Rather than snipe in with comments about substrates, I'll address your question
that excavator clay sure looks nice don't it? I allowed myself to get tempted by it when it first came out.
An UTH won't work. The clay is too good an insulator. It simply reflects all the heat back towards the underside of the tank, while above the clay is basically room temp. You could work around this by using a lamp for heat or by placing your UTH on the side of the tank rather than the bottom.
But
IME as soon as the surface of the clay gets wet, either from an animal plowing through it's water bowl or from spraying, the color leaches out. Your gecko's belly will get stained reddish, along with anything he/she crawls across while it's still wet. I couldn't deal with that, so went back to my old substrate. It's a shame, because the stuff is really easy to work with, and you can create very naturalistic looking bluffs and canyons/ridges with it. Hopefully Zoo-Med addresses this issue, as they said they were aware of it.