text: In 2013, Stephen Flower wrote that when dropped on buildings fortified with reinforced concrete the Grand Slam tended to break up on impact or go off too soon. Flower wrote that it was not surprising that the Grand Slam was limited in effect when dropped on concrete, when it was designed to fall on earth and then explode under the foundations of buildings, creating a  for the structure to subside into. The Grand Slam and Tallboy bombs did cause damage that conventional bombs were incapable of and had a great disruptive effect on the means of resistance available to the Germans, limiting the development of new weapons with which to continue the war. In 2013, Stephen Flower wrote that the earthquake bombs rarely caused mass civilian casualties and were the most effective bombs used in the war, until the advent of atomic bombs later in 1945.
  Operation Front Line/Project Ruby
  Beginning in March 1946, Operation Front Line (British title) / Project Ruby (US title), was a plan to investigate the use of penetration bombs against reinforced concrete targets. The  U-boat pens near Bremen were chosen, having been abandoned after the attack on 27 March 1945. Since 9 Squadron and 617 Squadron had been transferred to the Far East to join Tiger Force, Grand Slams were carried by Lancasters from 15 Squadron, into whose C Flight, the 617 Squadron crews not sent abroad were moved with their B.Mk 1 (Specials) and their SABS bomb sights. The Watten bunker, in the Pas de Calais, which has been used for Royal Navy Disney bomb tests, was bombed with Grand Slams.