The drainage methods are commonly used for a prevenfive measures against the landslide. The drainage works at Hirayama landslide area had been practiced by the drainage tunnels and weep holes.
The quantity of ground water, 400l/min, from the tunnels have been flowing for three years, but the sliding is occured by every heavy rain.
Therefore, it is the purpose of this paper to point out that it is difficult to prevent the landslide by the drainage works. The following became clear after the investigation on the effect of prevention by the drainage works at Hirayama landslide area. The purpose of the drainage works is to promote the consolidation of the disturbed clay layers by rubbing action on the landslide surface. When the degree of consolidation exceed a certain numerical value, the angle of internal friction increases little by little. As the consolidation proceeds, the effect of variation in the pore water pressure appears slowly.

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Debris movement is supposed to depend on the form of the base rock surface. And so, four landslides in which base rock surfaces are well investigated, are picked up. The form of the base rock surface is devided into two; type I and type II in schmatic solid figures.
Considering on the relation between the form and debris movement, it is deduced that the mass in motion and its moving direction correspond to their concaved form.
In order to analize phenomenon of the mass movement from 50 to 100 years more accurately, it is important to know the form of the base rock surface.

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There are several landslide areas along the axial trace of the Yamanaka anticline which is composed of the Miocene to Pleistocene formations. Landslides are well developed in the hinge of the anticline, but not in the limb, even though the strata are of the same horizon. Especially large landslide areas are situated where the anticlinal axis is curved or warped. It suggests that folding has influence on the development of landslides.
Mudstone and sandy mudstone of the Pliocene formations, collected from the hinge and the limb, are deformed and fractured under condition of triaxial compression at the strain rate of 1×10^-4/sec. Rocks from the hinge have lower strength, smaller Young's modulus and higher ductility than those from the limb. Longitudinal strain under the hydrostatic pressures of 1, 000kg/cm² is greater in the rocks from the hinge. Namely the rocks of the hinge can be strained more easily than those of the limb. It also shows that the rocks of the hinge is weaker to deformation and/or failure including landslide formation. It may be attributed to their textures disturbed by the tectonic flow in folding.
There are different kinds of fractures in the different parts of the anticline. Transversal extension fractures or gashes are well developed in the hinge. As weathering is accelerated by ground-water permeation through these fractures, landslides are well developed in the hinge of the anticline.

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[in Japanese]

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