Evaluating Uplift Restrictions for Shallow Onshore Wind Turbine Foundations
Abstract
Shallow foundations are the preferred design choice for onshore wind turbines, due to their cost-effectiveness and simplicity. The existing research on shallow foundations promotes the concept of "rocking foundations" which has culminated in a design philosophy beyond conventional failure limits. In this design philosophy, uplift is permitted as a means of energy dissipation in bridge and building applications. However, the design guidelines for wind turbine foundations allow no uplift for operational conditions and restrict the uplift region to a portion of the foundation width under extreme loads. This thesis extends the rocking foundations philosophy to the design of wind turbine foundations to enable repurposing of foundations of wind turbines at their end of life to support new larger wind turbines, provided that the foundation is inspected for cracks and damages.
Finite element models are developed for wind turbine foundations in undrained clay, dry sand, and saturated sand. Computer codes are developed to implement a constitutive model for undrained clay and to simulate the pore pressure in saturated sand. For each soil profile, a foundation designed for a 5 MW wind turbine is analyzed under the loads of 5 MW, 8 MW and 10 MW turbines to evaluate the effects of uplift. Moreover, the numerical modeling methods are validated by experimental data.
Results for the modeled soil profiles indicate that the zero-uplift rule for operational conditions governs the design when this rule is obeyed. For the cases studied in this thesis, relaxing this rule enables a foundation designed for a 5 MW wind turbine to support an 8 MW wind turbine, without compromising its safety or serviceability. However, the same foundation cannot support the larger 10 MW wind turbine because different types of failure occur: exceedance of the tilting limit for the foundations in dry sand and undrained clay, and liquefaction in saturated sand.
Allowing small uplift areas under operational loads leads to a more cost-effective design and facilitates repurposing existing foundations for larger wind turbines. This can translate into major cost savings and reduction of construction impacts on the environment.