Structural modelling of wind turbine shallow foundations on granular soil strata deals with combined complex phenomena, such as soil-structure interaction, material nonlinearities, local stress concentration in both reinforced concrete foundation and soil. Numerical finite element analyses are performed employing three-dimensional elements for both concrete foundation and soil and one-dimensional elements for reinforcement. The results of the nonlinear static simulations are validated with experimental results of recent tests carried out on 1:15 scaled models having reinforcement ratios and stress conditions similar to the prototype foundations designed according to current codes. The validated modelling strategies are also applied to investigate the role of nonlinearities in both soil and concrete foundation, emphasizing the parameters and the nonlinear constitutive laws that significantly affect the foundation response. Three different reinforcement layouts are considered in both testing and simulations of the prototype: (a) complete prototype reinforcement, (b) prototype without shear reinforcement and (c) prototype without shear reinforcement and with 50% of remaining reinforcement replaced by steel fibres. A critical comparison is also made with models employing elastic two-dimensional shell finite elements lying over elastic supports, typically adopted for design purposes.

The design of massive structures, like cast-on-site foundations for wind towers, is not an easy task for design. In fact, this kind of structure is not specifically taken into account by Eurocode 2 standard. The huge concrete volume requires a huge amount of reinforcement, even if only the minimum amount is selected, and it is not clear if the lateral confinement exerted by the truncated cone geometry, mainly loaded along only one radial direction, corresponding to that characterized by the fastest wind direction, really requires the huge transversal reinforcement conventionally introduced in current wind towers. Within the research programme developed in 2019 between Enel Green Power, the Enel Group company dedicated to the development, construction and operation of renewables across the world, and Politecnico di Milano, an experimental investigation, carried out on prototypes characterized by reduced scales (1:15 for the whole structure and 1:4 for the investigation of the only core foundation, where the cylindrical stem is anchored), was conceived to calibrate a reliable modelling with the aim of being extended to full-scale structure, optimizing the required reinforcement. In this perspective, the use of fibre reinforced concrete can significantly improve the construction time by giving a specific toughness to the whole foundation, limiting also the crack opening and the correlated durability problems in the serviceability limit states.

The current effort towards the progressive switch from carbon-based to renewable energy production is leading to a relevant spreading of both on- and off-shore wind turbine towers. Regarding reinforced concrete shallow foundations of onshore wind turbine steel towers, possible reductions of reinforcement may increase their sustainability, speed of erection, and competitiveness. The article presents the results of an experimental program carried out at Politecnico di Milano concerning both cyclic and monotonic loading, simulating extreme wind conditions on 1:15 scaled models of wind turbine steel towers connected by stud bolt adapters to reinforced concrete shallow foundations embedded in a sandy soil. Three couples of foundation specimens were tested with different reinforcement layouts: (a) similar to current praxis, (b) without shear reinforcement, and (c) without shear reinforcement and with 50% of ordinary steel rebars replaced by steel fibers. Additional vertical loads were added to the small-scale models in order to ensure similarity in terms of stresses. The test results allowed to (i) characterize the mechanical behavior of the foundation element considering soil-structure interaction under both service and ultimate load conditions, (ii) assess the foundation failure mode, (iii) highlight the role of each typology of reinforcing bars forming the cage, and (iv) provide hints for the optimization of these latter.