Effects of liquefaction-induced lateral spreading on pile foundations
Horne, John C. (John Charles)
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Earthquakes cause structural damage in many ways. A particularly devastating combination occurs when pile foundations are situated in loose, saturated soil deposits that are prone to liquefaction. If such a deposit is situated on a shallow slope or near a river channel, a state of nonzero initial shear stress exists which may induce significant lateral movements within the liquefied soil. Piles that extend through these deposits may also move laterally and sustain extensive flexural damage.A two-step methodology was developed to analyze the structural demands placed on pile foundations due to lateral spreading. The first step involved the development of a one dimensional, time domain ground response model that computes the displacement of an arbitrary soil deposit during and after strong shaking--both at and below the ground surface. A high resolution Godunov scheme was used in order to capture the extreme strain gradients that may exist in lateral spread regimes. Additionally, an effective stress formulation considers the influence of excess pore pressure on the strength and stiffness of the soil.The second step involved the development of a one dimensional pile-soil interaction model. A nonlinear rheologic model composed of separate near-field and far-field elements replicates hysteretic and radiation damping associated with pile-soil interaction. Properties of the rheologic model are updated continuously based on the effective stresses computed via the ground response model.An extensive parametric study revealed the dominant trends of the proposed methodology. It was found that lateral spread displacement--and hence pile response--is very sensitive to ground surface slope and standard penetration resistance, but not very sensitive to groundwater table depth. Furthermore, it was found that pile response was also strongly dependent on flexural stiffness, but not very sensitive to pile diameter or p-y curve stiffness.The model was then applied to well-documented case histories to assess its effectiveness. These include a direct comparison between the proposed model and an existing empirical method in addition to hind-cast analyses of two Niigata case studies. This effort verified that the model is useful to assess the effects of liquefaction-induced lateral spreading on pile foundations.
- Civil engineering