Maxam?s senior technical adviser Mark Dowd was first approached by Peter Allitt, the manager of Holcim?s Bli Bli Quarry, in October 2012 with a simple request: to provide some guidance with a non-standard blast design.
Maxam has supplied this Holcim site on the Sunshine Coast for a number of years and has been active in the site?s drill and blast process. As part of Holcim?s commitment to sustainability and rehabilitation of sites, the company is undertaking a project at Bli Bli to develop a wetlands environment in an old stockpile area. This required removing a portion of material to allow the development of a shallow water area. This might not seem like a large problem in a working quarry but the location determined that this was not a typical blast.
{{image2-a:r-w:450}}The blast was within 60m of critical infrastructure on the site, as shown in Figure 1. There were five sensitive locations in this radius as well as numerous other items of infrastructure within 150m critical to the operating quarry that any blast design and execution process had to consider. The area to be blasted is highlighted in red.
DEFINING BLAST AIMS, RISKS
Allitt and Dowd discussed the options for the blast and it was emphasised by all parties that for a successful outcome it had to be conducted with the utmost care and attention to detail. The initial discussions centred on defining the aims of the blast as well as identifying the risks that would have to be minimised to acceptable levels. The aims were established as being the conducting of a safe process that produced blasted rock of sufficient size and dig ability to enable a dozer to move and shape the material to the required level.
The need to meet Bli Bli?s community expectations of minimal environmental impact and minimise potential damage of the sensitive infrastructure from vibration, air overpressure or impact (flyrock) were stated as requirements for the blast to proceed. As in all drill and blast processes conducted by Maxam on Holcim sites, this was achieved by:
1. Adhering to all legislative requirements and Holcim and Maxam procedures.
2. Conducting detailed risk assessment prior to design plus the required Maxam technical services and Maxam blast loading risk assessments.
3. Minimising risks identified in each stage of the process, ie design, drilling, explosive loading and firing processes.
{{image3-a:r-w:250}}The detailed risk assessment identified that control of the process and clear communication of the process were essential to all involved. Controls were identified and put in place to achieve a successful outcome.
The site also committed to building a dirt barrier on the southern side of the blast area to offer protection to the gas tanks and asphalt plant. Because this blast was located close to numerous work places, special care on evacuation to safe distances of these personnel was planned.
The risk assessment process also identified that one of the main risks to a successful outcome was the management of the settlement pond adjacent to the blast. It was decided to dewater this pond prior to blasting for two reasons: to enable scanning of the face to ensure burden control could be established; and to minimise displacement of the water into adjacent areas during the blast.
BLAST DESIGN, CONTROL PROCESS
The design was commenced and greater than normal front row burdens and stemming were used to control movement and remove potential for flyrock creation. In this blast flyrock was defined as any rock leaving the blast area. It has always been acknowledged that rock properties play a significant role in determining blast outcomes and Blast 820 was no exception, with variable rock expected to be encountered that had to be incorporated into the design process.
Previous blasting in adjacent areas had shown that the material required powder factors of more than 1.0kg/m3 to be dug easily. This was considered to be high for this blast, plus the fact a dozer was to be used to push the material. The material was required to be broken but to just heave and remain in place. A lower energy level and a timing design that could achieve this was used in the design and firing of this blast. Electronics were considered but Maxam determined that all risk levels would not increase and the goals of the blast could be achieved with the RIONEL non-electric system at a considerable cost saving to Holcim.
{{image5-a:c-w:600}}As control of the process was required, adjustments to the blast and timing design did occur as the process progressed, especially on the day of loading and firing to account for variability in rock and blast hole conditions encountered. Any well executed blast process must be robust enough and the personnel involved must have the required knowledge to allow for variability or change.
This was only possible with clear communication between the Maxam blast designers, drillers and Holcim site personnel. In Blast 820, this was found to be the case. The ground was more variable than first thought and as such care was taken by the driller in the collaring of blast holes, as well as passing on detailed drill logs to Maxam that allowed explosive loading to be varied where required.
BLAST OUTCOME
The blast was fired successfully with no damage or effect to the surrounding infrastructure. The blast heaved well but no rock left the blast area. Site environmental measurements, vibration and air overpressure were well within limits and no community concerns were communicated to the site. The result of the blast was excellent for the site because the fragmentation and heave achieved allowed the removal of the material in less than scheduled time. The result also allowed the dozer to shape the area easily with minimal effort.
{{image4-a:c-w:600}}The blast process was reviewed after firing and excavation to ensure all the aims had been achieved. It was recognised that the design and execution of the blast achieved a result at the upper ends of expectations. No infrastructure was affected and Allitt said he particularly noticed the awareness of all parties of their requirements and risk management responsibilities. It was evident that clear communication had occurred and where changes were required to manage risk these had happened. This has allowed Maxam and Holcim to achieve their mutual goals in Blast 820 at the Bli Bli Quarry.
Source: Maxam Australia (with thanks to Peter Allitt, Holcim Australia)