Structural health monitoring of an onshore wind turbine piled-raft foundation

Over the years, wind turbine design has substantially changed to reduce cost and maximise performance, resulting in increased wind turbine height, blade size and power rating, seeking steadier wind at higher elevations while capturing more wind with longer blade lengths. This resulted in not only the vertical load but also the already dominating horizontal load and overturning moment caused by the wind acting on these turbine foundations, increasing significantly. Onshore turbine structures are often built on piled-raft foundations, which were historically not designed for dominant horizontal loads. Thus, with the increased popularity of these larger wind turbine models, the need arose to physically determine the response of these foundation types to large horizontal loads and overturning moments. In this article, a full-scale operational piled-raft foundation supporting a 117 m high onshore wind turbine in South Africa was instrumented and monitored for an extended period of time, focusing on the foundation’s response during turbine installation and the first year of operation. A good correlation between the wind data and foundation strains existed and is presented, with load sharing between the different foundation components also highlighted. The cyclic loading of the foundation from the wind resulted in the piles carrying increased loads over time compared to the loads carried after turbine installation (increasing from 35% to roughly 60% after 1 year of operation). The permanent bending moments in the piles also increased with time. Additionally, seasonal temperature variations caused significant strain changes in the raft, which cannot be ignored.