A History of Research and a Review of Recent Developments
A History of Research and a Review of Recent Developments
A History of Research and a Review of Recent Developments
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Table 2.3<br />
Below-ground explosions 39<br />
Chadwick, Cox <strong>and</strong> Hopkins used a theory <strong>of</strong> spherical plastic-elastic flow<br />
in ideal soils to study the formation <strong>and</strong> size <strong>of</strong> the camouflet chamber in deep<br />
explosions <strong>and</strong> produced numerical values for the variation <strong>of</strong> camouflet radius<br />
with time during the first expansion phase for explosions at a depth <strong>of</strong> 100 feet<br />
for three idealized soils: fully saturated clays, partly saturated clays or mixed<br />
soils, <strong>and</strong> dry s<strong>and</strong>s. The first <strong>of</strong> these has a cohesion >0 <strong>and</strong> an angle <strong>of</strong> internal<br />
friction=0; the second has values <strong>of</strong> both constants >0; the third has a cohesion=0<br />
<strong>and</strong> angle <strong>of</strong> internal friction >0. The numerical results for the explosion <strong>of</strong> a 1<br />
lb spherical charge (radius 0.137 ft) at a depth <strong>of</strong> burial <strong>of</strong> 100 ft were as given<br />
in Table 2.3 (figures are given for the end <strong>of</strong> the first expansion).<br />
These results show that the main influence on camouflet radius is the angle<br />
<strong>of</strong> internal friction, because when φ>0 the figures coincide, irrespective <strong>of</strong> the<br />
cohesion. When φ=0 there is a noticeable increase in camouflet size.<br />
There is a good description <strong>of</strong> the underground nuclear explosion in the<br />
book on the effect <strong>of</strong> nuclear weapons, compiled <strong>and</strong> edited by Glasstone <strong>and</strong><br />
Dolan [2.9]. In their analysis <strong>of</strong> shallow explosions they point out that the<br />
ground surface moves upwards in the shape <strong>of</strong> a dome, which disintegrates<br />
<strong>and</strong> throws soil <strong>and</strong> rock fragments upwards. Much <strong>of</strong> this material falls back<br />
into the crater, entraining air <strong>and</strong> dust particles which are carried downwards<br />
<strong>and</strong> then outwards to produce a ‘base surge’ <strong>of</strong> dirt particles. It is carried<br />
downwind, <strong>and</strong> can occur several minutes after the original explosion.<br />
Meanwhile the ‘main cloud’ <strong>of</strong> the explosion, containing exp<strong>and</strong>ing gases<br />
<strong>and</strong> vapours, rises to a height <strong>of</strong> many thous<strong>and</strong>s <strong>of</strong> feet.<br />
They describe an underground nuclear explosion <strong>of</strong> 1.7 kilotons in 1957 at a<br />
depth <strong>of</strong> 790 feet. During the hydrodynamic phase <strong>of</strong> the explosion, when the<br />
shock wave is initiated <strong>and</strong> begins to exp<strong>and</strong> outwards, the original chamber in<br />
which the bomb was placed exp<strong>and</strong>ed outwards to form a spherical cavity 62<br />
feet in radius. Eventually this cavity could not support the overlying soil <strong>and</strong> the<br />
ro<strong>of</strong> fell in to form a tall ‘chimney’ extending about 400 feet upwards from the<br />
burst point. Very roughly, the volume <strong>of</strong> the cavity was proportional to the cube<br />
root <strong>of</strong> the energy yield (assumed proportional to the equivalent weight <strong>of</strong> the<br />
charge in TNT). For contained explosions the constants for granite are 35 feet/<br />
KT 1/3 , <strong>and</strong> for limestone 25 feet/KT 1/3 . These figures apply to burst depths below<br />
2000 feet. From the rough data assembled in this way it was concluded that in<br />
deep underground tests, where radioactive gases must not be allowed to escape,