Seismic response of stepped variable section pile-cap system installed in sandy ground

Variable section pile foundation has been increasingly employed in engineering practice due to their ability of high vertical bearing capacity and strong bending resistance, while its seismic behavior has yet to be well investigated. In this study, a series of 1-g shaking table model tests were carried out on variable section pile-cap systems installed in sandy bed. The test results indicated that the using of variable piles could effectively reduce the acceleration response at pile cap and pile curvature, although the pile bending moment became larger. A validated three-dimensional (3D) finite element (FE) modelling procedure was employed for performing a suite of numerical parametric analyses, accounting the effects of variable diameter ratio, variable thickness ratio and pile flexural rigidity. Parametric numerical analyses suggested that there were optimal variable diameter ratio and variable thickness ratio of about 1.2 and 3, respectively, at which both the maximum pile curvature and curvature at pile top were significantly smaller than that of equal section pile. Besides, the improving effect of using variable section pile against seismic shakings was found to be dependent on the pile flexural rigidity, which was more significant for relatively flexible piles. The results obtained from this study can provide useful reference for the using of variable section piles against seismic risks.