Yoshio Tobita

Influence of fabric on liquefaction and densification potential of cohesionless sand

Abstract For simple shearing under constant pressure, the effects of fabric on liquefaction and densification potentials of saturated cohensionless granular materials are examined theoretically and experimentally. The fabric is identified with the distribution of the dilatancy angles (the angle between the sliding and the macroscopic shearing directions), and the influence of prestraining on this distribution and hence on the macroscopic sample behavior is studied. It is shown that prestraining with zero residual stress can reduce resistance to liquefaction by one or even two orders of magnitude, although the sample density and other conditions are kept the same. The micromechanical features responsible for this and related behaviors are examined in some detail. Finally, some tentative results on the effect of the inherent anisotropy that is produced during sample preparation are reported, showing that a method which yields samples more resistive in triaxial cyclic tests may provide samples less resistive in cyclic shearing.

Contact Tensor in Constitutive Model for Granular Materials

Summary The roles of the contact tensor in the constitutive model for granular materials are considered. After deriving the micromechanically based contact tensor and the modified stress tensor, we give possible physical meanings of them. The constitutive model with these tensors and their rates can account for more features of deformation of granular materials. The anisotropic hardening feature and the non-coaxiality between the rate of deformation and the stress tensors are given special attention in this paper.

Importance of Incremental Nonlinearity in the Deformation of Granular Materials

The property of incremental nonlinearity indicating that the tangent stiffness/ compliance depends on the direction of strain/ stress rate has received much attention in recent years. Experimental results by the triaxial and hollow cylindrical triaxial tests among others have clearly shown that the incremental nonlinearity is a common feature of deformation of granular materials like sand under complex loading paths including the rotation of principal stress axes. The incremental nonlinearity may be considered as a natural consequence from the view of micromechanisms for inelastic deformation of granular materials. Responding to experimental observations, many types of constitutive equations have already been proposed with different concepts and mathematical structures.

Damages of Hillside Embankments in Sendai City During the 2011 Great East Japan Earthquake

The 2011 Great East Japan Earthquake resulted in severe damages to housing and housing lots. In particular the hillside embankments for residential use surrounding the downtown of Sendai city suffered from serious damages. Many hillside lands which had been damaged during the 1978 off Miyagi-Prefecture earthquake were subjected to damages again. Typical damaged hillside embankments in Sendai city were investigated and discussed in this paper. The main cause of the damages of housings is not the seismic motion but the failure of fill embankment. Comparing the damages during the 2011 earthquake with those during the 1978 earthquake, the countermeasures constructed after the 1978 earthquake performed well for preventing the large landslide type failure; however, they were not successful in the reduction with the damages of housing and housing lots, which were attributed to the ground displacements such as cracks, differential settlement, and shallow slips.

Modified Double Slip Model with Fabric Anisotropy for Hardening Behavior of Granular Materials

Abstract Double slip model was modified to include the effect of fabric anisotropy. For the mathematical expression of the effects of fabric anisotropy in the constitutive modeling, the modified stress method was used. The anisotropic stress strain behavior in the hardening regime was numerically simulated; and the simple analysis of the onset condition of shear band was also discussed

Modified double slip model for anisotropic hardening behaviour of granular materials

Abstract The double slip model was modified to include the effect of fabric anisotropy. For the mathematical expression of the shear was numerically simulated resulting in a satisfactory correspondence with experimental observations; while for the simulation of the behavior under the tation of principal stress axes, further modifications were found to be necessary.

Description of Cyclic Deformation Behavior of Sand by an Anisotropic Hardening Model

Abstract A simple anisotropic hardening model for granular material is proposed to describe cyclic behavior of sands during earthquake excitations. Special emphasis was placed on the formulation of unloading and reloading behavior of the model. Some numerical results of response of a sand element under repeated loading condition are presented in order to demonstrate some features of the present model. Considerations are made on the calculation results by comparing to the laboratory test results and possible alternative formulation of the constitutive model is suggested.

Sand boils along the Naruse River and the Yoshida River by the 2011 Tohoku-Region Pacific Coast Earthquake

4月3日から5日にかけて，東日本太平洋沖地震における地震被害把握のために宮城県の中部に位置する鳴瀬川および吉田川の河川堤防の調査を行い，液状化した14地点の噴砂から試料を採取し，粒度試験を行った。ここでは，噴砂地点の記録と粒度試験の報告をする。噴砂地点は，旧河道などの自然地盤，ないしは河道を埋め立てた自然地盤が大部分である。また，平均粒径Ｄ50と均等係数Uを試料採取地点と河口からの河川経路に沿った距離で整理したところ，相関性が見られた。

The Significance of the Fabric Tensor in the Deformation Behavior of Saturated Sand in Cyclic Shearing

Abstract The deformation behavior of saturated sand in the cyclic shearing apparatus is influenced to a great extent by the fabric state and its change during previous stress history as well as over consolidation ratio. In this paper, the effects of previous shear stress history on the deformation behavior of saturated sand are summarized and discussed based on the state of the microstructure in a granular assembly and its change, which are observed in the bi-axial compression test. The microscopic properties are incorporated into the fabric tensor defined by the spatial distribution of the contact areas between grain particles. A modified stress tensor for granular materials is defined for the new modified plane, which is a transformed plane through the fabric tensor. The derivation of the well known stress dilatancy equation from continuum mechanical point of view is also presented.