I think the Grand Slam was approx 22.000 Lbs, but I'll look it up and post it as soon as I can.
It made a huge crater at times, up to 60 feet in diameter and dozens of feet in depth. It would knock down buildings from their foundations from "earthquake" effects rather than blast, using the bombs deep penetration to serve as a pressure holder for the explosion, - a bit like a combustin really.
In AH, the Grand slam and the Tallboy could both be of use if the damage model for structures was to be revised, i.e. Hardened against blast or not. As it is now, it's a bit senseless, for everything counts, - you can even strafe down a Shore battery if you have enough bullets.