抄録
Landslides are one of the most important processes supplying debris flow and bedroad materials into channels. Therefore, we need to predict occurrence of landslides for better estimation of the bed deformation of mountainous rivers. Spatial and temporal changes in the pore water pressure determine timing, location, and volume of landslides. However, most prior studies simply substitute the hydrostatic pressure for the pore water pressure and ignore existence of hydrodynamic pressure. In addition, depth profile of the pore water pressure and the effective stress in the slopes with multi-layer soil structure are poorly discussed. We, therefore, derived the pore water pressure and the effective stress on the basis of the equations for the vertical infiltration process（i.e., continuity equation and Darcy's law）in the multi-layer soil structure. We also discussed their depth profiles by considering boundary conditions. Our study clarified that the water velocity as well as depth profile of the pore water pressure are affected by depth profile of the hydraulic conductivity in the saturated zone. Pore water pressure agrees with the hydrostatic pressure when saturated zone develops on an impermeable soil layer. In case that the hydraulic conductivity is almost constant throughout the saturated zone, pore water pressure is much lower than the hydrostatic pressure. On the other hand, pore water pressure approaches hydrostatic pressure when the hydraulic conductivity in the saturated zone changes significantly in the depth direction. Peak of the pore water pressure locates at bottom of the soil layers in which ratio of the flow velocity to the hydraulic conductivity exceeds -1. When two separate saturated zones develop at different depths, existence of the upper saturated zone does not affect pore water pressure of the lower saturated zone ; upper saturated zone just increases effective shear stress of the lower saturated zone as increases in degree of saturation.
