# Regression Equations for Estimating Landslide-Triggering Factors Using Soil Characteristics
###### Abstract  
Landslides, which often occur on natural slopes of mountainous areas and artificial slopes around urban areas during summer in South Korea, are mostly caused by localized heavy rainfalls and typhoons. A survey was conducted, and the characteristics of landslide occurrences in different geological conditions--in this case, granite soils in Sangju area and gneiss soils in Yangu area--were analyzed. Soil characteristics in the landslide and non-landslide areas and the surroundings of each geological condition were also evaluated. Triggering factors, namely permeability coefficients (\(k\)), shear strength with cohesion (\(c\)), and internal friction angle (\(\varphi\)) of soils that are closely linked to landslides around weathered soil layers were extracted based on the examined characteristics and a statistics method. The study used regression analysis to formulate equations to estimate the permeability coefficients and shear strength. Ultimately, the permeability coefficients showed significant results in terms of void ratio (\(e\)), the effective size of grains (D\({}_{10}\)), and uniformity coefficient (\(c_{u}\)), while shear strength correlated with the proportion of fine-grained soil (_Fines_), uniformity coefficient (\(c_{u}\)), degree of saturation (\(S\)), dry weight density (\(r_{d}\)), and void ratio (\(e\)). landslide-triggering factors; weathered soil; permeability coefficient; shear strength; regression equation +
Footnote †: journal: Medical  
in the lower areas. On the same day, debris from landslides swept away houses and accommodations in Chuncheon in Gangwon Province, causing 13 deaths and 25 injuries [2; 3]. These two cases were similar in terms of the localized heavy rainfall during the summer season around the cut slopes, which were formed after the natural slopes have been cut [4]. Rainfall is considered as one of the major causes of landslides. The penetration of stormwater through the soil layers increases the weights per unit volume of soil particles; thus slope stability rapidly decreases with the shear stress reduction of the soil [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. Other direct and indirect causes of landslides include topography, geological features, and vegetation [16; 17; 18; 19]. In most cases of natural slopes, weathering soil layers located at the upper part of bedrocks are formed with the erosion of parent rocks, and their physical and engineering characteristics are determined by weathering intensity and geological conditions [20; 21]. Moreover, weathering soil layers have different properties depending on the mineral components of the bedrocks where they are created. In particular, the different geological types of rocks in slopes affect the factors that may cause landslides, such as weathering, shearing, cutting of bedrocks, directional nature of discontinuity surface, and water permeability [22; 23; 24; 25]. The slip surface of landslides is commonly found around the boundary between the bedrock and the weathering soil layers. Hence, the physical characteristics of soil in these layers are closely linked with the occurrence of disasters [26; 27]. More importantly, heavy rainfall influences the change in permeability and shear stress of weathering layers. It triggers the penetration of stormwater through the layers and decreases the shear stress of soils, increasing the possibility of disasters along the slope [28; 29; 30; 31; 32]. Therefore, the soil's permeability coefficient and shear strength are important variables for the direct assessment of slope stability considering infiltration of rainwater into the ground [33; 34; 35; 36]. The study suggests that influential factors are related to slope disasters, which were identified by comparing the soil material properties of landslide and non-landslide areas to select the components that play significant roles during slope disasters and to derive a statistical formula for calculation.