Climatic Influence of chemical weathering on the particle size distribution of the slope soil layers in granitic bedrock areas

Granitic rocks tend to turn into sandy sediment, grus, through weathering, which makes them prone to collapse even on relatively gentle slopes. Such weathering of rocks is broadly classified into chemical weathering and physical weathering, with chemical weathering dominant in warmer regions and physical weathering dominant in colder regions. Many weathering indices have already been proposed for chemical weathering, and it is possible to estimate the chemical weathering process by using these indices. However, it has not been clearly stated how the weathering process affects the properties of the weathering product, the sediment. Therefore, in this study, we examined the characteristics and regional characteristics of weathering of the soil layer, the collapse reserve material, by examining the relationship between grain size composition, the simplest physical property of the soil, and chemical weathering index values. The samples studied were weathered granite and granodiorite collected near the slip surfaces of slopes where slope failures have occurred in the past in several areas with different climates in Japan. First, each weathering index of CIA (Chemical Index of Alteration, Nesbitt and Young, 1982) and the Si/Al ratio (Marbut, 1935) were calculated based on chemical composition values. The percentage of clay and silt is analyzed and calculated by using Wentworth's (1922) scale. The relationship between CIA values and the clay-silt ratio is positive and proportional. A negative proportional relationship was obtained between Si/Al ratio and the clay-silt ratio. The relatively cold region is more affected by physical weathering than other regions, due to the smaller difference in chemical weathering index values between the surface soil and base rock and the higher percentage of coarse-grained weathering products.