Tatsuya Shibasaki

Experimental evidence for shallow, slow-moving landslides activated by a decrease in ground temperature

In order to understand the trigger mechanism of slow-moving landslides occurring in the early cold season from late autumn to winter, we investigated the effect of temperature on the shear strength of slip surface soils. Displacement-controlled and shear stress-controlled box shear experiments were performed on undisturbed slip zone soils under residual strength conditions. Test results conducted at temperatures from 9 to 25°C showed remarkable shear strength reductions with decreasing temperature. Creep-like slow shear displacements were induced by a decrease in temperature. These temperature-dependent shear behaviors are attributed to the rheological properties of hydrous smectite that dominantly compose the soil material along the failure surface. Our experimental results imply that ground temperature conditions influence slope instability, especially for shallow landslides occurring in smectite-bearing rock areas.

Temperature‐dependent residual shear strength characteristics of smectite‐bearing landslide soils

This paper presents experimental investigations regarding the effect of temperature on the residual strength of landslide soils at slow-to-moderate shearing velocities. We performed ring shear tests on 23 soil samples at temperatures of 6–29 °C. The test results show that the shear strength of smectite-rich soils decreased when temperatures were relatively low. These positive temperature effects (strength losses at lower temperatures) observed for smectite-bearing soils are typical under relatively slow shearing rates. In contrast, under relatively high shearing rates, strength was gained as temperature decreased.. As rheological properties of smectite suspensions are sensitive to environmental factors, such as temperature, pH, and dissolved ions, we inferred that temperature-dependent residual strengths of smectitic soils are also attributed to their specific rheological properties. Visual and SEM observations of Ca-bentonite suggest that slickensided shear surfaces at slow shearing rates are very shiny and smooth, whereas those at moderate shearing rates are not glossy and are slightly turbulent, indicating that platy smectite particles are strongly orientated at slow velocities. The positive temperature effect is probably due to temperature-dependent micro-friction that is mobilized in the parallel directions of the sheet structure of hydrous smectite particles. On the contrary, the influence of micro-viscous resistance, which appears in the vertical directions of the lamination, is assumed to increase at faster velocities. Our results imply that if slip-surface soils contain high fractions of smectite, decreases in ground temperature can lead to lowered shear resistance of the slip surface and trigger slow landslide movement.

TXT-tool 4.081-1.1: Mechanism of Large-Scale Deep-Seated Landslides Induced by Rainfall on Gravitationally Deformed Slopes: A Case Study of the Kuridaira Landslide in the Kii Peninsula, Japan

In September 2011, heavy rainfall brought by Typhoon Talas triggered 72 large-scale deep-seated landslides in Nara and Wakayama Prefectures, the Kii Peninsula, Japan. Most investigated landslides on the gravitationally deformed slopes were preceded by pre-existing small scarps along or near the head of the slopes. This study seeks to clarify the mechanism of the huge rainfall-induced Kuridaira landslide by simulating the increasing of pore water pressure with undrained high-stress dynamic loading ring shear apparatus. The authors also examined how gravitational deformations of upland slopes contribute to the mass movement under shear deformation. Laboratory experiments were conducted on two samples of the sliding plane taken in a site investigation, namely sandstone-dominated materials and shale materials. The pore water pressure control tests and shear displacement control tests clearly indicated that the rapid landslide was initiated due to high excess pore pressure generation and significantly shear strength reduction in the progress of shear displacement. The critical pore pressure ratio (ru) was about from 0.33 to 0.36 while shear displacement at the starting point of failure (DL) had a threshold value ranging only from 2 to 6 mm. More specifically, the high mobility of the landslide was in tests on shale sample due to a significant loss of shear strength. In addition, the authors observed the landslide occurrence associated with the sliding surface liquefaction behavior for both samples. The evidence of liquefaction phenomena in the tests was in accordance with the findings in the field survey and previous studies.

Seasonal transition of hydrological processes in a slow-moving landslide in a snowy region

Graduate School of Science, Kyoto University, Kyoto, Japan Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan Forest Research and Management Organization, Forestry and Forest Products Research Institute, Tsukuba, Ibaraki, Japan 4 Japan Conservation Engineers & Co., Ltd., Fukushima, Fukushima, Japan Snow and Ice Research Center, National Research Institute for Earth Science and Disaster Resilience, Niigata, Japan Correspondence Hikaru Osawa, Kyoto University, Graduate School of Science, Gokasho, Uji, Kyoto 611‐ 0011, Japan. Email: osawa@scs.dpri.kyoto‐u.ac.jp

Fluctuations in the Pore-Water Pressure of a Reactivated Landslide in a Snowy District

Pore-water pressure fluctuations and water that reaches the ground surface (MR) were monitored in a reactivated landslide which is located in a heavy snow district of Japan. Observations of pore-water pressure inside the moving landslide mass revealed that changes in pore-water pressure during snow cover periods were quite different from those in other periods. These results suggest that the hydrological properties of a moving landslide mass are strongly affected by snow load.

Development of a Cyclic Box Shear Apparatus to Elucidate Mechanisms of Earthquake-Triggered Landslides

In this study, we have developed a cyclic box shear apparatus for elucidating the mechanisms of earthquake-triggered landslides. We conducted a cyclic box shear test, on undisturbed slip-surface clay samples and obtained excess pore water pressure behavior which can provide dynamic strength under seismic condition.