Last May an important research project started up near Sudbury, Ont. An array of accelerometers and hydrophones (highly sensitive, electronic listening devices) were installed near four blasthole stopes 2,750 ft underground in Falconbridge Ltd.’s Strathcona mine. They began collecting important bits of electronic infor mation about the structural strength of the rocks in the mine. The information w ill be compiled by a computer to form a geotomographic image of the footwall roc ks adjacent to the stopes after each successive blast in the stopes. This full-s cale project, the first of its kind anywhere, will give mine engineers their fir st look at how fracture zones around stopes develop in response to mining. Knowing how these zones propagate as an orebody is mined should help mine engineers design safer mine openings in the future and help minimize dilution of val uable ore. The project, co-sponsored by the Natural Sciences and Engineering Res earch Council, Noranda Research and Fal conbridge, is being directed by Dr Paul Young of Queen’s University in Kingston, Ont. It is just one of many research pr ojects underway in Canada in the field of rock mechanics — projects which could revolutionize this rather esoteric science. Of the 38 projects listed in the Canada Centre of Mineral and Energy Technology’s annual index of mining technology projects, rock mechanics is the most popular topic. It accounts for 21 projects.
Rock mechanics is becoming an essential part of the mine planning process in both underground and open pit mining. Mineral Development Agreements, negotiated by the federal and most of the provincial governments in wake of the 1982 recess ion, are providing significant research funds to several mining companies (about $4.25 million over a period of 2-3 years. The money will be used to conduct research which should result in a number of new tools to be used by mine designers from coast to coast.
The work is expected to be of greatest benefit to companies engaged in underground mining, which is taking place at ever increasing depths in Canada. Companie s engaged in open pit mining — mostly coal companies heavily concentrated in we stern Canada — will benefit as well.
The eight major mining universities in the country are gearing up for the acceleration of research work too. Two new chairs in rock mechanics have been establ ished at Canadian universities. Ontario’s Ministry of Northern Development and M ines has endowed one at Laurentian University in Sudbury. Dr Peter Kaiser, professor of civil engineering at the University of Alberta in Edmonton, who has agreed to fill the position, will also become an adjunct professor at Queen’s Univer sity. The other new rock mechanics chair, at the University of Toronto, will be filled by Dr Evert Hoek, a long-time senior principal of Golder Associates in Va ncouver. This position is being funded by the federal government through the Nat ural Sciences and Engineering Research Council of Canada and by Campbell Red Lak e Mines, which operates one of the deepest gold mines in the country. A laborato ry to study the mechanics of mine backfill has also been established at Laurenti an Unversity in Sudbury.
“Momentum has been building over the past six months to the point where we should begin to see some tremendous strides in the science of rock mechanics in the not-too-distant future,” Malcolm Scoble, director of mining at McGill Universit y told delegates at the annual meeting of the Canadian Institute of Mining and M etallurgy in Toronto last May. The kinds of things we can expect to see emerg e from this research work in the next 1-2 years include:
* a design manual for Vertical Retreat Mining specifically for steeply- dipping orebodies;
* a universally acceptable and applicable, 3-dimensional, numerical modelling software package (probably written in fortran or c and run on a unix operating system) that takes into account the plasticity of rocks;
* a critical evaluation of the many existing rock mass classification systems;
* computer programs to simulate and predict the performance of various blasting patterns and agents;
* new techniques to deliver high- density backfill to mined-out stopes and new techniques to de-water hydraulic backfill before it goes into the open stope;
* a set of recommended engineering specifications for backfill for specific ground conditions;
* techniques for testing the in situ characteristics of backfill;
* a clearer understanding of the dynamic behavior of densified backfill;
* a selection of methods for monitoring and modelling the behavior of sill pillars;
* new ways to predict subsidence resulting from mining potash;
* a microseismic monitoring system for potash mines; and
* better ways to predict violent rockbursts in hardrock mines.
With these developments will likely come increased market opportunities for manufacturers which supply mining companies and universities with soil-and rock-monitoring instruments, microseismic monitoring equipment, drilling and rock-bolt ing machines, scaling machines, floculants for backfill, cement additives, and fast, powerful computers.
The mining of ore left behind in pillars after the first pass of mining, is contributing more and more to the total output of many older underground Canadian mines. For example, an estimated 70% of ore production from Inco Ltd.’s Levack m ine, north of Sudbury, comes from pillar mining. In Ontario alone, the nation’s biggest mining province, two-thirds of the 40 underground mines in the province use backfill in mined-out stopes, according to a survey conducted by the Mining Health and Safety Branch of the Ontario Ministry of Labour. Mining the ore in pi llars transfers rock loads to the backfill. This means the load-carrying capacit y of the backfill must be sufficient to hold up the mine. Only 13 underground mi nes in Ontario do not use any backfill.
When you consider that 144,500 tonnes of ore are taken out of the ground each day in Ontario, that leaves room for 42,000 tonnes of backfill and 1,300 tonnes of Portland cement every day. Almost half of all backfill is cemented mill taili ngs — typically a very fine-grained, wet material averaging more than 20% minus 200 mesh and containing 12%-20% moisture by weight. Nine Ontario mines use flocculants in their backfill to aggregate particles into small clumps.
In the far north, where rock temperatures dip below freezing, even at depths of 500 m, frozen backfill is becoming an increasingly important structural materi al as well. For example, at Cominco’s Polaris mine, on Little Cornwallis Island, 75 miles from the magnetic North Pole, mining alternatives are being evaluated to remove high-grade lead-zinc ore in pillars up to 120 m wide. Two smaller pill ars, between two backfilled sublevel blasthole stopes, have been mined so far. T he method used in these pillars was to drill 76-mm fanned downholes with rings b lasted into a 1.8-m slotraise. These raises are driven between a 5×5-m undercut level and are 25 m long. Walls of frozen backfill, 25-30 m high and 55 m long, r emained stable for up to five months, the company says. The backfill consisted o f shales and limestone quarried on surface and mixed with underground developmen t rock and with a moisture content of 11%-13%.
The first pillar was mined over a period of seven months. Backfill walls were exposed without any signs of sloughing or deterioration. The second pillar was o ne third mined out when about 1,000 tonnes of rock fell from the hangingwall. Th is failure was attributed to existing geological structures rather than mining-induced stresses.
To preserve the integrity of the rock at Polaris, a 450-tonne refrigeration plant operates during the short Arctic summer to keep mine temperatures below free zing.
In surface mining the main areas of concern for the rock mechanics experts include: the design of pit wall slopes, drilling and blasting design, design of was te dumps, and tailings pond design. At an international symposium held in Calgar y last November, 52 paper
s were presented to a capacity crowd of open pit miners representing the industry, universities, consultants, and private and government research organizations. If the attendence at this and other symposiums are any indication, it would appear that we are in for some exciting times in the field of rock mechanics.
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