Jinchi Lu

Mitigation of Liquefaction-Induced Lateral Deformation in a Sloping Stratum: Three-dimensional Numerical Simulation

Finite-element (FE) simulations are increasingly providing a versatile environment for conducting lateral ground deformation studies. In this environment, mitigation strategies may be assessed in order to achieve economical and effective solutions. On the basis of a systematic parametric study, three-dimensional FE simulations are conducted to evaluate mitigation by the stone column (SC) and the pile-pinning approaches. Mildly sloping saturated cohesionless strata are investigated under the action of an applied earthquake excitation. For that purpose, the open-source computational platform OpenSees is employed, through a robust user interface that simplifies the effort-intensive pre- and postprocessing phases. The extent of deployed remediation and effect of the installed SC permeability are investigated. The influence of mesh resolution is also addressed. Generally, SC remediation was found to be effective in reducing the sand stratum lateral deformation. For a similar stratum with permeability in the silt range, SC remediation was highly ineffective. In contrast, pile pinning appeared to be equally effective for the sand and silt strata permeability scenarios. Overall, the conducted study highlights the potential of computations for providing insights toward the process of defining a reliable remediation solution.

Parallel finite element modeling of earthquake ground response and liquefaction

Parallel computing is a promising approach to alleviate the computational demand in conducting large-scale finite element analyses. This paper presents a numerical modeling approach for earthquake ground response and liquefaction using the parallel nonlinear finite element program, ParCYCLIC, designed for distributed-memory message-passing parallel computer systems. In ParCYCLIC, finite elements are employed within an incremental plasticity, coupled solid-fluid formulation. A constitutive model calibrated by physical tests represents the salient characteristics of sand liquefaction and associated accumulation of shear deformations. Key elements of the computational strategy employed in ParCYCLIC include the development of a parallel sparse direct solver, the deployment of an automatic domain decomposer, and the use of the Multilevel Nested Dissection algorithm for ordering of the finite element nodes. Simulation results of centrifuge test models using ParCYCLIC are presented. Performance results from grid models and geotechnical simulations show that ParCYCLIC is efficiently scalable to a large number of processors.

Liquefaction-induced settlement of shallow foundations and remediation : 3D numerical simulation

Calibrated Finite Element (FE) simulations are increasingly providing a reliable environment for modelling liquefaction-induced ground deformation. Effects on foundations and super-structures may be assessed and associated remediation techniques may be explored, within a unified framework. Current capabilities of such a FE framework are demonstrated via a simple 3-dimensional (3D) series of simulations. Ground liquefaction and settlement under the action of a surface load is investigated. Liquefaction hazard mitigation is explored by soil compaction and/or increase of permeability below and around the applied surface load. The potential and limitations of numerical simulation are discussed, along with future research needs and challenges towards more reliable practical application.

Numerical Study of Shear Stress Distribution for Discrete Columns in Liquefiable Soils

AbstractDiscrete columns, such as stone and soil-cement columns, are often used to improve the liquefaction resistance of loose sandy ground potentially subjected to strong shaking. The shear stress reduction in the loose ground resulting from the reinforcing effect of these stiffer discrete columns is often considered as a contributing mechanism for liquefaction mitigation. Current design practice often assumes that discrete columns and soil deform equally in pure shear (i.e., shear strain–compatible deformation). In addition, because the discrete column is stiffer than the soil, it is assumed to attract higher shear stress, thereby reducing the shear stress in the surrounding soil. In this paper, shear stress reduction in liquefiable soils and shear strain distribution between liquefiable soil and discrete columns along with the potential of development of tensile cracks is investigated using three-dimensional linear elastic, finite-element analysis. Parametric analyses are performed for a range of geom...

A Framework for 3D Finite Element Analysis of Lateral Pile System Response

Linear and nonlinear three-dimensional (3D) finite-element (FE) simulations are becoming increasingly feasible on ubiquitous personal computers. Particularly suited to seismic applications, the open-source computational platform OpenSees provides such 3D simulation capabilities. With the aid of the graphical user interface OpenSeesPL for pile-ground analyses, routine 3D computations for this demanding situation are greatly facilitated. Through an analysis of an idealized pile- system embedded in clay, the reported investigations provide an overview of the involved numerical capabilities and user interface. To start, a single pile FE model is calibrated based on an available 3D linear analytical solution. The computed pile behavior for linear and nonlinear ground situations is then contrasted with that of the corresponding 3 x 3 pile group for two different pile-spacing scenarios. On this basis, assessments of lateral pile-group efficiency can be made. Further research and related practical applications are also discussed.

Design of DSM Grids for Liquefaction Remediation

AbstractDeep soil mixing (DSM) to form in-ground shear walls has been used to remediate against the potential effects of earthquake-induced liquefaction on many projects. A grid pattern of soil-cement walls act as a confined shear box, which can provide additional shear stiffness and strength for sites to withstand liquefaction. Current design practice for DSM grids commonly relies on the strain compatibility assumption, where the DSM walls and confined soil are assumed to experience the same shear strain. In this paper, the distributions of shear stresses and strains in liquefiable soil deposits treated with DSM grids are investigated using three-dimensional linear elastic finite-element analyses of unit cells. Parametric analyses are performed for a range of geometries, relative stiffness ratios, and dynamic loadings. These linear elastic results provide a baseline against which future nonlinear modeling results can be compared, but they are also sufficient for demonstrating that shear stress reductions...

A Framework for Performance-Based Earthquake Engineering of Bridge-Abutment Systems

A graphical user interface is developed to combine nonlinear dynamic time history analysis of bridge-abutment-ground systems with an implementation of a performance-based earthquake engineering (PBEE) framework. The user interface builds upon earlier code that allows for analysis of three-dimensional pile foundations in a soil domain under nonlinear monotonic and dynamic loads (OpenSeesPL). Functionality was extended for analysis of multiple suites of ground motions and combination of results probabilistically using the performance-based earthquake engineering framework developed by the Pacific Earthquake Engineering Research (PEER) Center. Definition of a two-span bridge, abutments, and underlying ground configuration is greatly facilitated using this new interface. In addition, all stages of the involved analyses are executed in a systematic fashion, allowing the end user to conveniently conduct extensive parametric investigations. In this paper, the analysis framework and the main components of the graphical user interface are presented. Abutment passive earth-pressure resistance to excessive bridge longitudinal displacement is modeled by a newly proposed hyperbolic force-displacement relationship. Using the interface, a representative bridge model is studied under a large ensemble of input ground motion excitations. Overall system response is investigated and the PBEE results are discussed.

Effect of DSM Grids on Shear Stress Distribution in Liquefiable Soil

Site improvement by Deep Soil Mixing (DSM) to form in-ground shear walls has been used to remediate against the potential effects of earthquake-induced liquefaction. The grid pattern of soil-cement walls act as a confined shear-box which can provide additional shear stiffness and strength for sites to withstand strong ground motion. Current design practice for DSM grids commonly relies on the strain compatibility assumption, wherein the DSM walls and confined loose soil are assumed to experience the same shear strain. In this paper, the distribution of shear stresses and strains in liquefiable soil deposits treated with DSM grids are investigated using 3-D linear elastic finite element analyses of unit cells with the OpenSeesPL platform. From the analyses, it is found that the assumption of shear strain compatibility can be unconservative because portions of the DSM system can deform in flexure and the shear strains within the enclosed soil can exceed those within the DSM walls. The effects of input motion frequency content, wall stiffness, and area replacement ratio are evaluated. A revised design equation is proposed that accounts for the identified limitations in the strain compatibility assumption.

Pilot 3D Numerical Simulation of Liquefaction and Countermeasures

A pilot study is presented to assess the influence of compaction and increased permeability on settlement below a surface load. Emphasis is on 3-dimensional (3D) Finite Element (FE) simulation. A relatively coarse mesh is used in order to conveniently obtain a general picture as to efficiency of a particular ground improvement approach and its spatial extent. Success of such efforts hinges on availability of constitutive soil models that allow for simulation of liquefaction induced deformations. High fidelity models may be studied for increased accuracy when needed.

Large-Scale Simulation and Data Analysis

Information Technologies (IT) are increasingly allowing for advances in monitoring and analysis of structural response. Sensor networks can provide real-time data streams, as a basis for system identification and decision-making. Fusion of video-derived information along with motion and strain sensors is already showing much promise. An integrated analysis framework encompasses data acquisition, database archiving, and model-free/model-based system identification/data mining techniques, towards the development of practical decision-making tools. Within this framework, data from experiments continues to provide much needed physical insight, as a basis for calibration of appropriate numerical models. In this regard, large-scale parallel computing and powerful visualization tools of soil-structure systems are a necessity.