In 4.8 & below, there is a "Refine Deflections" option in Tower Outputs for increasing accuracy of foundation deflections. It's been replaced by improved functionality in 4.9 and above called "Use finite element deflections for foundation loads".
In 4.9 and above, there is a checkbox called "Use finite element deflections for foundation loads" in the Additional Items screen, and there is no longer a "Refine deflections" checkbox in the Tower Outputs screen. Although superficially similar, these are completely different implementations and give different results. The earlier “refined deflections” can give erroneous results for non-linear foundations and in general we recommend that it should not be used. It doesn’t take account of the PY forces from FE deflections in the dynamic solution. The old method tries to calculate a more accurate foundation load after each time step, but the correction is not taken into account in the dynamic solution. This can lead to inconsistent and sometimes inaccurate results. "Use finite element deflections for foundation loads" includes the non-linear PY forces calculated using FE deflections in the dynamic solution, and therefore includes the dynamics from the non-linear foundation correctly. This method is documented in section 3.7 of the Theory manual for 4.9 and later versions.
We therefore strongly recommend that you use Bladed 4.9 and later versions for accurate modelling of foundations that include non-linear PY forces.