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ETL 1110-8-11(FR)
15 Jul 91
anticipated damage caused by airblast at various
h. Water shock levels can be significantly
overpressure levels.
reduced by the use of bubble screens between the
area to be protected and the explosion source. This
j. The water plume created by underwater explo-
mechanism has been studied to prevent fish kills and
sions can be quite destructive to overhead structures;
structural damage to nearby marine structures
so much so, in fact, that the water plume is one
(Strange 1963, Keevin et al., to be published 1991).
Reductions of peak pressures by a factor of 5 have
mechanism recommended for military use in bridge
demolition (Armstrong 1979). Special attention must
been observed. The reduction mechanism appears to
be paid to this risk when overhead structures exist in
be two-fold: (1) the initial part of the direct wave is
the vicinity of underwater detonations.
strongly attenuated by the bubble screen, and (2) the
remaining portion of the direct wave is slowed down
so that the surface relief wave can catch up and
4. Water Shock Monitoring Equipment
reduce the effect of the direct wave.
a. Water shock pressures at a point of measure-
i. Care must also be taken to avoid nuisance
ment are equal in all directions (i.e., they are hydro-
damage from airblast generated by the detonation of a
dynamic). Thus, the orientation of a water shock
high explosive charge in shallow water. Figure 5
pressure gage is not important. The tourmaline
(from Swisdak 1975) provides isopressure contours of
crystal pressure gage is the preferred transducer for
the airblast generated by underwater explosions as a
function of scaled depth (D/Wl/3), scaled range
measuring water shock pressure because of its proven
(R/W1/3), and at a constant scaled height above the
reliability and accuracy, ease of use, high-frequency
water surface of 0.25 ft/lb1/3. In all cases, the charge
response, and low sensitivity to temperature effects
(Cole 1948, Tussing 1982). Tourmaline crystal is
should first be converted to its TNT equivalency
piezoelectric, which enables it to convert pressure
before scaling. The airblast data is available only
(force per unit area) to electric energy. It should be
down to 0.25 psi. Because of scatter in the airblast
noted, however, that piezoelectric crystals, are not
data used in constructing the isopressure contours, the
able to detect static or very slowly changing pressure
contours are considered accurate only within plus or
levels.
minus 30 percent. Table 2 (Johnson 1971) provides
Figure 5. Airblast isopressure contours at a height 0.25 ft/lb1/3 above the water surface from TNT spheres
fired at various scaled depths (from Swisdak 1975)
5
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