Ikuo Towhata

Shaking table tests on subgrade reaction of pipe embedded in sandy liquefied subsoil

An interest in the behavior of liquefied sand during seismic flow failure led the authors to conduct shaking table tests in which an embedded pipe was pulled laterally and the required drag force was monitored. Test results showed that the amplitude of shaking acceleration affected the behavior of sand in both dry and water-saturated conditions. In dry sand, the induced inertia force decreased the shear strength and consequently the magnitude of the drag force. When the sand was saturated, a special consideration was made of the similitude of dilatancy between 1-G model tests and the in-situ situation. This goal was attained by employing very loose sand in model tests. The rate-dependency in which the drag force increased with the rate of pipe movement was focused on, leading to an apparently viscous behavior of sand. This is consistent with what several former studies reported.

Shaking Table Model Tests on Pile Groups behind Quay Walls Subjected to Lateral Spreading

This paper presents experimental results of 1-g shaking table model tests on a 3×3 pile group behind a sheet-pile quay wall. The main purpose was to understand the mechanisms of liquefaction-induced large ground deformation and the behavior of the pile group subjected to the lateral soil displacement. The sheet-pile quay wall was employed to trigger the liquefaction-induced large deformation in the backfill, and a study was made of the effect of several parameters such as soil density, amplitude and frequency of input motion, pile head fixity, and superstructure on the magnitude of soil lateral displacement and the maximum lateral force of liquefied soil. Furthermore, distribution of the maximum lateral force within the group pile was thoroughly studied. It was found that the force varies depending on the position of individual piles in the group. To evaluate the contribution of each pile in the total lateral force, a new two-dimensional parameter that is called contribution index was introduced and recom...

Experimental Study of Dry Granular Flow and Impact Behavior Against a Rigid Retaining Wall

Shallow slope failure in mountainous regions is a common and emergent hazard in terms of its damage to important traffic routes and local communities. The impact of dry granular flows consisting of rock fragments and other particles resulting from shallow slope failures on retaining structures has yet to be systematically researched and is not covered by current design codes. As a preliminary study of the impact caused by dry granular flows, a series of dry granular impact experiments were carried out for one model of a retaining wall. It was indirectly verified that the total normal force exerted on a retaining wall consists of a drag force (Fd), a gravitational and frictional force (Fgf), and a passive earth force (Fp), and that the calculation of Fd can be based on the empirical formula defined in NF EN Eurocode 1990 (Eurocode structuraux. Base de calcul des structures, AFNOR La plaine Saint Denis, 2003). It was also indirectly verified that, for flow with Froude number from 6 to 11, the drag coefficient (Cd) can be estimated using the previously proposed empirical parameters.

Numerical simulation of ground flow caused by seismic liquefaction

Abstract Flow failure of sandy subsoil induced by seismic liquefaction is known to cause significant damage to structures. It is induced not only by the dynamic forces exerted by seismic acceleration but also by the static gravity force in consequence of the topography of the ground. The ground flow may sometimes continue after the end of the seismic loading and finally the ground is significantly deformed to cause a failure. This paper numerically predicts the magnitude of flow that could occur when soil liquefaction continues for a sufficiently long period. It is considered that liquefied soil behaves like a viscous liquid, and hence, ground flow is governed by the principle of minimum potential energy. In the calculation, liquefied sand is assumed to be a viscous liquid that deforms in undrained conditions with its volume remaining constant. To consider the non-linearity due to large displacement, the updated Lagrangian method is used to solve the equation of motion. The Newmark β method is employed to calculate the time history of the ground motion. Finally, a simulation using this calculation method shows that the proposed method gives reasonable results for the conditions indicated.

Dynamic Properties of Municipal Solid Waste

Assistant Professor, Dept. of Civil and Environmental Engineering, University of Michigan, USA; Associate, Geosyntec Consultants, Huntington Beach, California, USA; Assistant Professor, Faculty of Engineering, Cairo University, Cairo, Egypt; Professor, Dept. of Civil Engineering, University of Tokyo, 7-3-1, Bunkyo, Tokyo 113-8656, Japan; Professor, Department of Civil and Environmental Engineering, University of Catania, Italy

Geotechnical aspects of the 2004 Niigata Ken Chuetsu, Japan, earthquake

The Niigata Ken Chuetsu earthquake induced significant geotechnical and geologic failures throughout the affected region. The most prevalent geotechnical observations from this earthquake are related to ground failure, including landslides in natural ground, failures of highway embankments and residential earth fills, and limited liquefaction in alluvial deposits. The absence of considerable levee deformations and surface faulting was noted. This paper documents the geotechnical aspects of the Niigata Ken Chuetsu earthquake as related to earth structures, liquefaction, and surface faulting; landslides are discussed in an accompanying paper.

Seismic behavior of a quay wall without and with a damage mitigation measure

A series of 1-g shaking table and 50-g centrifuge model tests have been planned to investigate the effectiveness of damage mitigation techniques for existing gravity type caisson quay walls. In this paper, results from 1-g shaking table model tests QW-01D and QW-06 are reported. Model QW-01D is a benchmark model without any mitigation measure and model QW-06 is to simulate a sheet piling in seaward side of quay wall as a mitigation measure. Within the limit ation of 1/50 scaled down model, QW-01D has replicated the field behavior. Reported mitigation measure has been found to reduce the seaward movement and vertical settlement of wall and more importantly, prevented seaward rotation at 10 Hz shaking whereas to limit the wall rotation is the main criterion for quick restoration works. Yet, some frequency dependency of caisson displacements has been noticed.

Assessment of Liquefaction Potential

Resistance of sand against liquefaction is determined by running undrained cyclic triaxial tests on undisturbed soil specimens (Sect. 18.15). Cyclic stress with a constant amplitude is loaded repeatedly (similar to torsion shear tests in Sect. 18.8), and the number of cycles are counted until 1) excess pore water pressure equal to the initial effective stress (σ′c) and (2) peak-to-peak (i.e., double amplitude) axial strain equal to 2.5%, 5%, 10%, etc. The 100% development of excess pore water pressure is called “initial liquefaction” and the strain upon initial liquefaction may not yet be as large as those mentioned above. Thus, liquefaction in laboratory tests are defined in different ways; by pore pressure rise or development of strain. When sand is loose, the number of cycles needed for pore pressure rise and large strain amplitude are not much different.

Dynamic Response Analysis

Clay has a creep behavior; its deformation develops slowly with time under constant magnitude of stress. Therefore, the stress-strain curve of clay varies with the rate of loading. When loaded faster, clay reveals greater modulus and higher strength. These observation suggests that clay is a viscous material.

Developments of Soil Improvement Technologies for Mitigation of Liquefaction Risk

Studies on liquefaction have a history of more than 40 years since Niigata and Anchorage were attacked by disastrous earthquakes. The topics of interest have changed with time and recent interest is focused on advanced mitigation which can achieve cost-effectiveness and/or is useful for existing structures. The present text describes recent experimental studies in both laboratory and field. The concerned mitigation technologies consist of blasting for low cost, grouting of colloidal silica for existing and sensitive structures, and drain pipes which are feasible in a small space under an existing structure. The conducted tests showed how they have advantages over other methods if they are used in appropriate conditions.