You know what's even better than dirt at stopping radiation? Water. And we have lots of it. 10km of water at the bottom of the Mariana trench should stop any amount of radiation that could hit Earth.
However, if you assumed that we were able to go the bottom of the ocean, transporting supplies there would be much easier than transporting supplies to Mars.
Except we reached it four times since 1960 : https://en.wikipedia.org/wiki/Mariana_Trench#Descents