## 4 Test Results and Discussion
### Chemical Weathering Indices  
The XRF analysis for the samples retrieved from each test site was performed to estimate the weathering indices presented in Table 1. The value of a weathering index depends on the weight percentage of major oxides in the samples measured by XRF; therefore, analysis of the major oxide composition is important to select an appropriate chemical weathering index [16]. As described in Section 3.2, the composition of alkali and alkaline oxides used in the four chemical weathering indices (Vogt's Ratio (VR), Chemical Index of Alteration (CIA), Mobiles Index (I\({}_{\text{mob}}\)), Modified Weathering Potential Index (MWPI)) generally decreased with a change in weathering grade at each test site (Figure 6). However, the composition of Na\({}_{2}\)O and K\({}_{2}\)O did not show a consistent decreasing tendency with weathering (Table 3). In particular, the ratio of K\({}_{2}\)O increased with weathering at the S site. This tendency could be explained by the abundance of illite, which is a secondary clay mineral of granite. The composition of K\({}_{2}\)O is reduced because of the decomposition of K-feldspar with weathering, but the leached K ions are used to generate illite; therefore, the variation in K\({}_{2}\)O cannot solely represent the weathering intensity [21; 29]. Therefore, it is suitable to use chemical weathering indices composed of various mobile and immobile major oxides. On the other hand, the weight percentage of Fe\({}_{2}\)O\({}_{3}\) increased with weathering intensity, which is a general tendency of weathering process of granite [10; 12; 21; 22]. The increase of Fe\({}_{2}\)O\({}_{3}\) resulted from a decomposition of biotite, whose secondary minerals are chlorite, kaolinite, halloysite, and goethite. Especially, it is well known that goethite causes a partial cementation of clayey matrix in the groundwater fluctuation zone. However, since the weathered granite at all test sites in the presented study were more than 5 m below the groundwater level, the goethite effect on the increase of friction angle might be insignificant. Even though the goethite was generated by weathering, the majority of the secondary minerals might result in the decrease of friction angle. [PERSON] et al. [26] reported that kaolinite, which is a main secondary mineral of granite, surrounds the origin grains of granite and causes microfracture of the rock texture, which results in the decrease of friction angle. Figure 6: Weight percentage of the alkalis and alkaline oxides: (**a**) used in Vogt’s Ratio (VR); (**b**) used in Chemical Index of Alteration (CIA) and Mobiles Index (I\({}_{\text{mob}}\)); (**c**) used in Modified Weathering Potential Index (MWPI). The average values of the chemical weathering indices evaluated in the present study are summarized in Table 4. Each index showed a distinctive tendency with weathering grade presented in Table 2 (i.e., increase or decrease); the indices defined as the molecular ratio of relatively immobile oxide to alkali and alkaline oxide (VR, CIA, I\({}_{\text{mob}}\)) increased. On the other hand, the MWPI value decreased because of the mobile oxides used as the numerator. Figure 7 shows the distribution of normalized chemical weathering indices with depth at each test site. All of the indices showed an overall tendency with depth (VR, CIA, and I\({}_{\text{mob}}\) are decreasing, MWPI is increasing); however, in spite of a deeper depth, the opposite tendency indicating a more weathered state was shown at a particular range. [PERSON] and [PERSON] [22] found a similar weathering profile within their study, and they also considered that the chemical weathering indices used in their study still presented an appropriate tendency (i.e., ideal tendency).