The company said concrete pedestals at the 74MW Hardscrabble wind project in Herkimer County, New York failed to meet standards during routine construction testing.
Iberdrola has suspended turbine installation to conduct further testing.
Hardscrabble will comprise 37 Gamesa 2MW turbines, and was scheduled to be completed by the end of the year.
Contrary to newspaper reports, the company has not had to remove any towers, Iberdrola spokesman Paul Copleman said.
Iberdrola does not yet know how the foundation problems will affect the project’s schedule, Copleman said.
Iberdrola plans to spend $6 billion developing US wind power by 2013.