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AGAPITO
ASSOCIATES, INC. Mining & Civil Engineers & Geologists |
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Project Summaries - Mineral |
High-strength backfilling in the B-NORTH Zone at the Cannon Mine is being used in sublevel bench-and-fill mining of gold ores. The high-strength backfill is required to minimize surface subsidence and to provide safe operating conditions during pillar mining between backfill pillars that are carrying portions of overburden weight. Pillar recovery between the backfill pillars has been very successful. Undercutting of the B-NORTH Zone between July 1987 and November 1988 has resulted in measured backfill stresses of 2.28 MPa, or 68 percent of full overburden weight. Low water content and high proportions of coarse aggregate allow in-place strengths of 5.66 MPa to be achieved, with cement contents between 5 and 6 percent.
Construction and Geological Engineering of an
Underground Ore Bin A 16-m by 4.7-m by 24.5-m (52-ft by 15.5-ft by 80.5-ft) underground ore bin was constructed at an 810-m (2660-ft) overburden depth in poor quality, faulted, and altered intrusive rocks at Molycorp's block caving molybdenum mine. Geological engineering incorporated detailed structural mapping, rock quality classification, in situ stress field determinations, numerical stress and failure analysis, and convergence monitoring. The ore bin was mined by conventional methods from the top down and utilized a bored raise for mucking. The initial ground support of rock bolts, lightweight steel sets, and reinforced concrete was later supplemented with an array of 6-, 12- and 21-m tensioned, grouted rock bolts when wall closure was noted in the upper ore bin area. The supplementary support was optimized by computer analysis and monitoring. |
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Project Summaries - Capability |
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Depth and Horizontal Stress
Challenges at White Pine Room-and-pillar mining at White Pine started in 1953 from a sub-outcropping ore body that gently dips to 1000 m depths in a distance of about 6 km. High horizontal stresses were recognized in the mid-1960s, which had obvious stability impacts when viewed in combination with the increased overburden stresses. Modifications in conventional room-and-pillar design were introduced through the years on a trial-and-error basis to help overcome stability and resource recovery problems. These modifications were based on pressure arch concepts using pillar yielding and/or pillar recovery, and on mine layout orientation to minimize stress impacts. While some of these modifications were successfully applied in some areas, they failed in others. Through the years, visual observations played a major role in panel design, with contributions from stress measurements and analyses. However, the down slide in the copper economy during the 1970s and early 1980s resulted in a loss of interest in the application of these concepts. This paper reviews ground control problems and presents the results of recent stress measurements and computer analyses. They shed light on the importance of using yield pillars and mine layout orientation to control stresses at depth.
Design of Pillars with Backfill
Interaction—A Case Study Multilift, sublevel bench-and-fill mining is being used at the Cannon Mine in central Washington, USA, for complete recovery of high-grade gold ores in a shallow, wide ore body. Tall, narrow rib pillars are formed in multiple lifts with high-strength backfill placed after each lift to provide lateral confinement of the pillar during primary extraction and to provide full overburden support during 100% recovery of the pillars. Unusual requirements are placed on the backfill, which must provide stable walls during recovery of the pillars and must support overburden weight both during pillar mining operations and over the long term to minimize subsidence. By placing the backfill tight against the hanging wall, the backfilled primary stopes are converted into backfill pillars whose support function is mobilized by vertical displacement of the overburden. The Cannon Mine, a joint venture of Asamera Minerals (US), Inc. (operator) and Breakwater Resources, is an interesting case study of pillar and backfill performance. Full extraction of the wide, shallow ore body has resulted in nearly full overburden loading of primary pillars, followed by transfers of overburden weight during pillar extraction. Rock mechanics played an important role in this project, both in developing design specifications for safe spans, pillar dimensions and backfill strength, and in predicting the global mine structural response to full extraction of the ore zone. Gold values at the Cannon Mine were high and warranted a high level of engineering evaluation. As mining of the primary pillars progressed, large portions of the overburden load was transferred into remaining pillars and later into the backfill adjacent to active mining operations. The ability to predict pillar and backfill loading through the use of computer models, and confirmations of the predictions by monitoring instrumentation, was very important information used for making decisions on mining sequence and for risk management during recovery of the pillars. The purpose of this chapter is to illustrate that the empirical and analytical tools currently available for rock mechanics design can provide a practical basis for complex mining problems. The geological environment at the Cannon Mine was complex and provided ample opportunity for simplifying assumptions, typical of rock mechanics design, to be wrong. Estimates of rock mass physical properties, based on laboratory testing and field characterization of rock mass quality, were reasonably close to values back-analyzed from f ield instrumentation data. Computer models based on boundary element formulations provided good predictions of pillar stress levels. Background information on the geology, ore zone geometry, geotechnical properties, and mining method are summarized in Section 27.2. Global analyses to determine primary pillar loading and the effect of local structural features on global stability are presented in Section 27.3. Backfill loads and stability aspects of pillar mining are the subject of Section 27.4. Global analyses using the EXPAREA code were performed to estimate load transfer during pillar mining and to evaluate the pillar mining sequence. This is followed in Section 27.4.2 by detailed microanalyses of pillar and backfill interactions to predict local wall and roof stability. Global mine response, as indicated by instrumentation results showing the transfer of overburden loads to pillars and backfilling during mining of the pillars, is discussed in Section 27.5. Instrumentation data provided verification of the analyses and gave information on backfill loading and in-place strength. Finally, the adequacy of the design assumptions is presented in Section 27.6.
Evaluation of Ground Control Requirements for D Orebody Load-Haul-Dump Block
Molycorp (click to view entire paper in PDF
format) Molycorp, Inc.’s Questa Mine plans to switch from gravity draw in Block 1 to a load-haul-dump (LHD) draw system in the East and West blocks of the D Orebody. Changes in mining method require modifying the support design. This paper summarizes the numerical modeling effort and findings in evaluating abutment stress conditions and ground support requirements for LHD mining. A three-dimensional, elastic-plastic, FLAC3D model was constructed comprising a detailed mesh representing the excavation geometry of the LHD drawlines encapsulated within a coarse, global mesh representing the entire D Orebody. The mining sequence was simulated to output stress changes at the LHD Level during advance of the caving front. The drifts at the LHD Level were mined before or after several key caving steps to represent pre-undercut and post-undercut conditions. A fictitious support pressure was then applied to the walls of the drifts and incrementally relaxed to measure convergence as a function of support pressure. LHD Level support requirements were determined from these ground-support interaction analyses. Conclusions were that a thick shotcrete liner would be required for the drifts in weak andesite, while light shotcrete and rockbolts would be sufficient in areas of strong aplite (granite porphyry).
Ground Control Design Challenges at the El Boleo Copper Project
(click to view entire paper in PDF) The El Boleo Copper Project is currently in the final design stages for development of a 3-MM-tonne/yr underground copper mine located in Baja California, Mexico. The nearly horizontal mineralized zones are amenable to exploitation by conventional mechanized coal mining technology using continuous miners, shuttle cars, and belt haulage. The mine design resembles a typical full-extraction coal mining operation utilizing a sequence of mains, sub-mains and production panels. Due to numerous faults, mining will be conducted as several small adjacent mines accessed through adits from large arroyos cutting through the mine area. Up to four seams may be mined in some areas with a maximum overburden depth of about 250 m. The ore zone is relatively weak and plastic, subject to extensive creep over time. The mine workings will pass through areas of historical shortwall mining panels where the ground has flowed into old entries and gob areas, and then reconsolidated. A small test mine was developed to evaluate mechanical cutting performance, roof-bolt anchorage capacities, roof and pillar stability and caving behavior. In-mine measurements were combined with two- and three-dimensional FLAC models to evaluate alternative development and production pillar designs. The paper will discuss the various factors influencing ground stability at the El Boleo project and the strategies employed in developing a suitable ground control design.
Ground Stability and Support in Block
Caving Operations at Molycorp's Questa Mine Evaluation of ground stability and support requirements during block caving operations at Molycorp's Questa Mine was conducted with the help of a simple rock mechanics program consisting of underground mapping and classification of ground conditions, and stability and support monitoring by extensometer measurements. This paper describes how rock quality evaluations based on the Q system, convergence measurements, and computer-aided analysis have proven useful in helping establish the support for a wide variety of rock conditions. The type of supports used consisted basically of three systems, which were generally installed according to the location with respect to the cave, as follows: (1) cast concrete and steel beneath the cave; (2) shotcrete and fully grouted bolts in the peripheral areas; and (3) shotcrete and split sets outside the cave abutment areas. Exceptions to this general rule occur in some locations. Support performance assessment was based on Q evaluations and convergence measurements, as well as roof fall experience. Results indicated that areas of very poor to poor ground need to be supported with steel and concrete or shotcrete and 6-m-long, untensioned, fully grouted bolts, depending upon the location in relation to the cave. Shotcrete and split sets performed well in areas of fair to good quality rock outside the cave abutments. Roof falls were structurally controlled, wedge-type failures, usually associated with water. No falls have occurred where the shotcrete is 50 mm or more in thickness.
Ground
Support Design Using Three-Dimensional Numerical Modeling at Molycorp,
Inc.'s, Block Caving Questa Mine (click to view entire paper in PDF
format) The Molycorp, Inc., Questa Mine, located in New Mexico, currently mines using a gravity-draw panel cave to extract molybdenum sulfide ore from the 600-m-deep D Orebody. Prior to initial development, geotechnical studies were undertaken to predict ground response for the design of entry support on the Grizzly and Haulage levels and in transfer raise connections. Heavy abutment pressures were anticipated ahead of the undercut, followed by significant stress relief as a consequence of a post-undercutting mining sequence. Detailed three-dimensional continuum modeling was conducted to predict changing stress states during the undercutting sequence and to evaluate the performance of various concrete and steel liner designs. Lithologic variation across the orebody was simulated and proved meaningful for identifying different stress transfer mechanisms and liner pressures in different types of squeezing ground. Recommendations for concrete liner thickness, concrete strength, reinforcement, and steel liner thickness were developed from modeling and, ultimately, were implemented during construction. Since the cave was initiated in October 2000, ground support has performed reliably, with only occasional compression cracking and minor tensile separation of the Grizzly Level liner in response to passing abutment loads. Observations to date corroborate model predictions and validate initial support design for the new deep orebody.
Geotechnical Analysis of Underground Mining Methods for the
Copper-Nickel Orebodies of N.E.
Minnesota A series of analyses of possible mining excavations associated with development of the copper-nickel deposits of northeastern Minnesota is presented. The majority of these analyses were conducted using a boundary element method, but some cases were also analyzed using the finite element method. Comparable results were obtained using the two methods, but the boundary element method required less data preparation time and computer run time. Results of the analyses, as well as indicating the capabilities of geotechnical numerical models, provide some insights into behavior around multiple excavations in jointed rock.
Horizontal
Stresses as Indicators of Roof Stability
(click to view entire paper in PDF
format) High horizontal stresses were recognized to impact roof stability more than 60 years ago. Since then, numerous measurements associated high horizontal stresses with difficult ground conditions. This paper presents case histories illustrating the practical usage of roof stress determinations for helping assess stability, not only in the case of high horizontal stresses but also of low stresses. Examples are given of high stresses associated with faults, mine design changes, quantification of stress shadow effect, and anistrophy. The paper concludes with a comparative evaluation on the effects of various stress fields on ground support requirements.
Hydrologic Stability Study of the Crown
Pillar, Crandon Deposit, in Support of Mine Permit Application This paper presents the results of an assessment of the hydrologic stability of the crown pillar that will remain after extraction of zinc and copper ores at the Crandon deposit in Forest County, Wisconsin. The crown pillar, defined as the rock remaining between the uppermost extent of mining and the base of surficial glacial deposits, is to be left in place to protect the surface and the glacial aquifer system. Hydrologic stability refers to potential changes in vertical hydraulic conductivity resulting from mining. Changes in hydraulic conductivity could result from mechanisms which include collapse of the crown pillar, joint movement, or strain changes in the crown pillar. The issue of hydrologic stability of the crown pillar has been raised by the Department of Natural Resources (DNR), State of Wisconsin, as a potential concern because of solute release to the groundwater regime after reflooding the backfilled mine. To address this issue, a series of two- and three-dimensional numerical analyses were conducted to predict the impact of mining to the crown pillar for the worst possible scenario. A three-dimensional simulation of the entire mine area was first conducted to evaluate the stress and strain changes in the crown pillar. A detailed analysis for selected locations within the crown pillar was then used to investigate the potential of joint shear and joint interaction. Maximum allowable strain, safety factors against joint slip, and change of hydraulic conductivity in the crown pillar were evaluated. A minimum crown pillar thickness of 100 ft was established based on the analysis results. The numerical analyses concluded that there will be no measurable impact to the hydraulic regime due to mining.
Importance of Geotechnical Field
Mapping in Assessing the Stability of Underground Excavations Stope instability at Ashanti Goldfields Company, Ltd. (AGC) is a major concern. Using information from field mapping, major discontinuity sets were defined, Rock Quality Designation (RQD) was determined. The stability of wedges in the stope walls was assessed using the commercially available computer program UNWEDGE (Rock Science, 1992). Analytical results indicate that the presence of graphite infilling in joints contributes to instability, while steeply dipping joints control wedge slabbing and wedge toppling in the hanging wall. Stable stope widths and dimensions were determined to range from 8.1 to 30 meters (m).
Mine Design at the Cannon Mine: Integration of
Operational Planning and Geomechanical Design A multi-lift benching cut-and-fill mining method was adopted at Asamera Minerals (U.S.), Inc.'s Cannon Mine to balance the goals of high productivity and near 100 percent recovery with the need to minimize subsidence. High strength backfill was required to provide effective long-term overburden support and safe working conditions during the pillar recovery operations. Operational designs were evaluated using numerical modeling to predict global mine stability and backfill strength requirements. These models incorporated sequential mining of primary stopes, backfilling of primary stopes and recovery of pillars, and have been transferred to the mine operations group for use in evaluating different production sequences. The modeling has provided a better understanding of the structural response of the mine to excavation of the primary stopes and removal of the pillars. Predictions about the transfer of overburden weight to the abutments, pillars, and cemented fill pillars have provided a criterion to evaluate mining plans and production sequence. Comparison of these predictions with actual mine structural performance has developed confidence in the design of the mining method and in the geomechanical design methods being employed. Primary mining of the B-North zone is nearly complete, and the stopes have been generally stable, as projected in the original design. Some exceptions have occurred where locally stope pillars have failed due to the presence of adversely oriented sheared beds. Recovery of the pillars between the high-strength backfill is underway, and will account for large proportions of daily production very soon. Recovery of the first pillar has been accomplished with relatively few problems. The high-strength backfill formed a stable highwall and supported the undercut block immediately overhead.
Old Mines – New Regulations: The Re-opening of the Gordonsville,
Elmwood, and Cumberland Mines Under New DPM Regulations The Mid-Tennesee Zince (MTZ) mines, comprised of the Gordonsville, Elmwood, and Cumberland sites, last operated 4 years ago and are undergoing rehabilitation to begin production in 2008. The mines utilize a random room-and-pillar mining method that results in complex ventilation networks. Upon decommissioning, the mechanical ventilation units were removed and disposed of and several shafts sealed. In an effort to re-open the mines and operate in compliance with imminent Mine Safety Health and Administration (MSHA) diesel particulate matter (DPM) regulatory limits of 160TC μg/m3, a new ventilation system was required. The new ventilation system was developed primarily based on the diesel equipment required to meet production. Particular emphasis was placed on the DPM, diesel equipment utilization, main fan sizing, and fan locations to enable the mines to meet MSHA’s ventilation requirements in a cost efficient manner.
Recent Experience Using Telltale Roof Monitoring Systems (click to view
Ground Control paper in PDF
format) Extensive arrays of telltale roof monitoring instruments were recently installed in a Mexican copper mine and a large underground mine storage facility in the eastern U.S. The data were used to evaluate roof stability during development and retreat mining and after installation of supplemental supports. Both manual-reading and automated systems were employed and consisted of up to 64 instruments. This paper will discuss the practical issues involved with installation and monitoring, including reliability and maintenance. Examples will be given for telltale response in relation to known events affecting roof stability, including nearby pillar extraction roof caving, installation of supplemental cable bolts, and separation of the immediate roof layer. The strategy for processing the large quantity of data, presenting the data for review, monitoring the system remotely, and identifying and reporting critical events will be described.
Rock Movement Induced by Bench Blasting A substantial number of Great Basin gold and silver ore bodies mined by open-pit methods in the Western United States are sediment-hosted, structure-controlled, disseminated deposits. Of the sediment-hosted deposits discovered in the United States, the majority are in Nevada, accounting for 70 percent of Nevada gold production and 44 percent of all U.S. gold. Typically, the mineralization is fine-grained, low-grade, and offers little to no visual distinction between ore and waste. Under these circumstances, ore is discriminated from waste by conceptual criteria, often by economic cutoffs, which have no corresponding observable physical periphery in the field. Conventional and geostatistical ore reserve models are generally the principal, if not the sole, means of defining ore blocks. In order to establish the ore-waste boundaries, assay values from regularly spaced blast hole samples are used to interpolate the borders of digging polygons based on their pre-blast bench locations. Open pit bench blasting typically results in varying degrees of rock mass displacement, which is a practical necessity for adequate rock fragmentation. The resulting horizontal movement of ore beyond its pre-blast boundaries has the potential to severely diminish the accuracy of even the most precise ore production estimates. Blast movement will typically dilute ore with waste, i.e., economic material replaced by sub-economic material, causing waste to be mined as ore and ore as waste. Economically detrimental consequences include:
The current industry practice of re-establishing post-blast ore-waste digging polygons based on their pre-blast locations fails to consider the possibility of ore migrating beyond its originally defined boundaries. Production optimization in these situations must first consider the potential for blast-induced movement to assure the proper application of reserve models to in-pit ore grade control practices. In order to overcome the deleterious effects of blast-induced displacement, detailed studies of movement involving a number of blasts are required to formulate a realistic model of blast movement for a specific mine in its unique geological setting. For this purpose, blast movement monitoring was conducted at the Coeur Rochester open-pit silver and gold mine near Lovelock, Nevada during the period from May to August 1994. During this investigation, a detailed set of movement data and support information detailing the geologic and blasting environments was collected. Analyses of results from the Coeur Rochester mine were applied to the formulation, calibration, and evaluation of an empirical blast movement model in conjunction with a computer simulation of the blast movement phenomenon. With an understanding of the movement mechanism, grade dilution was quantified and site-specific recommendations developed to mitigate future production losses.
Rock Reinforcement Longevity
Rock reinforcement has been accepted in mining and construction since the 1950s. The longevity of rock reinforcement is often questioned in engineering design because it's historical performance is measured in just decades, relies on natural systems to function, and is not easily inspected. The elements of a typical rock reinforcement fixture include an anchoring device, a bar or tube, a plate, a head bolt, and possibly grout.
19th Conference on Ground Control in Mining, Morgantown, West Virginia,
August 8–10, 2000 Rock reinforcement has been in widespread use and generally has been accepted in underground mining and tunneling since the 1950s. The first rock reinforcement technologies employed were mechanical anchors such as split wedges or expansion shells with ⅝-inch-diameter steel bolts. Failures occurred in weak strata which provided poor anchorage or in ground with corrosive waters. Friction rock fixtures consisting of relatively thin-walled tubes have been in use for about 15 to 20 years. While generally performing adequately, longevity problems have developed from corrosion in water bearing ground. Longevity of rock reinforcement is much enhanced with grouted bolts. Portland-cement-grouted rock reinforcement has been in use since the mid-1950s, primarily in tunneling and other civil engineering underground construction. Tests of decades-old installations have revealed few problems except in ground with aggressive waters. Polyester-resin-grouted rock reinforcement was introduced in the United States to mining in the late 1960s and to tunneling in the early 1970s. Experience from 30 years of resin-grouted bolt installations and field tests have identified longevity problems associated with degradation of steel reinforcing bars, but in generally unusual situations. Improvements continue in resin chemistries, packaging, corrosion protection, grout quantities, and mixing and distribution in the drilled hole to achieve long-term performance. Specific case histories cited with resin- or Portland-cement-grouted rock reinforcement longevity or performance problems, upon close examination, reveal that the causes of the problems were quality control procedures being inadequate and not in accordance with good practice. Manufacturers' recommendations and engineering specifications, if followed in the field, and with competent inspection and supervision, would have prevented most, if not all, reported longevity or performance difficulties.
Stability Assessment of an Underground Limestone Mine--A Case Study
The stability of an underground limestone mine became a concern following numerous roof falls. Factors influencing roof behavior were identified and a program of investigation initiated to understand the causes of the roof falls and to develop recommendations for changes in mine layout and ground support. Factors of concern included high horizontal stress, the presence of water or gas in the roof, and the influence of jointing and fracturing in the roof rock. The investigations included site visits, stress measurements, and three-dimensional numerical analysis. Mine roof falls identified on the mine map were tabulated and categorized according to size and orientation and, where possible, by other distinguishing features such as jointing or water pressure. The vertical in situ component of stress was determined to be approximately 11.72 MPa, about double the magnitude from gravity loading alone. The depth of cover for the mine ranges from slightly more than 167.64 m to more than 243.84 m. The numerical simulations indicated that reduction in span widths from 15.24 to 12.19 m (E-W) and from 12.19 to 9.14 m (N-S) while maintaining center dimensions did not appreciably change the stability factors, and that the orientation of the mine layout was near optimum.
Stability Evaluation During Bench
Cut-and-Fill Mining of the B-NEATH Zone at the Cannon Mine Bench cut-and-fill stopes with heights between 85 and 105 ft are being excavated in the lower levels of the B-NEATH Zone. Estimates of pillar strength and pillar loading indicated a high probability of pillar failure. Some changes in mining practice were made to minimize impacts of pillar failure, and a rock mechanics instrumentation program was employed to monitor stability during mining of the first stopes. Limited pillar failure, in the form of destressing, was observed in the initial stopes; however, global stability during mining was good. Instrumentation data indicated that predictions of pillar strength and stress were quite close to field results.
Stability of Open Stopes Intercepted by Graphitic Shears at the Ashanti
Goldfields Ltd., Obuasi Operations in Ghana
A geotechnical study was conducted to identify the cause(s) of stope instability at the Ashanti Mine (Obuasi, Ghana). As part of the investigation, laboratory testing of rock was performed and the rock mass was classified according to the Q-System (Barton et al., 1974). Numerical analyses were performed using the computer program FLAC (Itasca, 2001). The observed failure zone around the stopes was simulated using both Mohr-Coulomb and Hoek-Brown (Sheorey, 1997) models. Yielding along peripheral graphitic bands and the tensile failure were clearly identified to be the major contributors to stope instability.
Ventilation Planning for the Agnico-Eagle Pinos Altos Gold-Silver Project The design of a major underground ventilation and environmental control system is a complex process with many interacting features and, therefore, must not be treated in isolation to avoid costly retrofits later in the life of the mine. To begin underground production, it became necessary for the Agnico-Eagle Mines Ltd. Pinos Altos Gold-Silver project, located in the State of Chihuahua in Northern Mexico, to estimate the air volume required to ventilate the proposed mine while taking into consideration the type, number and size of underground equipment, as well as the required rate of mineral production. Mineralized zones at Santa Niño are up to 800 m long by up to 700 m deep for a distance of almost 5 km. The zones named from east to west are El Apache, Oberón de Weber, San Eligio, Santo Niño, and Cerro Colorado. Ventilation simulation for Santa Niño for a set of life-of-mine plans involving two ramps (one high and one low) and a series of raises with a ramp connection to the Oberón de Weber were analyzed using VnetPC™. In addition, the Oberón de Weber was evaluated as a stand-alone mine and the Santa Niño evaluated independently for the early operating years of the mine. Based on the ventilation simulation options analyzed within the constraints of the mine plans, the quantities required to ventilate the mine were estimated, the main fans were selected, and control device locations and power requirements specified. |
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