A History of Research and a Review of Recent Developments

Introduction xxvii
explosions/ structure interaction over a vast range of conditions. In an earlier
book, Buried Structures, the author has reviewed their test rigs and the results
of their research on the effect of local explosions on underground thick-walled
and thin-walled structures.
Much of the experimental work has been aimed at reinforced concrete
structures, because so many of the protective structures that are built to
withstand the effect of conventional bombs or shells are constructed from
this material. In earlier times the structures of castles and forts were exclusively
of stone, or of combinations of stone and earth. The Romans, for example, in
their city fortifications used two strong walls of masonry separated by a gap
of six metres, and the gap was filled with well rammed earth and loose rock.
By the fifteenth century the advent of the cannon firing balls over 30 lb in
weight meant that a wall of masonry two or three metres in thickness could
not resist the effects of a battery discharging three to five hundred balls over a
surface about eight metres square. This led to the development of ‘relieving’
arches within the body of the wall and internal abutments buried in the adjacent
earthworks. Often the external surfaces of the walls were further protected
by ramparts of earth and wood. This increase in strength proved effective
until the coming of the exploding balls and shells, which was discussed earlier.
The local explosion in the midst of the ramparts caused havoc and disorder,
and this led to the replacement of castle and wall by the bastioned fortress.
During the seventeenth century there was an absorbing conflict between the
developers of such protective structures as the ‘star’ fortress and the creators
of systematic methods of attack. The genius of the age was Sebastien le Prestre
de Vauban, Marshal of France, who became the outstanding technical director
of siegecraft and fortress defence at the same time. He also built bridges,
roads and canals, and created the French Corps of Engineers in 1676. In the
eighteenth and nineteenth centuries warfare was brought more into the open,
and commanders such as Napoleon and Frederick the Great avoided attacks
against permanent defences. However, the invention by the French at the end
of the nineteenth century of smokeless powder and high-explosive artillery
shells greatly increased the attacking power and the need for strong land
fortifications. This coincided with the rapid growth of concrete as a replacement
for stone, earth and masonry.
Later, the use of thick reinforced concrete construction was vital in resisting
the effects of direct hits from aerial bombs weighing 1000 kg and above. The
French carried out field tests during the construction phase of the Maginot
Line before the Second World War, and they deduced that 3.5 m of reinforced
concrete could withstand three direct hits at the same point of impact. The
ultimate fortifications of this period were the massive U-boat pens built by
the German Army in Brittany to protect their submarines against aerial bombs.
After the end of hostilities in 1945 the USA conducted a Strategic Bombing
Survey, and the reports of this work contain a record of interviews, reports
and photographs, including the effects of the conventional bombing of German