Quarrying & Mining Magazine
Quarrying

Hard rock, tricky blasting

A new blasting strategy at its Bombay quarry south of Auckland is paying substantial dividends for quarry operator Holcim. Hugh De Lacy explains.

Cutting blasting costs by 10 percent was just one of the benefits that explosives supplier and contractor Red Bull Powder was able to bring to Swiss multinational Holcim’s Bombay quarry, which supplies the Auckland market with roading and concreting aggregates.
Last year Holcim was having issues with its blasting at Bombay in relation to fly-rock, fragmentation, unfired explosives columns, blast response and vibration.
Red Bull had been talking to Holcim about the latter’s problems for some time before getting the call to come up with a revised blasting strategy.
Over a couple of weeks in August/September last year Red Bull formulated a draft plan, then sat down with Holcim to knock it into a working relationship that was “as much about getting a better result as it was about costs,” Red Bull’s technical manager, Nick Bastow, told Q&M.
Commissioned in 1997, Bombay is one of the biggest quarries serving the Auckland market, with production running at around 60,000 tonnes a month and 700,000 tonnes a year.  Its resource consents include strict noise limits, which meant that a complaint by neighbours of low-frequency noise coming from the site’s five crushers required a swift response.  Panels of autoclaved aerated concrete and cement-wood fibre had to be installed as noise barriers, though they were able to be assembled in a way that allows easy removal for maintenance.  Noise and vibration are the particular concerns of neighbours, and they get to communicate them to Holcim by way of a community liaison group the company set up when the quarry first went into operation.
Specific responses to the community’s concerns about noise have included beepers being replaced by flashing lights on reversing machines, the screen into the vibrating feeder of the primary crusher being modified, and over-size rock-breaking technology, such as choke-feeding of the primary crusher, being implemented.
Some metal screens were replaced with plastic or rubber ones, and a loader, rather than a dump truck, is used to load the crusher bins.
Bombay differs from most of the other big Auckland quarries –the likes of Stevenson’s Drury quarry, and Winstone’s Hunua – in being of basalt rather than greywacke.
Basalt requires a higher powder factor in explosive charges than does greywacke, and the 200-hectare Bombay site is further complicated by a mix of columnar, plate and scoriaceous rock and clay within the deposit.
“We have to adjust the blast patterns and powder factors between the two,” Bristow says.
“For this site we took a technical approach to blasting with significant emphasis on blast design, particularly in regards to vibration control.
“At Bombay this involves blast-specific timing plans and identifying the maximum instantaneous charge by the number of holes firing per delay.
“We then relate this to site-specific rock constants that define the vibration response.”
The first signature blasts allowed Red Bull to carry out vibration studies to define the most appropriate timing options and delays, and to optimise fragmentation.
bombay-quarry
A strong emphasis on safety ensured that all product was fired with no fly-rock.
Red Bull used face-profiling – laser scanning – for accurate mark-out and hole placement, following it up with video analysis of all blasts to identify any issues.
The explosive used was Red Bull’s high-energy, highperformance RedStar Bulk Emulsion, and electronic detonators ensured greater accuracy of the blast timing options.  This allowed the blasting team to fire bigger shots when space was available, helping to solve the over-size issues, with fewer overlaps and more flexibility in blast timing.  The net result was about a 10 percent reduction in drilling and blasting costs, attributable to an increased blast pattern from Red Bull’s high-energy emulsion and efficient use of the explosives.  “While that was a positive, it was not the main intent of the exercise,” Bastow says.
“Instead, one of the biggest side-benefits of the new blasting strategy has been the estimated 50 percent reduction in over-size rock from the column rock shots.
“This has halved the usual incidence to about 10 percent of the total material blasted.”
This increased the quarry’s production, with less product going to waste, and also increased digability and crusher throughput.  Other benefits are the improved site Relative Effective Efficiency (REE), and also better structure and communication between client and contractor, resulting in more streamlined operations, more emphasis on vibration control and a reduced average vibration rate.
The latter is an important factor in the quarry’s community relations, with the terms of the resource consent setting a maximum vibration (peak particle velocity, or PPV) of 5mm per second for 95 percent of the time.
redbull

Red Bull Plant and bulk truck, or Mobile Mixing Unit (MMU) – a 10 tonne delivery capacity and the ability to blend a range of products on the quarry bench.

The current site maximum designed vibration is 3.5mm a second.
To date the average PPV is 1.35mm a second across all monitoring locations for all Red Bull shots on site – well below the consented limits.
With a client base stretching from the Auckland Harbour Bridge to Pokeno, the Bombay quarry, which opened a second pit in 2008-2009, employs about 20 full-time staff to run the five rock-crushers, the Caterpillar 374 that is the main excavator, a couple of 41-tonne Caterpillar 771 dump-trucks, and a range of supplementary equipment.
As an overseas-owned company, Holcim had to apply in 2006 for
Overseas Investment Commission approval to add 61 hectares to the Bombay quarry and start the second pit that has since largely replaced the original one.
The commission had no trouble agreeing to the purchase, citing the new job opportunities it would provide, the increased competition in the sector, and the fact that Holcim introduced new capital to the country to acquire and operate it.  Given the notoriety of Auckland’s NIMBY (Not In My Backyard) reaction to quarrying developments – the city’s massive and ongoing aggregate requirements notwithstanding – Bombay’s blasting regime is of direct significance to the continuation of the Holcim operation.
And Bastow and Red Bull are confident their newly entrenched blasting strategy will mitigate rather than inflame the local community’s sensitivities at having a working quarry.
 

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