Blasting Methods

There are hundreds of explosive materials which are usually a strongly reactive substance containing a very large amount of chemical potential energy which can produce a chemical molecular energy avalanche.
Explosives are categorised as Primary, Secondary or Tertiary.
Primary explosives such as Nitroglycerin and TNT are highly unstable compounds to be handled with considerable care. These can be used as detonators.
Secondary explosives are unstable substances which require a detonator to initiate an explosion.
ANFO is one of the most commonly used primary blasting materials. It is made from Ammonium Phosphate fertiliser mixed with liquid deisel petroleum and can produce a very large explosion when detonated.
This became common in agricultural and mining industries and is used to either remove unwanted rock or extract fragmented rock.

Blasting at Mt Coot-tha Quarry

To be able to blast the strong rock in the Mt Coot-tha Quarry, requires strong explosives.
100 or more holes are drilled deep into the rock in a lattice or array format with consideration for the finished structure.
Explosive material is loaded into the holes with detonators which are electrically activated using a Shot Firing computer device.
Each Blast design specifies the correct shot fire sequence, to allow a large section of rock to be fragmented.
This design uses a travelling shock wave to assist with the shattering process. Hence the timing of each hole explosion is critical, not only to the fragmentation process but also to reduce any damage to residential or business premises.

The blasting causes a shock wave which travels through the ground for some kilometres and often continuing into residential areas. Sometimes following seams of bedrock which act as a duct.
When the shock wave reaches the surface it can be measured using a " Geophone " which is a 3-dimensional microphone-transducer system translating vibrations into electrical signals which are be measured.
This measurement equates to the velocity of ground particles with blast ground vibrations measured in mm/second (peak particle velocity).
Associated with the blast there is also an atmospheric effect commonly known as a sonic boom, the blasting industry calls this an overpressure blast sound wave which is measured in Decibels.

Residential Area Effects

Blast Effects on residential areas are limited to a specific particle velocity such as 5mm/second (ppv) plus an overpressure sound level below 115dB.
At Mt Coot-tha, all the blasting is mostly done below ground level and hence they never exceed their overpressure limitation (which we normally ignore).
There is also a 2mm/second vibration limit for QLD TMR Department Historic Building and Cemetery Monuments and buildings of special value or significance. Plus 10mm/sec absolute maximum for all residential homes..

Blast Vibration and General Noise Limits for the Mt Coot-tha Quarry

Without any community consultation, the Brisbane City Council Mt Coot-tha Quarry life was extended until 2015 for the purposes of constructing the Inner City Bypass general traffic roadway system, which was completed in 2002.
In spite of all efforts for consultation, the BCC totally ignored all the residents and Cr Magub by extending the Quarry mining life till 2032.
The original SR0041 approval had no expiry date and never lapses.
As defined in Schedule F, this approval allows the Mt Coot-tha Quarry to blast at levels without any legal upper limit on every 10th blast.
So the quarry could do 9 medium sized blasts of any size or kind, and then let rip with a gigantic blast on the 10th occasion and still remain within their environmental approval.
There was no limit to the number of blasts that could be done in any time period and no limit on the total number of blasts.
However whilst there is a noise limit which is exceeded on almost every blast, the BCC and MCQ appers to totally ingnore this regulation.

Current Blast Vibration Limits - Maximum Allowed Particle Velocity in mm/second

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Blasting Methods   Blasting at Mt Coot-tha Quarry
Residential Area Effects;   Blast Vibration and General Noise Limits for the Mt Coot-tha Quarry
Example Maximum Blast Vibration Limits