Observational Study on Landslide Mechanism in the Area of Crystalline Schist (Part 1) -An Example of Propagation of Rankine State-

Propagation of Rankine state was analyzed on the basis of the observation with a high time-resolution of surface and subsurface deformations in a crystalline schist landslide area. The differences in the onset time of deformation at different sites revealed the direction and speed of the propagation. Vertical propagation from the slip surface to the ground surface was clarified by the examination of internal strain at different depths. The records of the extensometer set up over a head cliff and those of the internal strain meters near the depth of the slip surface suggested the duration of the deformation

Observational Study of Landslide Mechanism in the Area of Crystalline Schist (Part 2) --An Application of 2-D F.E.M.--

In order to explain the behavior of landsliding observed in an area of crystalline schist in September 1980, numerical simulation has been conducted by the finite element method under the effects of self-weight and five different pore-water pressures; corresponding to 0.25, 0.5, 0.75, 1.0 or 1.25 times the observed groundwater level. The finite element method is supplemented by joint elements as introduced by Goodman. The joint elements are so devised to express the shear stress to follow the Mohr-Coulomb's failure criterion and the normal stress not to be transmitted across a joint once the joint element has been opened by the tensile stress. The joint elements are arranged along the slip surface determined by measurements using the pipe strain-meters and an insert type of strain meter. The soil above the slip surface is assumed to be linearly elastic. By the initial position and the progression direction of rupture, crystalline-schist landslides can be classified into 3 types. This simulation explains a case where the rupture starts at a point in the slope and progresses downwards at an average velocity of a few meters per hour. It is pointed out that the progression of the rupture has been controlled essentially by rise and fall of the pore-water pressure accompanied by rainfall during the slide

結晶片岩地すべり地における地下水脈・傾斜分布と崩壊箇所の関係

結晶片岩の地すべり地において，地温探査，安定解析および水質分析結果から斜面崩壊と地下水脈の分布の関係について検討した。その結果，崩壊発生箇所は地下水脈の分布に関連すること，この水脈は浅部のみを流れる地下水，山体内での貯留時間が短い深部地下水，この時間が長い深部地下水であることが判明した。また，安定解析結果と水脈分布情報を併用したところ，崩壊危険斜面をある程度絞り込めることが示唆された。The present study is concerned with the assessment of the distribution of thepotential considered about the location of slope failures in a crystalline schist areatriggered by heavy rainfall based on the results of one-meter depth undergroundtemperature survey, slope stability, and water quality analysis. The slope failures appearto concentrate around the vein of groundwater and the veins of groundwater rising fromdeep layers. There are two types of groundwater from origination the deep layers, one isshort storage time the other is long storage time. By combining the results of stabilityanalysis and distribution of veins of groundwater, it is suggested that the localization ofzones of high potential for slope failure can be narrowed down to some extent.結晶片岩の地すべり地において，地温探査，安定解析および水質分析結果から斜面崩壊と地下水脈の分布の関係について検討した。その結果，崩壊発生箇所は地下水脈の分布に関連すること，この水脈は浅部のみを流れる地下水，山体内での貯留時間が短い深部地下水，この時間が長い深部地下水であることが判明した。また，安定解析結果と水脈分布情報を併用したところ，崩壊危険斜面をある程度絞り込めることが示唆された。The present study is concerned with the assessment of the distribution of thepotential considered about the location of slope failures in a crystalline schist areatriggered by heavy rainfall based on the results of one-meter depth undergroundtemperature survey, slope stability, and water quality analysis. The slope failures appearto concentrate around the vein of groundwater and the veins of groundwater rising fromdeep layers. There are two types of groundwater from origination the deep layers, one isshort storage time the other is long storage time. By combining the results of stabilityanalysis and distribution of veins of groundwater, it is suggested that the localization ofzones of high potential for slope failure can be narrowed down to some extent

Some fluidized landslides triggered by the 2011 Tohoku Earthquake (Mw 9.0), Japan

The 2011 Tohoku Earthquake off the Pacific coast of Japan generated a large tsunami and many landslides, resulting in a great number of casualties. Although almost all casualties resulted from the tsunami, some long-travel, fluidized small-scale landslides also killed 13 people. After the earthquake, we surveyed seven of these catastrophic landslides triggered by the earthquake. We found that most of them have nearly identical geological features, with slopes consisting of pyroclastic deposits formed at different times, and with a palaeosol layer that outcropped in most cases after the landslide. Above the palaeosol there are layers of pumice and scoria. The palaeosol had a natural moisture content of ~ 160%, and the pumice and scoria had a moisture content of ~ 145%. From field observations we concluded that the sliding surface originated in the very upper part of palaeosol, and liquefaction occurred in both layers, resulting in the fluidization of displaced landslides. To examine the trigger and movement mechanism of these landslides, we monitored the ground motion of one landslide area during the many aftershocks, and compared the results with records obtained by a national seismic station nearby. We inferred that strong seismic motion occurred in the landslide area during the main shock. We sampled the palaeosol and pyroclastic deposits, and performed undrained static/cyclic shear tests on the materials both in a saturated state and at natural moisture content. The results indicate that high pore-water pressure was generated, resulting in decreased shear strength even in samples with the natural moisture content. The shear strength of the palaeosol lowered to a very small value with continuous increase of shear rate, enabling the high mobility of the displaced landslide materials