March/April 2014

Built to break through

New requirements at mining operations are opening the door to mechanized tunnelling

By Correy Baldwin

The trend toward bigger mines with lower grade deposits, along with the improvements in tunnelling technology, is opening up more space for tunnel boring machines to do their work.

Since the advent of industrial mining, engineers have relied on drilling and blasting for tunnel development. Although mechanized systems like tunnel boring machines (TBMs) have long been used for civil engineering projects and to some extent in the coal industry, they can only be found at a handful of hard rock mines. This could be changing, however. New requirements at mining operations are opening the door to mechanized tunnelling.

“With ore bodies becoming deeper and more complex, and with industry mineral grades decreasing, such being the case with copper, the need for very large underground mines is increasing, and with it the amount of tunnels and shafts,” says Fred Delabbio, general manager for underground mining innovation at Rio Tinto. “Some future mines will need hundreds of kilometres of tunnel and shaft just for one operation.” This means that mining companies will increasingly be looking for tunnel and shaft construction options that go beyond drill and blast.

“Mechanical excavation will never fully replace drill and blast,” says Delabbio, “but it will provide mining companies with construction options we previously didn’t have. As a result, mine layouts and construction sequences will change based on the options the new technologies will offer.” He equates it to the way longwall technology impacted the coal industry.

Current TBMs, however, may not always be appropriate to mining operations. “TBMs have been specifically developed for civil tunnel applications and not specific to mining applications,” says Delabbio. “As such, they have uses, but are not ideal, for a majority of excavations. The key question is: Will the mining industry be able to create and support technologies that leverage the aspects of TBM and civil tunnelling and create new mine tunnelling and shaft construction technologies?” Delabbio says he believes so.

The right tool for the right job

When appropriately used, the advantages of tunnel boring are numerous. Whereas the advance rate for drill and blast is generally between three and nine metres per day, TBMs can average between 15 and 50 metres depending on rock conditions and provide control over the tunnel profile. Initial capital costs may be higher, but are quickly recovered in tunnels of greater length, typically within two kilometres. They are also much safer than drill and blast, eliminate hazardous blast gasses, and can require fewer operators, resulting in labour savings. TBMs cause less damage to the surrounding rock, lessening the need for ground support. (In fact, with TBMs, ground support can be installed simultaneously with excavation). Boring provides a smoother and more consistent tunnel profile, making the approach optimal for ventilation and transportation of equipment, and allowing for what Delabbio calls a factory-like process within the mine.

But to deliver all those benefits, TBMs must compete with the cost-effective and proven drill and blast technique. “The underground hard rock mining industry has been excavating tunnels for hundreds of years with drill and blast methods,” says Delabbio. Drilling and blasting, on the one hand, allow miners to create any excavation shape, with an unlimited turning radius, and all for a low capital equipment requirement. TBMs, on the other hand, have a large turning radius, have difficulty on steep grades and backing up, have difficulty creating service turnouts and turnaround points, and require a major upfront investment.

Under the right circumstances, however, TBMs result in a better-functioning operation, says Benjamin Künstle, deputy division manager mining at Herrenknecht. “Mechanical excavation systems are designed to increase the level of health and safety, provide a better and more modern work environment, improve production and have a positive impact on the overall mine development including an increased net present value.”

All this is encouraging interest from the industry. Künstle says he has observed growing demand from Australia, Chile, the United States, Canada, Russia, and South Africa, and more recently from China. He notes an increased use of TBMs in block caving and cut-and-fill operations to create inclined access ramps, which he says are becoming the preferred option over deep vertical shafts. Even large open-pit mines are considering TBM applications, he notes, in order to bring ore and waste rock directly to the surface through a system of vertical passes and tunnels.

Application will drive innovation 

Delabbio suggests future operations will place new requirements on technology and equipment, which will necessitate more innovation. “As with any new technology, usage will start with specific niches,” he says. “Once mechanical excavation concepts and prototypes are proven, the usage will probably be based around the speed of tunnelling, the reduced rock damage, safety and the improvement in NPV.”

TBMs were first used in mining operations in the late 1950s, including at the Steep Rock iron mine in northwestern Ontario and their use continued through the 1960s and 1970s. They achieved high rates of excavation in soft rock, but their applicability was limited and their costs too high.

Since then, technological advances have increased their applicability. Some of the machines are designed for tunnelling in different ground conditions, from harder to less-competent to heterogeneous rock, including open-face machines, various and improved shields, and earth pressure balance technology. There have been advances in automation and continuous mining, and in hydraulic and electrical systems, while other innovations aimed at increasing power and advance rates, through changes in cutter design, have increased cutterhead revolutions per minute (RPM) as well as thrust and torque.

Attention has also been paid to greater maneuverability, specifically in decreasing turning radius and allowing for steeper inclines and declines. Current prototypes are pushing the boundaries of maneuverability as well as testing innovations for vertical shaft tunnelling, non-circular tunnelling, ground support and further automation.

“Contractors were adventurous in the 1960s and 1970s, when profits were high and there wasn’t as much competition,” says Joe Roby, vice-president of business development at TBM manufacturer Robbins. “Some of those radical concepts really advanced the technology and some went in the waste can of history. Today, the changes are evolutionary rather than revolutionary. Contractors want a TBM that is very conservatively designed. They want high performance, but first they want reliability.”

The future for tunnel boring in mining may be bright, but for right now, equipment suppliers are still looking for a breakthrough. Roby suspects much of the resistance is historical: “There were some TBMs used in mines in the 1960s and 1970s that didn’t perform that well in hard rock. But the machines today have so much more power and thrust than those machines, and ground support systems have significantly improved as well.”

Künstle agrees that there is an unfounded distrust of TBMs. “Choices were sometimes made based on wrong or incomplete figures and maybe guided by some skepticism of an unknown methodology, and because of positive experiences with alternative methods,” he says. “It is our goal to inform all involved parties about the real capacities and limits of the TBM technology.”

Suppliers point out that TBMs are mature products, with hard rock machines having been around for 60 years. “The majority of existing equipment is not fundamentally different than the units used 40 years ago,” says Delabbio, “other than being larger and with incremental improvements in fuel efficiency and ergonomics.” The machines often come from civil engineering, adapted to meet such requirements from the mining industry as weight and bulk limitations, and higher maneuverability for tight curves and steep alignments.

Vertical excavation 

The next big thing in TBM technology may come in vertical excavation for sinking shafts, which under current methods creeps downward at around three metres per day. Shaft boring machines have been developed for civil engineering but none that can create large enough diameters or go deep enough to be applicable to mining projects, optimally 10- to 12-metre diameters and depths of over 2,000 metres. Other shaft-sinking concepts like raise boring and vertical moles have specific applications but are limited in tunnelling size.

“The problem is removing the muck quickly from the full-faced boring head so that you’re not regrinding and causing a lot of wear on the cutter head,” says Roby. “When someone solves that problem, there will be a great deal of interest from the mining industry.”

Herrenknecht’s solution is a prototype with the cutting wheel rotated 90 degrees. Rio Tinto is working with Aker Wirth in the testing of shaft and tunnel boring systems at its Northparkes copper and gold mine in Australia as a part of the company’s Mine of the Future program. Robbins is also working on shaft excavation, alongside developments to its TBM technology, including the McNally continuous rock support system, a remote cutter monitoring program, and earth pressure balance systems. All this represents a great interest in a broader applicability of TBM technology.

“Recently in Australia there has been an increase in the number of TBMs considered for applications around the construction of coal mines. Anglo American’s Grosvenor mine is one example,” says Delabbio.

Roby says Robbins TBMs are at work in the Stillwater PGM mine in Montana and in the decline development for Oz Minerals’ Carrapateena copper-gold project in Australia.

“TBMs can drive declines, run access tunnels along an ore body or put in conveyor haulages a lot faster,” says Roby. “I’d love to work with a block caving mine to design a fully TBM-excavated draw-point level. I think we could make a machine that would excavate all the tunnels in a large draw-point level at an unprecedented rate. That would get a lot more metals to market much quicker, making some mine a lot of money.”

“It’s only a matter of time before it’s done,” he says. “It’s just a question of who gets started first.”

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