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Title: Explosions in snow
Authors: Livingston, Clifton W.
Keywords: Cratering
Blast effect
Publisher: Cold Regions Research and Engineering Laboratory (U.S.)
Series/Report no.: Technical Report;86
Abstract: Studies were made to establish means of predicting and optimizing the results of blasts in snow of the Greenland Ice Cap. A total of 141 test blasts were fired above and below the snow surface using three types of explosives. Seismic measurements were made of all shots, and 32 were instrumented for measurement of air-blast and/or under snow shock pressure. Crater cross sections were then mapped to beyond the limits of complete rupture. All crater data are presented in an Appendix. The change in texture of the snow was studied. Density samples of the disturbed and undisturbed snow were taken. The failure process in snow differs from that in glacier ice, frozen ground, rock, and certain types of soil. Characteristic features of this failure (referred to here as "viscous-damping failure") are: 1.) damping of the disturbance during the rise to peak pressure, and 2.) substantial recovery of stored potential energy during unloading. Both features result because air is trapped within the voids in snow. The snow is first compacted and driven outward as the gas bubble expands, and a primary cavity is formed. Melting at the cavity wall converts the skin of the compacted, fractured, and expanding zone to ice. The next event is implosion and disturbance of the original cavity. The walls of the primary cavity are displaced inward, and both the zone of skin-surface melting and the zone of compaction are destroyed. A sensitively balanced transition condition appears to exist at critical depth. The balance determines under what conditions fractures during the rise of pressure and the outward expansion of the gas bubble predominate over fractures formed as a result of implosion. Implosion is closely followed by a vortex motion within the snow and scouring action as the gas bubble emerges from the rising column defined by the vortex. This scouring motion largely determines the final shape of the apparent crater. Viscous-damping failure differs markedly from shock-type failure, which is characteristic of brittle materials, and from shear-type failure, which is characteristic of more plastic materials. During loading of the snow, a substantial proportion of the energy of the explosion is expended to compact and deform the material; during unloading much of the energy expended to compress air in the voids is recovered and re-expended in both fracture and flow. Tables, curves, equations, and example problems presented in the report make it possible, within the range of the experiments, to accurately predict any desired dimensions of the limit of complete rupture. Limits of complete rupture and limits of extreme rupture in snow are correlated empirically using cube-root scaling as a first approximation. Ranges of similar behavior and transition limits between ranges for blasts with small HE charges in 1958 surface snow are discussed.
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