An approximate finite element method is presented for the transient unconfined flow of porous media combined with un-saturated zone. Free surface water line is progressed by the changing procedure of element situation according to the condition of flux values. This leads to an approximate but simple calculation scheme because of no-remesh element in spite of un-homogeneous media condition. The solution is compared with other solution. It shows the technique is capable of the giving meaningful results when the slope stability analysis carries out on the submerged landslide slope.

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The effects of the horizontal drains on the ground water level are predicted using the three-dimensional finite element analyses of water flow through unsaturated-saturated soils. The slope stability is evaluated by the total factor of safety, using the three-dimensional elasto-plastic finite element analyses based on the shear strength reduction method. In the elasto-plastic analyses, the initial effective stress in the slope is evaluated by the distribution of pore water pressure wich has been given by the seepage flow analyses.
As indicated in this paper, in cases where the horizontal drains are constructed in inappropriate areas in the slope, the improvement of the total factor of safety is not satisfactory. It is confirmed that the analytical method presented in this paper is expected to be very effective for the evaluation of the performance of actual horizontal drains in slopes.

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In the southern part of Kyushu, Japan, covered with volcaniclastic materials, landslide and debris flow disasters have occurred frequently during rainy and typhoon seasons. Recently, two types of landslide which have rarely taken place were caused by a rise in the groundwater level in association with heavy rainfall. The one is sliding of talus slope at the valley head scarred on Shirasu (pyroclastic flow deposits) plateau in heavy rain in August 1993, and the other one is a deep-seated landslide on the western flank of the old volcano of Mt. Yahazudake in Izumi City in July 1997.
In this paper, the mechanism of these landslides, the geological and hydrogeomorphological characteristics of the landslide slope sites, and the possibility of predicting the potential landslide sites are examined based on field study.

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Rainfall and melting snow infiltrate fissures and aquifers, often triggering landslides on slip surfaces. This paper presents five typical landslide areas for which the depth of slip surface was determined and time series data of groundwater level and slide displacements. The relation between the displacement of the slide and the pore water pressure was examined especially closely. The useful results obtained are summarized below.
(1) The relations between the displacement of slide and the pore water pressure are classified into three types.
The first is a type of rock mass landslide that is sensitive to the pore water pressure the whole year round. In the relation curve of the velocity of displacement versus the pore water pressure, there is practically no hysteresis for the pressure increasing period and the pressure decreasing period. The second is a type of rock mass landslide that moves mainly in the snow melting period. The curve of the velocity of displacement versus the pore water pressure indicates hysteresis for the pressure increasing period and the pressure decreasing period. The third type is the detritus landslide consisting of pelitic-schist. In this type, the displacement peak occurs after the pressure peak.
(2) Comparing the rock mass landside and the detritus landslide, there are some differences in the mechanism of the slip surface and the form of the displacement of the landslide. These differences are due to the two different ways of transmitting the pore water pressure to the slip surface and the related differences in features and mechanisms.

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Generally speaking, the groundwater flow in landslides is a complex phenomenon to understand. In order to get some insights on this subject, the gloundwater logging is wildly and effectively used.
The author has been attempted measuring the flow velocity in up/down direction within a borehole by use of the micro-flow meter for many years. In particular, at two active landslides the measurements of flow velocity were carried out using the micro-flow meter last years. The results show that a considerable variation in the flow velocity can be clearly observed by the micro-flow meter. Thus, the in situ permeability and pore pressure for each zone along the borehole can be determined. On the basis of hydraulic information obtained from the measurements, the characteristics of groundwater flow in the two landslides can be understood well.
In conclusion, the micro-flow meter is a powerful instrument for investigation of groundwater flow in landslides.

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Oxygen is dissolved in the water as a free gas and is frequently treated as an index of contamination during water quality studies. This value is affected by the weathering of rock and is reduced by the decomposition of organic material. It is, therefore, assumed that the deeper the groundwater and the longer it has been in the aquifer, the lower its dissolved oxygen content. Other good points about dissolved oxygen are that it is easily diluted and once dissolved in water its vertical motion is small. For these reasons, a method of exploring groundwater flow strata using it as a tracer was carried out in a landslide area and a groundwater flow stera model.

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In Yachi landslide area at south-eastern part of Akita Prefecture, many drainage wells of groundwater have been installed. There, groundwater from the drainage wells as well as the surface waters were monthly collected, and analyzed for the oxygen and hydrogen isotopic ratios, to discuss origin of the groundwaters and mixing process in the groundwater reservoirs.
One of two cold springs has almost constant isotopic ratios of δD=-57‰, δ¹⁸O=-9.9‰ through the year, and the other shows large variations of isotopic ratios, δD=-47--70, δ¹⁸O=-8.7--11.2‰, of which seasonal patterns are similar to those of precipitation in Akita City. It is suggested that the former is precipitation well mixed for long duration enough to smooth its seasonal variation of isotopic ratios, while the latter, precipitation not well mixed. The groundwater from drainage wells shows three different degrees of mixing, Groundwater drained after infiltration of precipitation about 300m in shale layer shows small seasonal variation similar to the cold spring having almost constant isotopic ratios, while groundwaters drained in 30m distance from the shale outcrop shows seasonal variation of which extent is about half of the cold spring having large isotopic variation. Groundwater in colluvial soils shows intermediate variation of above 2 types of the groundwaters. These differences are able to explain by the difference in mean residence time of groundwater. As the result of a model calculation based on well mixed reservoir in a steady state, mean residence times of groundwaters were estimated as; about one year for the groundwater of almost constant isotopic ratio from shale layer, about six months for the those from colluvial soils, and about two months for those drained near the shale outcrop, respectively. As for the groundwater near the shale outcrop, also, certain discord of seasonal variation phase was observed between the model calculation and the observation. This suggests that the infiltration of precipitation includes some piston flow like process.

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[in Japanese]

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Residual pore water pressure due to drawdown of reservoir influence on safety of reservoir limb landslide. This paper propose a method on estimation of residual pore water pressure and apply of it. I investigate the behavior of residual pore water pressure of 16 landslides. I examine non-saturated permeability properties of landslide and apply non-saturated seepage analysis. And then I apply this method to landslide (rock slide) of high permeability befor filling, and estimate residual pore water pressure of it rationally.

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During the snow melting season some large landslides cause severe damage in heavy snowfall regions due to the increase of pore water pressure. It is important to clarify the properties of groundwater flow in these landslides during the snow melting season in order to understand the mechanism of landslide occurrence.
The long-term piezometric response of deep groundwater in the Dozangawa landslide, which is in a Tertiary area covered by pyroclastic material, was observed. A geological model for simulation was constructed using hydrogeological parameters obtained by electrical prospecting. Groundwater flow analysis by finite element method was then performed on this model and simulated groundwater flow for the snow melting season and for the rainy season.
Field observation and groundwater analysis showed that numerical simulation of groundwater flow is useful for clarifying the characteristics of groundwater flow in the pyroclastic material layer that influences the pore water pressure in a deep slip surface, and how the geomorphological characteristics of a landslide affect the piezometric response from snow melting water.

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For the purpose of explaining the flow mechanism of groundwater in landslide area, chemical analysis of groundwater and vertical groundwater logging were carried out in the Kuroshio landslide area. The Imari fault passes through the Kuroshio landslide area, so the fault may affect the flow of groundwater.
As a result, we realized that there are two aquifers in this area. One is unconfind groundwater flowing near ground surface, another is confind groundwater flowing on a slip surface.
We considered that; the groundwater flowing on a slip surface causes the landslide. And groundwater flowing near ground surface is supplied to that groundwater through the Imari fault.
Such survey will give us important information when we design groundwater drainage works.

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