Slope failures caused by earthquakes have had a significant impact on the design of slopes and earth retention systems in areas of strong seismicity. For example, several massive landslides occurred within natural clay soil formations during the 1964 Alaskan earthquake that caused loss of life and significant financial loss to infrastructure and private property in Anchorage. Consequently, design methods for seismic stability developed after this event take an overly conservative approach by assuming small values of strength that are appropriate for fully disturbed clay, resulting in designs that systematically ignore the substantial differences the behavior of the disturbed and the natural clay soil. This over conservatism results in designs that are significantly more expensive than one which accounts for actual behavior of the natural clays. Designs based on the results of this research will be more economical yet result in adequate margins of safety for the public. Therefore, this research will benefit the U.S. economy and society. The societal and educational impacts of the proposed research are broad, because its results will impact the basic science needed for predicting the occurrence of natural hazards and for developing a sustainable geotechnical design. This research involves aspects of engineering mechanics, geotechnical engineering, earthquake engineering and engineering geology. The multi-disciplinary facets of the research will help broaden participation of underrepresented groups in research and positively impact engineering education. This Grant Opportunity for Academic Liaison with Industry (GOALI) research is a collaboration between industry and academe, which has the benefit of rapid application of research results to practice.
This research will develop a new approach for quantifying the strength degradation and destructuration of natural clays exposed to cyclic loading. It will combine experimental and theoretical findings to predict the onset of seismically-induced failure in natural clay deposits. The experiments will be conducted on high quality samples of Bootlegger Cove Formation of varying degrees of sensitivity. Very sensitive clays in this formation triggered the catastrophic slides that took place in Anchorage during the 1964 earthquake. Northwestern University will collaborate in this research with GeoEngineers, Inc., who will coordinate drilling and sampling of BCF specimens. The experimental program will include monotonic and cyclic tests on intact and reconstituted samples. The experiments will elucidate the role of incremental nonlinearity, stress-paths, consolidation history and cyclic loading. The modeling activities will use this evidence to formulate an incrementally non-linear constitutive model reproducing the effect of destructuration in pore pressure build-up and cyclically generated failure. Although several models have been proposed to cope with the complex mechanics of natural clays, such models often ignore the rich variety of possible failure modes. Engineering mechanics theories for fluid-saturated soils provide insights, but to capture the complex processes taking place during seismic shaking, these theories must be enhanced to accommodate cyclically-decaying stiffness and strength processes. The approach will include the development of specific bifurcation criteria for assessing the cyclic strength degradation as a function of stress conditions and number of cycles. The model will be numerically implemented in computer programs for coupled dynamic analysis of geotechnical problems.
