Sumio Matsuura

Characteristics of the displacement of a landslide with shallow sliding surface in a heavy snow district of Japan

The displacement of a landslide with shallow sliding surface in a heavy snow district was automatically monitored for over 4 years by a newly developed displacement gauge. Meteorological properties, meltwater and/or rainfall (MR), water equivalent of snow and other factors were simultaneously observed. Pore-water pressure was monitored but only for a limited period of time since the displacement of the landslide was too large. Statistical analyses showed that maximum daily displacements of each year were observed not in snow melting periods but immediately before or at the beginning of a snow cover period. There are three types of annual variation pattern with peaks in mid-November to mid-December, mid-April and mid-July. The variation pattern of displacement from the beginning of March to the beginning of June showed the least total displacement of those three types. Therefore, snow cover is likely to affect the displacement of a landslide. Detailed investigation of the landslide displacement during snow cover periods showed that the rate of displacement slowed as the snow cover deepened, meaning a decrease in MR and increase in water equivalent of snow. The displacement ratio for the unit antecedent MR effect during snow cover periods dropped when the snow started to accumulate on the ground in all 4 years. The reduction was likely attributable to snow load since it was found to have a negative logarithmic relation with the water equivalent of snow. Changes in pore-water pressure and the relationship between pore-water pressure and displacement during the snow cover periods differed from those in the other periods. Therefore, the displacement of a landslide that has a shallow sliding surface in a snowy region was considered to be affected by snow accumulation conditions, especially water equivalent of snow.

Fluctuation of the seasonal snowpack in a mountainous area of the heavy-snow district in the warm-temperate zone of Japan

The seasonal snowpack and meteorological factors associated with the accumulation and ablation of the snowpack were monitored for 11 years in a mountainous area in the warm-temperate zone of Japan. No notable rise was observed in mean wintertime air temperature, but an increase was seen in the difference between the maximum and minimum air temperatures. Precipitation exhibited annual variability but no notable reduction over the measurement period. The length of the continuous snow-cover period increased slightly over the 11 years, but no trend in variability was observed. The maximum snow depth and maximum water equivalent of snow varied greatly from year to year, depending on the amount of snowfall. In a heavy-snow year, about 1600 mm of water, which is almost the mean annual precipitation for the whole of Japan, was found to be temporarily stored in the snowpack.

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.

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

Introduction: Landslides in Cold Regions

The areas we called the cold regions refers to permafrost, seasonal frozen soil and the short-term frozen soil. Their total area has accounted for about 50 % of the whole land area, and permafrost distribution accounts for about 25 % of the Earth’s land surface. In Wlf3-C7 session—landslides in cold regions, for same study area and same study object in cold regions, researchers adopted different method, began with different perspective. In the context of climate change, their research contain landslides mechanism, geomorphologic identification and classification, environment geological conditions change, even the plants-soil-permafrost systems in landslide zone.

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.

Sediment-Related Disasters Caused by the Nagano-ken Hokubu Earthquake in the Heavy Snow Season

Mountain slopes covered in deep snow were hit by seismic motions during the Nagano-ken Hokubu Earthquake on March 12, 2011, resulting in avalanches, slope failures, and deep-seated landslides. Sediment-related disasters occurred at places which had geologically weak structures such as faults and fractured rock masses and/or topographical features that were prone to receiving concentrated earthquake acceleration. However, the key characteristic was the interaction between snow cover and sediment-related disasters during and after the earthquake. Fast-moving layers of air and suspended snow particles were also produced when fast debris flows crashed into accumulated snow, destroying peripheral areas along the path of the debris flows. Furthermore, a landslide occurred about 1 month after the earthquake on a slope that was likely to have been loosened by the seismic motions. The numbers of slope failures and landslides were very small compared to the Niigata-ken Chuetsu Earthquake of a similar scale, which was probably attributable to the physical and/or mechanical properties of snow cover.

Observation of a Snow Environment and Snowmelt Process at Two Sites of Different Altitudes in the Same Catchment

多雪地帯に位置する山地流域内で,標高差が667mとなる2カ所の気象観測施設を設置し,積雪環境や融雪要因に関する連続観測を実施した.その結果,積雪底面からの流出量(MR(a/o))のタイミングや強度が大きく異なることが明らかとなった.特に融雪期では,上部のMR(a/o)は下部に比べて14日程度遅れて観測されるとともに,上部の強度は下部よりも大きくなる傾向が見られた.流域上部では風速が大きくなることから,融雪期には顕熱および潜熱交換量による融雪量の占める割合が相対的に大きくなるものと推定される.

Pore Water Pressure Fluctuation on the Landslide Slip Surface Based on Controlling Drainage Experiment of Drainage Wells

Abstract The purpose of this paper is to examine the mechanism of pore water pressure fluctuation and to grasp the environment of underground hydrology in the landslide by controlling drainage experiment. At first in this paper, the results of experiments on the pore water pressure fluctuation at the slip surface are explained. The second, the drainage effects of horizontal borehole are confirmed as quantitative grasp in drainage wells. Finally, the environment of underground hydrology in Tairasawa landslide area is cleared. The data was collected at Tairasawa landslide area during 1986-1987. The results are as follows:1) In this natural slope, the geological structure of hydrology around the No.1 drainage well has three confined groundwater strata with a three-stratum (upper, middle and lower) underground structure. It is proved that the confined groundwater in the upper and middle stratum is fluid mutually, but exclusively in the lower stratum. Furthermore, the dropping velocity of pore water pressure in the lower stratum is slow than other strata, and the pore water pressure fluctuation in the upper and middle stratum responds to rainfall.2) To compare with the upper stratum, the dropping volume of pore water pressure in the middle stratum (strong weathering rock stratum) at No.1 drainage well is larger, but the drainage volume is smaller. It is also found out that the increase of drainage volume and pore water pressure fluctuation due to rainfall are difference from weathering degree of rock stratum and depth of the observation borehole.3) The No.1 drainage well is a three-stage structure design. Each stage drains groundwater away from horizontal boreholes. The distribution of drainage volume has a tendency to concentrate groundwater at the lower stage. The drainage effects of No.2 drainage well is partial at some horizontal boreholes of lower stage, and the drainage volume of each horizontal borehole is clearly different.