The geological structure of the Japan islands is complicated. The crustal movement is active and the relative relief of the mountainous areas is conspicuous. The mountainous areas reflect the existence of faults, the relative hardness of rock and the upheaval of the ground. The striking development of the structural relief and the tectonic relief is sometimes seen. The landslide is a kind of landform process which proceeds rapidly. The landslides in the mountainous areas accelerate the development of the slope landform; the response to the structural relief is rapid. We have surveyed the landslides occurred at the Tertiary zone in the northern mountainous districts of Nagano Prefecture and have summarized the following interesting characteristics.
(a) The formation of gentle slopes in the mountain side is accelerated by the occurrence of landslides. The formation of gentle slopes is noticeable in the folding areas, in the back slope of homoclinal ridges and in the surroundings of monadnock areas composed of the Tertiary pyroclastic rocks. Repetition of landslide occurrence forms the rounded wavelike gentle-slopes in the mountain side. The wavelike gentle-slopes produced in this manner are the unique ones which are not formed by the river erosion process due to the common running water.
(b) In the rock slide areas, abnormal landforms are often produced by the movement of the huge rock and soil mass. Sometimes, large cracks run across the ridge behind the mountain side and part of a mountain mass including the ridge slides. And the deformation process in the surroundings of the sliding rock and soil mass is abnormally accelerated.
(c) The most important in surveying the ground structure of the sliding area is to decide whether the landslide in question are primary or secondary. The landslides in the landform with the character of (a) are often secondary and those in the landform with the character of (b) are usually primary.

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[in Japanese]

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It is the drain work that has usually been carried out under control works of landslides. On a drainage plan, the lowered amount of the ground water level is generally reckoned by the analysis for stability of the cross section of landslide.
However, the kind, the scale and the arrangement of equipments to lower the ground water level to the planned one have been decided only from experiences. To control landslides by the safety factor, then, plans for lowering the ground water level must be formed by means of the data obtained from the actual survey of the precipitation, the ground water level fluctuation, the coefficient of permeability etc.
We examined it at Toyomaki Landslide in the Mogami-gawa basin. In our plan, we used the finite element method and intended the designed safety factor to come to 1.1. On forming the model, we adopted the outcomes of our actual drilling survey and coefficient of permeability, the amount of drainage and the boundary condition obtained from the past records of constructions of two water catchment wells. The result proved the model to be reliable because it reproduced the ground water level fluctuation when the equipment had not been constructed yet, and because there was correspondence between the outcome of the simulation and the influence of two water catchment wells which had been constructed by now.
From these points of view, our conclusion is as follows.
1) It is possible to form a model of simulation in a landslide area.
2) A reproduction by the model is well worth being reliable.
3) It is possible to examine the drainage scale and place with simulation of the model.

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[in Japanese]

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This is a study of flowing mechanism of the groundwater on the basis of the hydrological data obtained from the measurements in 1971-1978 at the Yachi Landslide, a typical rock block slide in Japan. The consideration was made from the viewpoint of geological environment, specifically from the relationship between the geologic structure and the physical properties of the Tertiary strata as the bedrock of the landslide area.

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