Hiroyuki Kyokawa

Simple modeling of stress-strain relation for unsaturated soil.

Though stress-strain characteristics of unsaturated soils are complicated and rather different from those of saturated soils, it should be described properly by a constitutive model for soils because soil usually stays under an unsaturated condition in actual field. In the current study, a simple elastoplastic model for saturated soil is extended to one applicable to unsaturated soils. The proposed model is formulated using the Bishop’s effective stress and the residual strength is, therefore, assumed to be constant. In the proposed model, the decrease (or increase) in the degree of saturation is linked with upward (or downward) movement of normally consolidated line in the compression plane of mean effective stress and void ratio, by which the typical volumetric and distortional behaviors of unsaturated soils are properly described. In addition, a simple method to extend classical water retention curves such as van Genuchten’s equation to be able to incorporate the influences of suction histories and density is proposed and applied to the proposed model. In the present paper, the outline of the proposed model is explained and applicability of the model is discussed through typical results of simulations.

Shaking Table Test of Scaled 1/5 EPS Embankment Model

A large earthquake in the Kumamoto district in the south of Japan in April 2016 occurred twice within 28 h with a magnitude class 6.5. Due to this earthquake many houses and roads collapsed. In order to investigate the behavior of EPS embankments when a large-scale earthquake acts consecutively, an EPS embankment model with a scale of 1/5 was built and a shaking table experiment was conducted using the shaking table (3 m × 2 m) at the University of Tokyo. The EPS embankment was found to cause rocking phenomenon due to seismic motion. As a countermeasure, in addition to the effect of an improved Joint Metal Binder (JBM), the effect of applying a larger number of JMBs was also investigated.

Ultimate Lateral Resistance of Piles in Soils Based on Active Pile Length

Simulation of the in situ behavior of pile foundation is necessary in the seismic design and assessment of piles for target structural integrity and performance during earthquakes. Having the mere presence of the soil and the pile in this foundation system, the complex behavior of piles is generally captured by the soil-pile interaction. In this research, a simple parameter called the active pile length, L a, which is reflective of the deformation of pile relative to the stiffness of the soil, is explored to describe the ultimate lateral resistance of the soil. The idea is based upon the deformation of flexible piles commonly used in engineering practice. When piles are induced by a lateral load, the pile deforms significantly in the region near the ground surface and decreases with increasing depth. This region of significant deformation down to the negligible point along the pile depth is defined as the active pile length, L a. During the event of nonlinear excitation, a soil wedge is formed in the passive region along this active pile length. This soil wedge is indicative of the ultimate side soil resistance, and thus can be inferred to be described by L a. To simply investigate, a simple plane strain condition using 2-D finite element method in nonlinear analysis is done to obtain the behavior response of a single pile embedded in a homogeneous soft soil. The elasto-plastic behavior of the soil is modeled using the subloading t ij model and the pile is modeled as a 2-D continuum based beam element. Deformation of the pile and corresponding surrounding lateral soil deformation are analyzed. The potential of this simple concept of active pile length to describe the nonlinear response of piles embedded on soft soils is presented for more practical approach in the seismic design and assessment of piles.

Simple Modeling of Time-Dependent Behavior for Structured Soils

A simple model to describe time-dependent behavior of various soils in 1D stress conditions is presented in this paper. The model is formulated not using the usual viscoplastic theories such as over-stress type and non-stationary flow surface type but utilizing the subloading surface concept by Hashiguchi (1980), and paying attention to the experimental results that the normally consolidation line (NCL) on the e–ln σ plane shift depending on the strain rate. The present model can describe various time-dependent behaviors not only of normally consolidated soil but also of over consolidated and naturally deposited soils in the same manner without violating the objectivities. The 1D model can easily be extended to the 3D using the t ij concept (Nakai and Mihara 1984).

A Simple Method to Consider Density and Bonding Effects in Modeling of Geomaterials

A simple method to describe stress-strain behavior of structured soils under normally and over-consolidated states in one-dimensional stress condition is first presented by introducing a state variable to represent the influence of density. To describe the one-dimensional stress-strain behavior of structured soils, attention is focused on the density and the bonding as the main factors that affect a structured soil, because it can be considered that the soil skeleton structure in a state which is looser than that of a normally consolidated soil is formed by bonding effects. The extension from one-dimensional model to three-dimensional model can be done only by defining the yield function using the invariants of modified stress ‘tij′ instead of one-dimensional stress σ and assuming the flow rule in modified stress space tij.