Project Title/ID Number 3D Soil Simulation Models in OpenSees—2212002
Start/End Dates 10/1/02—9/30/03
Project Leader Boris Jeremíc (UCD/Faculty)
Team Members Zhaohui Yang (UCD/Post Doc), Kevin Murakoshi (UCD/Undergrad Student), Qing Liu (UCD/Grad Student), Feng Xiong (UCD/Post Doc)
Project goals and objectives

The goal of this project is to develop computational tools for seismic analysis of soil-foundation-structure interaction (SFSI). In particular, three dimensional elastic-plastic material models, integration algorithms and coupled formulations are in development and are being used to assess the effects SFSI has on Humboldt Bay Bridge. To this end, a number of elastic-plastic material models have been developed and implemented into OpenSees. Moreover, a fully coupled solid fluid formulation has been developed and implemented into OpenSees as well and is currently being validated.
Role of this project in supporting PEER’s vision

This project supports PEER strategic plan by providing simulation tools to be used in the performance evaluation of bridges when subjected to seismic events. Accurate and robust simulation capabilities (within OpenSees) are essential to the PEER methodology. While this project is particularly connected to the Humboldt Bay Bridge., the models and implementations developed are quite applicable to other testbeds as well.

  


Figure 1. Load factors within a pile group
Larger View

  
Methodology employed

This project is developing models and implementation for numerical simulations of geomaterials. In addition to that, the project in general provides implementation support to OpenSees in terms of code quality verification and validation.
Brief description of past year’s accomplishments and more detail on expected Year 6 accomplishments

We previously completed the development and implementation of the Template Elastic Plastic Framework. This framework is now being used in assessing the performance of deep foundations. In particular a study is underway into the performance of single piles in layered soils and the interaction effects for pile groups. Initial part of the study is conducted for static pushover loads while we are currently building and analyzing models that will be dynamically loaded. One of the important findings of the single pile study (see Figure 1) is that the effects of soft layers on stiff layers are felt further away from the interface than previously thought. In addition to that an initial study of interaction effects for piles in a pile group was conducted. The different load factors taken by the piles in a pile group are presented in Figure 2. One of important findings is that in addition to the in-plane bending, piles will undergo significant out-of loading plane bending, depending on the type of soil around them. In addition to that, the unequal distribution of loads is far more significant than previously thought and that also is a function of the type of soil surrounding the pile group. A number of additional studies is under way.

Even more important that the results obtained is the fact that all the finite element models used in this study were put in public domain. The intentions behind sharing of finite element models are twofold. First. It is hoped that practitioners will be encouraged to start using advanced computational tools in design, by giving them the tools (OpenSees) and the models (input files for finite element models). Secondly, other researchers are using our models and can improve them, generate new models for their problems, and hopefully provide them to other as we did.

  

 


Figure 2. Pile sectional forces in layered grounds
Larger View

  
Also aimed at the seismic behavior of deep foundations is the development and implementation of the fully coupled u-p-U formulation for analyzing liquefaction problems. The formulation is quite general and is more robust and versatile than traditionally used u-p formulation. Implementation is undergoing verification and validation at this time. Figure 3 shows a response of a soil column to vertical load. Viscous effects, stemming from coupling of pore fluid and the soil are obvious.
Other similar work being conducted within and outside PEER and how this project differs

Finite element analysis of piles and pile groups was rarely performed in the past. Relevant work is described in one of our papers. During this project we are performing a detailed study using sophisticated material and finite element models, and providing those models to the public for further use and critic.

As for the coupled formulation, there is some similar work being conducted by a group from San Diego, however, it is generally agreed that u-p-U approach is more robust and versatile. The other formulation and implementation provides an opportunity for verification and validation and that will be pursued during this project.

We collaborate with a number of BIP members (Tom Shantz, Cliff Roblee) and they provide valuable guidance and comments for our project. In addition to that, the hope is that Caltrans Engineers will start using some of our tools (OpenSees) in near future.
Plans for Year 7 if this project is expected to be continued

We anticipate further development of layered and pile group models, as well as the coupled models used to simulate liquefaction condition for seismic numerical tests. In addition to that, the implementation of the Plastic Bowl Method (from other project on I-880 simulations) allows for a full seismic simulations of the soil (liquefiable or non-liquefiable), foundations (piles that will be modeled with inelastic beam elements to simulate damage to piles) and the bridge structure. With this capabilities we will be able to investigate the performance of bridge-foundation-soil system to various types of earthquakes, and estimate the performance of the complete system taking into the account various types of damages.

In addition to that we anticipate creating a set of simple models (through the graphical user interface) that will help practicing engineers tackle very specific problems (like ground motions for a given elastic-plastic layered soil profile, or behavior of standard Caltrans piles in layered soils...) by using OpenSees. The idea is to hide the complexities of the inelastic finite elements and the input files and just create a very user friendly interface for these very specific problems. The Background computations will rely on OpenSees but the end user will not need to know that or to know much about the finite element method (or actually will not need to know anything at all). This graphical / simulation system can then be connected to the data base of test results (including photos of components at various stages of inelastic deformation) and be used in decision making after earthquakes or to validate the modeling of components.
Describe any instances where you are aware that your results have been used in industry
Our work has been recognized by Caltrans and we will start seeing the use of both results (in terms of practice oriented diagrams) and the models (to be run on OpenSees) in near future. This is coordinated with BIP members from Caltrans.
Expected milestones
  • Publication of final PEER report on pile layering and pile group effects (Fall 2003)
  • Publication of preliminary results and models for the liquefaction analysis of pile foundations (in relation to the testbed problems) (Fall 2003)
  • Continuing publication (on the web) of models used in our analysis (sequentially as they become available)
  • Further improvements to the OpenSees platform, particularly in terms of code quality assurance (continuously)
Deliverables
  • Implementation of models and formulations into OpenSees,
  • Finite element models (OpenSees) for pile foundations,
  • Study results for layering effects, pile group effects, liquefaction effects,
  • OpenSees implementation testing.