A Sand Plasticity Model for Earthquake Engineering Applications PDF Download

Author: Aikaterini Ziotopoulou
Publisher:
ISBN:
Category : Earthquake engineering
Languages : en
Pages : 0

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Book Description
The development of the sand plasticity model PM4Sand (Versions 2 and 3) for geotechnical earthquake engineering applications is presented. The motivation behind this work is twofold. The first goal is to have a constitutive model that is applicable to the wide range of conditions encountered in geotechnical engineering practice, is easy to calibrate utilizing the limited data that are usually available and can reasonably approximate the behaviors of interest. The second goal is to provide an extensive documentation of the capabilities and limitations of the model. The PM4Sand plasticity model is built upon the basic framework of the stress-ratio controlled, critical state compatible, bounding surface plasticity model for sand presented by Dafalias and Manzari (2004). A series of modifications and additions to the model were incorporated by Boulanger (2010; Version 1) and further herein (Version 2 and Version 3) to improve its ability to approximate the stress-strain responses important to geotechnical earthquake engineering practice; in essence, the model is calibrated at the equation level to provide for better approximation of the trends observed in empirical correlations commonly used in practice. Each stage of its development has been guided by consecutive cycles of formulation, validation and calibration against the whole body of available data (case histories, lab, centrifuge and shake table tests) on the cyclic behavior of sands both at the element and at the system level. The formulation of PM4Sand Version 2 for geotechnical earthquake engineering applications is presented followed by its calibration and implementation. A generalized calibration of the constitutive model is presented which attempts to produce drained and undrained, monotonic and cyclic responses under a broad range of stress conditions that are reasonably consistent with the behaviors expected based on engineering correlations to commonly available in-situ test data (i.e., SPT, CPT and Vs data). Simulated single element responses are compared to design correlations to illustrate the efficacy of the model modifications and evaluate the model’s performance. The constitutive model is shown to be relatively easy to calibrate and provide reasonable responses for key liquefaction behaviors. The numerical implementation as a user defined material for use in a two-dimensional explicit finite difference program is described. The formulation of PM4Sand Version 3 for improved modeling of post-liquefaction reconsolidation strains is presented. Modifications are introduced that provide a means for improved modeling of post-liquefaction reconsolidation strains after the end of strong shaking in a nonlinear deformation analysis. The methodology of the approach along with the calibration is presented, followed by a comparison of the response computed for a field case history. A review is presented on the performance of a number of constitutive models in reasonably predicting liquefaction effects under sloping ground conditions. Example results from undrained cyclic DSS tests on clean sands under zero and nonzero static shear stress ratio conditions are presented and findings from past experimental studies for these types of loading conditions are reviewed. Examples are presented of the performance of selected constitutive models in modeling the observed experimental results. It is shown that they all have certain limitations and that the effect of sloping ground conditions is particularly difficult to simulate. Finally, the formulation of PM4Sand Version 3 is further updated to better account for the effects of sloping ground conditions and irregular cyclic loading on liquefaction behaviors and thus improve simulations of liquefaction-induced deformations of sloping ground. Existing laboratory test data are reviewed and new experimental data from undrained cyclic Direct Simple Shear (DSS) lab tests of liquefiable sand under sloping ground conditions subjected to irregular cyclic loading are introduced. Evidence from the tests shows that it is the effect of loading history on the dilatancy and stiffness characteristics of the response that is not properly captured by the model. The modifications made in Version 3 include a revised dependency of dilatancy and plastic modulus on the fabric tensor and its history. These modifications are introduced using the irregular cyclic DSS testing to illustrate the motivations for the changes in the constitutive equations. Finally, two examples of calibration are presented: one against a specific lab test result for a single sand and one against an engineering correlation describing trends observed for many sands across a broader range of relative densities, confining stresses, and loading conditions. The updated formulation in Version 3 of the model is shown to better approximate liquefaction behaviors for sloping ground conditions.