The effect of large earthquake loading on fine-grained foundation materials: determining residual undrained strengths at large strains and corresponding embankment deformations

Project ID: 23024
Principal Investigator: Carolyne Bocovich
Research Topic: Improving Geotechnical Infrastructure Reliability
Funded Fiscal Years: 2023 and 2024
Keywords: None

Research Question

Realistic dynamic analysis, such as for large earthquake loading, is dependent on having accurate residual undrained strengths of soils at large strains. Currently there is uncertainty in the state of practice on how to determine the reduced undrained strengths through laboratory testing. This research intends to develop laboratory practices to determine residual undrained strengths and to determine the effect these residual undrained strengths have on dynamic analyses by answering the following questions.

1.a. How do constant volume ring shear, monotonic direct simple shear, post-cyclic direct simple shear strengths and strengths from field methods such as peak, remolded, and residual vane shear strengths compare?
1.b. What are the most reliable test methods to determine residual undrained strengths?
2. How do the residual undrained strengths affect final deformations calculated using numerical dynamic analyses in 1-element and full-scale models?
3. What are the most realistic ways to incorporate residual undrained strengths into numerical deformation models?

Need and Benefit

The Reclamation Design Standard on Seismic Analysis and Design of Embankment Dams (Design Standards No. 13, Chapter 13), expresses the critical need to accurately determine residual undrained strengths for post-earthquake stability and the lack of consensus within the industry on how to determine these strengths. The design standard continues with methods to analyze deformations due to large strains such as comparing case histories with similar loadings (though the material may be different), simple sliding block analysis, and nonlinear FLAC analysis. However, to get reasonable results out of these methods, it is important to understand the strength of the site-specific soil at large strains and the effect of residual undrained strengths on estimated deformations. Without addressing these concerns, Reclamation will continue to be very conservative and potentially over design embankment dams for seismic and post-seismic stability. One on-going example is that of BF Sisk, which as currently designed requires over approximately 27 million cubic yards (MCY) of material placement to address seismic issues including liquefaction of alluvium and weak, fine grained, foundation and embankment materials.

This research addresses the concern outlined in the Reclamation Design Standard by determining the best method to measure residual undrained strength in the lab and relating differences in measured residual undrained strengths to changes in calculated deformation. This research will result in more confidence in values used for residual undrained strength and a greater understanding of how residual strength affects estimated deformations.

Urgency for this research is great as ongoing projects, including BF Sisk, Scoggins, Ochoco, Conconully Dam, Keene Creek and Starvation Dam will move into modifications. Additional embankments, not listed, will continue to move into issue evaluation, corrective action study, and modification.

This research will have direct impacts to TSC Design groups and Geotechnical Laboratory group, and future benefits to Reclamation's Dam Safety Program and Regions. Direct impacts include clear methods to capture residual undrained strength, improved laboratory capabilities, and improved deformation analysis and design methods for seismic-related modifications. Clear methods to capture residual undrained strength is critical and could save several weeks of re-analysis and re-running deformation analyses (i.e., sensitivity studies), or about $25,000 worth of staff time per cross section per project. Improved laboratory capabilities will result in better, more realistic residual undrained strengths, critical for deformation analyses. Improved analysis and design of seismic modifications from well defined, realistic residual strengths results in higher confidence in embankment design and risk estimation. These results would have direct impacts on jobs such as BF Sisk, in which the first of three contracts was awarded for about $120M requiring 2.5MCY of soil placement. BF Sisk contracts 2 and 3, not awarded at this time, require about 25MCY of soil placement. In projects like this, optimizing the design by even just 10% could save tens of millions of dollars.

Future benefits include benefits to Reclamations Dam Safety Program, Reclamation's Regional offices, and the TSC. These include reduced cost compared with current business practices by providing more realistic strengths for seismic analysis, design, and modification. Results will increase confidence in design, reduce conservative design and potentially reduce over designing embankment dams for Reclamation for seismic and post-seismic stability. This will conserve costs, materials, and resources for the significant number of embankment dams with seismic probable failure modes with fine-grained material in the embankment or the foundation.

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Last Updated: 6/22/20