## 4 Test Results and Discussion  
### Shear Strength of Grade IV and V Granite  
The cohesion and friction angle, which are the shear strength parameters defined by the Mohr-Coulomb failure envelope, represent the shear strength of soils and rocks. However, it is known that the shear strength of grade IV and V granite dominantly depends on its friction angle because weathering results in a reduction of the cohesion. [PERSON] and [PERSON] [26] explained this tendency in their study using the failure envelopes of weathered granites and a simple model that illustrates the microfabric consequences under different degrees of weathering. They mentioned that the main factor developing cohesion is intergranular bonds and that of a friction angle is interlocking texture. When the weathering progresses severely, such as for grade IV and V granite, grain boundaries open because of microfracture, which derives a remarkable decrease of cohesion. On the other hand, since the internal weakening of the grains are small and the minerals are rarely changed, the granite maintains a high interlocking texture, resulting in a slight decrease in the friction angles. [PERSON] et al. [27] analyzed the geotechnical properties of grade IV and V granites collected from in situ and laboratory test results performed at 17 sites in South Korea, and most of the granite showed relatively low cohesion from 0.03 to 0.09 MPa. [PERSON] et al. [5] evaluated the shear strength of grade IV and V granite in Hong Kong by interpreting PMT curves, and only friction angles were used for the analysis because the cohesions measured from laboratory tests were close to zero. Therefore, in this study, it is regarded that the friction angle represents the shear strength of grade IV and V granite, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Chemical Weathering Index** & **Equation** & **Ideal Tendency with Degree of** \\  & & **Weathering** \\ \hline Vogt’s Ratio, VR [23] & \((Al_{2}O_{3}+K_{2}O)/(MgO+CaO+Na_{2}O)\) & \(\uparrow\) \\ Modified Weathering Potential Index, & \(100(K_{2}O+Na_{2}O+CaO+MgO)/(SiO_{2}+Al_{2}O_{3}+\) \\ MWPI [24] & \(Fe_{2}O_{3}+FeO+TiO_{2}+CaO+Na_{2}O+MgO+K_{2}O)\) & \(\downarrow\) \\ Chemical Index of Alteration, CIA [25] & \(100A_{2}O_{3}/(Al_{2}O_{3}+CaO+Na_{2}O+K_{2}O)\) & \(\uparrow\) \\ Mobiles Index, I\({}_{\text{mob}}\)[10] & \((I_{\text{fresh}}-I_{\text{unatthend}})/I_{\text{fresh}}\), \(I=(K_{2}O+Na_{2}O+CaO)\) & \(\uparrow\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Applied chemical weathering indices. \(\uparrow\): increase, \(\downarrow\): decrease. Figure 4: Retrieved sample of grade IV and V granite at G site. and the friction angles (\(\phi^{\prime}\)) were estimated using the relationship with limit pressure (\(P_{L}\)), as shown in Equation (1), as suggested by [PERSON] (2018). \[P_{L}=b\times 2^{(\phi^{\prime}-24)/4} \tag{1}\]  
where \(P_{L}\) is the limit pressure, and \(b\) is a soil condition constant: 1.8 (dry condition) and 3.5 (wet condition). The wet condition was applied because all grade IV and V granite were under the groundwater level (see Figure 2). Figure 5 presents the distribution of the friction angles evaluated using limit pressures from the PMT at each test site, and Table 2 summarizes the maximum, average, and minimum values of the friction angles. Compared to the friction angles of grade IV and V granite in South Korea presented in a previous study by (2018) (30\({}^{\circ}\)-55\({}^{\circ}\)), the friction angles evaluated at each site seem to be appropriate; the friction angles in the present study ranged from 33.6\({}^{\circ}\) to  
It is shown that the friction angles generally increase with depth at all the test sites with minor exceptions, but the tendencies were present at a particular depth. This minor exception between friction angles and depth indicates that weathering is not linearly developed with depth, indicating that using weathering indices to estimate the weathering profile and geotechnical properties is more reasonable than using a simple model with depth.