Backfill for Bench Stoping Operations

Transcription

1 Backfill for Bench Stoping Operations E. Villaescusa Professor of Mining Geomechanics, Western Australian School of Mines K. Kuganathan Senior Backfill Research Engineer, Mount Isa Mines ABSTRACT This paper describes a new methodology that can be applied to compare several bench extraction strategies requiring backfill. The method is applicable at the planning or operational stages and can be used to maintain dilution within design parameters and improve the overall economics of bench stoping operations. The parameters influencing bench performance have been empirically rated based on economical, geomechanical, operational and backfill properties. Four different extraction strategies have been considered and rated from most preferred to less preferred using an integrated approach to bench extraction. INTRODUCTION The success of mining by the bench stoping method largely depends upon the level of understanding of critical wall exposure, usually unsupported behaviour, the application of remote mucking technology, drilling and blasting optimization and the appropriate use of backfilling technology (Villaescusa et al, 1994). The economics are influenced by the effectiveness of the adopted bench design and also on having an extraction strategy that matches the site conditions. Bench design is controlled by the geometrical dimensions such as sublevel interval, and the exposed stable lengths likely to match the expected rockmass conditions. Stability charts such as the HSR method (Villaescusa et al, 1997) or the Modified Stability Graph (Potvin et al, 1989) can be utilized during the planning and design stages to calculate the required bench dimensions. An extraction strategy related to the maximum stable length that can be safely exposed, and the type of backfill to be used is usually identified during the design stages. In most cases, permanent infra-structure such as ramp access configurations are also fixed within an initial mine design stage, leaving the extraction strategy as the only flexible (and most important) parameter to be optimized during the subsequent production stage. THE ROLE OF BACKFILL Backfill can be generally described as any material that is placed underground to fill the voids created by the extraction process. In up-dip bench extractions, the backfill provides a working floor for mucking and also helps to stabilize the exposed spans by minimizing deformation and dynamic loading of the excavated walls from blasting. Following extraction of an economic

2 length of a steeply dipping orebody, the void created by a bench stope can be filled with hydraulic fill or dry fill (waste) to the floor of the drill drive which becomes the new extraction horizon on the next lift as indicated on Figure 1. Dry rockfill can be used to minimize deformations (and optimize stability) while the benches are being extracted, provided that the backfill can be kept sufficiently far away to minimize dilution of the broken ore by fill at the interface. Empirical stability charts such as the HSR (Villaescusa et al, 1997) and the Modified Stability Graph Method (Potvin et al, 1989) can be used to determine the maximum unsupported strike lengths which can be safely exposed during continuous backfilling operations. An optimal use of the 'critical strike length' concept would ensure that excessive dilution does not occur during production blasting, where the blasted material may be thrown on top of closely located backfill rills, contributing to contamination of the ore during mucking. Production Blasting Filling Maximum Unsupported Span (Critical Strike Length) Bench Limit Mucking ORE BACKFILL Backfill Previous Bench Figure 1. Schematic of continuous bench backfilling techniques. The beneficial support provided by the backfill is very important in order to minimize deformations experienced by the exposed unsupported s as the stope is being extracted or following bench completion. Hangingwall deformation data collected from properly located multiple point extensometers has shown that backfill effectively stops the large scale deformation of the unsupported layers during bench stoping (See Figure 2). Deformation (mm) stope 30 blastings FP1 Exto 1 Backfill introduced here anchor depth into H/ W A1-0.5m A2-1.5m A3-2.5m A4-3.5m A5-7.5m A6-Ref 0 2/ 22/ 93 3/ 14/ 93 4/ 3/ 93 4/ 23/ 93 5/ 13/ 93 6/ 2/ 93 6/ 22/ 93 Date

3 Figure 2. Influence of backfilling on deformations. Instrumentation has also been used to determine the dynamic response of the stope walls as a bench stope is extracted and filled progressively. Table 1 presents a frequency analysis of instrumented walls using triaxial arrays of geophones indicating that the wall of a filled stope (using dry fill) behaves like a closed wall (i.e., intact solid ground, where no void has been created). All the blast vibration data was collected at approximately 5, 9 and 13m into the of the stope (Villaescusa et al, 1997). The beneficial impact of backfill to stabilize the rockmass surrounding a stope void is very clear from the data presented in Table 1. Promptly placed backfill appears to reduce the dynamic loading caused by blasting, thus enhancing the overall regional rockmass stability. Table 1. Dynamic response of a rockmass as backfilling proceeds. Backfill Status Dominant Frequency (Hz) Average Frequency (Hz) Number of Data points None Closed walls (1.5m burden) None Closed walls (3m burden) None 6m open span None 9m open span None 15m open span stope empty 15m open span Stope ½ filled Stope 3/4 filled Stope 5/6 filled Stope filled Stope filled EXTRACTION STRATEGIES In mining operations where the bench heights are fixed during mine development, the extraction strategy is the only variable that can be used to optimize the economics of bench stoping. The extraction strategies considered within this study include: 1) Extraction using a continuous dryfill mass (waste rock having a rill angle between degrees) that follows an advancing bench brow at a fixed distance (not exceeding a critical unsupported strike length) along the entire bench length (as shown in Figure 1).

4 2) Extracting a bench to a maximum stable unsupported strike length, followed by backfill using hydraulic fill in conjunction with brick bulkheads. This is followed by pillar recovery and the process is repeated along entire bench length (See Figure 2). Although this strategy is primarily linked to hydraulic fill, the use of cemented fill would ensure that minimal fill dilution would be experienced following pillar recovery. Temporary pillar (drilled) New Slot Production recovered pillar Blasting hydraulic fill Maximum strike length (void filled) Bench Limit Mucking ORE Bulkheads Backfill Figure 2. Hydraulic fill and pillar recovery 3) Leaving (planned) permanent pillars between independent (unfilled) spans along the entire bench length. Backfilling is done at bench completion using either dry or hydraulic fill (See Figure 3). On this strategy, it is critical to establish the optimum distances between the pillars in order to minimize the number of pillars required, especially in high grade orebodies. Pillar dimensions are a function of the ground conditions, the expected stress levels and the optimum extraction of the adjacent long hole winzes. Blast monitoring programs can be implemented to determine optimum cut lengths to be taken during long hole winzing in order to minimize blast damage to the adjacent areas and specially the narrow pillars (Villaescusa et al, 1997). In weak rockmasses, the stability of un-filled spans may be affected by blasting in adjacent spans along the strike of the orebody. permanent pillar New Slot Production Blasting permanent pillar Mucking ORE Maximum unsupported strike length (void to be filled at bench completion) Permanent pillar Bench Limit Backfill Backfill Figure 3. Non recoverable permanent pillars in conjunction with backfill. 4) The introduction of full Avoca techniques, where the bench is extracted along the entire length by a repetitive process which requires the bench to be

5 extracted to a maximum stable length, the void tight filled to the brow, and the subsequent bench blasting to be taken with no free face (See Figure 4). Filling Filling Production blasting (no free face) Continuous AVOCA fill Bench Limit Mucking ORE BACKFILL Backfill Previous Bench Figure 4. Full Avoca extraction method. The option of extracting a bench beyond its stable limits and then leaving a (unplanned) pillar to arrest a failure has not been considered in this analysis, because it does not represent good design or operational practices. Option 3 above is related to extracting the bench using pillars that have been designed at the very early stages, and it is assumed that the spans between pillars are stable and independent (from a deformational point of view) of each other. In order to assess the effectiveness of each of the bench extraction strategies, several parameters were considered (See Figure 5). Geomechanics is related to the ability of a system to provide adequate support and to minimize dilution and the effects of blast damage to the adjacent rockmass. Fill performance is related to optimal selection of fill material and the placement method in order to reduce cycle time, dilution, ore losses and to enhance the stability of the fill itself as well as the surrounding rockmass. The economics of any of the selected strategies is controlled by factors such as cost of backfilling (materials and placement), dilution, ore loss, poor fragmentation, etc. all contribute to the success and applicability of each of the bench extraction strategies considered. Fill performance 1. Material selection 2. Placement & compaction 3. Cycle time 4. Stability/failure 5. Dilution/oreloss Bench Performance Economics 1. Cost of fill material 2 Cost of placement 3 Cost of dilution 4 Cost of ore loss 5 Cost of operational issues (mucking fragmentation, etc) Geomechanics 1. dilution 2. support 3. blast damage Figure 5. Parameters influencing bench performance

6 Optimization of any bench extraction strategy can only be achieved by analyzing the problem in an integrated manner. Any attempt to analyze benching based on a single, isolated issue could lead to erroneous and misleading conclusions. Effective, preventative measures to improve stability and reduce rock mass damage should be provided in the design stages to prevent fill mucking and ore contamination in the extraction stages. This will automatically minimise the problem of dilution. The best approach to dilution control is not the measurement of dilution, but to concentrate in prevention at its sources by due diligence at the design and extraction stages. Experience suggests that is very difficult to estimate the amount of waste or fill material mucked out either to gain access to broken ore or in the process of removing the ore. Any attempt to estimate dilution by relating it to a solid ore volume or bucket count in the mucking process may not quantify dilution with the required accuracy. Cavity monitoring using the Optech system may be the only way to quantify ore loss or fill dilution within single digits. Once an inadequate design has been implemented (for example an excessive bench height in poor rockmass conditions), it is very difficult to optimize any extraction strategy. Furthermore, when the excavations are extracted to instability due to indifferent or inefficient operational practices, excessive dilution to the ore stream is likely. Remedial measures such as arresting failures with unplanned pillars can be effective but this can lead to orelosses and blast damage to the surrounding rockmass. EMPIRICAL RATING OF EXTRACTION STRATEGIES The paper proposes an empirical selection procedure for bench and filling extraction strategies (See Table 2). The classification rates the most significant controlling parameters and can be used to determine the most suitable extraction strategy for a particular mine site. The preferred bench extraction strategies are linked to the use of conventional dry fill and hydraulic fill options. In mine sites where a hydraulic fill plant is not available, the classification would be limited to three extraction strategies (all linked to dry fill). The selection procedure presented in Table 2 can be used in conjunction with an economic model of backfill that accounts for stope dimensions, the cost of backfill material and transport to the stope, number of bulkheads, filling rate, etc. Figure 6 shows the results from a model developed by Mount Isa Mines and suggest that high lift benches should be filled using hydraulic fill. The strategy to fill short lift benches will depend upon the rockmass quality (maximum stable length) i.e., for short lift benches dry fill or wet fill may be recommended (See Figure 6). In high lift benches (24-45m high), adequate stabilization of the unsupported s is very difficult using continuous dry fill as shown in Figure 7. The shallow rill angle (38-42 degrees) of the dry fill means that a significant portion of the exposed walls remain unsupported, even if the rill reaches the

7 advancing bench brow. The backfill mass is likely to interact with the broken ore muckpile, thereby contributing to dilution. 50 One way tramming distance = 200m bench width = 7m Bench Height (m) high lift benches dry fill recommended most likely stable length hydraulic fill recommended short lift benches Bench Length (m) Figure 6. A conceptual model of backfill material and transport cost. Filling Production blasting Bench height > 24m Backfilled Unsupported Hangingwall area Most likely critical strike length ore mucking Figure 7. Schematic long section view of a high lift bench stope. The selection criteria presented in Table 2 suggest that full Avoca is the least recommended method of extraction. This is particularly true in high lift benches where blast damage, backfill stability and excess dilution are likely to become an issue. High lift benches require larger diameter blastholes, compared with short lift benches, in order to minimize hole deviation. However, the large blasthole sizes are likely to increase the level of blast damage, especially when the detonation occurs under full confinement from the Avoca backfill. Table 2 identifies seven key parameters (blast damage, support, fill stability, dilution, ore loss, material cost and operational issues) likely to control the economics and the performance of bench stoping operations. The total rating for each extraction strategy is simply calculated by adding the numbers on each column.

8 The results on Table 2 were calculated assuming that each of the controlling parameters had equal weighting. Alternatively, the most suitable extraction sequence can be determined by weighting the parameters in order of importance for a particular mining site. An example from the Lead Mine at Mount Isa Mines is used to illustrate the methodology (Table 3). The results indicate that hydraulic fill is the recommended option for that particular set of parameter weightings. Similar exercises can be undertaken for any mine site, provided the weighting of the parameters controlling bench performance is determined. In all cases, the chosen extraction strategy is the one with the maximum number of points. Table 2. Empirical rating of bench extraction strategies Strategy weight (most preferred = 4, least preferred = 1) Parameters Extraction Strategy to be optimized Hydraulic fill Permanent pillars Continuous Dry fill Minimize blast damage Maximize support Maximize fill stability and minimize fill dilution Control of dilution from failures Minimize ore loss at stope boundaries Minimize backfill (material & transport) cost Operational issues repeated Long Hole Winzing (LHW) to create pillars, but support provided by hydraulic fill (H/W) deformations minimized by tight filling moisture content in hydraulic fill likely to allow steep angle of exposure against recovered pillar potential for failure before hydraulic fill, repeated LHW detrimental, but rockmass supported by HF experience with cut&fill mining and during earlier benching indicates that minimal oreloss is expected lowest material cost, provided significnat runs can be achieved and the number of bulkheads minimized bulkhead & pipelines set-up, repeated longhole winzing, pillar recovery repeated LHWinzing to create pillars (unfilled while blasting) rockmass may continue to deform between pillars fill is not placed or exposed within extraction sequence potential for failure between pillars, repeated LHW detrimental, but failure arrested by the pillars ore left behind in pillars to enhance stability of independent unfilled spans. This ore will never be recovered less backfill material required due to pillars left in place. Hydraulic fill or dry fill can be used. repeated longhole winzing, backfill at bench completion conventional blasting with a free face along bench length H/W deformations arrested only on backfilled portion fill is not required to stand steeper than its natural angle in order to provide support, but close to blast (dilution) potential failures within the unsupported areas can not be arrested and likely to follow each blast ore left behind at the top of the fillmass, where is thrown by blasting. Ore left in any unfilled gaps near the bench more expensive than hydraulic fill, requires mucking units to be used. a single slot followed by a repetitive process of extraction and dry backfilling Full AVOCA repeated blasting without free face likely to create damage due to confinement H/W deformations arrested by earlier placement of backfill low moisture content fill required to stand at very steep angles close to confined blastings minimal H/W lengths exposed, but confined blastings may cause instability ore wedged into fillmass and ore left at the toe of the fill inorder to achieve a steep fill rill angle similar to conventional, but additional stop logs needed in filling horizon, spilling of material in blastholes a single slot followed by a repetitive process of extraction and tight dry backfilling General comments Blast damage likely to control up to 15% of overall behaviour Support to unsupported (not cabled) span is critical to overall bench stability Significant dilution may occur at the fill/muck rill interface during mucking operations Hangingwall material can not be easily separated from ore during horizontal mucking Broken ore loss (unmucked within the bench stope likely to be detrimental to stope economics (difficult to measure) The cost of fill productioin and reticulation likely to increase unit cost Distruption to routine operations likely to decrease extraction rate Recommended:

9 Total Rating 23 (most preferred) (least preferred) Table 3. Empirical rating of bench strategies, Mount Isa Mines Lead Mine. Parameter weight (most important = 7, least important = 1) Parameters Extraction Strategy to be optimized Hydraulic fill Permanent pillars Continuous Dry fill Full AVOCA minimize blast damage repeated Long Hole Winzing (LHW) to create pillars, but support provided by hydraulic fill repeated LHWinzing to create pillars (unfilled while blasting) conventional blasting with a free face along bench length repeated blasting without free face likely to create damage due to confinement 1). Conventional dryfill for short lift benches. 2). HF for high lift benches General comments Blast damage likely to control up to 15% of overall behaviour subtotal H/W rockmass may H/W deformations maximize deformations continue to deform arrested only on minimized by tight between pillars backfilled portion (H/W) filling support H/W deformations arrested by earlier placement of backfill subtotal maximize fill stability and minimize fill dilution subtotal (7) control of dilution from failures moisture content in hydraulic fill likely to allow steep angle of exposure against recovered pillar fill is not placed or exposed within extraction sequence fill is not required to stand steeper than its natural angle in order to provide support, but close to blast (dilution) low moisture content fill required to stand at very steep angles close to confined blastings potential for failure before hydraulic fill, repeated LHW detrimental, but rockmass supported by HF experience with cut&fill mining and during earlier benching indicates that minimal oreloss is expected potential for failure between pillars, repeated LHW detrimental, but failure arrested by the pillars ore left behind in pillars to enhance stability of independent unfilled spans. This ore will never be recovered potential failures within the unsupported areas can not be arrested and likely to follow each blast ore left behind at the top of the fillmass, where is thrown by blasting. Ore left in any unfilled gaps near the bench minimal H/W lengths exposed, but confined blastings may cause instability subtotal (6) minimize oreloss at stope boundaries ore wedged into fillmass and ore left at the toe of the fill inorder to achieve a steep fill rill angle subtotal less backfill material more expensive than minimize required due to hydraulic fill, backfill pillars left in place. requires mucking (material & Hydraulic fill or dry units to be used. transport ) fill can be used. cost lowest material cost, provided significnat runs can be achieved and the number of bulkheads minimized similar to conventional, but additional stop logs needed in filling horizon, spilling of material in blastholes subtotal (5) operational issues bulkhead & pipelines set-up, repeated longhole winzing, pillar recovery repeated longhole winzing, backfill at bench completion a single slot followed by a repetitive process of extraction and dry backfilling a single slot followed by a repetitive process of extraction and tight dry backfilling Support to unsupported (not cabled) span is critical to overall bench stability Significant dilution may occur at the fill/muck rill interface during mucking operations Hangingwall material can not be easily separated from ore during horizontal mucking Broken ore loss (unmucked within the bench stope likely to be detrimental to stope economics (difficult to measure) The cost of fill productioin and reticulation likely to increase unit cost Distruption to routine operations likely to decrease extraction rate

10 subtotal Global Rating (most preferred) (least preferred) Recommended: hydraulic fill or dry fill As explained earlier (See Figure 6), a model that accounts for the volume of material to be used, the cost of the material and the transport to the stope must also be considered. On that particular case in the Lead Mine, continuous dry fill is used for short lift benches, while hydraulic fill is used for high lift benches. CONCLUSIONS The most economical bench extraction strategy can be recommended by considering a series of seven controlling parameters that can be rated according to their local importance in a particular mine site. Geomechanical, economical and operational issues that can be linked to backfill are likely to influence the overall bench performance and economics. The methodology developed can be applied at the planning or during the operational stages of bench extraction. REFERENCES Potvin, Y., M. Hudyma, and H. Miller, Design Guidelines for open stope support. CIM Bulletin, 82. Villaescusa E., L.B. Neindorf, and J. Cunningham, Bench stoping of lead/zinc orebodies at Mount Isa Mines Limited. Proceedings of the MMIJ/AusIMM Joint Symposium, New Horizons in Resource Handling and Geo-Engineering, Yamaguchi University, Ube Japan, Villaescusa, E., C. Scott and I. Onederra, Near field blast monitoring at the Hilton Mine. Mount Isa Mines Technical Report, No. Res Min 78. Mining Research, Mount Isa Mines Limited. Villaescusa E., D. Tyler and C. Scott, Predicting underground stability using a stability rating. Proccedings of the 1 st Asian Rock Mechanics Symposium, Environmental and Safety Concerns in Underground Construction (H.K. Lee, H.S. Yang and S.K. Chung, Editors), Seoul Korea,