Wambo Coal AEMR 2011-2012 - Peabody Energy

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frequency limits but starts at a lower PPV with a “plaster” cosmetic limit of 12.7 mm/s from 2.5 Hz to 10 Hz, before
rising to a PPV of 50 mm/s at 40 Hz also.
1.2.2 APPLICATION OF LIMITS
One key factor in all these limits is that they apply either to maintaining comfort levels for occupants or guard against
even the slightest cosmetic damage in a pristine occupied building. The limits have also been set with no consideration
for the vulnerability of a particular building and are therefore an envelope of a “worst case” scenario: the “worst
case” would be when the vibration frequencies generated by a blast coincide with resonant frequencies in the building,
leading to substantial amplification.
The application of these standards has been explored in recent papers 1,2,3 . The work leading to the papers started with
detailed finite element structural modelling, but with limited monitoring of buildings; it was then extended so that the
induced vibrations of elements of the building could be directly measured, in the first instance with velocity-recording
instruments. Whilst this has led to more robust results in assessing the vulnerability of the buildings, the difficulties
of setting up such instruments on delicate fabric was limiting in its application.
1.3 The basis for the new approach
The long term monitoring at Wambo has shown few, if any, indications that vibrations were affecting the buildings,
with the exception of a single incident when there was possibly some slight ceiling plaster cracking in the main house
(“New House”) which could have been caused by resonant vibrations.
Building structures react to ground vibrations caused by blasting in a similar manner to their reaction to earthquake
ground vibrations. Vibrations are transmitted to the building from the foundation and the reaction of the building
depends upon the size and frequency of the activating movement and the inertia and frequency response of the
building. Air blast limits are such that buildings would normally not be affected even to the extent of what might be
experienced in strong winds. There is little or no evidence that air blast has caused damage in buildings at the distances
relevant to Wambo Homestead.
In design for earthquake actions, the vibration is normally considered in terms of acceleration units and these can
be readily transformed into building forces from Newtonian mechanics: this is the simplest, quasi-static approach
often used in the past for smaller and less important structures. For larger structures the response of the building
is taken in account by evaluating the response spectrum in accordance with the Australian Standard AS 1170.4: this
can be readily done for a new building built from modern materials with known properties and of a form which can
be readily modelled and analysed. Historical masonry buildings can be analysed approximately if full dimensional
details are known, but the unknown properties of the materials and the connections between elements do no yield
satisfactory results.
The inability to obtain usable results by calculation has led to the approach of directly measuring the vibrational
characteristics of a building.
As explained above, the previous building monitoring work was limited in application due to the type of instrument
available. New lightweight and very sensitive accelerometers have now been sourced and coupled to a seismic recorder,
so giving the best results yet obtained for building vibrational behaviour.
1.4 Application to Wambo
1.4.1 GROUND VIBRATION
The response of a building is dependent upon the magnitude, direction and frequencies of the ground vibration and
the orientation and characteristic frequencies of the building and its individual components. In general, it is found that
for a 19 th century building such as Wambo, the following characteristic frequencies are present:
• single storey building as a whole — 5 Hz to 10 Hz (new house);
• two-storey building as a whole — 2 Hz to 5 Hz (kitchen wing);
1 Jordan, JW, Sutcliffe D and Mullard J, Blast Vibration Effects on Historical Buildings, Australasian Structural Engineering Conference, Melbourne, June 2008 (revised and
published in the Australian Journal of Structural Engineering, Vol. 10 No. 1, 2009).
2 Jordan, JW, Monitoring Blast Effects on Historical Buildings, Australasian Structural Engineering Conference Sydney, August 2010
3 Jordan, JW, Mine blasting vibration and its effects on buildings and structures – implementing a frequency-based approach, Australian Earthquake Engineering Society
Annual Conference, Barossa Valley, November 2011.
Wambo Homestead
Vibration vulnerability
April 2012