Many landslides with Neogene siliceous mudstone beds in Tohoku district are characteristically observed in the form of largescale weathered-rock slide or bedrock slide. The main purpose of this study is to clarify the anomalous sedimentary structure of the siliceous mudstone beds distributed around Ou mountains and Yokote basin, and to identify if there exists any relationship between landslide occurrence and such structural formation. We added a new observation result to the previous researches done by Abe et al.(1994) and Abe (1996). It was verified that the anomalous sedimentary structure of the siliceous mudstone formation was mainly formed by marine slumping at the time of piling up of Sannai-formation layers . Moreover, stratum of the mountain range is found to be characterized by remarkable crushing. It was clarified that the main cause of landslide in the mountain range was the existence of ‘weak stratum’ according to the anomalous sedimentary structure of the siliceous mudstone formation . Since these types of landslide do not often exhibit clear surface topography, analysis of underground sedimentary structure becomes important for the forecast of these landslides.

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Newmark sliding block models are commonly used for seismic stability analysis of slopes. However, estimation of permanent displacements in conventional Newmark methods is based on a distinct and fixed slip surface which is usually located using a pseudostatic limit equilibrium analysis. By combining a finite element seismic analysis and dynamic programming, this paper first shows that critical slip surfaces within a slope greatly change their shapes and locations over a wide area with time during an earthquake. Then, a modified Newmark analysis is presented in order to consider the effect of variable critical slip surfaces on estimated seismic displacements of slopes. Results of analysis for a hypothetical slope and an actual slope failure caused by an earthquake indicate that the neglect of variation of critical slip surfaces during an earthquake may result in an improper sliding mass and as a result incorrect estimation of permanent displacements. It is also shown that permanent displacement may occur along a relatively wide slip band rather than a distinct slip surface.

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In order to develop the evaluation method for the drainage effect by drain tunnel, monitoring of water content using neutron moisture logging in the drainage boring pipe for 19 months and environmental isotope concentrations (Tritium and Radon: ²²²Rn) in groundwater from the drainage boring pipes for five years were carried out after the construction of drain tunnel. The drain tunnel is 1.25m (diameter) × 500m (length) at 50m underground with drainage boring pipes, which are expected to drain superfluous water around the sliding surface of an active landslide in Tertiary mudstone area, Niigata Prefecture. Consequently, the moni-, toring of water content showed a remarkable decreasing with about 8% of water content around the sliding surface for the first four months and subsequently slight decreasing. The tendency of decreasing of water content corresponds with the fluctuation pattern of discharge water volume from drainage boring pipes. The monitoring of tritium and ²²²Rn concentrations revealed that the groundwater around the tunnel was recharged from rain before 10 years and the unsaturated zone around tunnel was occurred. These change of groundwater flow after construction of drain tunnel certifies that the monitoring techniques are useful methods for evaluation of the effect by the drain tunnel.

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This paper represents the change in shear strength of clay for repetitive pore water pressure loading with the use of both experimental tests and model simulation. Pore water pressure was increased under constant deviator stress to simulate the soil behavior in landslide. Yield stress in pore water pressure was obtained constant for repeats of pore water pressure loading, but failure stress was obtained to decrease. Failure stresses constituted the failure envelopment line in void ratio and effective mean stress space. This exhibits the process of change in both shear strength and state variable of soil caused by repetitive failures during pore water pressure loading cycles. In pore water pressure loading, the strain softening of clay develops due to dilation and the shear strength decreases. On the contrary, the shear strength recovery develops by consolidation effect in unloading. Failure stress varies due to two opposite effects of pore water pressure loading cycle. It was found the failure stress in pore water pressure approached to the balanced one between two opposite effects in repetitive pore water pressure loading cycles.

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A limit equilibrium method is presented to evaluate the stability of slopes reinforced with two stabilizing walls (e.g. sheet pile walls). The basic idea of the method is to allow presence of three different slip surfaces in upslope, midslope and downslope earth masses separated by the two walls. Contrary to conventional limit equilibrium analyses which predict a safety factor for a given sliding mass, the proposed method estimates forces acting on the stabilizing walls by specifying a target factor of safety for the stabilized slope.
Estimation of interactive forces between earth masses and walls can be made using an existing method of slices. In this paper, the Bishop simplified method is applied, and a repeated trial procedure is established in order to find the most dangerous state for the walls and the corresponding set of three critical slip surfaces. An illustrative example is presented to demonstrate the effectiveness of the proposed method.

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The features of slope failure at the sea cliff of the Muroto peninsula, Shikoku district, were analyzed by the aerial photo interpretation and the field investigations. Some essential characteristics of the slope failure became clear. Those slope failures at the sea cliff often occurred at the time of great earthquake. While the slopes of terraces, which are covered by forest keep being stable, though abandoned terraces which had been used for cultivation will often collapse on the occasion of the heavy rainfall because of the lack of maintenances. As for the slopes which consist mainly of unconsolidated terrace deposits, the cycle of the slope failure is about 20 years. The erosion rate of basement rock, Shimanto groups, is about 5. 6cm/year.

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On January 5, 2000, a landslide of debris avalanche type (Cruden and Varnes, 1996), took place on the steep slope along the Tozawagawa river, upper tributary of Aganogawa river in Niigata Prefecture, Japan. This landslide blocked the Tozawagawa River, forming a natural dam 500m long temporarily. This landslide has been changing in form in short times just after the catastrophic event on January 5, 2000. In this paper, we are describing the sequence of the landslide using the aerial photographs taken in 1947 up to 2001, and are showing the scale and volume calculation using the aerial photographs.

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