Question: how to convert RootOf to a proper radical form

I have a result:

u1 = RootOf(4*_Z^2 + (4*RootOf(60*_Z^3 - 60*_Z^2 + 15*_Z - 1) - 4)*_Z + 4*RootOf(60*_Z^3 - 60*_Z^2 + 15*_Z - 1)^2 - 4*RootOf(60*_Z^3 - 60*_Z^2 + 15*_Z - 1) + 1)

I tried: convert(u1,'radical');

but it reported error Error, (in sprintf) too many levels of recursion

how can I convert it to this form?

Thanks!!

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