Browsing by Author "Dong, JJ"

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A micromechanics-based methodology for evaluating the fabric of granular material
(1999-12-01)
Pan, YW
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Dong, JJ
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交大名義發表
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National Chiao Tung University
A micromechanics approach can successfully model the stress-strain-strength relation of a granular material once the microcharacteristics of the material have been obtained. However, it is usually difficult to determine the fabric of natural granular materials, Using a stress-dependent micromechanics model, the elastic properties can become a function of the geometric and kinetic fabric. Also, the wave velocity can be related to the elastic properties, Consequently, this paper proposes a methodology for evaluating the fabric of a granular assembly from a set of measured wave velocities. The methodology contains three elements: (a) stress-dependent micromechanics elastic model, (b) an anisotropic elastic wave propagation theory and (c) an optimization procedure. It is verified by calibrating available wave velocity data of a glass bail assembly and washed mortar sand, The methodology is further applied to study the microstructural evolution of the washed mortar sand under biaxial stresses. Two aspects of fabric change can be observed: (a) concentration of contact normals in the major principal direction and (b) a residual fabric after subsequent loading/unloading.
Hierarchical model of rough rock joints based on micromechanics
(1996-02-01)
Dong, JJ
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Pan, YW
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土木工程學系
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Department of Civil Engineering
A rough rock joint model is proposed in this study on the basis of micromechanics concepts. In the model, the global behavior of a rough joint depends on the microfeatures of the contact planes on the joint. The contact mechanics on contact planes controls the mechanical behavior of the joint via a homogenization process. Also, the complex mechanical behavior of a joint is associated with simple microfeatures of the joint, including the frictional properties and the structure of contact planes. To capture the feature of scale dependency of joint roughness, a hierarchical representation of a joint profile is proposed in the form of a multi-level-asperity model. Moreover, the constitutive relation of the multi-level-asperity model is derived through a recursive homogenization process. The proposed model's framework is general and systematic. Furthermore, major deformation mechanisms of joint asperities, i.e. interlocking, wearing, shearing-off sliding, separation and degradation, are taken into account.
Micromechanics model for elastic stiffness of non-spherical granular assembly
(1999-09-01)
Dong, JJ
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Pan, YW
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土木工程學系
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Department of Civil Engineering
This paper presents a micromechanics model for the elastic stiffness of a non-spherical granular assembly. The microstructural continuum model of ideal spherical assembly is extended for non-ideal assembly. The presented work takes the effects of gradation, shape, and preferred orientation into account by introducing a directional distribution function of branch-vector length. The microstructure of a granular assembly is described by the distributions of packing structure, branch-vector length, and particle number per unit volume. These distributions account or the random nature of a realistic granular material. The microfeatures relevant to the description of non-deal particle assembly are elaborated. The influences of various direction-dependent and direction-independent microfeatures on the elastic stiffness are demonstrated. Hypothetical non-ideal granular assemblies are used to study the effects of gradation, shape and preferred orientation. Based on the proposed model, the paper discusses the inherent anisotropy in a non-ideal granular assembly. The presented work also makes use of a generalized static hypothesis to estimate the contact-force distribution for specific microstructure and stress state. With the estimated contact-force and the Hertz-Mindlin contact theory, the elastic stiffness of a particulate assembly can be evaluated. Hence, the effects of geometric fabric and anisotropic stress state on the elastic stiffness can be deliberated. Consequently, the effects of geometric fabric and kinetic fabric of a natural granular material can be evaluated independently. It is shown that the proposed model can reasonably capture the phenomena of inherent anisotropy and stress-induced anisotropy of a non-spherical granular assembly under small strain. Copyright (C) 1999 John Wiley & Sons, Ltd.
The influence of surface ruptures on building damage in the 1999 Chi-Chi earthquake: a case study in Fengyuan City
(2004-01-01)
Dong, JJ
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Wang, CD
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Lee, CT
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Liao, JJ
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Pan, YW
In addition to the main surface rupture along the Chelungpu fault associated with the 1999 Chi-Chi, Taiwan earthquake, numerous secondary or branch ruptures on hangingwall were also observed. These secondary surface ruptures are parallel or sub-parallel to the main rupture within a distance of a few meters to 1-2 km. The rupture length of these secondary ruptures varies from a few tens of meters up to 5 km. The surface deformation resulted in serious damages of buildings. The present work studied the features of surface deformation on the hangingwall around the Chung-Cheng Park, Fengyuan, Taichung. Three distinct surface ruptures, minor ruptures and tension cracks were observed in this area. The observed distribution and types of building damage on the hangingwall are demonstrated. Due to the difference in geological condition and complex pattern of surface deformation, the resulted building damages on the hangingwall vary. A series of site investigation including field survey, drilling, seismic prospecting, P-S logging tests and laboratory tests were carried out in the interested area. A geological structure model was proposed on the basis of the results of site investigation. Numerical simulation was carried out to model the surface deformation as well as subsurface potential damage zone of an active fault. It reasonably explains the observed pattern of surface deformation and indicates that the surface deformation zone during a catastrophic earthquake is predictable. (C) 2003 Elsevier Science B.V. All rights reserved.