Failure-mechanism for wind turbines supported by pile groups at the Kugino wind farm in the Kumamoto earthquake

The Kugino wind farm at Japan was seriously damaged in the severe Kumamoto earthquake, characterizing as all three pile group cracks but only one tower buckling. This study aims to reveal the failure mechanism underlying such damage pattern through the Beam on Nonlinear Winkler Foundation (BNWF) analyses, where the soil-footing interaction is considered with a new q-z model (QzSimple6). It identifies three parameters in a hyperbolic function to match any desired modulus reduction curve, whereas adjusts the unloading-reloading curves iteratively with the Ishihara-Yoshida rule to achieve site-specific soil damping curve. The QzSimple6-based BNWF analyses quantitatively reproduces centrifuge test results of a pile group foundation system, and newly reveals the soil-footing interaction does not influence pile bending moments but reduces the point mass acceleration. A parametric study is conducted on the full BNWF model with identifying pile group supported wind turbine, but with scaling soil stiffness and strength. The thrust force is attracted from the aero-elastic analysis in OpenFAST and the free-filed seismic displacement are calculated with the site response analysis in OpenSees. The simulation shows consistency with site observations that the No.2 wind turbine tower is destined to buckling at the height of around 13.9 m due to the sudden reduction of tower thickness, while No.1 and No.3 towers could remain safe potentially because soil properties under them are softer than that under the No.2 tower. In contrast, all three pile groups are found to be cracked under the Kumamoto earthquake intensity since the pile bending moment relies on the footing rigidity rather than the footing-soil interaction.