It’s not unusual at this time of the year for Australians from all walks of life to spend time in the country’s alpine regions, predominantly to enjoy the snow and the breathless beauty of an Australian winter.
Mount Hotham, in the Victorian Alps of the Great Dividing Range, is one such destination. The mountain, whose summit is about 1862m above sea level, is located 357km northeast of Melbourne, and is the fourth highest point in the Bogong High Plains and the sixth highest in Victoria.
The earliest European settlement in the Hotham area dates back to the 1830s, when the open plains of the foothills were initially utilised for grazing. In the 1850s and 1860s, Mount Hotham became part of the Victorian gold rush, when rich deposits were found on both sides of the Great Divide. In 1853, up to 8000 miners were drawn to the Buckland Valley. {{image2-A:R-w:250}}
In turn, the miners’ arrival led to the first signs of commercial infrastructure and road construction in the region and – indirectly – of a tourist industry (the miners originally used skis on the high plains to cross the divide to Harrietville and later mail men would ski the plains to make their deliveries).
Today, the Mount Hotham Alpine Resort covers 3450 hectares (ha), most of which is on Crown Land and is bounded on all sides by the Alpine National Park.
The resort constitutes less than 0.6 per cent of the Alpine National Park’s total area. The ski field accounts for 245ha (or seven per cent) of the total resort area and features 13 lifts servicing 76 runs.
The resort has spent the past five summers constructing three new walking trails to boost tourism outside the winter season (what the resort board calls the “green” season) and former mine sites that have been rediscovered have the potential to be integrated into walking trails and tours.
In 2011-12, there were more than 411,000 visitor days to the region, with more than 264,000 visitor days in the first half of 2012-13.1
CAPITAL WORKS
It’s during the “green” season that the resort board commissions capital works programs to improve its infrastructure and develop attractions that can be utilised all year round.
In 2010, after a period of high intensity rainfall and floods, it was discovered that the Swindlers Valley pipeline had deteriorated and was suffering from a loss of structural integrity.{{image3-A:L-w:250}}
The resort board made the replacement of the pipeline a priority, and after extensive consultation with the Victorian Government and the departments of environment and primary industries, and work with Coffey Geosciences, preliminary work was conducted in the first quarter of 2014. The pipeline replacement – which will see the installation of 2.1m diameter concrete-reinforced pipes – will start in October this year.
As part of the preliminary work, engineering fill that will be used in the bedding of the new pipes in Swindlers Valley has been drilled, blasted, crushed and stockpiled in the resort, ready to be deployed this October.
The fill itself came from Whitey’s Car Park, overlooking the alpine plains. In addition to being used as bedding for the replacement pipeline, the recycled aggregate from the site was also intended for expanding and resurfacing the same car park for the ski season.
The size of the carpark, which originally covered 4200m2, was to be increased by 83 per cent – to almost 7700m2.
FLYROCK MITIGATION
Traditionally, blasting is not conducted at Mount Hotham, and the work will mainly consist of digging and trenching. However, for this job, blasting was conducted.
Tom Pelly, the manager of technical services for the Mount Hotham Resort Management Board, stressed that the most important aspect of the blasting was asset protection, meaning Drop Shot Blasting Services and Flintstone Mining Services “had to design the blast carefully and execute it in such a way that they don’t have any collateral damage to the infrastructure”. {{image4-A:R-w:250}}
This included the nearby Hotham Village, underground telephone and electricity lines and storage facilities such as the Slatey Cutting bus depot, which was within 6m of the blast zone.
The other consideration was the main highway that runs through the mountain and was only 25m from the blast zone. To ensure the Great Alpine Road was not littered with flyrock and was not closed to traffic, blast mats from Flintstone Mining Services were employed.
Blast mats are made from recycled truck tyres and have been predominantly used throughout Europe and North America. They are not common in Australia, despite being readily available and indeed recommended by various state regulators.
Flintstone Blast Mats has been importing blast mats for use in urban blasting and sensitive areas. The mats are constructed from the steel-belted treads of used truck tyres, woven together with steel cable to make mats of various sizes but with sufficient weight to resist the upward movement of rock, and with tough resistance to penetration.
A typical mat weighs about 60kg per square metre (or between 400kg and 1200kg) and its maximum size is 6m x 3m. For the Mount Hotham assignment, which marked the first time rubber tyre blast mats had been utilised in Australia, Flintstone Mining Services and Drop Shot Blasting Services used slightly smaller blast mats, measuring 2.9m x 2.4m and weighing 450kg over an area of 3500m2. One mat was placed per blast hole.
In an article in Quarry earlier this year2, Flintstone managing director John Butchart emphasised the importance of the science in flyrock prediction and the setting of exclusion zones.
The flyrock model Butchart follows was devised by Alan Richards and Adrian Moore of Terrock Consulting in 2006.3 This model was developed to determine the maximum distance flyrock is thrown for a given burden, stemming height and charge mass per metre of blast hole length.
In the same editorial, Butchart explained that using blast mats to cover the face or surface of a blasting area had the same effect as increasing the burden or the stemming while mitigating the risk of flyrock.
“Quarry professionals are advised to divide the weight of the mats per square metre by 80 to get an estimate of stemming equivalence, ie at 60kg/m2, the stemming equivalence is 60/80, or 750mm,” he wrote. “An increase in stemming from 3.25m to 4.0m will halve the recommended clearance zone from 200m to 100m.” {{image5-A:L-w:250}}
TIGHT CONTROLS
There were 10 blasts at the Whitey’s Car Park site, all within 25m of the highway and directly opposite both Slatey depot and two street light towers, all of which were unaffected by the close proximity blasts.
Between each of the blasts, 28-tonne and 22-tonne excavators with buckets and chains would crawl over the shot rock and raise and relocate the mats for the next blast.
Steve Martyn, from Drop Shot Blasting Services, which manages blasting for quarrying and civil construction projects, explained that strict controls were needed for a blast that did not utilise blast mats.
“We always had to keep in mind that this material has to be crushed, so we’re trying to create maximum fracture without flyrock,” he said.
Alluding to the Terrock flyrock model, Martyn explained that in conjunction with Flintstone, “we came up with a formula for this rock type and for flyrock control. There was a zone where the mats had to start and the blasting stopped without mats. That’s all worked very, very well.”
As he neared the completion of the blasts, Martyn added: “We’ve had excellent results. I don’t think I’d try a job of this type without the blast mats.”
Butchart, who was also present for the blasts, echoed Martyn’s sentiments.
“The job’s reaffirmed my total confidence in the efficiency of blasting mats,” he said. “In fact, if you asked me to come do a job like this and there were not going to be any mats, I’d refuse to do it. That’s the equivalent of blasting 6m away from somebody else’s building without protection!”
Once the blasting was complete, crushing contractor Extons crushed the shot rock over a six-week period, installing a mobile crushing and screening circuit.
The mobile circuit processed about 16,800 bank cubic metres, or 42,000 tonnes, of excavated rock at a rate of up to 2000 tonnes per day, producing a variety of quarry products, including 20mm minus, 40mm minus and ballast, all for inclusion in the pipeline replacement project and other infrastructure on the mountain as needed.
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Damian Christie visited Mount Hotham, at the invitation of Flintstone Mining Services, on 28 February, 2014.
REFERENCES & FURTHER READING
1. Mount Hotham Alpine Resort Management Board. Annual Report 2012-13: 14. www.mthotham.com.au/all-about-hotham/why-hotham/corporate/publications/images/Mt_Hotham_Annual_Report_2012_20131.pdf
2. Butchart J. Flyrock prediction – From mystery to science. In: Quarry 22(2), February 2014: 14-18.
3. Richards AB, Moore AJ. Effective strategies for controlling flyrock. In: Quarry 14(11), November 2006: 26-27.