Onan,
That's quite true and it was the intention to use them as deep penetration bombs to create a localised earth quake. The decision was taken to use them directly against hardened targets such as U-boat pens as they were the only weapon capable of defeating them. The Tallboy actually proved more effective as the Grand Slam tended to detonate prematurely or breakup. In the case of the Valentin bunker, a direct hit by a Grand Slam (which did not penetrate) caused so much concussion that it brought down the internel crane and overhead gantries that it rendered the facility useless. A look at U-boat pens at Brest will deliver similar results. You can see where holes have been filled or simply covered up.