Qiushi Chen

CPT-Based Evaluation of Liquefaction Potential Accounting for Soil Spatial Variability at Multiple Scales

AbstractUnderstanding and assessing the spatial extent of liquefaction requires that the spatial dependence of soil properties to be taken into account. In this work, a cone penetration test (CPT)-based approach for the evaluation of liquefaction potential is presented where the soil spatial variability is explicitly considered through internally-consistent probabilistic models developed at multiple scales. The novelty of the proposed work comes from the integration of the classical empirically-developed liquefaction criteria with tools in geostatistics and novel multiscale random-field models. A unique feature of the proposed work is its ability to refine and obtain higher resolution random fields for soil properties in critical areas, such as those adjacent to important infrastructure or in areas with detailed small-scale field data. An illustrative example assessing the liquefaction potential at various shaking levels in the Marina District of San Francisco is used to demonstrate the capability of the ...

Predicting liquefaction probability based on shear wave velocity: an update

The simplified methods based on the cone penetration test (CPT), standard penetration test (SPT), and shear wave velocity (Vs) test are prevalent in liquefaction potential evaluation. In this study, new case histories with the shear wave velocity measurements and the liquefaction phenomenon observations are compiled from the 22 February 2011 Canterbury earthquakes in New Zealand. The new case histories are combined with the existing Vs database for assessing and updating probabilistic models. The widely used logistic regression models, as well as other probabilistic models, are examined in the framework of generalized linear models (GLMs). To this end, the maximum likelihood estimation (MLE) principle is used to determine the model parameters. Then, the developed generalized linear models are ranked using three model assessment criteria. Based on the assessment criteria adopted, the log–log and logistic models are recommended for both the existing and the combined database. The updated log–log model and logistic model are recommended for shear wave velocity based liquefaction potential evaluation.

Automatic Differentiation for Numerically Exact Computation of Tangent Operators in Small‐ and Large‐Deformation Computational Inelasticity

Advances in computer software tools and technologies have transformed the way in which finite element codes and associated material models are developed. In this work, we propose a numerically exact approach for computing the sensitivites required to construct local consistent tangent operators in computational inelasticity applications. The tangent operators that come from the derivatives of constitutive equations are necessary for achieving quadratic convergence in integrating material models at the integration point level. Unlike finite difference-based numerical methods, the approach proposed in this work is based on an exact differentiation technique called automatic differentiation (AD). The method is efficient, robust and easy to incorporate. Numerical examples in both small- and large-deformation inelasticity problems with complicated material models are presented to illustrate the efficiency and applicability of the proposed method.

Centrifuge model test and limit equilibrium analysis of the stability of municipal solid waste slopes

We have investigated the stability of a municipal solid waste (MSW) slope by the centrifuge model test and limit equilibrium analysis. The mechanical behavior of artificial MSW was studied using consolidated drained triaxial tests, and the shear strength was obtained and described by a power form failure criterion. The deformation pattern and failure mode were obtained from the movement of the monitoring point of the slope during the acceleration process in the centrifuge model test. The safety factor of the slope was calculated by the limit equilibrium method with a power form nonlinear strength criterion. Comparison of the results of the centrifuge model test and the calculated results using the Mohr–Coulomb criterion proved that the nonlinear strength criterion was a reasonable method to determine the location of the slip surface and safety factor. The Bulbul landfill failure was used as a case study to verify the applicability of the proposed method in the real-world case, and reasonable results were obtained.

Random field-based regional liquefaction hazard mapping — data inference and model verification using a synthetic digital soil field

Geostatistical tools and random field models have been increasingly used in recent liquefaction mapping studies. However, a systematic verification and assessment of random field models has yet to be taken, and implications of various random field-based mapping approaches are unknown. In this paper, an extremely detailed three-dimensional synthetic digital soil field is artificially generated and used as a basis for assessing and verifying various random field-based models for liquefaction mapping. Liquefaction hazard is quantified in terms of the liquefaction potential index (LPI), which is mapped over the studied field. A classical CPT-based liquefaction model is adopted to assess liquefaction potential of a soil layer. Different virtual field investigation plans are designed to assess the dependency of data inference and model performance upon the level of availability of sampling data. Model performances are assessed using three information theory-based measures. Results show that when sampling data is sufficient, all random field-based models examined capture fairly well the benchmark liquefaction potentials in the studied field. As the size of the sampling data decreases, the accuracy of predictions decreases for all models but to different degrees; the three-dimensional random field model gives the best result in this scenario. All random field-based models examined in this paper yield a slightly more conservative prediction of liquefaction potential over the studied field.

Finite Element Analysis of Hydro-mechanical Coupling Effects on Shear Failures of Fully Saturated Collapsible Geomaterials

The fully coupled diffusion-deformation processes occurring within porous geomaterials, such as sand, clay and rock, are of interest to numerous geotechnical engineering applications. In this work, a stabilized enhanced strain finite element procedure for poromechanics is integrated with an elasto-plastic cap model to simulate the associative and non-associative hydro-mechanical responses of fluid- infiltrating biphasic collapsible porous geomaterials. We present a quantitative analysis on how macroscopic plastic response caused by pore collapse and grain rearrangement affects the seepage of pore fluid, and vice versa. Finite element simulations of shear failure problems will be presented to study the effect of pore pressure dissipation on the stress path and plastic response of the porous geomaterials.

NEAMS VLTS project

The objective of the U.S. Department of Energy Office of Nuclear Energy Advanced Modeling and Simulation (NEAMS) Very Long Term Storage (VLTS) Project is to develop a simple, benchmark model that describes the performance of Zry4 d-hydrides in cladding, under conditions of long-term storage of used fuel. This model will be used to further explore the requirements of hydride modeling for used fuel storage and transport. It is expected that this model will be further developed as its weaknesses are understood, and as a basis of comparison as the Used Fuel Disposition (UFD) Campaign explores more comprehensive, multiscale approaches. Cladding hydride processes, a thermal model, a hydride model API, and the initial implementation of the J2Fiber hydride model is documented in this report.

Bi-objective Optimization of Site Investigation Program for Liquefaction Hazard Mapping

The development of a liquefaction hazard map generally requires field data from site investigations. In this study, a bi-objective optimization framework is proposed for selecting an optimal site investigation program considering both the accuracy of the liquefaction hazard map and the site investigation efforts. To validate the proposed framework, a three-dimensional synthetic soil field with extremely detailed soil properties is generated and the corresponding liquefaction hazard map is used as the benchmark. Both regular and random sampling-based site investigation programs with varying site investigation efforts are considered and are used to infer input parameters of the subsequent random field-based liquefaction hazard mapping. It is found that the random field-based liquefaction hazard maps generally overestimate the hazard when validated against the benchmark liquefaction hazard map. When site investigation efforts (quantified by the number of sounding sites) are the same, regularly spaced site investigation programs yield more accurate hazard maps than those by random sampling-based investigation programs. An optimal site investigation program is recommended for the study site and the proposed framework can be applied to optimize site investigation of other sites.

Probabilistic Assessment and Mapping of Liquefaction Hazard: From Site-Specific Analysis to Regional Mapping

Probabilistic methods have been increasingly used in liquefaction hazard assessments for purposes of considering the substantial uncertainties in both the liquefaction case histories and the model development process, and for the risk assessment and performance-based earthquake engineering. In this paper, a review on the probabilistic methods of site-specific liquefaction assessment, including logistic regression, the Bayesian method and various performance-based methods, is first undertaken. Another important topic in the liquefaction hazard assessment is to understand its spatial extent, leading to mapping of liquefaction hazard over a region. The regional liquefaction hazard maps are being employed as planning tools and provide guidance to assess the need for site-specific evaluations. The second focus of this paper details a review of methods for regional liquefaction hazard mapping, including geology-based, geotechnical data-driven and geostatistical methods as well as multiscale methods. The review of the site-specific probabilistic methods and regional mapping methods involves a discussion of their formulations, key assumptions, advantages and applications in liquefaction assessment. The challenges and the need for further research in these areas are also mentioned.

Image-Based Shape Characterization and Three-Dimensional Discrete Element Modeling of a Granular Martian Regolith Simulant

Particle size distribution (grading) and particle shape are two of the most salient factors that affect the mechanical behavior of a granular material. In this paper, a machine learning and level set-based method is proposed to obtain the particle size distribution and three-dimensional particle shape information of a granular Martian regolith simulant from its X-ray computed tomography (CT) images. The extracted realistic particle shapes are characterized by various shape descriptors and are then incorporated into the development of a shape and grading-dependent three dimensional discrete element model. Numerical repose angle tests are conducted to demonstrate the capability of the DEM model and to study the mechanical behavior of the Martian regolith simulant. Validated against conventional laboratory tests, the numerical analysis provides insights of the material behavior from the fundamental level.