Seismic performance of buffer-equipped anchored piles considering end restraints

Abstract

During seismic events, slopes buttressed by anchored piles can experience instability when piles convert to perilous cantilever systems upon anchor cables failure. Implementing buffer devices in anchoring structures has the potential to reduce the seismic forces in the anchors and consequently improve the seismic performance of the supported structures. Taking advantage of the excellent ductility of flexible piles, this study investigates a buffer device installed in series with the anchored piles. Shaking table tests were conducted on a slope stabilized by buffer-equipped anchored piles considering two typical pile tip conditions: either hinged or free end restraints. Three ground motions with different characteristics were applied to the model slopes fitted with springs with varying stiffness and its seismic response was evaluated in terms of soil accelerations, pile displacements, earth pressures, and anchor-cable forces. In addition, the cable loading reduction mechanism was investigated through the force Fourier spectrum. The results indicated that the system dominant frequency decreased for anchor with hinge support and the pile displacement increased and the dynamic earth pressure decreased as the spring stiffness decreased. Most importantly, implementing the buffer device (i.e., the spring) reduced the response energy significantly over the whole frequency range, especially in the middle range, which resulted in a marked reduction in the anchor cable stress and fatigue damage. The reduction of anchor force was found to be inversely correlated with the spring stiffness. Based on the maximum pile displacement and spring stroke, a stiffness optimization strategy was proposed considering the foundation conditions and seismic excitations. For major earthquakes, stiff springs are advisable to mitigate load effects and to help maintain pile displacement within permissible bounds, while the spring stiffness can be reduced appropriately by implementing a hinge support. Meanwhile, the design should focus on critical displacement rather than loading reduction for seismic hazard characterized by near field earthquakes or high peak ground acceleration.