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Estimating Free Field Seismic Settlement History in a Saturated Layered Soil Profile
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Existing methods proposed for estimating the earthquake-induced one-dimensional free-field settlement in saturated soil, focus on the post-liquefaction permanent volumetric strain. These methods rely on undrained cyclic laboratory tests that subject the sandy samples to uniform cycles. A new method is presented to estimate the free-field one-dimensional settlement histories induced by random base motions. This approach uses an existing incremental volumetric strain model to determine the settlement history before and after liquefaction. The shear strain history of a layered profile was obtained from an effective stress-based response analysis available as an option in DEEPSOIL and was subsequently used as an input to the proposed method. A simple two-parameter porewater pressure generation model available in DEEPSOIL was employed. This model, which is capable of simulating strain-softening, was calibrated to an existing probabilistic liquefaction potential curve and acceptable rate of porewater pressure generation. The proposed method was then used to calculate the settlements at the Marina District after the 1989 Loma Prieta earthquake, at Port Island after the 1995 Kobe earthquake, and at 6 different sites located within Christchurch and Ferrymead after the 2011 Christchurch earthquake. The estimated settlements were compared to the measured values along with those estimated by existing methods.Subsequently, the proposed incremental volumetric strain model is used to calculate the earthquake-induced one-dimensional free-field settlement in liquefiable three-layered profiles subjected to a variety of earthquake ground motions. The shear strain histories of these profiles, obtained from the effective stress-based response analysis using DEEPSOIL, were used as an input to the incremental volumetric strain model. A new simplified equation has been developed to estimate the free-field one-dimensional settlement based on the analytical results from the three-layered profiles. This equation includes the influence of important earthquake parameters and soil layer configuration and properties. The proposed equation is compared to the predictions made by an established model. This equation is then used to estimate the free-field settlements at 7 locations in the Marina District after the 1989 Loma Prieta earthquake and at 3 different sites after the 2011 Christchurch earthquake. The evaluated settlements were found to be consistent to those observed at the site and the ones estimated by an existing method.