Blast vibration dependence on charge length, velocity of detonation and layered media

• Mach waves are produced by most commercial explosives initiated in most rock types.
• These waves introduce a high near-field directivity of vibration within a uniform viscoelastic medium.
• Contrary to all previous predictions, this directivity implies that the blast vibration is not always dependent upon charge weight.
• Layered media can destroy the directivity. Under this condition, vibration could be dependent upon charge weight.
• The current understanding of vibration remains quite rudimentary.

Charge weight scaling laws predict that the peak blast vibration will always increase with charge weight. However, this prediction is not consistent with analytical models of vibration from a blasthole in an infinite viscoelastic medium. These models show that for a blasthole of fixed diameter, there are both near-field and far-field regions within the volume for which the peak vibration is independent of the charge weight provided the charge length is greater than approximately 0.25 times the blasthole diameter. For smaller charge lengths, the vibration approaches zero as the charge length approaches zero, which is logically expected. At first sight, these model predictions appear to be counter-intuitive, especially for far-field regions. However, the model also shows that increasing the charge length will increase the vibration amplitude, but only in highly directed regions within the medium-field; this could have implications for blast damage. Large sets of measured vibration data from underground and surface blasting were also analysed. It was found that the peak vibration measured underground due to underground blasting is not significantly dependent on charge weight, and this is consistent with model predictions for an infinite viscoelastic medium. However, the peak vibration measured at the surface and due to surface blasting was found to be strongly dependent upon charge weight, and this might well be due to geological influences such as near-surface layering. The present work illustrates that the current understanding of blast vibration remains quite rudimentary.