As our mines grow older, our miners are working in stopes which are deeper and deeper underground. Some stopes are 6,000 ft below surface and two new base metals discoveries in the Sudbury Basin — the Lindsley and Craig orebodies — begin at 2,000 to 4,300 ft below surface. Intuition tells us stress increases with depth, and so ground support must be designed to reflect the change in ground conditions with depth. As miners go deeper and bulk- mining methods are used more widely, health and safety officials across the country are preparing themselves for the next round of ground control catastrophies. Because of the mysterious nature of mining-induced stresses, mine engineers must select the appropriate type of rock bolt to use and decide at what spacing they should be installed in order for newly exposed ground to be adequately supported. Then they must develop a control program to ensure miners install bolts properly.
Normally, changes in ground conditions are subtle ones, referred to by geotechnical experts as rock mass modulus. This modulus takes many different physical properties of a rock mass into account. Mass modulus changes as stess conditions around mine openings change, as the pattern of discontinuities in the rock change, as ground water conditions change and as openings advance through lithologies of different strength. Because of the wide variety of possible ground conditions encountered in mining, there is no one best form of ground support — the type of bolt and the spacing between each installed bolt will depend on the mode of failure the mine engineer anticipates in each new rock type encountered. So says Dr Pierre Choquet, assistant professor of rock mechanics at Laval University. Successfully predicting that mode of failure is the key to designing adequate ground support. A number of publications are available to assist mine engineers design ground support systems. In 1986, a 267-page guide to the use of rock-bolting was published (in French) by the Quebec Ministry of Energy and Resources. And Trans Tech Publications of Germany published a 145-page handbook (in English) entitled Rock Bolting.
There are basically two types of rock bolts:
* those which are anchored in the hole by a mechanical device located at the end of the bolt, and
* those which are anchored by friction along the entire length of the bolt, either by cement, resin or by simple friction between the bolt and the rock wall.
Each has its advantages and disadvantages. A vast majority of mines use more than one type of rock bolt. It is estimated that close to five million rock bolts are installed in Canada every year. Denison Mines alone uses 400,000 bolts a year (in Elliot Lake). In Ontario, the country’s largest mineral- producing province, 90% of all underground mines use the least expensive rock bolt type — those which are anchored in a hole by mechanical means, according to the Ontario Ministry of Labour (OML). But column- anchored bolts (resin or cemented rebar, Swellex* or split set stabilizers), although more expensive than mechanical bolts, have several advantages. They are being used by more and more mine operators, often in special ground control situations, according to Grant McIntyre of Atlas Copco.
Fewer than 10 companies supply mechanical rock bolts to the Canadian mining industry. Three are in Sudbury, including Mansour Mining Supply. It manufactures a wide range of mechanical rock bolts, cable bolts and resin bar and is the national distributor of FAS-LOC, a resin cartridge system manufactured by Dupont Canada. Skip Smith, sales manager for Mansour, says sales for mechanical bolts are fairly steady. But he expects to see a trend towards more resin bar and resin cartridges because of the increase in mining activity at greater depths.
The design of the mechanical anchors marketed by each supplier varies, but researchers at the oml in Sudbury have found no significant difference in the quality of the steel used in the various rock bolt types available. The mining health and safety branch of the oml has developed standard testing procedures for the mechanical variety of bolt. The branch also trains mine personel in the use of these testing procedures and closely monitors mine quality control programs.
“Some mines do a better job than others,” says Peter Campbell, of OML, “some do pull tests weekly while others do it yearly.”
With an increase in column- anchored bolts, the OML is now developing the necessary tools and standards for testing these bolts. “If we are to enforce provincial legislation, we have to have firm standards,” Campbell says. One method involves using a boltometer — a portable ultrasonic unit which transfers a compression and flexural wave from a piezo-electric transducer to the bolt. A sensor head detects the reflected waves and analyses them electronically. The quality of the reflected signal indicates the length of the bolt and the overall condition of the bolt’s contact with the surrounding rock. The unit is manufactured by Geodynamik ab of Stockholm, Sweden. Several features limit the widespread use of this unit, according to James Cluff of the ground control committee of the Mines Accident Prevention Association of Ontario. That committee has therefore recommended that a different method be developed — one that would not require grinding the head of the bolt. Atomic Energy of Canada (AECL) has expressed an interest in developing such a unit. It would be more rugged and lighter and have a longer-lived power supply. Funding is being sought for development.
As of yet, there exists no testing procedure for cable bolts.
Split set stabilizers are one of the relatively new type of column-anchored rock bolts. They are manufactured by Ingersoll-Rand in three diameters: 33, 39 and 46 mm. These high- strength steel tubes are tapered at one end and have a slot along the length of the tube. The slot allows the tube to be compressed when forced into a slightly smaller hole with a hand-held pneumatic drill or a roof-bolting jumbo. Once in the hole, the compressed tube exerts pressure against the rock over the entire length of the bolt. A variety of lengths, ranging from 0.91 m to 3.66 m, are available. They can be installed with conventional drilling equipment by inserting a simple driving tool into the drill chuck of any jack-leg or stoper or by adapting rotary roof-bolters. About 30 million have been installed worldwide, according to Eric Nelson, sales manager for Ingersoll-Rand Canada.
Swellex, another column-anchored bolt was introduced in Canada in late 1982. Since then, demand has increased steadily, with more than 45 mines and a number of contractors using the new bolts, according to Grant McIntyre, product specialist for Atlas Copco Canada in Dorval, Que. Because of the price of the Swellex bolts, their use is limited to special applications. The bolts require a small water pump or intensifier with 4,350- lb-per-sq-inch pressure, to inflate the folded steel tube. The tube is manufactured from a special type of mild sheet steel. For a bolt 2.1 m long, a pressure of 200 bar is required. Most mines use water to inflate the tube, but some use either clycol or saline solution to prevent freezing.
“Since the price of the metals used in the alloy are going up, I expect prices will begin to stabilize,” McIntyre says.
Swellex is available in two basic types: standard (for 31- to 38-mm holes) and super Swellex (for 41- to 51-mm holes). Lengths range from 0.6 m to 7.3 m. But longer Swellex should be available within a year, according to McIntyre, for supporting open-pit and rock cut slopes in surface applications. To protect Swellex bolts from corrosion, the bolts are coated with a special zinc compound which has been developed to protect the steel in rock and soil.
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