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Blasting challenges in advanced vibration management

Orica recently completed a blasting project in the Brisbane CBD, working within tight environmental constraints. The civil development project serves as an interesting case study to the construction and quarries sectors alike, due to the application of blasting knowledge and specialist products and the use of sophisticated environmental measurement and online reporting techniques.

When Orica partnered with Mainland Civil on the 77-storey high-rise development Infinity Tower, it was realised that the project would face challenges related to advanced vibration management. These included strict compliance with legislative requirements, environmental impacts and considerable consultation with the site?s urban neighbours.

The Infinity Tower stands 236m tall, making it Brisbane?s tallest residential tower. The foundations are equally impressive, sinking more than 30m below street level. Neighbouring buildings are both close and sensitive, including a heritage listed medical forensics building, glass facade office buildings and other residential towers.

Civil engineering contractor Mainland Civil was responsible for the excavation and shoring of Infinity Tower?s basement and foundations and engaged Orica to partner with it during the phase of blasting the foundations after geotechnical information indicated that a very hard, massive phyllite would be encountered during the excavation.

Mainland Civil contemplated traditional non-explosive excavation techniques. Due to the massive nature of the phyllite, these were discounted because of the potential for extended public nuisance (noise from rock breakers) and the slow progress of excavation.

Mainland Civil invited Orica to conduct a desktop blast evaluation. This blast evaluation process identified a number of key requirements in minimising disturbance to neighbouring buildings due to ground vibration.

Following this, Mainland Civil and Orica developed an advanced construction methodology that would, on the basis of a seismic survey, facilitate the use of explosives in the excavation process.

{{image2-a:r-w:250}}?Once limits were established in consultation with neighbours and in compliance with legislative requirements, a seismic assessment was conducted to determine the ground vibration transmission characteristics of the site,? said Mark Della Sabina, senior technical services engineer with Orica. ?This involved firing a number of small explosive charges and monitoring the vibration waveforms generated at key monitoring points around the site.?

Using this technique, Orica was able to create a site-specific vibration model to calculate the maximum instantaneous charge (MIC) weights to be used in the excavation.

This information was key to designing blasts that would not exceed the vibration limits.

?We used state of the art environmental monitors around the site to capture all vibration data and also installed a crack monitor on the rear shoring wall to gather as much information as possible about the environmental impacts of the blasting,? he said. While this was an especially sensitive project, Della Sabina said many quarry operators faced similar challenges and could use similar strategies to limit environmental impacts and monitor their operations.

Along with its specially formulated small hole diameter bulk products, Orica used its most technically advanced initiating system in this sensitive project ? the i-kon electronic blasting system.

?I-kon provides complete timing flexibility coupled with a total burning front whereby all detonators receive the ?fire? command at the same time and thereafter operate on internal power for their designated time,? Della Sabina explained.

?This meant that multiple separate charges could be loaded into each blast hole as the risk for cut-offs is eliminated.

?By deck loading, we could maximise the size of each blast while controlling vibration output through limited MICs in each deck.?

A total of five blasts were fired at the excavation site. The monitoring equipment established by the Orica team recorded the vibration data from each blast event and automatically uploaded it to the NCVIB (Nitro Consult) web portal. This allows 24-hour access to the results from anywhere in the world. All the blasts performed were in line with the vibration model created during the blast evaluation process.

?We refined the vibration model by adding the data from each blast to allow an increase in maximum charge weight as the job progressed,? said Della Sabina. ?All of the blasts came in well under the imposed vibration limits.?


Della Sabina said a quarry could benefit from using Orica?s monitoring system, thus permanently monitoring vibration, air blast or other environmental factors 24 hours a day, seven days a week, if required.

?This can be very useful in providing neighbours and the community with information about your operations,? he said. ?Not only can it show that you are operating within the limits, but we often find that natural events, such as wind, thunder or other businesses and events, are having a greater impact on perceived overpressure and vibration levels than the blasts themselves.

?This can give people a realistic perspective of the limitations placed on your operations when it comes to blasting.?

Della Sabina encouraged quarry operators to consider how similar strategies could be used at their sites to limit environmental impacts and monitor operations.

?Our approach would be the same for quarry operators ? starting with planning and evaluating the site and project, applying the most appropriate blasting techniques and technologies and then monitoring the results,? he said.

Source: Orica Quarry Services

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