Pillar power THE RIGHT ROCK BOLT FOR YOUR MINE

Four years ago the Quebec government contracted Laval University in Quebec City to conduct a study of the rockbolt types used in the province’s mines. The objec tive of the project was to produce a guidebook that mine planners could use to s elect the appropriate rock bolt for a given set of ground conditions. Quebec min es use an estimated 750,000 rock bolts every year. The book would be used by min ing engineers and mine planners throughout the province. Pierre Choquet, assistant professor of rock mechanics in Laval’s mining and metallurgy department, visited 25 mines in 1983-84 and met with the major rockbolt manufacturers supplying bolts to mines in North America — namely Stelco Ltd. ( Hamilton, Ont.), Stewart Mining Products (Noranda, Que.), Ingersoll Rand Canada (Montreal, Que.) and Atlas Copco Canada (Point Claire, Que.). His findings, compiled in a 267-page report, have been available to mines in the province since November, 1986. Unfortunately there is no English version of the guide.

A brief summary of the results of the study were presented (in English) at the annual meeting of the Canadian Institute of Mining and Metallurgy, held in Toronto last spring. A summary follows:

Mechanical rockbolts with bail-type and R-type expansion shells and steel plates are by far the most common type of rockbolt used in Quebec mines , capturing 7 2% of the market. The pull-out strength of these bolts depends on the diameter o f hole in which they are installed. If the diameter of a hole exceeds the tolera nce of a bolt by as little as 1/16 of an inch, a drastic loss of strength can re sult. Tests have shown that the amount of torque a miner applies to these bolts with his stoper when they are installed has very little influence on the pull-ou t strength of the bolt.

Higher-cost and higher-load-carrying, resin- and cement-grouted rebar is the second most popular type of artificial roof support used in Quebec mines. Some 13 5,000 are installed each year. With the introduction of cement cartridges, many of the disadvantages of using pumped cement grouting have been eliminated.

Two relatively new forms of roof support are slowly gaining popularity in Quebec mines: Swellex bolts, manufactured by Atlas Copco, and Split Sets, manufactur ed by Ingersoll Rand. Both offer the dual advantages of rapid installation and r apid pull-out strength. The pull-out strength of the Swellex bolts depends on th e type of rock in which they are used, hole diameter and the pressure at which t hey are inflated with the use of water. The optimum hole diameter is 38 mm. For medium and hard rocks with compressive strengths greater than 42 mpa, tests show the minimum bonding length needed to support the rock is 0.5 m. In weak er rocks the minimum recomended bond length is 1-2 m.

The pull-out strength of Split Sets depends on hole diameter and ground type. Tests have shown that the pull- out strength increases with time as the ground shifts and corrosion increases friction between the bolt and the rock. The minimu m bonding length of the Spit Set bolts is 3.4 m, which is much higher than other bolts.

If you are experiencing frequent rockbolt failure in your mine the study recom mends you change to a bolting system which is less stiff. Details on the recommended procedure are given in the published report.