect the friction angles of grade IV and V granite. Considering the effects of weathering and site-specific characteristics, the chemical weathering indices evaluated in the present study and the friction angles of grades III and IV are used as parameters determining the relationship, as suggested by [PERSON] et al. [16; 30]. Following [16; 30], the presented study built correlations between the friction angle and weathering indices based on three assumptions:  
1. The friction angles of grade III and IV granite represent site-specific characteristics. 2. The friction angles of grade III and IV granite affect those of grade IV and V granite. 3. The friction angles of grade III and IV granite vary with the degree of weathering, which the chemical weathering indices quantitatively represent. In this study, the suggested relationship was used to determine the relationship between the friction angles of grade IV and V granite and chemical weathering indices (Equations (2) and (3)). \[\phi_{HW\&CW}=\left(\frac{WI_{MW}-WI_{HW\&CW}}{WI_{MW}-WI_{RS}}\right)^{k} \phi_{RS}+\left\{1-\left(\frac{WI_{MW}-WI_{HW\&CW}}{WI_{MW}-WI_{RS}}\right)^{k }\right\}\phi_{MW} \tag{2}\]  
\[\left(\frac{\phi_{MW}-\phi_{HW\&CW}}{\phi_{MW}-\phi_{RS}}\right)=\left(\frac {WI_{MW}-WI_{HW\&CW}}{WI_{MW}-WI_{RS}}\right)^{k}\ \Rightarrow\ R_{\phi}=(R_{W})^{k} \tag{3}\]  
The subscripts _MW_, _HW_, _CW_, and _RS_ refer to grade III, IV, V, and VI granite, respectively, and _WI_ is the chemical weathering index used in the analysis. \(R_{\phi}\) and \(R_{W}\) are the normalized friction angles and weathering indices, respectively, and \(k\) is a curve-fitting  
Figure 7: Distribution of normalized chemical weathering indices: (**a**) A site; (**b**) G site; (**c**) S site. parameter defining the relationship between \(R_{\phi}\) and \(R_{W}\). The chemical weathering index of grade III and VI granite (i.e., \(W\!I_{MW}\) and \(W\!I_{RS}\)) and the friction angle of grade III and VI granite (i.e., \(\phi_{MW}\), \(\phi_{RS}\)) are boundary conditions of weathering and friction angles. Therefore, the curve-fitting parameter \(k\) values for each chemical weathering index can be obtained using given sets of friction angles and weathering indices for grade IV and V granite. The curve-fitting parameter \(k\), which defines the best-fit curve between the friction angle and weathering index, are presented in Table 5. The coefficient of determination (\(R^{2}\)), except for the case of MWPI, was larger than 0.5, which is a moderate correlation [15]. The coefficient of determination was not so great; however, considering the high variability of geotechnical properties, it is acceptable and within values presented by previous research studies [13; 14; 15; 22]. Table 5 implies that the simple estimation of the friction angle using chemical weathering indices is possible for the weathered granite for which SPT is not feasible. The friction angles of grade IV and V granite were also predicted using Equation (2) and compared with the measured value. As shown in Figure 8, the predicted friction angles were generally closed to the measured ones, although some particular values were over- or underestimated. In particular, data of the G site are relatively more scattered than those of the other sites. It is because that the measured friction angles from 29 to 33 m seem to be a median value, but the CIA values in that range of depth showed a similar value of grade III granite. However, since grade IV and V granite have a much larger SPT-N value than 100, the present method based on the chemical weathering indices is more reasonable compared to the estimation of the friction angle based on the SPT-N value, which is generally used in practice.