Assessment of Liquefaction Potential of an Intermediate Foundation Supporting an Onshore Wind Turbine Considering Dynamic Behavior during Emergency Shutdown and Seismic Events

Wind energy turbines are a key technology in the effort to decarbonize the energy grid and fight the ongoing climate crisis. To meet society's current and future clean energy demands, wind turbines continue to grow in both size and complexity. This growth has required the parallel development of advanced support structures and foundation systems for these turbines. In onshore applications, the increasing size of turbines has led geotechnical and foundation engineers to explore alternative designs that deviate from past practices of shallow foundations for these structures. These alternative, intermediate-type foundations more efficiently leverage the strength of the surrounding soil body compared to shallow foundations while avoiding the costs and potential schedule delays often associated with deep foundation solutions. However, this class of foundations requires a more exacting understanding of the dynamic behavior of the foundation, especially during extreme loading events caused by natural hazards. During extreme load events, fail-safes within the mechanical components of the turbine may initiate an emergency stop of the turbine rotor. Emergency braking creates a significant demand on the support structure and foundation and can often be a controlling load case for the foundation system. In a seismic event, the arrival of ground motion can trigger an emergency stop condition. The combined turbine-tower-foundation system then must withstand the structural demand caused by the turbine braking while also withstanding the ground accelerations for the remainder of the seismic event. As with any structure founded in cohesionless material, liquefaction of the adjacent soil body is a concern for wind turbines. While current design practice for wind turbines and their foundations considers liquefaction through analyses of varying complexity, designers typically do not directly analyze the true dynamic behavior of the entire turbine-tower-foundation-soil system. It is especially rare to consider the impacts of an emergency stop during a seismic event. To assess the impact of this loading, a typical wind turbine foundation-soil system is modeled in a three-dimensional finite element software. The soil body is modeled using constitutive model that captures excess pore water pressure development, liquefaction potential, and hardening behavior. Loading conditions are applied to the model to simulate the load sequence of a seismic event triggering an emergency stop. Liquefaction of the soil is then monitored along with displacement of the foundation.