Dear @Tareq.AbuAgolah,
This question has been discussed in several other forum topics, e.g., see: Turbine-soil interaction. Influence on the mode shapes. - #11 by Jason.Jonkman. Basically, the translation and rotation of the tower at its base, interacting with the soil, should come through the platform degrees of freedom (DOFs) in ElastoDyn, rather than through the tower DOFs. The tower mode shapes should be cantilevered (starting with the a_2 term) to the platform, meaning that the mode shapes should be found such that the shape is tangent to the tower at its base. If you are using the NREL provided utility ModeShapePolyFitting.xls to fit the polynomial coefficients, this process is handled automatically.
Best regards,