Abstract
Spall, as identified by minus 1 g dwells on vertical accelerometers followed by impulsive rejoin signals, has been observed in ground motion data recorded in alluvium. These data resulted from explosions ranging from fully contained to surface burst configurations. The variability of the loading environments and the observational data support the conclusion that the phenomena result from multiple mechanisms. The data show that relatively low accelerations (<20 g) accompany the process, but owing to the long dwell times involved, resulting displacements may be large. Five spall models are discussed and shown to operate in some aspect of the data base. These mechanisms include (1) simple tensile reflection from the free surface, (2) Rayleigh/shear waves from surface sources, (3) spherical and cylindrical wave field divergence, (4) positive and negative phase airblast effects on porous materials, and (5) material properties of porous particulate matter.
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