二,三の花崗岩山地の表層崩壊地における土層形成速度の推定と崩壊周期

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To examine recovery time and formative process of soil layer, Doken-type simplified dynamic cone penetration hardness (N_<10>) and dry unit weight of soils (γ_d, gf/cm^3) were measured at different times after failure in soil-slip scars on slopes underlain by four kinds of granitic rocks (Ghb, Gb, Gi and Gs) in Taga, Mikawa and Shibi mountains. The soil-forming model is described by the following diffusion equation: (∂_<γ_d>)/(∂t)=D/(cos^2i)(∂^2_<γ_d>)/(∂z^2) where D is the diffusion coefficient (cm^2/year), t is the time after failure (year), i is the slope angle, z is the depth of soil layer from ground surface (cm). For this model, the N_<10>-values are converted to γ_d-values using γ_d=0.964 N^<0.161>_c and N_c=0.46 N_<10>, where N_c is Tsukuba-Maruto-type simplified dynamic cone penetration hardness. The formative process of soil layer in the granitic slopes is classified into diffusion type and lateral flow type depending on lithology. The process of the diffusion type acts in the soil-slip scar on slopes underlain by more mafic granitic rocks (Ghb, Gs) having large D-values (18.3-20.9cm^2/year), and the soil layer are formed by weathering process described by diffusion equation progressed from the ground surface. The boundary between soil and weathered bedrock becomes unclear. On the other hand, the process of the lateral flow type acts in the soil-slip scar on slopes underlain by more felsic granitic rocks (Gb, Gi) having small D-values (14.0-15.4 cm^2/year). The soil layer is seemed to be formed not only by the weathering process controlled by the diffusion theory, but also by soil-forming process caused by saturated lateral flow. The boundary between soil and weathered bedrock becomes sharp. The recovery time of the soil layer (the return period of soil slips) on the soil-slip scar is estimated to be 160-240 years on Ghb slope, 90-150 years on Gb slope and 120-140 years on Gi slope.
日本地形学連合の論文
2008-10-25

二,三の花崗岩山地の表層崩壊地における土層形成速度の推定と崩壊周期

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