# Regression Equations for Estimating Landslide-Triggering Factors Using Soil Characteristics
## 5 Conclusions  
This study investigated different characteristics of landslide occurrences in areas with gneiss and granite soils. In addition, regression equations for landslide-triggering factors using soil properties were developed to estimate the permeability coefficient and shear strength in relation to the cohesion and internal friction angle of different weathered soils that affect landslides.  
The occurrence of landslides is directly affected by large porosity and minimal density of soil materials as soils in landslide areas have poor soil particle size distributions and loose ground conditions. Therefore, these types of soils are relatively more vulnerable to landslides under similar geological conditions.  
When the correlations between the soil parameters of gneiss and granite regions were analyzed, the permeability coefficient was found to have a significant correlation with the void ratio, the effective diameter, and the uniformity coefficient, while shear strength had a significant correlation with fine grain content, uniformity coefficient, saturation, dry density, and void ratio.  
As such, the regression equations were developed to easily calculate the permeability coefficient and shear strength (cohesion and internal friction angle), which were considered important in landslide  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Soil variable**} & \multirow{2}{*}{**Type**} & \multicolumn{2}{c}{**Gneiss Soil**} & \multicolumn{2}{c}{**Granite Soil**} \\ \cline{3-6}  & & **MSE** & **RMSE** & **MSE** & **RMSE** \\ \hline \multirow{2}{*}{**Cohesion, kN/m\({}^{2}\)**} & Landslide & 1.643 & 1.282 & 0.722 & 0.850 \\ \cline{2-6}  & Non-landslide & 0.692 & 0.831 & 0.559 & 0.748 \\ \hline \multirow{2}{*}{**Internal Friction Angle, \({}^{\circ}\)**} & Landslide & 0.987 & 0.993 & 1.633 & 1.278 \\ \cline{2-6}  & Non-landslide & 0.761 & 0.872 & 1.854 & 1.361 \\ \hline \end{tabular}
\end{table}
Table 7: Results of RMSE analysis of cohesion and internal friction angle in soils per geological feature.  
Figure 8: Comparison of internal friction angle factor estimations for landslide and non-landslide soils in areas of study.  
areas, using only the properties that had a significant correlation with the soils. These estimation results were then analyzed to gain a correlation to the actual test results of at least about 81%. The regression equations were deemed suitable for the study and were used to calculate the permeability coefficient and shear strength with cohesion and internal friction angle in unsaturated soils of natural slopes under certain geological conditions using simple soil properties.  
The current study is limited to a statistical analysis to estimate landslide-triggering factors using soil properties for the study area comprising of granite and gneiss. Therefore, it is likely that these results may have limited implications beyond this particular study area. Future research is therefore needed to assess the wider application of this approach to other areas, and to also consider the inclusion of unsaturated characteristics for different soil types along with the physical relationships between the soil properties and landslide triggering mechanisms.  
Conceptualization, K.-S.K. and E.-S.H.; Data curation, K.-S.K. and M.-S.L.; methodology, M.-I.K. and M.-S.L.; writing--original draft preparation, K.-S.K. and M.-I.K.; writing--review and editing, K.-S.K., E.-S.H. and M.-I.K.; All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Korea Institute of Geoscience and Mineral Resources (KIGAM) Research Project (19-3413). Furthermore, the reanalysis of the research data and paperwork was supported by the Korea Agency for Infrastructure Technology Advancement under the Ministry of Land, Infrastructure, and Transport of the Korean government (Project Number: 20 SCIP-C151408-02).  
The authors declare no conflict of interest.