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INSTRUCTORS:
Hoang Bao Khoi Nguyen, Ph.D.
Chenying Liu, Ph.D
Renmin Pretell
Salman Rahimi, Ph.D., PE
Purpose and Background
These presentations were recorded at the Geo-Extreme 2025 conference.
Investigating Undrained Behaviour of Partially Saturated Granular Materials Under Cyclic Triaxial Loading Using DEM (14 minutes)
This presentation examines the cyclic undrained response of partially saturated granular soils using the Discrete Element Method (DEM). The study focuses on how reducing the degree of saturation influences pore pressure generation and liquefaction resistance under cyclic triaxial loading. Soil desaturation is modeled numerically to represent practical mitigation techniques such as air sparging. DEM enables direct investigation of particle-scale mechanisms, including contact forces, coordination number, and fabric evolution during cyclic loading. Results show that partially saturated soils require significantly more loading cycles to reach liquefaction compared to fully saturated soils. The findings demonstrate DEM’s capability to link micromechanical behavior with macroscopic liquefaction response.
Modelling Undrained and Drained Triaxial Shearing Behaviour of Realistic Sand Particles Using DEM (13 minutes)
This presentation explores the use of DEM with realistic sand particle shapes to model undrained and drained triaxial shearing behavior. Particle geometries are reconstructed from micro-CT scans and approximated using bonded sphere clusters to capture angularity and interlocking effects. A sensitivity analysis evaluates the number of spheres required to accurately represent real sand behavior while maintaining computational efficiency. Simulation results demonstrate that both drained and undrained loading paths converge to a unique critical state line, consistent with critical state soil mechanics theory. Key parameters such as state parameter, instability stress ratio, and phase transformation are evaluated. The study confirms that DEM with optimized particle shapes can realistically capture sand behavior.
Modeling path effects due to 3-D Velocity Structure for Non-ergodic Ground Motion Models: A Case Study Using Turkish Ground Motion Data (14 minutes)
This presentation addresses spatially varying path effects in ground motion prediction using non-ergodic ground motion models. Traditional ergodic models assume spatial averaging and neglect repeatable source, path, and site effects. Using a comprehensive Turkish ground motion database, the study incorporates Gaussian process models to explicitly capture path-dependent effects related to both attenuation and 3D velocity structure. Two advanced correlation models are evaluated, including one that accounts for path geometry and azimuthal effects. Results show significant reductions in model uncertainty, particularly at short distances and long periods. The work highlights the importance of explicitly modeling 3D velocity effects for seismic hazard analysis.
Relationship between factors of safety against soil liquefaction triggering and liquefaction effects on ground motions (15 minutes)
This presentation investigates how liquefaction triggering, quantified by factor of safety, relates to changes in ground motion characteristics. Numerical site response analyses are conducted using both total stress and effective stress methods across a wide range of soil profiles and earthquake records. Liquefaction effects are evaluated in terms of both soil softening and dilation-induced acceleration spikes. Results demonstrate that liquefaction can lead to either amplification or deamplification of ground motions, depending on soil density and oscillator period. As the factor of safety increases, ground motion response from effective stress analyses increasingly resembles total stress results. The findings provide guidance on when simplified analysis approaches may be appropriate.
Methodology and Validation for Tri-directional 1D Site Response Analysis in LS-DYNA (17 minutes)
This presentation presents a validated methodology for conducting tri-directional (two horizontal and one vertical) 1D site response analysis using LS-DYNA. The study addresses limitations of conventional site response tools by incorporating both shear and volumetric damping mechanisms. A new formulation allows independent control of volumetric damping, which is critical for modeling vertical ground motions. The methodology is applied to a deep soil site in the Santa Clara Valley using spectrally matched three-component ground motions. Results are validated against nearby earthquake recordings, demonstrating improved agreement when volumetric damping is properly defined. The approach provides a practical framework for projects requiring full three-component seismic analysis.
Benefits and Learning Outcomes
Upon completion of these sessions, you will be able to:
- Explain how partial saturation affects cyclic liquefaction resistance based on DEM simulations.
- Describe how realistic particle shape modeling influences critical state behavior in DEM simulations.
- Discuss how non-ergodic modeling improves representation of path effects in ground motion prediction.
- Identify how factor of safety against liquefaction influences amplification and deamplification of ground motions.
- Explain the importance of decoupling shear and volumetric damping in tri-directional site response analysis.
Assessment of Learning Outcomes
Students' achievement of the learning outcomes will be assessed via a short post-test assessment (true-false, multiple choice, and/or fill in the blank questions).
Who Should Attend?
- Geotechnical Engineers
- Structural Engineers
- Civil Infrastructure Designers
- Researchers and Academics
- Risk and Resilience Analysts
- Construction and Project Managers
How to Earn your CEUs/PDHs and Receive Your Certificate of Completion
To receive your certificate of completion, you will need to complete a short post-test online and receive a passing score of 70% or higher within 1 year of purchasing the course.
How do I convert CEUs to PDHs?
1.0 CEU = 10 PDHs [Example: 0.1 CEU = 1 PDH]