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NEESR: Reduction of Seismic Shaking Intensity on Soft Soil Sites Using Stiff Ground Reinforcement

Project funded by:   NSF #1208117 (October 2012)

PI: Guney Olgun, Virginia Tech
Co-PIs: Kenneth Stokoe, James Martin, Binod Tiwari, Daniel Wilson
Description: The primary objective of this project is to develop a new seismic design concept for reinforced ground that can be used to reduce the intensity of strong ground shaking on soft sites. The study involves centrifuge and shake table testing as well as numerical modeling to demonstrate that stiff soil-mix panels, installed in lattice-type grids, can reduce the amplification of seismic energy up through soft soil profiles. In most cases, such ground reinforcement is used to increase bearing support, limit permanent deformations, and/or reduce liquefaction potential. Additional benefits, such as favorably altering the dynamic transfer function of the soil profile and thereby reducing the shaking levels input into the superstructure are not considered in current design provisions. Such reductions in ground shaking can lead to safer and more economical designs. This detailed study utilizes model testing with the centrifuge facility at the University of California Davis and a shake table at California State University Fullerton to demonstrate the effects of reinforcement geometry and layout, stiffness ratio, layering and properties of the soil profile, and other factors. Advanced three-dimensional nonlinear finite element modeling will be used to extrapolate the model test results to a wide range of field and site conditions so that design curves can be developed. This work represents an unprecedented approach to design ground improvement to reduce surface ground motions and base input into structures at seismically vulnerable soil sites. Data from this project will be archived and made available to the public through the NEES Project Warehouse/data repository (http://www.nees.org).

The mitigation of the earthquake damage potential of soft soil sites remains one of the leading challenges in earthquake engineering. It is well-established through observations of earthquake damage patterns that structural damage is strongly correlated with local geological and soil conditions. Soft, weak sites generally fare worse than stiff sites. This is because soft sites tend to amplify the intensity of ground shaking and often undergo large deformations that can cause major structural damage. Earthquake damage potential can be significantly reduced by reinforcing and effectively stiffening such sites. Preliminary studies suggest that mixing cement with the soft soils to form stiff soil-cement panels in the ground (i.e., ?deep soil mixing?) can be an effective ground reinforcement technique for such purposes. Therefore soft soil sites can be transformed to behave like stiff sites, thereby greatly reducing earthquake vulnerability. The project is organized around an integrated plan of physical and analytical modeling to investigate and validate the effectiveness of various ground reinforcement techniques and develop a practical, economical design approach. This work has the potential to improve building performance, increase safety margins, and reduce development costs in earthquake prone regions. The findings will have implications not only for the engineering community, but also for business owners, developers, communities, and other stakeholders. As such, there is a well-developed plan for transferring knowledge into practice through partnership with the National Institute of Building Sciences (NIBS), the Deep Foundations Institute (DFI), and soil improvement contractors who are collaborating on this research. Results will be also be disseminated via conferences, journals, the project website, and incorporated into academic courses and professional shortcourses for groups such as American Society of Civil Engineers (ASCE), Federal Emergency Management Agency (FEMA), Federal Energy Regulatory Commission (FERC), and U.S. Army Corps of Engineers. There is also an aggressive outreach plan for the involvement of students from a predominantly undergraduate and minority serving institution, as well as from area K-12 schools. This award is part of the National Earthquake Hazard Reduction Program (NEHRP).

Project Links

NEES Hub: http://nees.org