"Even despite the size of these bombs they still weren't powerful enough to penetrate some of the U-boat pens due to the thickness of the concrete used in their construction (e.g. Hamburg and Bremen). I used to work in Germany and visited a client in Bremen once who's office block was built on top of an old U-boat pen. The effort that would be required to demolish the old U-boat pen was not considered worthwhile and obviously it was considered a strong enough foundation to build on."
Glen,
The idea behind Tallboy, and the later Grand Slam was not to penetrate in that way. Barnes Wallis designed the bombs to deliver a seismic explosion - that is to penetrate the ground to a great depth and deliver a massive seismic shock wave like an earthquake - hence the name 'earthquake bomb'.
The best example of the correct use of the bomb was against the Bielefield Viaduct, when over 100m of the viaduct collapsed into the subterranean crater caused by the Grand Slam/Tallboy combination. The bombs were fitted with delayed action fuses to prevent an explosion before they had penetrated the ground fully.
"It was an extraordinary weapon, an apparent contradiction in terms, since it had at one and the same time the explosive force of a large high-capacity blast bomb and the penetrating power of an armour-piercing bomb. On the ground it was capable of displacing a million cubic feet (29,000 m?) of earth and made a crater which it would have taken 5,000 tons of earth to fill. It was ballistically perfect and in consequence had a very high terminal velocity, variously estimated at 3,600 and 3,700 feet (1,100 m) per second (1,100-1,130 m/s or about 2,500 mph / 4,000 km/h), which was, of course, a good deal faster than sound so that, as with the V-2 rocket, the noise of its fall would be heard after that of the explosion."
W. J. Lawrence
