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Precision blasting clears reservoir trench

Mechanical trenching or excavating machines are highly accurate but painfully slow. Blasting is a viable alternative for precision excavation projects, but it requires extensive experience and planning, and reliable execution. A miscalculation or failure to account for factors such as geology or environmental restraints can cost a construction firm financially and result in failure to meet deadlines. However, performed well, blasting can significantly reduce project costs, meet technical requirements and increase productivity to ensure projects remain on track.

In mid-2011, Orica Quarry Services faced the challenge of blasting sandstone to create trenches and valve pits for a new reservoir in southeast Queensland. Contractor Stirloch Constructions required Orica to minimise the risk of damage to infrastructure within 10 metres of the blast, promote fragmentation and maintain trench wall integrity.

The New Beith reservoir has been built to service the water infrastructure needs of fast growing communities in the southwest Logan region, in southeast Queensland. A population of up to 120,000 people is expected in the area over the next 20 years.

The Logan Water Alliance ? comprising Allconnex Water, Tenix, Cardno and Parsons Brinckerhoff ? contracted Stirloch Constructions to design and construct a 20-megalitre water reservoir, and connecting pipework at the site. The project progressed well, with works completed in December 2011, two months ahead of schedule.
Stirloch Constructions initially opted to use mechanical trenching and rock breakage machines to excavate the trenches and valve pits. However, the machines could not work fast enough; after only 30 metres of excavation work, pipe construction had caught up to the development of the trenches. Planning predictions indicated this problem would get worse as the trenches got wider and work began on larger valve pits.
The firm was also incurring high operating costs and facing challenges due to the limited excavation capabilities of its rock-digging equipment on-site. To keep the project moving and free up resources to perform other tasks, Stirloch Constructions decided to consider blasting.
The firm had concerns about the approach, including whether blasting would be as cost-effective as mechanical breakage. It needed to be sure the blasting would achieve the required accuracy and depth of trench, as well as yield the required fragmentation for increased productivity. Stirloch Constructions was also concerned that flyrock could damage equipment and infrastructure on-site.

In May 2011, Stirloch Constructions contracted Orica Quarry Services to blast the remaining 130 metres of trench, totalling 1065m3 of sandstone. The engagement included assessing, evaluating, designing and executing the blasting program.

?Our objectives were to deliver the project quickly to save Stirloch Constructions time and money, deliver an accurate blast that resulted in excellent fragmentation, limit downtime on the reservoir, and ensure there was no flyrock that could damage equipment or infrastructure,? said James Tiedgen, a mining engineer for Orica.
?To meet these objectives, we broke the project into assessment, evaluation, design and implementation phases.?

The Orica team surveyed the area in depth to understand the location and determine how they would meet the customer?s needs. To ensure alignment with the site crew, the Orica team spent considerable time establishing expectations and explaining the process. Orica provided in-depth detail about how it would undertake the blasting operations.

?Once we had completed the assessment phase, we conducted a formal evaluation of the project. The technical, commercial and operational teams worked together to develop a proposal and work methodology,? James explained.

Orica also wrote a paper that assessed the risks associated with the blasting approach and how it planned to mitigate these. These documents were then shared with Stirloch Constructions and the Logan Water Alliance.

The precision design had to ensure good fragmentation while mitigating the impact of blast vibrations and flyrock on nearby site infrastructure. Using Orica?s SHOTplus-i Pro blast design software, the engineer developed a full model of the blast and its complexities. The software enabled the engineer to import 3D survey plans of the site from an AutoCAD file to improve planning.

?Our blast design enabled us to fulfil Stirloch Constructions? requirements with a single, precisely calibrated blast,? James said. ?We
used a small crew of four to deliver the project and a mobile manufacturing unit that loaded most of the holes with bulk explosives in two hours. In addition, we used some packaged explosive product where accuracy was most critical.?

To minimise the possibility of flyrock, in addition to the design parameters, the team covered the entire blast with blasting mats ? five centimetre (5cm) thick interwoven strips of rubber. They also collaborated with Stirloch Constructions on minimising downtime and ensuring other events on-site, such as the arrival and departure of trucks to pour concrete slabs for reservoir walls.

For the blast, a precisely placed and weighted combination of Centra Gold bulk and Senatel Powerfrag packaged explosives were used.
The mobile manufacturing unit loaded charges of as little as two kilograms of Centra Gold GT ? a high-energy, water-resistant pumped emulsion blend ? at a precise rate into more than 300 blast holes. Centra Gold GT was used for its ability to resist the adverse weather conditions present in the area while delivering the strength and energy to fragment the rock to the required level.

The region?s geology also presented an obstacle to delivering the required fragmentation, as the massive sandstone geological structure was expected to produce significant cap rock at the collar of each hole. The design called for the use of strategically placed pocket charges of explosives in the collar, or stemming zone, of each hole. These charges had to be loaded in quantities that would keep vibrations and flyrock under control. To achieve this objective, Senatel Powerfrag packaged emulsion explosive was employed where a second deck of Centra Gold would provide too much energy for the necessary controlled blast. Senatel Powerfrag delivers excellent fragmentation, and its packaged cartridges allowed precise quantities to be loaded into each hole.
Finally, Exel Connectadet detonators were selected to produce an initiation sequence that would promote fragmentation and reduce the overbreak of trench and valve pit walls.

The single blast across all trenches and valve pits at the site was the culmination of a week-long project that only disrupted the site for two days. This fast completion saved the customer the time and expense of the delays it would have incurred by staying with its trenching and rock-breaking machines.

Tony Geurts, project manager at Stirloch Constructions, described the blast as ?exactly what we wanted, nearly to the millimetre?.
The controlled blasting using a combination of precise designs and blast mats ensured there was no flyrock or damage to existing pipes, trenches or site infrastructure. The blast also delivered excellent breakout along the trenches and valve pits, delivering walls that were as geometrically precise as the previously machined areas. The blast also left enough room around the structures for the required formwork, but eliminated overbreak and the need for expensive fill.
The charges produced fragmentation that Geurts described as ?like digging powder?, and enabled Stirloch Constructions? equipment
to easily remove the blasted materials.
By turning to Orica, a business with a long history of investing in tools to calculate the blast impacts, and with extensive experience in managing explosives, Stirloch Constructions achieved its project objectives without incurring delays and extra costs.?

James Tiedgen is a mining engineer for Orica Quarry Services.

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