Phase-the forgotten problem of blast vibration prediction
In terms of the environmental impact of blasting, regulators usually specify statutory limits on air overpressure (in terms of air blast) or particle velocity (in terms of ground vibration) either as absolute values that may not be exceeded or as a fixed percentage of allowable exceedances. These limits vary considerably and even within the same country, different authorities can apply different limits. The specified values are equivalent to the absolute maximum values achieved within the transient blast induced wave form, which in turn is known as the Peak Level. However, both Peak Particle Velocity and Peak Air Overpressure levels have historically been notoriously difficult to predict. All blast vibration time signals or transients are defined by having measurable components of frequency (amplitude and phase) which can be determined through a Fast Fourier Transform routine. It is impossible to define a vibration transient (either the ground vibration or even the much simpler waveform of air overpressure) from the amplitude of the frequency components alone; the phase of each frequency must also be known. In fact, it is not the absolute phase angle of each frequency component which is important, but the relative phase (or phase difference) of one frequency to another. Therefore, the relative phases of the frequencies within the transient have an impact on the amplitude of the transient. When two or more sources of vibration combine to form a single transient (such as an air overpressure signal produced by two or more blast holes), the resulting signal will not only depend on the amplitudes of the frequency components, but the relative phases of these components. This will determine which frequencies are amplified and which are attenuated, which in turn will have an effect on the amplitude observed at any point in the time signal or transient. This paper seeks to illustrate the problems associated with phase and to highlight the need for more research into this area. It will discuss the factors that relate both to the Peak Air Overpressure levels and to Peak Particle Velocity and discuss the similarities and differences that relate to these two separate environmental impacts of blasting.
|Conference||10th International Symposium on Rock Fragmentation by Blasting, FRAGBLAST 10|
Birch, W.J., Bermingham, L., Hosein, S., White, A, & Farnfield, R. (2013). Phase-the forgotten problem of blast vibration prediction. In Rock Fragmentation by Blasting, FRAGBLAST 10 - Proceedings of the 10th International Symposium on Rock Fragmentation by Blasting (pp. 425–432).