Motf
Grand Slam? - I think not. That was definitely sub-surface burst, usually about 80 feet below the surface, which was why it was so effective against difficult targets such as bridges. It just created a huge underground void and the whole structure, along with a fair amount of the surrounding scenery, collapsed into it. This sounds more like an extra large FAE (Fuel Air Explosive) device which can create a huge overpressure, much higher than that from a nuclear device.
There was a thread about FAEs some time ago. I think they have tried up to 10,000 lb versions so far, this could be the logical (?!) progression.