Abstract Observation on sliding surfaces in situ in several landslide areas has led to the recognition that sliding surface consists of a zonal structure. The zonal structure is composed of a fine clay seam of 3-8 cm of thickness and a conglomerate clay zone from top to bottom. In the fine clay seam, a piling of thin clay slices of 1-5 mm of thickness and many micro-cracks due to displacement shears are observed. These displacement shears are more dominant in the fine clay seam than are in conglomerate clay zone, to be well attributed to the principal displacement shears as has been defined so by Skempton, A. W.
Soil samples of fine clay seam and conglomerate clay zone were taken in collecter wells and put to soil tests. The characteristics thus found are summerized as follows:
(1) Fine clay seams are 0.5-16% higher in water content and 5-15% higher in clay fraction (<5μ) as compared with conglomerate clay zones.
(2) The sheared surface made with ring-shear-apparatus test had a zonal structure, the thickness being 0.5-1.5mm, and the water content being 0-20% higher than the upper and lower parts.
(3) The shear strength of fine clay seam was 1-7 degree smaller in the angle of shearing resistance than that of conglomerate clay zone. To note here is that, though the displacement be greatest in the fine clay seam, the displacement reaches to a certain height above (or below) the principal displacement shears, and that the displacement relative to the principal shear gets smaller as the height increases.

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The conventional quasi-three-dimensional method can not used for the actual problems such as the examination of groundwater fluctuation due to the construction of drainage well and tunnel excuvation because the discharge nature of vertical direction may not be considered. From the above reason, on this paper, a new quasi-three-dimensional analysis method containing the vertical flow amount is discussed from the view of the development of efficient and economical seepage flow simulation method. The presented method makes it to be possible to carry out the analyzation of saturated-unsaturated seepage flow containing the seepage face in porous media and radiant flow form. Especially, this method is carried out by the small memory computer because the horizontal and vertical direction flows can be calculated separately by using the each discretization procedure of triangle and vertical elements.

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Landslide phenomena are caused by many factors such as layer of soil, geological features, properties of bedrocks, properties of weathering products, water balance and others. The authors collected many data over more than 20 years, in order to analyze on.mechanism of the landslides called HASAITAI JISUBERI in Japanese occurred in Ehime prefecture.
In this paper, the following conclusions were obtained from the examination on foundation and ground water in landslide area.
1. 3-5 velocity layers are classified by the refraction seismic prospecting of drilling core samples, and these layers are well correlated to the grade of weathering. Overmore, all of these layers have the destruction zone.
2. Section velocity of ground water calulated from permeability is slow than velocity of actural ground water. Because, ground water flow pass through creacks of weathering products. Still more, ground water is easy to flow in boundary layers or destruction zone.
3. Landslides are occurred in boundary layers or destruction zone.

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Recently steel pile work has been developed as an effective countermeasure against landslides.
We introduce a general method of calculating the deflection, angle of deflection, bending moment, shearing force and anchor forces of the pile with anchors in the lateral layers under arbitary load conditions for the design of the cantilever type for landslide prevention.
We analize the characteristics of the pile under various conditions using a personal computer.

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Neogene Shimajiri mudstone is widely distributed in the central and southern areas of Okinawa Island and along the marine terrace in the northeastern part of Miyrko Island. Landslides are apt to occur in those areas controlled by the landform and geological structure of the mudstones. The fist-time slide occurs along the small fault and joint planes of the Shimajiri mudstone which is uplifted and exposed in the ground. In the mudstone which is caprocked by the Ryukyu limestone of Quaternary-age, slides occur in the colluvial deposits. Some distinctive features of the first-time slides in the Shimajiri mudstone are as follows: (1) structurally weak planes become a slip surface; (2) the shear strength along the slickensided surfaces of weak planes has lowered to near the residual strength; (3) fractured mudstone mobilizing the low peak strength exists in a part of the shear zone, and it, together with the structurally weak surface, resists the sliding force of the whole block; (4) the sliding is triggered by the artificial reformation of land and initiated by the rainwater seepage into the fractured shear zone.

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