Rainer:

There is a photo of a "hole-in-the-roof" at the U-boat pen at Brest on page 189 in
"WOLFPACK: U-boats at War - 1939-1945, by Philip Kaplan and Jack Currie - 1997

Click on: [home.aol.com] "The Man-Made Earthquake: A short history of very heavy conventional aerial bombs." page 12 of 18.

"The original pour of the reinforced cement roof was about 12 feet thick. Successive
additions had increased the thickness to about 16 feet by the end of hostilites. A "fangrost" screen of close-spaced inverted U-shaped concrete beams about three
feet wide and five to six feet above the roof had been constructed over most of the machine shop. These beams served to deflect or pre-detonate bombs in the void
underneath. The underside of the roof was covered by a corrugated metal scab mattress which prevented detached concrete from falling onto the areas below. The
concrete was generally of average to poor quality which explains why post-war
photographs show little concrete adhered to the tangle of reinforcement rods hanging from the areas where "GRAND SLAMS" penetrated the roof."

Click on: [www.globalsecurity.org]
"Direct Strike Hard Target Weapon/Big BLU." - Bottom of page:

"In one raid on 27 March 1945, against the U-bootbunkerwerft "Valentin" submarine
pens near Bremen, two "GRAND SLAMS" penetrated 7 meters (23 feet) of reinforced concrete, bringing down the roof."

"These "GRAND SLAMS" were 26'-6" (7.7m) long. They weighed 22,000# (9972kg); 3'-10"(1.17m) diameter. The first one was dropped on Germany on 14th March 1945. It hit the ground about 80' from the target, but it created a crater over 100' deep. This bomb was used with great effect against viaducts or railways leading to the Rhur and also against several U-boat pens near Bremen."

In early summer 1945, three Boeing B-29s were modified to carry a 22,000# "GRAND SLAM" on external bomb racks under each wing between the inboard engine and fuselage. After experiments at Elgin Air Force Base in Florida, an order for fifty modified aircraft was placed. If the war hadn't ended with the dropping of the atom bombs, dual "GRAND SLAM" B-29s might have been available to see action over Japan by the first week of September."

FYI,

ROBERT M.