More money is being spent on rock mechanics research than ever before. The Mining Research Directorate in Ontario estimates there is $100 million being spent in Canada this year and about $20 million in Ontario alone. The 15th Canadian Rock Mechanics Symposium, held in Toronto in early October, provided an opportunity for researchers and mine operators to prove they have something worth financing. For three days they exchanged views on the new developments in this maturing area of mining technology. Some new computer models, a new borehole probe, new theoretical methods and some interesting case histories which were presented at the symposium are summarized here. Hemlo’s Golden Pyramid Safety and profitability are the two hallmarks of any successful mine. You probably know operations that satisfy both these criteria. But mines often achieve this status by usually one of two means. Some mines are successful merely by chance (they happen to be run by operators who have experience in similar geological environments) while others succeed by strict, analytical design. Noranda Inc. has long recognized the potential benefits of improving the chances of building a good mine by combining the latest analytical design techniques with a wealth of previous mining experience, according to John Gordon, vice-president of Noranda’s Mining Corp. Group. Combining the two has led to improved mine design procedures. The company, which has a 51% interest in Hemlo Gold Mines, is using this design philosophy to mine the Golden Giant portion of the Hemlo orebody, near Marathon, Ont.
Over the past two years, Hemlo Gold has opened up stopes in two blocks of ore in the upper portion of the orebody (from 400 to 900 m below surface) using blasthole open-stopping methods with delayed fill. In the process, the company has compiled an extensive database of geotechnical information on the orebody and has used this information to develop an elaborate scheme to mine the much larger third block at depth. The scheme radically changes the sequencing of mining in the third block.
In block one, stopes 100 m high by about 25 m along strike were laid out and every other one was mined to the full width of the orebody in the first pass of mining. After each stope was mined, fill was introduced, usually within four to six weeks. In one stope (987), fill was delayed for an unusually long period and sloughing of some 8 m occurred, according to W. F. Bawden of Queen’s University, in Kingston, Ont. Usually, about one metre sloughs off the hangingwall, which is completely acceptable for dilution, according to senior mining engineer Ken Reipas of Hemlo Gold. When it came time to mine the stopes between the filled stopes in the second pass of mining, there were some ground control problems as well. As a result, the stopes in block two were mined in sequence (i.e. everthing was mined in the first pass of mining). According to Reipas, “this has worked quite well for us.”
So well, in fact, that the method has been adopted for the all-important third block, which is much larger than the previous two. Based on numerical modelling, the sequencing of the stopes in that block will resemble a huge pyramid, which will expand laterally with time. Once the pyramid opens up, there will be enough work areas so that there will be no delays caused by miners waiting for a stope to be filled before the stope next to it can be mined.
Fill at the Golden Giant mine consists of granodiorite waste, mixed with 60%, by weight, cement slurry in the box of a truck by feeding the two into the truck simultaneously.
Hemlo Gold is also planning to install a new microseismic monitoring system in the mine this fall or early in the first quarter of 1988. PC Power Numerical methods have long been a powerful tool for designing openings in rock. But these calculation-intensive programs require enormous computing power, which until recently has been available only on mainframe computers. Now, several numerical methods are available in programs designed for use on personal computers. One company that has applied most of the available programs to particular mine design problems is Bharti Engineering Associates, (BEA) a new mine consulting company which set up shop in May, 1988. Bea draws on the many years of mine design experience of seven former Falconbrigdge Ltd. employees in Sudbury, Ont. and the specialized knowledge of engineering consultants based in Minnesota, Arizona, Australia and the United Kingdom. They have applied most of the available numerical modelling programs on various mine consulting jobs throughout Canada.
One 3-dimensional, distinct element model, known as the 3DEC code, was used by Itasca Consulting Group of Minneapolis, Minn., a BEA associate, to investigate the problem of fault and dike slip associated with rockbursting at the Strathcona mine in Sudbury in 1985. By simulating rock movements along the fault on the computer, the consultants were able to determine friction angles which correlated favorably to fault motion observed in the field. While this was an important step in confirming the utility of 3-dimensional distinct element models for simulating underground mining, Barry Brady, technical director of Itasca, says they do not enable rock mechanics engineers to predict rockbursting in mines; it is only a starting point. A Caution on Computers Off-the-shelf numerical modelling programs are not the be-all and end-all when it comes to predicting ground failures in underground mines. The limitations and accuracy of such computer models should be considered carefully when using numerical models for mine design layouts, according to T. D. Wiles of Inco Ltd.’s Mines Research Dept. in Sudbury, Ont. Many of the programs should only be used with caution, Wiles says. His work, comparing extensive field measurements at Inco’s Copper Cliff South mine in Sudbury with numerical models, indicates that two computer programs, MINTAB and BEMDD, which use the displacement discontinuity method, underestimated the increase in pillar stress by as much as half. Furthermore, they were completely incorrect in predicting stresses tangential to the plane of the analysis at the mine. Programs using plane strain assumptions (such as NFOLD, DZTAB, MSEAMS and MINAP) overestimate increases in pillar stresses, Wiles found.
The analysis was done on pillar stresses in the 86.5 pillar, between a 350-m and 500-m depth below surface at South mine. Borehole Probe Explosives Technologies International (ETI) of North Bay, Ont. is in the process of developing a borehole probe, called ROCSCAN, to measure in situ velocity of detonation, pressure waves and seismic activity associated with blasting. But because mining companies are more interested in long-term stress changes as mining progresses, ETI has decided to develop a computer software package to calculate rock mechanics properties from data trans mitted by the probe.
The current prototype is 2 ft 8 in long and 23/4 in in diameter. The probe fits in NQ-size boreholes and locks tightly in place with a special locking mechanism. A number of metal buttons, in contact with the wall of the borehole, react to changes in the diameter of the borehole and translate that movement via a series of cantilevers to strain gauges which translate the movement into voltages that are proportional to borehole displacements. Electronic signals are transferred through a cable to a computer were they are recorded and processed.
Once the instrument is perfected the measuring range should be from 16.6 one-millionth of an inch to 10.5 one-thousandth of an inch and should be capable of taking as many as 20,000 readings every second.
Support software, also under development, will convert static borehole changes to in-situ stress changes and wave arrival times to material properties. The program will assume the material is homogeneous, isotropic and elastic.
The data collection and self-triggering feature were tested at a limestone quarry in Orillia, Ont. One test, using 27.5 lb of explosives, was conducted 15 ft from the probe. Another test was c
onducted in an underground mine. This test was designed to test the fast scan (3,000 HZ) and slow scan (one sample per min) modes and to test the instrument’s ability to analyse multiple-hole blasts.
“The results to date are very encouraging,” researchers C. J. Preston, et al, report. “We are confident the basic design approach is correct, and we have gained considerable operational experience. Although the field tests have indicated a number of areas that need refinement, at this stage ROCSCAN appears to be capable of monitoring blasting sequences and also of guantitatively tracking quasistatic stress changes. Future developments are aimed at improving the noise level in the electronic circuits; at calibrating the determination of stress change; at improving the dynamic response; and at generally gaining a greater confidence in the performance of the instrument.” Rockbursts in Cape Breton Rockburst phenomena are not restricted to hardrock mines. The coal mines off Nova Scotia’s Cape Breton Island, which slope out under the Atlantic to a depth of 720 m below sea level, have their fair share of bursts as well. It may be that these are lesser known because no one has been injured or trapped by such a rockburst. All have been initiated by blasting, while no one is in the mine. As it turns out, the fluvial nature of the environment, in which the coal was deposited some 300 million years ago, appears to have a direct influence on these unusual events.
Clement Yuen, an associate geotechnical engineer for Golder Associates, and three colleagues have been studying rockbursts at Cape Breton Development Corp.’s No 26 Colliery for several years. Although that mine was closed in April, 1984 because of an uncontrollable fire, the studies are important because the paleo-river sandstone channels which meander through the coal-bearing strata extend into the workings of the operating Lingan mine. These channels are the only bursting strata in the mine and miners at the Lingan mine are about to encounter the same type of ground.
Yuen says there were 37 outbursts in development roadways at No 26 in the 7-year period prior to the mine fire. Curiously, none of the earlier mild events were associated with methane gas. But as mine workings extended to greater and greater depths, the intensity of the events and the amount of gas detected increased dramatically. All events (except one) were initiated by blasting, so no one was injured. Predicting what initiates these events was an important part of the Yuen, et al, study. The model they have propos ed for the bursting event takes in situ stresses, material properties, blast-induced stresses and pore gas pressures into account and is consistent with the actual events. The next step is to develop methods of mining safely in burst-prone ground. The researchers recommend mine operators (the Cape Breton Development Corp., or DEVCO) map the location of the paleo-stream channels in detail, measure in situ gas pressures in the channels and core the sandstone prior to excavation to assess the amount of disking in the core. If 30 to 40 discs per metre of core are found, Yuen says, the strata has a high outburst risk; 20 to 30 discs per metre indicate a medium hazard and fewer discs indicate a low hazard. To prevent bursting, DEVCO should also consider installing upwardly inclined rock bolts to pre-support ground ahead of the face and development headings should be driven in the stress shadow of existing longwall panels, the researchers recommend. Perfecting Presplitting Advanced rock failure theories and their implementation in finite element computer programs have reached the stage where there is good agreement with field results. This development represents a giant step forward in helping mine engineers predict how far cracks will propogate in a given rock mass, permitting them to design appropriate borehole diameter and spacings and the amount of explosives to use in a given mining application. This is of particular importance to engineers designing presplitting blasts, used mostly in open-pit mining. Until now, the state of the art was defined by research work conducted by M. Mellor of the U.S. Army Corps of Engineers in 1975.
Researchers D. Frantzos of Acres International of Niagara Falls, Ont., and A. Bauer of Queen’s University measured pressures generated by detonating low-density explosives in boreholes of various sizes drilling in limestone and modelled the propagation of cracks on the computer using a theory of plasticity developed by Chen and Chen in 1975. They found that crack propagation is sensitive to tensile strength and compressive strength of the rock and is relatively insensitive to modulus of elasticity. Crack length per inch of diameter is also longer for small diameter holes than it is for large-diameter holes, they conclude. The ultimate objective of the work is to be able to accurately predict the effect on crack propagation of increasing the spacing between boreholes and decreasing borehole diameter and decreasing the amount of explosive. Mine engineers already know what effect those measures will have on mining costs. Wanted: Fresh Core, 30 cm Long, Experience Necessary The University of Toronto needs Tdrill core. Not just any old drill core, but drill core that has a known past. That is to say, core from an area where the in situ stresses are known. Ideally, the core should come from a conventional overcoring stress determination test. U of T wants the core so that researchers there can refine a technique of estimating stresses in rock where the field stresses are not known. With more data, researchers D. R. Hughson and Adrian Crawford hope to perfect the technique, which is easier, quicker, less expensive (but today less accurate) than using sophisticated instrumentation.
The technique centres around a puzzling phenomenon known as the Kaiser Effect: an acoustic emission of ultra-sonic pulses emanating from rocks under stress. In the 1950s, J. Kaiser noticed that the output of these pulses increases significantly as the stress rises from a level of stress previously experienced by the sample to a level of new stress experience.
By mounting a transducer on a drill core specimen, and loading the specimen at increasingly larger loads, the U of T researchers can record the emission of these acoustic pulses and determine at what level of loading the specimen is experiencing a new, higher level of stress. The point at which the acoustic emissions accelerate is known as the Kaiser Effect point. The researchers have found that this point deviates from the true previous maximum stress level, depending on the ultimate strength of the rock. The relationship between the true maximum stress and the Kaiser Effect point is called the Felicity Factor.
The Kaiser Effect point, adjusted by the Felicity Ratio, produces a measure which bears some similarity to stress measurements derived by conventional methods, the researchers conclude. But the hypothesis requires extensive testing. Thus the need for more core. The seven papers reviewed here were published by the University of Toronto, in a 248-page symposium volume entitled “15th Canadian Rock Mechanics Symposium”. Copies are available from Dr John Curran, Dept. of Civil Engineering, University of Toronto, Toronto, Ont. M5S1A4. Phone (416) 978-4611.
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