The 69-daylong rescue of 33 workers trapped in the collapsed San José underground gold-copper mine during the late summer and autumn days of 2010 accomplished several matters of differing importance: Most importantly, the miners were rescued without further serious injury or loss of life. Millions of TV watchers came away with a better understanding of the toughness and ingenuity of underground miners, and the dark, unforgiving world in which they spend their working hours.
The public was also given a brief but fascinating glimpse at the real-world difficulties associated with a seemingly simple task—drilling holes deep through the earth to reach a desired target hundreds or thousands of feet below the surface, usually within a tight time schedule.
Although raise boring technology wasn’t the final solution to the rescue attempt, it played a major role in the effort as one of three drill-based plans of attack the rescue crews employed to reach and extract the miners. Televised pictures of the 31-ton Strata 950 raise boring machine brought to the site gave viewers a striking visual lesson in the size, power and weight of machines built to bore through the earth.
The San José rescue didn’t quite make raise boring machines the rock stars of drilling, but it did call attention to the usefulness and efficiency of these machines—qualities that were already well known to the mining and underground construction industries and which continue to drive customer demands for even more powerful boring machines from the relatively small number of companies that build them, such as Atlas Copco Robbins, RUC Cementation, Aker Wirth, TUMI and others. And, to the extent that state-of-the-art engineering and manufacturing allows, the suppliers have answered the call. As our 2011 coverage of the sector explained, the limitations on raise boring machine power and capacity are mostly external factors such as drill string integrity, local ground conditions and the frequent requirement that these machines must be able to operate in confined spaces (See Speed and Safety: Raise Boring Provides the Solution, p. 32, April 2011).
As an example, Australia-based raise boring and tunneling machine builder Terratec reported in early February that it had successfully completed the factory acceptance testing of its first raise boring machine, a model TR3000, sold for the Mexican market. The completion of testing was marked by a ceremony held in Tasmania, Australia, six months from receipt of order, according to the company.
Terratec identified the client as Cau S.A. de C.V (CAUSA), founded in 1978 as a service company for development of mining and civil works. The company is regarded as the largest raise boring contractor in Mexico.
In North America, Terratec currently has RBMs operating in the U.S. and Canada and now, with delivery of the latest unit, in Mexico as well.
The TR3000, said Terratec, was robustly designed to be capable of boring a 3-m-diameter hole up to 600 m deep in hard rock. The nominal hole diameter for this size of machine is 3 m with a standard pilot hole diameter of 311 mm. The TR3000 provides torque of up to 78,000 Nm for pilot drilling, 237,000 Nm for reaming, 261,000 Nm makeup and 266,000 Nm for breakout. Down thrust is calculated at 1,600 kN with up thrust at 4,500 kN. Total installed power is 352 kW.
However, the dimensions of the machine—as necessary to fit within the client’s specified drift requirements—were a critical element. This model’s extended height is 4,500 mm; retracted, it is 3,815 mm.
That isn’t always the case, though. In what is regarded as the biggest raise boring project in mining history, Swedish mining services contractor Bergteamet was able to use some of the largest raise borers available to produce 55,000 m of ventilation shafts and ore passes as part of LKAB’s recent project to develop a new haulage level 320 m below the existing level at 1,045 m depth.
The contractor employed an Atlas Copco Robbins 73RM-DC and several larger 91RH Cs. The machines were used to bore raises ranging from 90–360 m long and up to 5 m in diameter. Weighing 33 mt, the Robbins 91RH C provides continuous torque of 450 kN, with a relatively compact design that measures 4,115 mm fully extended.
Dwarfing the 91RH C is Redpath’s Redbore 100, touted as the world’s most powerful raise boring machine, measuring 7,540 mm tall fully extended, and capable of providing maximum pilot hole force of 667.2 kN and reaming force of 15,569 kN. Nominal capacities range from 3.66 m up to 8 m) diameters with lengths of raises up to 1,000 m.
At MINExpo 2012, Redpath said it was expanding its fleet of RBMs and adding models, including three more Redbore 30 units and a second Redbore 90. A new model—the Redbore 60—is scheduled to enter service in the first half of 2013.
Also introduced at MINExpo was Atlas Copco Secoroc’s latest offering in pilot bit design, the PrimO pilot bit. The bit features a contoured shirttail to reduce heat on the seal area and carbide inserts on the shirttail to increase the life of the shirttail and seals. The PrimO also employs a dual seal arrangement, which includes a primary seal and an excluder for longer seal life.
The PrimO design also incorporates wear-resistant carbide on the cone gage bevels coupled with enhanced carbide on the gage of the bit to improve gage life, which translates into longer pilot holes with fewer trips for the customer and overall faster penetration rate as the insert retains its hape longer, according to the company.
The PrimO bits utilize proprietary synthetic lubricants for longer bearing life and increased rpm, and the combined features of the bit line have shown up to 30% better performance in field testing compared with standard bits in hard or difficult ground conditions.
