When we calculate the fracture time using Saito's equation,
log (t_r-t)=a-b logε,
we have assumed b=1
But, recently, the cases of b≠1 are shown by several observers.
This author calculated the correction factor in case of b≠1.

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Since, in Japan, the frequently occurring landslide area has been occupied by the people from the ancient times, the countermeasures for landslide to protect the people living at the sliding area from landslide disasters as well as to check the now-occurring landslide have been taken. Great stresses have been laid on the prediction of landslide occurrence. Investigation of the primary and provoking causes for landslide occurrence has, also, been performed together with that of the provoking causes changeable artificially. Considering at what slope of the mountainous area the landslides take place and at what part of the slope the villages are built up, it is important to study the relationship between the primary causes and the provoking ones changeable artificially. In this paper, we shall describe the relationship between the primary causes and the provoking ones, taking the case of the frequently occurring sliding area consisting of the tertiary layers along the downstream of the Sai River in the north of Nagano Prefecture.

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The Kaizuka landslide area is situated on the easternmost part of so-called Mineoka Mountainrange, which trends along a tectonic line running in E-W directions from Kamogawa City to Hota Town in southern Boso Peninsula. The area is composed of Oligocene Mineoka group, Serpentine and basaltic rocks which are intruded into them.
The author found a specimen of boring core which is constituted from different two kinds of rocks, although both are serpentine petrographically. The one is pale-green in color and rather soft and the other is serpentinized ultramafic igneous rocks, considered from existence of relic minerals internally altered into sezpentinized olivine and non-altered pyroxene.
As results, the author concluded that serpentine of the area is not a single geological unit but is divided into two units which are different in geological age. The one is serpentine proper and the other is ultramafic igneous rocks (basaltic rocks.) In addition, it is notable that accumulation of stress are observed along a contact zone between above-mentioned two rocks.

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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 modules of soil below the surface-stress fall in soil below the surface-vertical deformation of soil layer above the surface…subsrdence of the soil layer with failure in the sides.
“Consider Vertical Subsidence in Slope Unstabilization-I” researches the above-mentioned process by the means of indoor experiments. Part-II will research some examples in the field, and Part-III will research numerically the characteristics of the vertical subsidence, using the equilibrium equation for it.

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When we calculate the safety factor of landslide, we usually uses the ground-water level as pore water pressure. Since there are many landslides in Japan and the field observation needs a very long period, it is almost impossible economically for the landslide engineer himself to measure the pore pressure by piezometer in every landslides. Conveniently, we use the ground-water level as the pore pressure in stead of measuring it directly.
The author observed the variation of ground-water level precisely at Chausuyama landslide area from 1971 to 1973, and found that seepage force cannot be neglected in the case of using the ground-water level in stead of pore pressure. If we rely upon only the ground-water level as the indicator of the variation of the safety factor, it is very dangerous.

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