Osamu Nagai

Plenary: Progress in Landslide Dynamics

Landslide Dynamics is a relatively new field in Landslide Science. Reliable scientific knowledge to assess the motion of landslides including hazard area, speed and depth is needed to reduce human loss from landslides. However, the initiation and motion of landslides is not easy to explain quantitatively because of pore-pressure generation during initiation and motion, and continuing changes in grain size, grain shape and water content of the involved soils in the shear zone. An apparatus has been developed to physically simulate the formation of sliding surfaces and the post-failure motion of the involved soils under realistic stresses. It can simulate pore-pressure increase due to rain water infiltration and dynamic loading due to earthquakes in the field, and can monitor pore-pressure generation, and mobilized shear resistance together with shear displacement. The apparatus has evolved from the model DPRI-1 in 1984 through DPRI-2, 3, 4, 5, 6, to the model ICL-1 in 2011 and ICL-2 in 2013. This apparatus which is called the landslide ring-shear simulator is now in use in foreign countries. This paper presents the progress of the landslide ring-shear simulator and its application to earthquake-induced landslides, the 2006 Leyte landslide killing over 1,000 people, the 1792 Unzen Mayuyama landslide killing 15,000 people, and a hypothetical Senoumi (Stone flower sea) submarine megaslide using a cored sample from 190 m below the sea floor. A new integrated computer model (LS-RAPID) simulating the initiation and motion using soil parameters obtained from the landslide ring-shear simulator has been developed in parallel with the development of landslide ring-shear simulator. LS-RAPID was applied to the three earthquake-induced landslide cases mentioned above. The simulations included two triggering factors: pore-water pressure and three-component seismic waves. The combination of landslide ring-shear simulator and integrated landslide simulation model provides a new tool for landslide hazard assessment.

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.

A Landslide Monitoring and Early Warning System Using Integration of GPS, TPS and Conventional Geotechnical Monitoring Methods

An advanced comprehensive monitoring system was designed and used on the Grohovo Landslide in Croatia. Equipment selection was based on scientific requirements and consideration of possible ranges of monitored values and sensors precision. Establishment of an early warning system and defining of alarm thresholds is based on existing knowledge of the landslide behavior as well as collected comprehensive monitoring data. The focus of the early warning system establishment at Grohovo Landslide was on an effective combination of sensors (equipment fusion) with respect to detecting device malfunctions and reducing false alarms in the future. The weakest component in the Grohovo Landslide monitoring system is power supply based on solar devices, field data collecting and the data transmitting from the field PC to the control room at the University of Rijeka. This paper presents the main ideas and advances of the monitoring equipment fusion as well as weaknesses of the applied monitoring system at the Grohovo Landslide.

Landslide Inventory in the Area of Zagreb City: Effectiveness of Using LiDAR DEM

Preliminary results of landslide mapping in the City of Zagreb (Croatia), obtained in the frame of the Japanese-Croatian scientific project, are presented in this paper. The aim of this research is to develop a method for landslide delineation in order to enable land use officials to implement this data to create more useful measures for landslide risk management. Selected landslides in the hilly zone of Mt. Medvednica were identified visually using LiDAR bare-earth DEMs. The results of data analysis will be implemented to perform a more comprehensive study of landslides in the entire pilot area (total area is 180km2).

Simulation of a Rapid and Long-Travelling Landslide Using 2D-RAPID and LS-RAPID 3D Models

In this study, two process-based computer numerical models for simulating the generation and propagation of landslide are developed by integrating the initiation process triggered by rainfalls and/or earthquakes and the development process to a rapid motion due to strength reduction and the entrainment of deposits in the runout path. Among them, the 2D-RAPID model is a two dimensional model and LS-RAPID 3D Model is a three dimensional model. Both of them were developed from the geotechnical model for the motion of landslides and its improved simulation model and new knowledge obtained from a new dynamic loading ring shear apparatus. The aim of this study is to validate and compare these two models. For this purpose, the two models were applied in a rapid and long-traveling landslide, which occurred on 17 February 2006 in the southern part of Leyte Island, Philippines and caused 154 confirmed fatalities, and with an additional 990 people missing in the debris. For comparison, all the parameters used in the 2D landslide model are using the same values used in the 3D landslide model. As simulation results, the application of these two simulation models could reproduce well the initiation and the rapid long runout motion of the Leyte landslide. However, for the deposition area, the 2D landslide model resulted in a higher and narrower mass volume than the 3D landslide model. Moreover, the 2D-RAPID shows a simple process to handle the input and output database, which is easily understood and can be used in engineering application. In addition, the LS-RAPID 3D Model shows an excellent interface for rainfall or/and earthquake induced landslide with spatially distributed complex topographic data. The distributional information of soil parameter can be set and the 3D view of the calculated landslide initiation and runout can be successfully achieved in LS-RAPID 3D Model. Thus, each of these different dimensional landslide models has its respective advantages and disadvantages depending on the target study area and the type of the area.

TXT-tool 3.385-1.2: Deterministic Landslide Susceptibility Analyses Using LS-Rapid Software

This paper presents the landslide susceptibility analyses on flysch slopes in Istra, Croatia, performed using deterministic three dimensional analyses in LS-Rapid software. The area of investigation is in the Pazin Paleogene Flysch Basin in the northeastern part of the Istrian Peninsula. Using deterministic approach in landslide hazard and risk analysis includes gathering of fundamental data about geometry, soil strength parameters, cover thickness and groundwater level, as well as the application of numerical models in safety factor calculation. LS-Rapid uses 3D models for simulation of progressive failure phenomena, developed to assess the sliding initiation and activation of landslides triggered by earthquake, rainfall or their combination. Detail distribution of pore pressures or the groundwater level inside the slope is taken into account trough the pore pressure ratio ru, which gradually increases until the failure appears in a certain part of the slope. If this approach is applied on the wider area, in which it is possible to define the relative position of the sliding surface, it is possible to obtain the values of the critical pore pressure ratio that causes conditions in which failures occur in specific parts of the investigation area. Connecting the critical pore pressure ratio with distribution of rainfall it is possible to obtain the landslide susceptibility and landslide hazard. The model was validated through the interpretation of stereopairs and engineering geological mapping, and the results have shown that landslides inside the zones in the model that were characterized as highly susceptible, occurred in the nearest of farthest past.

TXT-tool 3.081-1.8: A New High-Stress Undrained Ring-Shear Apparatus and Its Application to the 1792 Unzen–Mayuyama Megaslide in Japan

Sassa and others in the International Consortium on Landslides (ICL) have developed a new series of undrained ring-shear apparatus (ICL-1 and ICL-2) for two projects of the International Programme on Landslides (IPL-161 and IPL-175). Both projects are supported by the Science and Technology Research Partnership for Sustainable Development Program (SATREPS) of Japan. ICL-1 was developed to create a compact and transportable apparatus for practical use in Croatia; one set was donated to Croatia in 2012. ICL-2 was developed in 2012–2013 to simulate the initiation and motion of megaslides of more than 100 m in thickness. The successful undrained capacity of ICL-2 is 3 MPa. This apparatus was used to simulate possible conditions for the initiation and motion of the 1792 Unzen–Mayuyama megaslide (volume, 3.4 × 108 m3; maximum depth, 400 m) triggered by an earthquake. The megaslide and resulting tsunami killed about 15,000 people. Samples were taken from the source area (for initiation) and the moving area (for motion). The hazard area was estimated by the integrated landslide simulation model LS-RAPID, using parameters obtained with the ICL-2 undrained ring-shear apparatus.