Akihiko Wakai

Earthquake-Induced Landslides

Investigation of recent and historical earthquake-triggered landslides and their impacts.- Characteristics, processes and mechanisms of earthquake-triggered landslides.- Physical and numerical modeling of earthquake-triggered landslides.-Instrumentation and monitoring technologies for earthquake-related landslides.- Risk assessment and management of earthquake-related landslides.- Stabilization and disaster mitigation of earthquake-related landslides.- Earthquake-related landslide dams and their risk assessment and management.-Monitoring, prediction and early warning systems for post-earthquake landslides and debris flows.- Other relevant topics concerning earthquake-related landslides.

Analysis of the Factors Affecting Slope Failures Distribution Within Northern Kanto Area due to The East Japan Great Earthquake

In this study, the factors affecting various types of slope failures in the Northern Kanto area, i.e., the input motion, the geological and geomorphological conditions, are analyzed by the observed distribution of the slope failures due to the East Japan Great Earthquake. The data for the location of each slope failure have been collected from the prefectural government offices as well as our site investigation for the study area. The observed data for the strong ground motion were referred to the K-net website managed by The National Research Institute for Earth Science and Disaster Prevention, Japan. The distribution of the acceleration response spectrum in the range of longer periods calculated from the above data was found to have a correlation to the deep-seated landslides distributions. It was also found that, the thicker the loam depth in the ground, the higher the landslide risks.

Numerical Simulation for an Earthquake-Induced Catastrophic Landslide Considering Strain-Softening Characteristics of Sensitive Clays

The dynamic elasto-plastic finite element method is used to simulate an earthquake-induced catastrophic landslide that occurred during the 2011 Great Tohoku and Kanto Earthquake in Japan. In the analysis, a strain-softening constitutive model for sensitive clays originating from weathered pumice is applied, where the material parameters of the strain-softening materials have been determined based on the results of a series of laboratory tests involving undisturbed samples. The observed phenomena such as a long-distance traveling failure triggered by degradation of the apparent shear strength of sensitive materials in the slope are appropriately simulated by the analysis.

The Recent Activities in the Earthquake-Induced Landslides Research Project Carried Out by the Japan Landslide Society: The Interim Report

The Japan Landslide Society has established a task force to carry out the special research project entitled “Development of a methodology for risk assessment of the earthquake-induced landslides”. Altogether 8 Working Groups, which are carrying out concrete investigations and analyses on individual items, were organized to cover all the objectives of the task force. In this report, a few of the essential research items treated in them are described in detail.

Session Introduction Earthquake-Induced Landslide

This session is devoted to the description of events, processes and mechanisms associated to the occurrence of landslides triggered by earthquakes. Fourteen manuscripts have been submitted to this session.

Finite Element Simulation for Seismic Ground Response in Mountainous Areas in Nepal

In this study, dynamic elasto-plastic finite element method is applied to simulate seismic ground response in mountainous districts in Nepal, which aims to reevaluate the earthquake-induced landslides occurred at the time of The 2015 Nepal Gorkha Earthquake. In the analysis, nonlinear material properties of the ground as well as 3D topography, geological conditions and input motion are taken into account appropriately. Those factors strongly influence the dynamic amplification effects relevant to slope failures. Throughout the comparisons of the results between the calculated one and observed facts in local areas, it can be concluded that the proposed numerical method has a sufficient ability to predict the phenomena and can be possibly utilized for predicting overall distribution of earthquake-induced landslide which would be helpful for developing landslide susceptibility maps in mountainous areas in Nepal.

Numerical Simulation for the Earthquake-Induced Deformation of a Residential Land Slope Reinforced with Landslide Prevention Piles

In this study, a numerical simulation for the earthquake-induced deformation of a residential land slope reinforced with landslide prevention piles is presented. The objective slope is a real residential land slope in Sendai city, damaged at the time of the 2011 off the Pacific coast of Tohoku earthquake. The focus of the study is placed on the effectiveness of the landslide prevention piles in the slope which have been constructed before the earthquake. Reproduction analyses for the slope which has been constructed these piles are performed with 2D dynamic elasto-plastic finite element analysis considering strain-softening characteristics of the soils. In addition, analyses which assumed the slope without these piles are performed for a comparison, and the effectiveness of these piles is considered.

Initial Trigger for Slope Failures in Volcanic Ash Layer of Hillside Surface in Izu-Oshima Island in Japan Due to a Typhoon Rainfall in 2013

In the year 2013, the typhoon Wipha passed through the neighbourhood of the Izu-Oshima Island from 15 to 16 October 2013, and on the early morning of 16 October, a large number of slope failures occurred at various sites in the island. The largest one occurred at a caldera external slope located in the western part of the island near Motomachi Town. In this study, the initial trigger for such huge failures in the upstream slope, as the beginning of this serious mud flood disaster, has been discussed in mechanical point of view. According to the results obtained from the experimental and analytical examinations including the centrifuge model tests, it can be concluded that the increase of the pore water pressure in the thin ash-fall deposits layer with high permeability has promoted the decrease of the effective confining pressure in the layer, and it caused the surface failures in the hillside slopes.

Finite Element Simulation for an Earthquake-Induced Catastrophic Landslide Considering Strain-Softening Characteristics of Sensitive Clays

In the present study, a strain-softening constitutive model for sensitive clays originating from weathered pumice is applied to simulate an earthquake-induced catastrophic landslide that occurred during the 2011 Great Tohoku and Kanto Earthquake in Japan. The upper part of the slope sliding about 200 m and 13 people were killed. The failure could be related to shear strength degradation of the sensitive clay layers in the slope during the earthquake.

Mechanism of Landslide Composed of Strongly Weathered Mudstone Induced by the 2004 Mid-Niigata Earthquake

This paper reports the mechanism of a landslide composed of strongly weathered mudstone induced by the 2004 Mid-Niigata Earthquake. The size of the landslide is 250 m long, 150 m wide and 15 ~ 20 m deep. Undisturbed soil samples of strongly weathered mudstone were obtained from the soil layer near the assumed sliding surface by boring. Cyclic undrained triaxial tests performed on the undisturbed soil specimens revealed the possibility of the landslide mechanism that the excess pore water pressure was induced in the soils near the sliding surface by the cyclic earthquake shaking and the safety factor of the slope became fairly less than 1.0, which led to the long-distance sliding of the landslide.