Keizo Ugai

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.

Two-dimensional base stability of excavations in soft soils using FEM

Abstract The 2D base stability of excavations is evaluated using FEM with reduced shear strength. The numerical results indicate that the base stability of excavations is significantly influenced by the ratio of the depth to the width of the excavations, the thickness of the soft soil layer between the excavation base and hard stratum, the depth of the walls inserted below the excavation base, and the stiffness of the walls around the excavation. Closed form equations considering embedment depth under the base of braced excavation has been proposed based on the upper bound analysis and compared with the FE results.

Assessment of 3D Slope Stability Analysis Methods Based on 3D Simplified Janbu and Hovland Methods

AbstractAn assessment of three-dimensional slope stability analysis methods in terms of safety factors using several idealized sliding masses composed of plane sliding surfaces was made. Three-dimensional safety factors were calculated and compared for different study cases considered in this study on the basis of the exact solution methods, the Hovland method, and the 3D simplified Janbu method. Parameters investigated in this study included the effect of water pressure, horizontal seismic force, the changing gradient of a sliding surface, the changing lateral gradient of a sliding surface, and anchor force. Results showed that the Hovland method gives smaller safety factor values compared to the exact solutions, especially in cases of narrow failure width and high water pressure along sliding surfaces whereas the 3D simplified Janbu method gives the same safety factor values as the exact solutions

A DESIGN METHOD FOR SLURRY TRENCH WALL STABILITY IN SANDY GROUND BASED ON THE ELASTO-PLASTIC FEM

Abstract Deep-trench excavation supported by slurry has become a widely used technique for the construction of diaphragm walls. It has been proved by numbers of field tests, scale model tests and numerical analyses that the supporting effect of the slurry in the trench will provide sufficient stability for the deep-trench excavations. However, there is still neither a generally accepted theory of the failure mechanisms of the slurry trench, nor a widely used design method. This paper gives suggestions for the practical design on the stability of the slurry trench based on the elasto-plastic finite-element method (FEM) analysis, which consists of two parts. The first part is concerned with the evaluation of slurry trench stability in terms of safety factor. The safety factor was calculated using the shear strength reduction technique in the FEM and verified by comparison with centrifuge tests. The second part suggests a method for choosing the preliminary slurry parameters, i.e. the height of slurry filling and slurry density, based on the active earth pressure FEM.

A simple lumped mass model to describe velocity of granular flows in a large flume

This paper presents a lumped mass model to describe the run-out and velocity of a series of large flume tests, which was carried out to investigate some propagation mechanisms involved in rapid, dry, dense granular flows and energy transformation when the flows encountered obstacles and reoriented their movement directions. Comparisons between predicted and measured results show that the trend of predicted velocities was basically matched with that of measured ones. Careful scrutiny of test videos reveals that subsequent particles with a higher velocity collided with slowed fronts to make them accelerate. However, this simple model cannot reflect collisions between particles because it treated released materials as a rigid block. Thus, the predicted velocity was somewhat lower than the measured velocity in most cases. When the flow changed its direction due to the variation in slope inclination, the model predicted a decrease in velocity. The predicted decrease in velocity was less than the measured one within a reasonable range of 10% or less. For some cases in which a convexity was introduced, the model also predicted the same trend of velocities as measured in the tests. The velocity increased greatly after the materials took a ballistic trajectory from the vertex of the convexity, and reduced dramatically when they finally made contact with the base of the lower slope. The difference between prediced and measured decrease in velocity was estimated to be about 5% due to the landing. Therefore, the simple lumped mass model based on the energy approach could roughly predict the run-out and velocity of granular flows, although it neglected internal deformation, intergranular collision and friction.

Enhanced mobility of polydisperse granular flows in a small flume

BackgroundA series of laboratory tests was conducted to investigate the influence of the interaction between coarse and fine particles on the mobility of granular flows in a small flume.MethodsThe angle of the upper slope was fixed at 45°, and the lower slope was inclined at angles of 0°, 5°, 10°, and 15° in different cases. Three monodisperse materials (gravel, coarse sand, and fine sand) were mixed, and the proportion of each material in each test was varied but maintained the same total mass of 3.0 kg.ResultsTest results show that the proportion of fine sand strongly influenced the run-out of polydisperse materials. With increasing proportion of fine sand, the run-outs of granular flows increased until its peak. However, the run-outs decreased with further more fine sand.DiscussionThe reason might be that a thin layer of fine sand acted as rollers for the rolling of the gravel, leading to the reduction of effective friction resistance during the movement; when excessive amount of fine sand was involved, these rollers were thrown into disarray so that the particles were either blocked or forced into sliding.ConclusionThis implies that an appropriate proportion of fine particles were partly responsible for the long run-out of rock avalanches.

Improvement Properties of Cohesion-Less Soil Using Recycled Bassanite

Solid waste management is a serious problem over the world. Therefore, reduction, re-use and recycling of waste have become major issues in recent days. Gypsum waste plasterboard is considered one example of these waste materials. This study evaluates the use of recycled bassanite, which is derived from gypsum waste plasterboard, to enhance the performance of two types of cohesion-less soil. Recycled bassanite was utilized as a stabilizing agent to improve both compressive and splitting strengths of the tested soil. The effect of bassanite content, soil type, water content and curing time were investigated to explore the behavior of treated soil with recycled bassanite. Test results showed that increase of bassanite content is associated with increase in optimal moisture content, while no significant increase in the dry unit weight was observed. Both compressive and splitting tensile strengths enhanced with the additives of recycled bassanite. The increase of bassanite content had a more significant effect on the compressive strength compared with the effect on tensile strength. The use of recycled bassanite to enhance the strength of sandy soil had a more significant effect compared with silty soil. The effect of curing time on the strength of treated samples was more significant in early curing ages compared with late curing ages. The strength decreased significantly in case of stabilized samples prepared with water content at the wet-side of the compaction curve. However, insignificant decrease in the strength of the stabilized sample was detected with moisture content at the dry-side of compaction curve. This research meets the challenges of our society to reduce the quantities of gypsum wastes, producing useful material from waste materials that will help to a sustainable society.

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.

Numerical Simulation of Granular Flows in a Large Flume Using Discontinuous Deformation Analysis

A long run-out rock avalanche is defined as an extremely rapid, massive, flow-like motion of fragmented rock derived from a bed-rock failure (Hungr et al. 2001). Due to their extraordinary mobility, rock avalanches have been extremely costly in terms of human lives and of engineering developments, and have interested many researchers. Numerical method appears a promising approach for analyzing most of the physical aspects of dry rock avalanches and for predicting the consequence of real events as well (Cundall and Strack 1979; Gonzales et al. 2002; Tommasi et al. 2008). The distinct element method (DEM) and discontinuous deformation analysis (DDA) are the two most popular discrete element methods used to tackle problems dealing with discontinuous rock mass. Limitations of the explicit DEM include its very small step for numerical stability and the use of artificial damping to absorb the energy generated from the relaxation analysis to maintain equilibrium, although some researchers (Calvetti et al. 2000; Valentino et al. 2008) have devoted to appropriately calibrate numerical damping in simulating run-out of granular flows. Shi (1988) claimed that the energy-based DDA can overcome the two limitations of the force-based DEM. Based on its advantages, this paper selected DDA as the analysis method to investigate the behavior of granular flows. In order to acquire confidence in the numerical simulations, it is vital to corroborate them by direct observation and to prove their efficiency. Due to the difficulty of reproducing the complexity of a rock avalanche on a reduced scale, few researchers tackled the problem through an accurate experimental work on physical smallscale models (Valentino et al. 2008). In this spirit, we performed a series of tests to investigate the effects of some factors influencing mass-front velocity and deposit characteristics of granular flows in a large flume. This paper compares DDA simulations and the experimental results in order to validate the efficiency of this numerical method. Q. Yang (&) Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 610031, China e-mail: yangqq_71@126.com

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.