The mechanism of unstabilization of slope which is generally used is; Soil mass starts to slide almost in parallel to slope by the decrease of shearing resistance with uplift of ground water level and decrease of effective stress. However, according to the results of indoor experiments and field observations by the authors, it has been nearly proved that another mechanism of slope unstabilization different from the above-mentioned exists; Soil mass subsides vertically at first, and it may start to slide, or it may be mobilized to flow, or it may change to real mud flow, or on the contrary it may only subside without any holizontal move.
The mechanism of subsidence is of which it occurs with uplift of ground water level, quite different from the settlement due to consolidation of clay. The mechanical process of subsidence is; uplift of ground water level-increase of water content below the ground water surface-decrease of Young's modulus of soil below the surface-stress fall in soil below the surface-vertical deformation of soil layer above the surface-subsidence of the soil layer with failure in the sides.
The above-mentioned process was already examined by the means of indoor experiments and field observation and measurement in “Consider Vertical Subsidence to Slope Unstabilization-I, II”. In this paper the authors examine how the strain of soil layer increases with the uplift of ground water surface, and with the increase of rigidity of soil due to weathering and erosion under the ground, and the other characteristics of the vertical subsidence numerically, using the data in Part I.

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