SUMMARYA granular material consists of an assemblage of particles with contacts newly formed or disappeared, changing the micromechanical structures during macroscopic deformation. These structures are idealized through a strain space multiple mechanism model as a twofold structure consisting of a multitude of virtual two-dimensional mechanisms, each of which consists of a multitude of virtual simple shear mechanisms of one-dimensional nature. In particular, a second-order fabric tensor describes direct macroscopic stressstrain relationship, and a fourth-order fabric tensor describes incremental relationship. In this framework of modeling, the mechanism of interlocking defined as the energy less component of macroscopic strain provides an appropriate bridge between micromechanical and macroscopic dilative component of dilatancy. Another bridge for contractive component of dilatancy is provided through an obvious hypothesis on micromechanical counterparts being associated with virtual simple shear strain. It is also postulated that the dilatancy along the stress path beyond a line slightly above the phase transformation line is only due to the mechanism of interlocking and increment in dilatancy due to this interlocking eventually vanishing for a large shear strain. These classic postulates form the basis for formulating the dilatancy in the strain space multiple mechanism model. The performance of the proposed model is demonstrated through simulation of undrained behavior of sand under monotonic and cyclic loading.
Liquefaction-induced ground deformations are permanent ground displacements resulting from earthquakes, which can extend over areas as large as a few square kilometers and have amplitudes ranging from a few centimeters to few tens of meters. This type of ground deformation caused substantial damage to lifelines and pile-foundations of buildings and bridge piers along the Kobe shoreline during the 1995 Hyogoken-Nanbu, Japan, earthquake. This paper presents a four-parameter multiple-linear-regression model for estimating the amplitude of liquefaction-induced ground displacement for both ground-slope and free-face conditions at a regional scale. The applicability of the model for mapping the amplitude of liquefaction-induced ground deformation is investigated over selected regions. The paper also presents a regional model for estimating the probability for the displacements to exceed some threshold amplitude, and to fall within confidence intervals. Both models are useful for risk assessment to spatially distributed lifeline networks resulting from future earthquakes.
SUMMARYThe strain space multiple mechanism model idealizes the behavior of granular materials based on a multitude of virtual simple shear mechanisms oriented in arbitrary directions. Within this modeling framework, the virtual simple shear stress is defined as a quantity that depends on the contact distribution function as well as the normal and tangential components of inter-particle contact forces, which evolve independently during the loading process. In other terms, the virtual simple shear stress is an intermediate quantity in the upscaling process from the microscopic level (characterized by the contact distribution and inter-particle contact forces). The stress space fabric (i.e. the orientation distribution of the virtual simple shear stress) produces macroscopic stress through the tensorial average. Thus, the stress space fabric characterizes the fundamental and higher modes of anisotropy induced in granular materials. Comparing an induced fabric associated with the biaxial shear of plane granular assemblies obtained via a simulation using Discrete Element Method to the strain space multiple mechanism model suggests that the strain space multiple mechanism model has the capability to capture the essential features in the evolution of an induced fabric in granular materials.
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