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Nonlinear behavior and modeling of piles in partially liquefied and layered soil conditions


Saturated soils, particularly cohesionless soils, may liquefy when subjected to earthquake excitation, resulting in reductions in strength and stiffness of the soil. The consequences have shown to be devastating to foundations in past earthquakes. For pile foundations, a reduction of vertical or lateral soil resistance may cause failure of the super structure. To study this issue, two unique 1-g soil-pile tests were conducted. The models were constructed in a laminar soil box which was subsequently placed on a uniaxial shake table. One experiment focused on characterizing the soil resistance of partially liquefied soil, while the other on multi-layered soil and the effect of its movement and load demands on the nonlinear behavior of piles. These two different experimental configurations illustrate scenarios commonly observed in the field, where the behavior of the soil and the pile merit further investigation. While evidence in the literature indicates that the lateral soil ("p-y") resistance of piles in liquefiable soils is significantly reduced, the shape and amplitude of the reduced p-y curve during pore pressure build-up is not well understood. To investigate this, a single steel pile embedded in homogeneous saturated Nevada sand was subjected to sequential dynamic shaking and lateral inertial equivalent loading. A key goal in the test program was to develop a data set capable of rendering insight into the characteristics of p-y resistance under developing and partial liquefied soil conditions. Test data were used to back-calculate p-y resistance curves for partially liquefied soils. These curves are subsequently adopted in a numerical study of the behavior of piles in partially liquefied soils considering different layering conditions. In contrast, in a multi-layered soil profile, if a loose layer liquefies, large localized plastic demands may be generated in the piles. In the case of concrete piles, these demands manifest flexural cracks in the concrete, which weakens the pile and exposes it to subsequent environmental degradation. During the experiments conducted herein to study this issue, plastic demands in the pile were characterized using curvature profiles coupled with back-calculation of the plastic hinge length and post-test physical observations. Numerical modeling studies demonstrate the applicability of current design- oriented tools to capture these key response parameters needed for design

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