In 1968, a supposedly stable, near-surface backfilled stope in Timmins collapsed. In 1981, an old stope caved to surface near the town of Malartic, Que., creating a depression 300 ft across and more than 100 ft deep. More recently, a section of Highway 11B that was laid over old silver workings near Cobalt, Ont., collapsed just south of town. In some locations, housing developments have been built over old workings that are unknown to the planners or even to the mining companies. With the need to revamp municipal services such as watermains and sewers and with the ever-increasing demand on prime land for new construction in areas of old mine workings, more and more attention is being focused on locating old workings that exist beneath potential development areas. From this demand stems the requirement to develop systematic procedures to locate old stopes, to assess their stability, to quantify the risk arising from their presence and to define remedial actions for stabilizing the remaining crown pillars.
The task of accurately locating old workings calls for the detective skills of Sherlock Holmes. Sifting through drawers and files of musty old reco rds to find clues to the mining methods and development history of individual stopes often leads to more questions than answers. For many areas, signs of surface subsidence or hearsay comments from old timers often constitute the only records relating to potential hazard areas. With recent experience in solving some of these difficulties, a systematic methodology has been developed for addressing the problem of delineating the extent and location of old mine workings.
Engineering geophysics can often be an aid in this task. Radar, gravity, magnetics and seismic methods, when used in a controlled manner and in the correct environment, are powerful adjuncts to drilling. Effectively integrating several approaches permits rapid location and characterization of old workings. With overburden thicknesses and crown pillar geometries known, assessment of the stability of near-surface workings can more readily be accomplished by application of appropriate mining, soil and rock mechanics principles.
The most practical and effective measures for identifying and categorizing such areas follow a well-defined sequence of steps, namely:
* initial plotting of zones of potential concern onto a map, through
* assembly of old mining records and interviews with local town officials, residents and former miners to set a priority of areas for further investigation, to
* detailed surface geological and geophysical mapping surveys accurately locating openings of concern, to
* in-depth appraisal and soil and rock mechanics analysis of overburden and crown pillar stability based on subsurface drilling data.
Often, on the basis of the data assembled from site surveys and old records, it is possible to build a coherent picture of the overall geology and of the mining practices used in developing the old workings. Areas of potential hazard can be iden tified, investigated and prioritized on the basis of risk assessment. A remedial action plan may then be devised taking into account both degree of hazard and governing economic considerations.
The whole process begins with assessing old records. In most mining areas, a plethora of records exist. Diligence in sifting through the records is key. In order to make such information useful, the mine geology and the original mining strategy have to be defined as early as possible in an investigation. The mining strategy provides a framework for rational sorting of miscellaneous pieces of information. The sources of information are many and often conflicting. However, in the process of sorting data to build a picture of the underground workings, areas of uncertainty become defined, allowing planning of more detailed investigation.
Initial surface investigations address the obvious features of the site, such as existing collapse features, old fenced-off areas and surficial geology. In areas of thin overburden cover, geology at surface may indicate the factors controlling mineralization and, in turn, reveal the manner in which mine development occurred. In such cases, systematically delineating areas most likely to have undergone previous mine development can be quickly accomplished.
In many instances, the bedrock geology is masked by overburden and there are no conspicuous surface manifestations of mining activity. In such instances, effective use can be made of the full repertoire of geophysical methods. Some methods can be effective for direct detection of underground openings. More often though, they help define buried bedrock geology with sufficient detail to allow a comprehensive program of subsur face drilling to be planned. The geophysical methods most effective in mapping crown pillar composition and geometry are magnetics, gravity, radar, electromagnetics and seismic. G eneral background on most geo physical methods appropriate for such work can be found in standard texts such as Telford et al (1976). Informa tion on the relatively new radar meth ods is given by Annan and Davis (1976) and by Cook (1975). In addition, limited but occasional require ments exist for radiometrics and elec trical resistivity methods.
A brief synopsis of the important advantages and limitations of each of the most appropriate methods follows:
* Magnetics are most effective for delineating bedrock geology under overburden. The magnetic method will not respond to open or water- filled voids unless ferrous metal mining infrastructure is present in the underground workings.
* Gravity is a means of directly detecting large open voids. Microgravity surveying techniques can provide a useful adjunct to other procedures and, in particular, can aid magnetics for geological mapping. Unfortunately, many mine stopes are too small to be detected above the gravitational fluctuations from either the geologic background or variations in overburden thickness.
* Radar is a very high-resolution sounding technique. The method responds strongly to voids in rock that are within its sensing range. The method is also useful for overburden thickness measurement and buried structure delineation. Penetration is limited in areas of electrically conductive rocks or overburden such as clays. In amenable environments, though, the method is unsurpassed in providing detailed information on subsurface conditions.
* Electromagnetic and electrical resistivity methods can be most effectively used for conductivity mapping to assist in delineating the geology and estimating overburden thickness. EM has a limited ability to directly detect voids. Electrical resistivity methods have been effectively used for void detection in some instances but they can be cumbersome to use.
* Seismic methods are most often used for overburden thickness estimation. While seismic techniques can detect large open voids, most narrow vein mine workings are too small for direct detection.
Alone, these surface geophysical techniques usually do not provide unique definition of subsurface workings. Sensible integration of geophysical data with other information is the key to effective use of geophysical survey methods.
Exploratory drilling can help locate and delineate crown pillar geometries. Airtrack drilling is perhaps the least expensive, but also the most difficult to interpret, because of its heavy reliance on the driller’s interpretation of encountered conditions. Other non- coring methods (including reverse circulation) suffer from the same problem. All are adequate for defining voids in the rock mass and, therefore, are useful in rapidly delineating the geometry; but all suffer from their inherent inability to allow assessment of rock quality and of overburden composition. Non-coring methods, although useful for rapid delineation of geometry, should not be construed as adequate for geotechnical characterization of a crown pillar of dubious stability. Coring with as large a diameter as practicable is essential.
Diamond core drilling and comprehensive soil sampling of overburden are necessary to obtain the information required for undertaking appropriate analyses of crown pillar stability. Core drilling can be more effective if surface information and non-coring methods guide the diamond drilling. Interpretations, however, must be continually updated as drill results are obtained. Geology, rock quality and groundwater conditions must be quantified throughout the drilling.
Although in many terrains overburden conditions can be problematic from a stability viewpoint, the crown pillar’s rock competence usually governs overall stability. Accordingly, core drilling efforts must be aimed at getting the maximum information on rock quality. For example, using core diameters of less than n-size to investigate rock conditions can lead to serious interpretation errors. Core quality for a given drill rig, operator and equipment improves appreciably with core size. It is important to keep drilling-induced fracturing to a minimum. The core size usually employed for exploration purposes (aq) frequently yields core that is significantly more broken and disturbed because of the drilling process than that produced at n-size or larger. Triple tube techniques allow drilling at a somewhat smaller size than n (for example, bq), and still yield core of sufficiently high quality that the informa tion necessary for undertaking a rational geotechnical appraisal is acquired.
Blast- and stress-induced fracture patterns existing in the rock mass can be almost identical in form and texture to those induced by drilling. Serious underestimation of the degree of actual rock fracturing can thus arise because of core logging difficulties. Geophysical borehole logging techniques can be beneficial in some situations. However, to date, the inability to confidently conduct such measurements cost effectively in small- diameter holes of dubious stability has precluded their widespread use for crown pillar investigations. Cross- hole tomographic techniques, employing seismic or radar methods, hold great promise for the future.
Beyond the question of defining crown pillar geometry, a thorough appreciation of rock quality and groundwater conditions is essential. On the basis of the results obtained from geotechnical logging and simple laboratory testing of diamond drill core, several rock quality indices can be computed (Barton et al, 1974; Bieniawski, 1974). These indices, in combination with an understanding of the existing stope geometry, groundwater conditions, the original mining methods and the overall structural geology of the area provide the basis for characterizing the crown pillar rock mass. For example, calculating the Rock Quality Designation (rqd), which is defined as the percentage of each N-sized drill core run comprised of core sticks greater than 0.1 m long, gives an immediate and ready clue to the degree of fracturing of the rock mass along the line of each borehole.
Rock material intact strengths can also readily be determined by means of a comprehensive program of simple, field-conducted point-load tests. This test can provide an index of the competence of the various rock materials comprising a pillar. The number of joint sets, their typical spacing and their condition (roughness, alteration and infilling) characterize the structural fabric of the rock mass forming the crown pillar. This structural fabric, in turn, defines the kinematics of controlling geological structures that could give rise to progressive caving of walls or crown or could govern chimneying of the stope. All this information, taken together with an appreciation of the groundwater and in- situ stress conditions within a crown pillar, allows computation of generic rock mass classification indices such as the ngi “q” index or the csir “rmr” parameter. With accurate field data, computing such indices can provide a rational basis for undertak ing the quantitative rock mass characterization necesssary for pillar stability analyses.
Rock quality classification parameters can be used with numerical or analytical modelling techniques to assess the stability and competence of a crown pillar rock mass. Simple approaches allow first-order assessment of problem conditions. Often simple analogies can be drawn between certain crown pillar geometries and, for example, simply supported, fixed-end or cantilevered beams. More sophisticated computer-aided methods can then be employed for more detailed quantification of crown pillar stability. If conditions are marginal and time is available, monitoring can be done by establishing settlement points/pins for routine repeat surveying, apply ing glass “tell tales” to cracks on buildings or across rock fractures or, with a higher degree of complexity, installing wire or rod extensometers into holes through the crown pillar
Diamond drilling investigations of numerous stope crowns has shown that a zone of damaged rock often exists immediately above a stope. Although in many cases this zone may reflect original blast damage induced at the time of mining, it also frequently indicates degradation of rock quality because of time-dependent rock deformation. Even gradual degradation in certain rock types, if coupled with inadequate original crown pillar thicknesses, can lead to uncontrollable caving. Because ore zones are invariably structurally controlled, simple rod or cable bolting is frequently inadequate, particularly for severely disturbed pillars. If it is absolutely critical that the pillar remain in place without undue continuing deformation, then such procedures must be integrated with other remedial actions. When a stope crown pillar is found to be marginal or its stability is critical, several options exist:
* Leave the stope as it is and fence off the area.
* Backfill to the crown and then collapse the crown pillar on to the backfill so that all future subsidence movements would be controlled without the risk of sudden collapse,
* Backfill the stope as far as practicable, then after a suitable period, fill the space between the backfill and the roof of the stope by cement/slurry infill grouting.
* Blast out the crown pillar and backfill the stope to the surface with crushed rock.
* Create a composite bridge across the pillar by integrating the rock mass with either a mass concrete infill cap or a reinforced concrete deck.
In all cases, remedial measures will be site specific; frequently, cost will be the ultimate factor dictating the final course of action. Quantitative risk assessment based on utilizing systematic investigation procedures can provide the best rationale for reconciliation of costs and benefits. The use of systematic investigation and an overall improvement in maintenance of readily accessible, up-to-date mining records are leading towards safe utilization of surface areas in the vicinity of old mine workings. Current improvements in the ability to interpret observations from indirect and direct subsurface investigation techniques are further enhancing the industry’s capability of dealing effectively with identified problem conditions. REFERENCES Annan, A. P. and Davis, J. L., 1976, Impulse radar sounding in permafrost. Radio Science, Vol. II, No. 4, pages 383-394. Barton, N., Lien, R., and Lunde, J., 1974, Engineering classification of rock masses for the design of tunnel support. Rock Mechanics, Vol. 6, No. 4, pages 189-236. Originally published as Analysis of rock mass quality and support practice in tunnelling. Norwegian Geotechnical Inst. Report No. 54206, June, 74 pages. Bieniawksi, Z. T., 1974, Geomechanics classification of rock masses and its application in tunnelling. Proc. Third International Congress on Rock Mechanics. ISRM, Denver, Colo., Vo. IIA, 1974, pages 27-32. Cook, J. C., 1975, Radar transparencies of mine and tunnel rocks. Geophysics, Vol. 40, pages 865-885. Hoek, E. and Brown, E. T., 1980, Underground Excavations in Rock, IMM London, 527 pages. Telford, W. M., Geldart, L. P., Sheriff, R. E., Keys, D. A., 1976, Applied Geophysics. Cambridge University Press, Cambridge, 843 pages. Dr T. G. Carter, P.Eng., is with Golder Associates and Dr A. P. Annan is with A-Cubed Inc. This article was prepared from several studies carried out for mining companies, consultants, muni cipalities and the Ontario Ministry of Northern Development and Mines. The intent has been to summarize proce dures that have proven effective for investigating crown pillars. Carter and Annan would like to thank their clients and colleagues for their support and patience during many of the investigations and for their permission to publish the data in this article. — 30 — SOME OBSERVATIONS OF GOLD ZONING By Chester J. Kuryliw A gold-related mineralization has been recognized by this writer to occur in a consistent pattern of zoning in some gold deposits and gold camps in widely scattered areas in Canada, the U.S. and Brazil. The consistent sequence of gold-associated mineralization is stated as an empirical rule. Five major gold-mineral zones are recognized in sequence. The key is the arsenical gold-bearing zone, which has recognizable sub-groups in the sequence. A recognition by prospectors and explorationists of the gold- zoning provides a valuable tool to direct the concentration of exploration towards richer gold mineralization within an extensive structure. Listed below are five major gold-related mineral zones:
* Gold-Tourmaline zone — (fair gold)
* Gold Pyrite (Sulphide zone) — (rich gold)
* Gold Arsenopyrite zone — (richest gold) (D)
Be the first to comment on "LOOK OUT BELOW"