State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences (2021) / Chapter Skim
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8 Use of Computational Mechanics to Predict Liquefaction and Its Consequences
8 Use of Computational Mechanics to Predict Liquefaction and Its Consequences
Pages 161-179
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From page 161... ...
• Research is needed on ways to implement advanced constitutive models and on incorporating mixed formulations to solve simultaneously for deformations and pore pressures. • Instability and flow problems characteristic of liquefied soil could be addressed with meshless and discrete element methods used in conjunction with multiscale approaches and depth averaged flow models.
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From page 162... ...
The elements of site characterization for case history assessments described in Chapter 3 are also appropriate to inform numerical modeling. Engineering-mechanics-based computational methods are accepted and used widely in structural, mechanical, materials, and manufacturing engineering, in the modern seismological prediction of earthquake ground motions, and in many geotechnical engineering applications not involving liquefaction.
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From page 163... ...
In engineering practice, one of the most widely used two-dimensional programs for modeling soil liquefaction and its consequences is FLACTM. FLACTM is a finite difference code that allows implementation of userdefined constitutive models, includes structural elements and energy absorbing boundaries, and can employ a Lagrangian large strain formulation to accommodate large displacements.
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From page 164... ...
FIGURE 8.1 Displaced shape and displacement vectors at the end of post-earthquake analysis of Upper San Fernando Dam in the 1971 San Fernando Earthquake. This figure shows the ability of a common commercial program to capture relatively large displacements.
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From page 165... ...
Hence, OpenSees is used much more commonly in research than in consulting engineering practice at this time. ISSUES IN THE COMPUTATIONAL MODELING OF LIQUEFACTION PROBLEMS Computational methods to analyze liquefaction problems solve the equations of motion and deformation for assemblages of solid particles containing fully or partially fluid-filled pore spaces.
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From page 166... ...
and finite elements (see, e.g., Hughes, 1987; Bathe, 1996; Zienkiewicz and Taylor, 2013)
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From page 167... ...
strains even when, in practice, they are used to simulate finite strains. Coupled and Uncoupled Solutions A fully coupled solution scheme is one in which the changes in pore fluid pressures and deformations in the soil particle framework are obtained simultaneously in each step of the marching procedure.
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From page 168... ...
Both finite difference and finite element computer programs are used in research and practice. Both methods can accommodate complex geometries, boundary conditions, and finite deformations.
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From page 169... ...
The continuum approach has shown promising results in capturing some of the basic features observed in the laboratory and physical models. Nevertheless, more laboratory and physical models are needed to validate computational models and thereby increase the confidence in simulation tools to address liquefaction problems accurately.
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From page 170... ...
For example, there is a need to develop interface elements that can account for void ratio redistribution and the large deformations resulting from the separation between the pile and the soil. Improved computational efficiency for three-dimensional analyses, increased accuracy and validation for three-dimensional constitutive models, and a better understanding of interface behavior and the interface elements that model that behavior are required to obtain more accurate representation of soil-structure interaction in liquefied soil.
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From page 171... ...
Independent peer review of the computational models and results should be incorporated into important projects. CONSTITUTIVE MODELING OF LIQUEFIABLE SOIL Solution procedures that contain the equations for motion and balance of mass for a coupled system (see Box 8.1)
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From page 172... ...
; (c) field-scale simulation of flow related to liquefaction for a bridge using realistic topography, seismic excitation, and finite elements, and an appropriate constitutive response such as that shown in (b)
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From page 173... ...
More efforts are still needed in this direction, especially at large deformations. RECENT COMPUTATIONAL RESEARCH DEVELOPMENTS APPLICABLE TO LIQUEFACTION ANALYSIS Advanced computational techniques may be able to address such soil liquefaction issues as void ratio redistribution and large displacement flow slides following the triggering of liquefaction, and separations and discontinuities that sometimes accompany ground displacement (e.g., separations between pile and soil and ground cracks in the upstream face of the Lower San Fernando Dam; shown in Figure 8.3)
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From page 174... ...
Figure 8.4 shows simulations of flow problems using the MPM and DEM platforms, displaying the capability of these methods to represent large deformations resulting from flow associated with liquefaction or other phenomena. The aforementioned methods provide alternative numerical solution architectures that may have advantages over the traditional finite element and finite difference methods in modeling liquefaction problems and are further described in this section.
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From page 175... ...
This technique, often used in the geosciences, invokes the Mohr-Coulomb failure criterion to limit the shear stress at the base of the deforming soil mass. Recent formulations also take into account the effective stress principle, observe concepts of critical-state soil mechanics, and account for evolving solid volume fractions and porewater pressures (Iverson and George, 2014)
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From page 176... ...
. Fluid saturation and pore pressures are simulated, and the formulation is more efficient than are other formulations such as those based on Eulerian finite elements.
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From page 177... ...
coarse-grid approaches in which the fluid flow component is simulated using a discretization much larger than the average particle size; and (c) fine-grid approaches in which the fluid flow component is simulated using a discretization finer than the average particle size (O'Sullivan, 2011)
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From page 178... ...
the interparticle contact forces using average effective stress tensor expressions such as the one proposed by Christoffersen and colleagues (1981) (see, e.g., Nicot et al., 2005; Wellmann et al., 2008; Andrade and Tu, 2009; Nitka et al., 2011; Guo and Zhao, 2014)
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From page 179... ...
In addition, a new generation of computational models needs to be developed that can reproduce physical tests across scales ranging from the laboratory to the field. This can be achieved by basing computational methods on physics and mechanics to establish a more intimate coupling between computation, modeling, and experimentation.
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