Earthquake-induced ground failure has resulted in billions of dollars of damage during recent earthquakes. For example, the Canterbury earthquake sequence resulted in $40 billion in losses (over 20% of New Zealand GDP) in Christchurch, New Zealand, with much of this loss attributed to liquefaction of loose saturated sandy soil. Entire communities were relocated following the earthquake sequence as "red-zones" were declared in regions of high liquefaction hazard. Soils can be broadly characterized as exhibiting either "sand-like" or "clay-like" behavior with respect to strength loss during earthquakes. Liquefaction of "sand-like" soils is due to reductions of interparticle contact stresses, and transfer of stress from the soil particles to fluid occupying the space between particles. A large number of case histories of liquefaction of "sand-like" soils has enabled engineers to develop procedures for predicting its occurrence. By contrast, the mechanisms of failure of "clay-like" soils are more poorly understood, though there are cases in which such failures have resulted in damage to structures. In fact, cyclic failure of "fine-grained" soils are often constrained to the regions beneath structures, and not in the free-field soils away from the structures. There is a significant need to better understand the conditions for which "clay-like" soils may fail during earthquakes. Forging such an understanding will require new knowledge in the fundamental cyclic behavior of "clay-like" soils combined with knowledge of the cyclic stresses imposed on the soil beneath structures during earthquake shaking. This project will enable engineers to better predict conditions for which cyclic failure of "clay-like" soil will and will not occur during earthquakes. This will enhance public safety by reducing the risk of future structural failures. The research project will result in a publicly accessible database containing all of the experimental data so that future researchers will be able to benefit from the experiments. Furthermore, the project team will engage under-represented students through the UCLA Center for Excellence in Engineering and Diversity (CEED) through a hands-on project-oriented course for engineering freshman, and through the RISE-UP (Research Intensive Series in Engineering for Under-represented Populations) program.
The project will achieve its technical objectives by developing an understanding of the influence of Soil-Foundation-Structure Interaction (SFSI) on cyclic ground failure potential. The project activities include: (1) development of elasto-dynamic solutions quantifying the amplitude and phase of stresses induced by SFSI, and deployment of these solutions in a web-based tool for public use; (2) laboratory testing of fine-grained soil samples to evaluate variations of liquefaction susceptibility with soil plasticity and more advanced indicators of soil behavior (i.e., strength normalization) for the purpose of designing the centrifuge modeling experiments, (3) centrifuge modeling studies involving single-degree-of-freedom structures resting atop variable-plasticity fine-grained soil deposits, and (4) a simplified procedure for analyzing triggering of cyclic failure and the resulting settlements. Currently, the influence of SFSI is nearly entirely neglected when assessing ground failure potential due to a lack of understanding of key aspects of behavior, and a lack of tools available for engineers to incorporate SFSI effects . The primary intellectual merit of the project will be to enhance understanding of these fundamental issues.
