Question: evalf fails to deliver RootOf numerical value

Dear All,

evalf(RoofOf(Z^6-3*Z^4+3*Z^2+Z-1,index=real[2]) fails to deliver a numerical value.

1. I can use fsolve to compute the real roots but I need to replace all the occurences of RootOf by its numerical value in lengthy expressions.
2. Also simplify(RoofOf(Z^6-3*Z^4+3*Z^2+Z-1,index=real[2],RootOf) does not work.
3. I read the other posts related but could not find any answer.

Thanks for any help,

S.