A History of Research and a Review of Recent Developments

Loads on underground structures 85
have surface structures above them, and in certain circumstances the presence
of such structures can affect the peak incident overpressure reaching the
underground structure for a large air burst.
It is clear that the loading on underground structures is influenced by the
shape and flexibility of the structure. For example, because of the support
that can be mobilized from well-compacted soil, it is possible to use thinwalled
metal construction for underground structures or for the linings of
temporary trench shelters or earthworks. Here, the structure is formed from
thin sheet elements that in themselves have low structural strength, but which
can support large overpressures if properly embedded in soil. Corrugated steel
culvert material used in civil engineering is a good example. The limitation to
load-carrying ability is often the inward buckling of the walls of the structure,
or the tensile rupturing of sheeting due to large membrane forces, rather than
collapse by the formation of ultimate moments in structural members.
For heavier, thick-walled construction, typical of reinforced concrete, there
are broad, approximate quasi-static loading conditions that can be used for
initial design calculations without recourse to complex analysis or personal
computers. For example, in a shallow-buried rectangular structure, with a
cover depth less than one-half the span of the roof slab, it can be assumed
roughly that the roof and floors are loaded with the ground surface peak
overpressure, p 0. In a dry soil the effect of attenuation might limit the side
pressure on the walls to 0.5 p 0, but in saturated conditions with a high water
table this would increase to p 0. The loading conditions on roof elements would
be the full overpressure, p 0, plus the dead load of the roof structure, the soil
cover, and any debris that might fall on the surface. For side wall elements the
static earth and water pressure would need to be added to the blast overpressure.
Examples of reinforced concrete underground shelters to withstand unclear
explosions, designed on this rough quasi-static basis, were given in the UK
Home Office publication, Domestic Nuclear Shelters [4.15]. It is interesting
to note that in the UK domestic nuclear shelter design of the 1960s and 1970s
the loading overpressures were limited to three atmospheres (315 kp a or 45
psi), because at pressures above this the effects of Initial Nuclear Radiation
would probably kill the occupants.
Perhaps we should pause for a moment and consider radiation doses, since
they are a fundamental part of nuclear explosions. They are measured in various
units such as roentgen, gen, rad or rem, depending on the precise kind of radiation.
We will only consider the roentgen here. Acute radiation sickness is produced
by a brief exposure of the whole body to 50 roentgens; for exposures between
50 and 200 there could be weakness and fatigue in addition to sickness, but
only about one person in twenty would need medical attention. Between 200
and 450 roentgens there would be moderately severe illness, and perhaps one
quarter of the people so exposed could die. Exposures of over 600 roentgens
would lead to death in less than 14 days in almost every case. Exposures in
excess of several thousands causes severe brain damage and death within hours.