Effect of liquefaction on the behavior of a retrofitted pile foundation subjected to cyclic loading

Abstract

Liquefaction is a major concern for bridge column foundations in earthquake prone regions. Although its effects are devastating to the structural integrity of foundations, there is little quantitative information to guide engineers in the design of bridge column pile foundations that may be subjected to liquefaction of the surrounding soil during a seismic event. In order to quantify the effect of liquefaction on the stiffness and damping provided by a pile foundation under the cyclic loading that occurs during a seismic event, a representative foundation often used in bridge design is modeled using linear finite elements. The case study foundation is a 3x10 pile group with thirty concrete piles that are each 15 m deep. The effectiveness of a common micro-pile retrofit is also examined through the use of the finite element model. Sixteen 30 m deep micro-piles are added to the perimeter of the 3x10 pile group and are connected to the original 3x10 pile foundation with an expanded pile cap. These slender piles are comprised of reinforced concrete partially encased with a steel pipe. Loading is applied to the finite element model at frequencies of 0, 2, 4, and 8 Hertz in three directions of translation and two directions of rotation. Liquefaction is simulated in the finite element model by decreasing the elastic modulus of various layers in the model to ten percent and one percent of their original value. The quantitative effects of different loading directions and frequencies are observed when both the retrofitted and non-retrofitted model are subjected to varying degrees of liquefaction at several different soil depths. The information obtained in these simulations is then used to develop stiffness and damping modification factors. This study shows that the effect of retrofitting and of liquefaction on the dynamic behavior of the pile group is highly dependent on load direction, load frequency, and liquefied layer location.