Xcel Energy retained Barr to complete a supplemental geotechnical evaluation for a wind repowering project located in Nobles County, Minnesota. This evaluation included an assessment of soil behavior subjected to cyclic foundation loading for the proposed repower locations. The field campaign focused on undisturbed thin-wall samples collected at a depth corresponding to the base of the turbine foundation, while the laboratory testing program centered around cyclic triaxial strength to assess the behavior of the soil under enhanced foundation loading associated with the repower.
Numerical modeling was performed using FLAC, a software that allows for the computation of stresses induced in soils in response of changing structural loads at the surface. Using FLAC, the cyclic behavior of the major and minor principal stresses and corresponding shear stress were determined. Based on FLAC modeling, significant variations were found in the major and minor principal stresses relative to a small corresponding variation in shear stress. Cyclic testing was completed using stress path sequencing in a triaxial chamber, in which the major and minor principal stresses underwent 250 full cycles (calculated via the damage equivalent method to approximate loading for the operational life of the wind turbine). The major and minor principal stresses were computed from the FLAC model described previously.
In this study, the following criteria were used:
Settlement: Differential settlement induced by cyclic loading is anticipated to be minimal based on measured cumulative axial strain.
Stiffness: Constant or increasing Young’s modulus results indicated either neutral conditions or an improvement of the foundation subgrade soil stiffness.
Strength: The undrained shear strength of soils for cycled specimen strength testing was found to exceed the design requirement.
The soil samples were found to not be susceptible to significant degradation, and therefore, costly foundation retrofits were deemed unnecessary for the repowering effort.