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From late June to mid-July, 2018, successive heavy rainfall hit a wide area of southwestern Japan, resulting in widespread sediment-related disasters and devastating floods. After the rainfall, we performed reconnaisense investigation on some disasters, and in this report we introduced one landslide occuring in Koubun area of Shisoshi, Hyogo Prefecture, and a cascading failure of irrigation ponds in Fukuyama, Hirosima prefecture. We found that the slope where landslide in Kubun area occurred presented creep deformation such that the landslide boundary could be clearly identified before the landslide occurrence. This suggests the importance of using similar precussor to predict the potential landslide areas. The failures of ponds in Fukuyama resulted in the slope failure occuring on landfill that was constructed on the most upstream of the valley as sports yard. The failure of these ponds revealed the problems in the management of ponds and also the risk of this kind of landfill in the upper stream of a valley.

平成16 年7 月25 日に発生した台風10 号に伴って, 徳島県南部地域では7 月30 日から8 月2 日までの4日間で連続降雨量が2, 000mm を超過する記録的な豪雨が発生した。この豪雨により，徳島県木沢村の大用知, 加州, 阿津江及び釜ケ谷地区で大規模な斜面崩壊が発生した。現地調査, 計測を行った結果, これらの斜面崩壊が高速長距離運動となったと考えられる。また, 阿津江地区の崩壊により崩壊地の背後に再滑動を始めた大規模な地すべりが生じたことが判明された。さらに, リングせん断試験機を用い, 阿津江と釜ケ谷崩壊地から採取した試料に対して降雨による斜面崩壊の再現実験を実施した。その結果, 地下水位が降雨により一定の値まで上昇することによって, 崩壊が発生し, その後せん断に伴って水圧が急速に増加し, せん断抵抗が低い値まで低下したことが分かった。, During Typhoon Namtheun, heavy rainstorm fell on Shikoku area, and triggered numerous landslides in Nakagawa basin of Tokushima Prefecture, Japan in August 1, 2004. Among them four large landslides were triggered in Kisawa village, causing great damage to properties and leading the missing of two persons. Preliminary investigation on them revealed that these landslides experienced rapid long runout movement. Ring shear tests on the samples from different landslides showed that shear resistance could be reduced greatly due to the build-up of high excess pore-water pressure after the shear failure was triggered by the increase in ground water table during the rain.

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.

In order to monitor the crustal movement in Kinki District, southwest Japan, we are continuing daily relative positionings with MINI-MAC2816 GPS receivers. The results show that a relative displacement of 5 cm or smaller will be successfully detected on the Uji-Shionomisaki baseline (162 km), which means that a linear strain of 3×10-7 is detectable. One of main errors in GPS observation is the excess path delay due to water vapor in the troposphere which is hard to be corrected to the level of a few cm by using only meteorological data on the ground surface. We introduced two water vapor radiometers and have started parallel observations with GPS receivers for developing effective correction methods for excess path delay due to water vapor.

During Typhoon Namtheun, heavy rainstorm fell on Shikoku area, and triggered numerous landslides in Nakagawa basin of Tokushima Prefecture, Japan in August 1, 2004. Among them four large landslides were triggered in Kisawa village, causing great damage to properties and leading the missing of two persons. Preliminary investigation on them revealed that these landslides experienced rapid long runout movement. Ring shear tests on the samples from different landslides showed that shear resistance could be reduced greatly due to the build-up of high excess pore-water pressure after the shear failure was triggered by the increase in ground water table during the rain.

In order to monitor the crustal movement in Kinki District, southwest Japan, we are continuing daily relative positionings with MINI-MAC2816 GPS receivers. The results show that a relative displacement of 5 cm or smaller will be successfully detected on the Uji-Shionomisaki baseline (162 km), which means that a linear strain of 3×10-7 is detectable. One of main errors in GPS observation is the excess path delay due to water vapor in the troposphere which is hard to be corrected to the level of a few cm by using only meteorological data on the ground surface. We introduced two water vapor radiometers and have started parallel observations with GPS receivers for developing effective correction methods for excess path delay due to water vapor.

Explanation of landslide mechanism is described from extensometer's and internal strainmeter's records. There are both a creeping landslide and a brittle landslide. Landslide surfaceseems to shift from one place to another. Analyzing the onset time of these records makes clearthat rupture occures in middle landslide slope and is propagated bilateral along a slope at a fewmeters per an hour or unilateral along a slope at dozens of meters per an hour. Extensometer'srecord put between head cliff makes clear that rise time in landslide is about ten and odd hours.

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.

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.

The authors carry out some observations on the rainfall, runoff, evaporation and groundwaterlevel since 1973 at Kuki landslide area, Tokushima Prefecture, Shikoku District in order to clarifythe relation between hydrologic factors and landslide. The results which are obtained are de-scribed as follows; Kuki landslide area is a closed system in regard to hydrologic balance. Drainagebasin of No. 2 has gathered much ground water. Coefficient rainfall and penetration in the area ofbed rock shallower than ten meters is about twice as large as one in the area of bed rock deeperthan ten meters. Upper groundwater level is above the slide surface.

Movement of soil mass is being observed by extensometers, tiltmeters and pipe strain metersin the Irahara landslide area from June, 1979. The authers were interested in detailed surveiesof features of the distributions of extension and compression of the ground surface and slip plane, and investigated the relationship between the state of movement of the ground surface and theform and movement of the slip plane. Then, they presumed the characteristics of the movementof soil mass, as shown in Fig. 10.

lt is indispensable for study of the mechanism of landslide to estimate the rupture velocityat slide surface. The array observations of extensometers and observations of internal strainmeters at Kuki landslide have been made since 1972. It is found out that the rupure at slidesurface is propagated upward along a slope at several tens of meters an hour, from analyzingthe onset times of these records.

Movement of soil mass is being observed by extensometers, tiltmeters and pipe strain metersin the Irahara landslide area from June, 1979. The authers were interested in detailed surveiesof features of the distributions of extension and compression of the ground surface and slip plane, and investigated the relationship between the state of movement of the ground surface and theform and movement of the slip plane. Then, they presumed the characteristics of the movementof soil mass, as shown in Fig. 10.

Explanation of landslide mechanism is described from extensometer's and internal strainmeter's records. There are both a creeping landslide and a brittle landslide. Landslide surfaceseems to shift from one place to another. Analyzing the onset time of these records makes clearthat rupture occures in middle landslide slope and is propagated bilateral along a slope at a fewmeters per an hour or unilateral along a slope at dozens of meters per an hour. Extensometer'srecord put between head cliff makes clear that rise time in landslide is about ten and odd hours.

The authors carry out some observations on the rainfall, runoff, evaporation and groundwaterlevel since 1973 at Kuki landslide area, Tokushima Prefecture, Shikoku District in order to clarifythe relation between hydrologic factors and landslide. The results which are obtained are de-scribed as follows; Kuki landslide area is a closed system in regard to hydrologic balance. Drainagebasin of No. 2 has gathered much ground water. Coefficient rainfall and penetration in the area ofbed rock shallower than ten meters is about twice as large as one in the area of bed rock deeperthan ten meters. Upper groundwater level is above the slide surface.

lt is indispensable for study of the mechanism of landslide to estimate the rupture velocityat slide surface. The array observations of extensometers and observations of internal strainmeters at Kuki landslide have been made since 1972. It is found out that the rupure at slidesurface is propagated upward along a slope at several tens of meters an hour, from analyzingthe onset times of these records.