A History of Research and a Review of Recent Developments

36
The detonation of explosive charges
Table 2.1
For explosives other than TNT it is necessary to multiply the expressions
for p 0 by a further factor E, where E is an energy factor determined by the
type of explosive. Lampson gave these for some common explosives of the
time (mostly outdated now) (see Table 2.2).
The coefficient f in Eq. (2.3) is related to the depth of burial, H, by another
curve given by Lampson in reference [2.12]. At shallow burial depths the
earth above the ‘chamber of compression’ formed by the explosive gases will
break up and be projected into the air, forming a crater. As a result the energy
available to push the shock front through the soil will be lowered, and the
value of p 0 from Eq. (2.3) will be reduced. However, if the pressure of the
confined gases is not sufficient to lift the overburden of soil the chamber will
be left intact, forming what is known as a ‘camouflet’. At the time of Lampson’s
review of experimental results the relationship between f and H for clay silt
was as shown in Figure 2.11, with an optimum value of f (=1) occurring when
H/W 1/3 =2 ft/lb 1/3 .
Thus a charge of 1000 lb of TNT would have to be buried 20 feet to ensure
the formation of a camouflet in this type of soil, whereas a charge of 500 lb
TNT would need to be buried 16 feet. At depths of burial greater than the
optimum there was a slight reduction in p 0 (i.e. f>1) because the effect of the
reflected shock wave from the underside of the soil surface began to diminish.
This reduction was often ignored in design calculations.
Table 2.2