## 1 Introduction  
Today, wind energy is developing at a record pace as the demand for renewable energy increases. In 2020, a wind power capacity of 93 GW was installed globally (including 87 GW onshore), which corresponds to the installation of tens of thousands of new wind turbines, and annual installations are expected to keep increasing substantially during the coming decade [1]. Onshore wind turbines' foundations are supported by massive concrete structures, which are commonly built in remote locations with tough weather conditions. Their construction represents a challenging task, which requires the coordination of a wide range of different and complex aspects (e.g., material, environment, equipment, human). It is important to conduct proper planning prior to the construction that encompasses these aspects. As reported in several other studies [2, 3], a large proportion of existing wind turbine foundations are cracked or have other types of defects. For instance, there have been numerous records of deficiencies in wind turbine foundations and towers, in particular of cracking in the concrete [4, 5]. The inspection of several thousands of wind turbine foundations in Germany a few years ago concluded that more than a quarter of wind turbine foundations examined need remedial measures due to inappropriate structural detailing or poor construction [6]. In China, corrosion and cracks are the main defects identified on towers and foundations, and it has been observed there that damages and failures are most often caused by fatigue under normal operating conditions [7]. Crackpropagation within a concrete foundation due to design deficiencies was identified as the cause of the collapse of a 3-MW wind turbine in Canada, leading to the costly replacement of the foundations for 50 turbines at the same wind farm [8; 9]. Frequent problems have also been reported concerning embedded anchor rings [2; 3; 5], which are commonly used to connect the tower to the concrete foundation. This is an extensively used solution worldwide. Excessive movements of up to tens of millimetres in the vertical direction have been observed. According to [PERSON] et al. [3], the problem is widespread but often not reported because of commercial reasons. Similarly, defects are seldom reported and assessed in detail in the literature and the causes of these defects and their relation to the construction process and design aspects have rarely been investigated. Additionally, it is worth noting that cracks in the concrete and other defects in wind turbine foundations may often not be detected as the foundations are often almost entirely covered and cannot be inspected visually after the wind turbine is installed. Defects or anomalies affecting the wind turbine foundations may affect the durability of the foundation and the serviceability of the turbines or even affect their stability. To preventively address these issues, which have important economic and safety consequences, it is important to gather information from wind farm construction projects and study possible deficiencies to prevent the resurgence of these anomalies. According to [PERSON] [2], a defective design process is the main cause of damage to wind turbine foundations, as construction solutions developed for smaller wind turbines are used and applied to larger structures. In fact, the size of wind turbines has increased exponentially in recent years, and the hub height of a wind turbine installed today can reach more than 140 m [10]. [PERSON] [2] also pointed out that investigations carried out should have been carried out to improve the design of new foundations. Despite the occurrence of damages and failures, structural health monitoring of onshore wind turbine foundations has attracted minimal interest so far and has been limited to a few single installations for research purposes [11; 12; 13]. Cracking in concrete structures is ineluctable and nearly all concrete structures exhibit a certain kind of cracking.