Wambo Coal AEMR 2011-2012 - Peabody Energy

• spectrograms were constructed using a Matlab procedure from both the ground monitor velocity
signals and the accelerometer results so that comparisons of activating and induced vibrations
could be made as seen in Figure 10.
7
Figure 10: For the event of 16/12/2011, the ground monitor spectrogram for the vertical recording is on the left and the resultant
ceiling spectrogram is on the right. Note that the scales of the two spectrograms are different. It will be seen that the ground vibration
had a strong vertical pulse at 16 Hz and that the ceiling resonated at about 13 Hz. The closeness of these values explains the large
amplification.
All the diagrams for the two monitored events are reproduced in the appendix.
It should be emphasised that the results may not explain the behaviour of the whole of the buildings. However, every
effort was made to make them representative and, in the case of the New House ceiling, the location was chosen to
coincide with fabric which may have been damaged previously by blast vibration.
The ground monitor geophone is mounted with axes north-south, east-west and vertical. The horizontal axes have
orientations similar to the walls of the house, making it easier to make comparisons.
2.2.2 NEW HOUSE
Readings taken in the New House are summarised in Table 1. The orientation refers to the direction of movement.
Event ident.
Channel
no.
Table 1 – New House 16/12/2011
Orientation Max ‘a’, Max ‘v’, Max. ‘s’, mm
mm/s² mm/s
Dominant
frequency,
Hz
House 1 E-W 676 8.4 0.21 30 2.3
2 N-S 712 8.7 0.14 13 4.6
3 V 1630 21.6 0.28 13 16.6
Velocity
amplification
in building
Grounds E-W 1.9 12.5
N-S 3.6 15.5
V 1.3 16
The dominant frequencies tabulated are not sharp peaks and, in some cases, there are a number of pulses at different
frequencies which probably relate to the blast design parameters. The spectrograms from which the frequencies are
taken are appended.
It can be seen clearly that significant movement occurs in elements of the house when resonance is present. The
movement in the ceiling has a particularly high acceleration, equivalent to 0.16 g (where g = acceleration due to
gravity). On its own, this value suggests that fastenings of the ceiling were subjected to a 16% increase in load but this
should not have led to adverse effects. However, if values increased proportionally for higher levels of blast vibration,
then a PPV of 5 mm/s might result in a ceiling load increase of 64%. Such an extrapolation is only very approximate
because of the many factors involved, but it does show that the ceiling is vulnerable at a low PPV of 5 mm/s if
frequencies are not controlled.
April 2012
Wambo Homestead
Vibration vulnerability