I have reported* that the cohesion and the angle of internal friction of the disturbed clay (a thin layer fromed by rubbing action at landslide face) are changed by ratio of consolidation.
But it is necessary to check whether shearing properties change or not with the time passed since consolidation is started at landslide field.
Thus we obtained the thrust of landslide from the measured value of earth pressure in the observation well, and from this we estimated the change of the shearing properties of slide face. According to the estimation, the calculation results are in approximate agreement with experimental value. Based on the investigation on the pore water pressure, it has become clear that the pore water pressure is divided into the following two classes: one produced by the depth of ground water and the other, namely, the hydrostatic excess pressure produced by the process of consolidation in the disturbed clay.
Accordingly when we calculate the safety factor, it is necessary in some cases to consider two kinds of the pore water pressure.
*B. Tamada: Studies on the Dynamical Properties of Landslide Face and their soil Test: Landslide, Vol. 10, No. 4 (1974)