The earth pressure working on the portal of the tunnel are quite complicated. The tunnel receives a widely changing earth pressure because of topography and geology as well as of difference of excavation method.
Loosening of ground after excavation progressed in general as time elapse and pressure increase accordingly.
It is especially so where the tunnel has a little earth covering at the top part and the topography has a very steep gradient or where the discontinuty of the layer are inclining against the cross section of the tunnel.
Past experience, also, teach us that many of the landslide, rockfall and falling accident of the tunnel during the construction were caused by the loosening of the ground.

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Fluctuation behaviors of the groundwater level at Washiodake landslide area, Nagasaki Prefecture, are dealed with in this paper.
(1) The aquifers here are rich in vertical cracks caused by repeated landslides and horizontal, porous coal beds accompanied with clayey layers, which seem landslide surfaces.
(2) Seventeen piezometers (of standpipe type) were settled in different depth of 7 boreholes. This style depends on a technique recently established by the author. Water injection tests were carried out with these devices and changes of the groundwater level were observed.
(3) A new model was proposed by the author for simulating fluctuation behaviors of the groundwater level. Hence, a close agreement between the observed and calculated values can be obtained. A descending rate of the groundwater level or a half life of ascending head by injection and a permeability of the soil were calculated.

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In the present paper, from the geological and geomorphological standpoints we discuss the landslides in the Sambagawa metamorphic belt in the Kawai area, the central part of Tokushima prefecture, which is one of the most famous landslide areas in Japan.
In this area, the Sambagawa metamorphic belt is widely ditributed and its stratigraphic sequence in ascending order is as follows:
Kashidaira formation; mainly composed of pelitic schist intercalated with basic schist, siliceous schist and calcareous schist.
Nonowaki formation; mainly composed of pelitic schist and siliceous schist intercalated with basic schist.
This belt is in contact on the south with the Mikabu green rocks or the Kenzan group, the Chichibu Palaeozoic formation, and by the Akui river fault or the Mikabu tectonic line.
A large number of landslides in the area occur in the Sambagawa metamorphic belt. Their characteristics can be summarized as follows:
(1) Most of the landslides occur in the pelitic schist area, and rarely occur in the siliceous schist area.
(2) A sliding body is mainly composed of debris deposites derived from the basement schists.
(3) Let θ denote the angle on a horizontal plane between the sliding direction of a landslide and the dip direction of a bedding schistosity plane. Then, most of the landslides move within the range 0°≤θ<90°, and are referred to as the “nagareban” structure type. The landslides within the range 0°≤θ<30° occur in the largest number, and the landslides within the range 60°≤θ<90° have the second highest frequency. On the other hand, there are a small number of landslides moving within the range 90°<θ≤180°, which are of the “ukeban” structure type.
(4) The landslides belonging to the “nagareban” structure type are generally large in scale, but the landslides belonging to the “ukeban” structure type are generally small in scale.
(5) The slope angles in the area are determined by the Horton method. The slopes in the landslide areas generally range between 25° and 30°, irrespective of the lithofacies of the basement schists.
(6) The pelitic schist area often forms slopes of 25° to 30° in slope angle. On the other hand, in the siliceous schist area the slope angles are 35° to 50°, in general. Consequently, from the discussion of slope angle it is concluded that the pelitic schist area has many hazardous slopes appropriate for landslide activity.

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It seems to be a recent trend to analyze landslides by the Swedish Circular Arc Method, and few engineers are re-examining the problem. Landslides usually occur on infinite slopes.
In this case, factor of safety of “circular arc slide” decreases as the radius of slip surface increases, and the lowest factor of safty is found in “plane surface slide” under the most extreme conditions.
Therefore, most cases of landslides should be analyzed by the Plane Surface Slide Method.
This paper presents, purely from the soil-mechanical point of view, the investigation of slide damage on slopes in order to obtain a more precise factor of safety to apply for design of a countermeasure against landslides.

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