Effective Shaking of Bridge Pier on Deep Foundation in Liquefiable Ground

Effective ground shaking is evaluated at the pile cap of a bridge pier by performing kinematic soil-pile interaction analysis, with and without including effects of liquefaction. The subject pier is supported on six 3-m diameter drilled shafts connected by a pile cap, extending through loose liquefiable sand, dense sand, and into bedrock. The computed effective pile cap shaking accounts for the depth-varying free field motions along the pile, and the near field soil-pile interaction, which are both affected by liquefaction. Global bridge modeling in practice typically utilizes soil springs (p-y), which cannot model pore pressure buildup to explicitly simulate liquefaction. To address this limitation, this study aims to bound the pier response by separately considering both liquefied and non-liquefied conditions. Depth-varying free field ground motions are computed at nodal depths along the piles by performing site response analyses, with and without accounting for liquefaction. Separate sets of p-y springs are generated to represent liquefied and non-liquefied soils. The computed depth-varying motions are then used as inputs through the corresponding soil springs to a massless model of the pile foundation. The resulting forces are computed at the pile cap, and the effective motions are then evaluated (referred to as kinematic motions). Response spectra of the kinematic motions are found to be higher for the non-liquefied condition over the period range relevant to the bridge in this study. As a result, the non-liquefied condition would govern seismic demands affecting structural member stresses and sizes. Lower kinematic motions for the liquefied condition are attributed to the softer soils near the mudline, which are not capable of driving the motion of the stiff pile foundation, effectively deepening the pile point of fixity.