Gen Furuya

Mechanism of creep movement caused by landslide activity and underground erosion in crystalline schist, Shikoku Island, southwestern Japan

Abstract The mechanism of creep movement of the Zentoku landslide in crystalline schist has not been studied in detail because of the steepness of the slope, very slow movement, low population density and complex topographic and geologic characteristics. Sassa et al. (1980: Proc. INTERPRAEVENT 1, 85–106) and Sassa (1984: Proc. 4th International Symp. on Landslides, Toronto, vol. 2, pp. 179–184; 1985. Geotechnical classification of landslides, Proc. 4th International Conference and Field Workshop on Landslides, Tokyo, pp. 31–40; 1989: Landslide News, Japan Landslide Society, No. 3, pp. 21–24) monitored landslide movement and groundwater level at the Zentoku landslide on Shikoku Island, southwestern Japan, and suggested that the mechanism may be caused by underground erosion. To study the influence of underground erosion at this site, continual monitoring of suspended sediment and water discharge from a groundwater outlet (i.e. a spring) was implemented. The locations of groundwater flow paths were determined, as were the amounts of discharged sediment. Slope deformation was monitored by means of a borehole inclinometer. The conclusions were as follows: (1) the flow paths were found to be on or above the shear zones in which underground erosion has occurred; (2) in addition to being a result of precipitation and groundwater discharge, sediment discharge is affected by landslide activity; and (3) the mechanism of creep movement is an interrelated chain process that combines underground erosion caused by landslide activity with landslide activity caused by underground erosion. Thus, landslide activity increases erosion susceptibility and transportation of soils within the mass, and underground erosion causes instability of the landslide mass, in turn. This mechanism can explain the observed phenomenon that the Zentoku landslide not only moves actively during heavy rain, but also continues to creep throughout the year.

Review of Monitoring Parameters of the Kostanjek Landslide (Zagreb, Croatia)

Since 2011, in the framework of the Croatian-Japanese SATREPS FY2008 Project, scientists have been working on the establishment of the Kostanjek landslide monitoring system in the City of Zagreb (Croatia). External triggers at Kostanjek landslide are measured with rain gauge and accelerometers. Displacements at the surface are measured by GNSS sensors and extensometers, while subsurface displacement is measured by vertical extensometers and inclinometer. Hydrological measurements consist of groundwater level measurements, discharge measurements, chemical and isotope analysis. Monitoring sensors recorded landslide reactivation due to external triggers in the winter period of 2012/2013. During the period from September 2012 to March 2013 the total cumulative precipitation was 793.7 mm and horizontal displacements were in the range of 9–20 cm. The installed monitoring sensor network proved to provide reliable data for the establishment of relations between landslide causal factors and landslide displacement rates aimed at establishing threshold values for early warning system.

Chemical Weathering and the Occurrence of Large-Scale Landslides in the Hime River Basin, Central Japan

The Hime River Basin is located in the northern part of Central Japan and is known as one of the areas where both erosional potential and sediment yield are extremely high in Japan. Landslides and debris flows triggered mainly by heavy rainfalls have frequently occurred in the basin. We have estimated the chemical weathering rates for nineteen watersheds in the Shirouma-Oike Volcano located in the western part of the basin. These rates have been simply estimated by the mass balance equation between solute fluxes of stream waters from each watershed and solute loss comparing fresh and weathered volcanic rocks and were calculated to be ranging from 0.15 to 3.24 mmyr-1. A watershed showing the highest rate of chemical weathering and solute flux corresponded to the area where the large-scale landslide occurred in 1911 and debris flows and landslides have continually occurred until now. Unstable sediments yielded by chemical weathering are thought to be an important factor of sediment disaster occurrences in the research area. Solute fluxes of each stream could be useful for susceptibility mappings of landslides and debris flows in each watershed.

Open image in new windowSimulation Model to Predict Landslide Speed Using Velocity-Dependent Viscous Damping

We propose a simulation model using viscous damping to predict the moving velocity (v) of a landslide before it reaches a strain limit. We call this model “Lumped mass damper model”, and it is based on a motion equation using a mass system model composed of a damper. The damper introduces a damping force in the opposite direction to the downward force (F) of the landslide, according to the landslide velocity. For slope-stability analysis, a simple model such as the Fellenius method is used. In the analysis, the resistance force (R) and driving force (D) are calculated in each individual slice and summed up for all slices. the safety factor (Fs) is indicated as Fs = R/D. Then, the equation of motion is shown as \( {\text{m}}\upalpha = {\text{F}} - {\text{A}} \cdot {\text{Cd}} \cdot {\text{v}} \). In the equation m is mass and α is acceleration of the landslide. After solving this motion equation, the formula ‘\( {\text{v}}{ \approx }{\text{F}}/{\text{A}} \cdot {\text{Cd}} \)’ is obtained. ‘A’ is the area of the slip plane of the landslide. ‘Cd’ is the viscosity coefficient of the damper, which exerts its effect on the slip surface of the landslide. Analytical results using this technique on the Kostanjek landslide in Croatia show that this Lumped mass damper model was able to reproduce the variations in landslide motion in response to the variation of groundwater level. Also, the Takino landslide of 4th August 1986 which was induced by embankment construction for a Japanese highway was successfully analyzed using this technique.

Application of the Newly Frequency Domain Electromagnetic Method Survey in a Landslide Area

Geophysical survey in a Landslide area is mainly perfomred by measuring the elastic wave velocity or electric resistivity. The electrical resistivity is well known to reflect the information in a slope, which can be obtained by the seismic prospecting or electrical resistivity survey. The commonly used electrical resistivity survey has disadvantage sometimes due to local ground conditions during the installation of electrodes and layout of electric cables on the hard material. On the other hand, the frequency domain electromagnetic method (FDEM), one of the electromagnetic methods, has advantages due to its nondestructive measurement nature and also its simple operation. In this study, we used a newly developed FDEM survey apparatus to examine its applicability in the exploration of landslide. The results showed good consistency with the structure of the landslide mass, which had been indentified by many other apporaches during the passing decades. Therefore, we concluded that FDEM is an effective method in exploring the area and also the inner structure of landsliding mass.

Groundwater Flow Behavior at Landslide Area in Crystalline Schist Mountains

In this study, we performed the 1-m-depth ground temperature survey and continuous ground temperature monitoring at the Zentoku landslide area and the Nishi-ikawa landslide in the crystalline schist mountains to clarify the actual groundwater flow in landslide slopes. Our results showed that the ground temperatures at differing locations of the slope were different, probably due to the influence of veins of groundwater flow. We also found that the ranges of these differences in ground temperature were different for different landslides in the same mountain or different mountains. To clarify the reasons for these differences, we compared the data obtained from these two different landslide areas and also those data obtained from other case studies with groundwater dating. We inferred that during a rainfall, the groundwater from deep layer, which is relatively cold in summer and warm in winter than shallow groundwater, and wasn’t formed just after heavy rain, but infiltrated previously (approximately two to nine years ago), might have also been expelled out, resulting the greater difference in ground temperature. Namely, the groundwater flowing in the crystalline schist slope involves not only the newly infiltrated rainwater, but also those water concentrated flow from deep layer with some residence time. Therefore, it is suggested that this kind of groundwater flow should be considered in the instability analysis of landslides on the crystalline schist slopes.

Vein of Groundwater Flow Behavior in a Land-Slide by Continuous Monitoring of Ground Temperature

To examine the dynamics of groundwater flow within landslide mass, we installed 29 thermocouples at the depth of one meter along three survey lines crossing the main body of Nishi-ikawa landslide in Tokushima, Japan, and continuously monitored the ground temperature since 30 April 2010. Through comparing the variation of these 1-m deep ground temperatures, we estimated the location of groundwater flow vein and found that: (1) characteristic of annual change in the 1-m deep ground temperature on Nishi-ikawa landslide was lower than the value estimated by Takeuchi’s equation, (2) some measurement points on Nishi-ikawa landslide read decreasing values, showing approximately the same tendency as the constant-temperature zone during heavy rain in summer season. We than inferred that these decrease in 1-m deep ground temperature resulted from the flow of groundwater from deeper layer such as the constant-temperature zone to the shallow layer through vein-like passes in the landslide mass during infiltration of heavy rain.

Numerical Simulations for the Possible Future Geohazard Events in the Rječina Valley, Croatia

Landslide, debris flow and rockfall are the main geohazards in the Rjecina River catchment. The Grohovo landslide is located on the northeastern slope in the central part of the Rjecina river valley just downstream of the Valici dam. As the effects of intense rainfall or earthquake, two potential geohazard events could occur. One scenario is that the slope deposits of Grohovo landslide moving towards the channel of the Rjecina River will form a landslide-dam. Another scenario is that the slope deposits mixing with water will form a debris flow arriving at the City of Rijeka. In this paper, the formation of landslide-dam and the debris-flow are simulated by two integrated models using GIS to represent the dynamic process across 3D terrains. GIS-based revised Hovland’s 3D limit equilibrium model is used to simulate the movement and stoppage of the slope deposits to form landslide-dam. The simulation result shows about 9 m landslide-dam formed. GIS-based depth-averaged 2D numerical model is used to predict the runout distance and inundated area of a debris flow. As a result, the simulated debris-flow takes about 16 min to travel about 6 km along the Rjecina River valley arriving at the City of Rijeka.

Shallow Landslide Susceptibility Mapping for Zagreb Hilly Area, Croatia

The aim of this study is to map the landslide susceptibility of the hilly area of Mt. Medvednica located in the northwestern part of the City of Zagreb, Croatia. Landslides in this region are mostly shallow movements of superficial deposits along contacts with fresh deposits of soil and cause significant economic losses by damaging houses and the urban infrastructure. The method used here is the deterministic slope stability analysis model SINMAP which is developed by Pack et al. (A stability index approach to terrain stability hazard mapping. SINMAP user’s manual, 1998; A stability index approach to terrain stability hazard mapping. SINMAP user’s manual, 2005). SINMAP is a raster based slope stability predictive tool based on coupled hydrological-infinite slope stability model. This approach applies to shallow translational landsliding phenomena controlled by shallow ground water convergence. The input data required for this model are (1) inventory of past landslides in a point vector format, (2) Digital Elevation Model (DEM) of the study area, (3) geotechnical data such as soils strength properties, thickness of soil above the failure plane, and (4) hydrological data such as soil hydraulic conductivity and the rainfall. Because the geotechnical data and hydrological data are highly variable in both space and time, the method does not require numerically precise input and accepts ranges of values that represent this uncertainly. The major output of this model is the stability index grid theme, which can be used as a landslide susceptibility map. The results also provided slope area plots and statistical summary for each calibration region in the study area facilitating the data interpretation. The landslide susceptibility map which is developed in this study is also compared with the results from the Analytic Hierarchy Process (AHP) method and aerial photo interpretation.