Evaluation for Soil Liquefaction Due to Long Duration Earthquakes with Low Acceleration
Past studies show that soil liquefaction due to a long duration earthquake with low acceleration, for example as observed in the Tokyo Bay area in the 2011 Great East Japan Earthquake, is largely affected by liquefaction strength for a large number of loading cycles. It is, therefore, extremely important to know the liquefaction resistance for a large number of loading cycles. This paper describes the results of the hollow cylindrical cyclic torsion shear tests that were conducted to ascertain the effects of grain size distributions on liquefaction resistance for a large number of loading cycles. The test results clearly show that the tendency of a decrease in liquefaction resistance for a large number of loading cycles is influenced by mean grain size, inclination of grain size distribution, and so on. Finally, the effect of liquefaction resistance curve characteristics on the assessment of liquefaction potential is examined using the cumulative damage theory.
Strain Space Plasticity Model for Cyclic Mobility
ABSTRACT The cyclic mobility model proposed in this paper is of a generalized plasticity—multiple mechanism type. The salient feature of the present approach is that the concept of the multiple mechanism, within the framework of plasticity theory defined in strain space, is used as a vehicle for decomposing the complex mechanism into a set of one dimensional mechanisms. The other feature is that the undrained stress path is idealized with the concept of liquefaction front, which is defined in the effective stress space as an envelope of stress points gradually approaching the failure line. Once coded into the finite element program, most of the existing models suffer from serious difficulty in the numerical solution process when effective stress path becomes very close to the failure line. A numerically robust approach is proposed in this paper by introducing a scheme for gradually enlarging scale of shear strain with progress of cyclic mobility.
SHEAR WORK AND PORE WATER PRESSURE IN UNDRAINED SHEAR
When a loose sand is subjected to a cyclic rotation of principal stress axes during an undrained test, excess pore water pressure is generated. To study the nature of pore pressure buildup, several series of laboratory tests were conducted using a hollow cylindrical torsion shear apparatus. Tests were conducted employing different kinds of cyclic loading schemes in which specimens were subjected to cyclic shear stresses with or without continuous rotation of principal stress axes. Based on the test results, accumulated shear work per unit volume was calculated at each stage of shear stress application, and its correlation with the pore water pressure was examined. It was found that, at each state of shear stress alteration, there exists a unique relationship between the shear work and the pore water pressure, independent of the shear-stress history. This observation enables contour lines of equal shear work to be drawn in the stress space. (Author/TRRL)