Assessment and Control of Ore Dilution in Long Hole Mining: Case Studies
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- Henning, J.G. & Mitri, H.S. Geotech Geol Eng (2008) 26: 349. doi:10.1007/s10706-008-9172-9
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Unplanned ore dilution has a direct and large influence on the cost of a stope, and ultimately on the profitability of a mining operation. This paper presents the results of an examination of factors influencing ore dilution in a blasthole stoping environment. For the study, a comprehensive database was established, incorporating information related to the design, construction, excavation and cavity surveys of 172 sequentially mined long hole stope case histories from two orezones. Through a review of the case studies, it was demonstrated that, in addition to stope dimension, the amount of unplanned dilution differed according to stope type. Five stope types were identified, based on their position within a tabular blasthole mining sequence. Measured overbreak varies with stope type, with secondary stopes generating a greater volume of hanging-wall dilution than do primary stopes. Furthermore, a case study is presented to demonstrate the role cablebolts installed in the stope hanging-wall play to control ore dilution. The study illustrates relationships between measured hanging-wall overbreak, cablebolt orientation and stope type.
KeywordsParametric numerical modelingOre dilutionRock mechanicsUnderground miningHanging-wall reinforcementStope typeStope case study
In a global competitive market, there is pressure on mines to maximize production and increase revenue. Unplanned ore dilution has a direct and large influence on the cost of a stope, and ultimately on the profitability of a mining operation. The economic impact of dilution is due to costs associated with the mucking, haulage, crushing, hoisting, milling, and treatment of waste or low grade rock having little or no value, displacing profitable ore and processing capacity. The additional time required for excavation and backfilling of the larger stope volumes produced by the extraction of waste rock can also lead to unscheduled delays and changes to the mining schedule.
Following a discussion of dilution terminologies and a review of a parametric numerical modelling study examining factors influencing dilution in a long hole stope setting, the outcome of case study ore dilution assessments are provided in this paper.
2 Assessment of Ore Dilution
Sources of planned dilution can be associated with the limitations of the mining method, when mining irregular, small thickness or narrow vein deposits, or when unsuitably sized equipment has been selected (Trevor 1991). The contact between ore and subgrade material cannot be followed in detail due to a lack of flexibility of the drilling machines and blasting utilized by the mining operation (Elbrond 1994), leading to planned dilution.
Unplanned dilution sources are associated with instability of the stope wall. Dilution takes place wherever the low mechanical strength of the wall results in additional material being dropped into the stope. When overbreak causes the rupture of an individual bed or structural feature on the stope hanging-wall it is difficult to prevent failure of the remainder of that feature into the open stope (Pelley 1994). Stope wall stability is also adversely affected by blasting damage (Valleé et al. 1992).
Other contributors to unplanned dilution include: drill set-up errors; material handling errors, such as dispatching of waste rock into the ore passes; backfill dilution due the failure of free standing backfill faces; and the age of the mine. At the late stages of mining, associated with pillar recovery and deteriorating ground conditions, the amount of dilution increases (Trevor 1991).
2.1 Quantification of Dilution
In their review of Canadian mining practices, Scoble and Moss (1994) reported that Eqs. 1 and 2 were the most widely used. Of these two, Eq. 1 was recommended as a standard measure of dilution (Pakalanis et al. 1995), as it was more sensitive to wall sloughage. For example, a 2:1 sloughage-to-ore ratio produces a 66% dilution factor according to Eq. 2, whereas Eq. 1 produces a dilution factor of 200%. Dilution is usually calculated as a percentage.
Narrow stopes mined by long hole method are generally victims of considerable dilution: the narrower the zone is, the more important the border effects, (Valleé et al. 1992). For example, if both the hanging-wall and footwall of a steeply dipping 1.5 m wide tabular deposit contributes 0.3 m of overbreak, then an unplanned mining dilution of 40% results. If this orezone was 3.0 m thick, mined in the same conditions, the resulting dilution factor becomes 20%. To express dilution independently of stope width, Dunne and Pakalnis (1996) suggested that dilution values be calculated in terms of average metres of wall slough per square metre of wall (m/m2), rather than percent dilution.
When assessing dilution, it is necessary to understand the way a stope operates. The measurement of mined stope profiles has traditionally been difficult, due to the non-entry nature of the open stope mining method. In recent years, accurate surveying of excavated stope surfaces has been made possible with the application of automated non-contact laser rangefinders, such as the Cavity Monitoring System (CMS), described by Miller et al. (1992). The CMS consists of reflectorless laser rangefinder, which when extended into the stope cavity, provides a volume-based technique for directly measuring stope performance. First implemented by the Canadian mining industry, (Anderson and Grebenc 1995; Mah et al. 1995; Germain et al. 1996), it is now adopted by mines world-wide (Calvert et al. 2000; Uggalla 2001).
Empirical design techniques, such as those described in Mathews et al. (1981); Potvin (1988); and Pakalnis and Vongpaisal (1998) have gained acceptance as a simple, ‘first-pass’ means of generating broad design guidelines for primary stopes. Empirical back-analysis of CMS stope stability data can provide useful predictions of dilution or sloughage for particular stope design (Connors et al. 1996; Milne et al. 1996; Germain et al. 1996).
2.2 Previous Work
A parametric numerical modelling study, described in Henning and Mitri (2006, 2007), was undertaken to examine relationships of hanging-wall dilution with depth, stope dimension, dip angle, stress setting and stope type. The parametric study considered two criteria for overbreak: (i) the volume of relaxed ground available for overbreak, assuming the rockmass has no tensile strength, represented by the σ3 = 0 MPa; and (ii) the σ3 = σt contour, which accounts for rockmass tensile strength. When considering the sensitivity of individual factors on potential overbreak, it was found that mine depth does not play a significant role in the extent of potential hanging-wall dilution associated with the σ3 = 0 MPa contour. However, overbreak associated with rockmass tensile strength contour increased with depth.
Other trends observed included: (i) Tall slender stopes with large vertical and short horizontal dimensions or squat broad stopes having long horizontal and short vertical dimensions are more stable than large rectangular stopes; (ii) Modelled overbreak increased as the hanging-wall dip angle decreased; (iii) The influence on hanging-wall dip angle on overbreak is more pronounced as strike length increases; and (iv) Modelled overbreak is reduced when the orientation of pre-mining principal stress (σ1) is parallel to the strike of the stope.
Type S1 stope refers to a secondary stope, with rock on one side and the opposite side wall against backfill. S1-type stopes are common to pillarless stope sequence extraction sequences and to longitudinal mining methods. The other secondary stope, Type S2, refers to a secondary stope having both side walls against backfilled primary stopes. S2-type stopes are common to transverse mining methods.
3 Ore Dilution at Bousquet #2 mine—Case Study
With this paper, the parametric modelling study is extended to field validation by way of case study. Using comparisons of stopes from two discrete orezones at the Bousquet #2 mine, referred to as Block 5 and Zone 3-1, influences of individual factors on dilution relationships were examined.
3.1 Mine Setting
Bousquet #2 mine is a trackless bulk-mining operation, with production rates of approximately 1800 tonnes per day. Mining methods, equipment, manpower and design used for mining the main Bousquet #2 orebody (including Block 5) were also employed for mining of Zone 3-1.
Typical transverse stope dimensions, Bousquet #2 mine
Vertical height (m)
Strike length (m)
11000 to 14000 tonnes
4000 to 5000 tonnes
Block 5 consisted of 97 stopes, distributed over five stoping horizons. The strike length of the Block 5 orezone was up to 330 m. Zone 3-1 consisted of 75 stopes, distributed over eight stoping horizons. The strike length of the 3-1 zone was up to 110 m.
Rock properties for Block 5 and Zone 3-1, Bousquet #2 mine (Henning 2007)
Host rock and orezone
Foliated rhyolite tuff
Foliated rhyolite tuff
Rock Quality Designation, RQD (Deere 1968)
Rock Mass Rating, RMR (Bieniawski 1984)
Tunnelling quality index, Q (Barton et al. 1974)
Uniaxial compressive strength, MPa
3.2 Empirical Database Description
The information for the database was collected for each individual stope from a variety of sources in the Production, Geology and Engineering departments of the mine. Stope setting refers to the location and terminology used to describe a specific stope within the mining block. Stope type defines the local stope setting within the mining sequence. Stope geometry provides specific descriptions of excavated dimensions of individual stopes. Stope construction refers to mostly man-made influences on stope mining and recovery, excluding designed stope dimensions. Stope construction factors are associated with the design of hanging-wall reinforcement, drilling and blasting. Stope recovery refers to measured overbreak. Descriptions of the distribution of data from Block 5 and Zone 3-1 case histories are provided in the following sections.
3.3 Stope Geometry and Setting
Stope geometry has a significant influence on hanging-wall stability and the amount of dilution that can be expected. Stope dimensions are a variable factor which can be established during initial mine design, that influences overbreak. Geometry parameters collected from each stope of Block 5 and Zone 3-1 include: hanging-wall dip angle (which influences true stope height), strike length, hydraulic radius of the stope hanging-wall, designed stope size (tonnes blasted), and the azimuth of the hanging-wall strike.
When comparing the geometries of Block 5 and Zone 3-1, similarities were found in hanging-wall dip angles (78o vs. 83o, respectively), and true stope height (31.4 m vs. 31.1 m). Similar orientation of the hanging-wall strike of the two stopes (strike azimuth) suggests that the orientation of in situ principal stress (95o vs. 97o) is approximately perpendicular for both Block 5 and Zone 3-1.
As a result of identified similarities between the stopes of the two orezones, it was determined that the difference in strike length between the two zones drives the empirical assessment.
3.4 Stope Type
Overbreak volume represents the volume of unplanned dilution from the hanging-wall of individual stopes, as measured by the CMS scan. Surface area exposed refers to the designed hanging-wall surface area, represented as true stope height multiplied by stope strike length.
Summary of surveyed stope dilution from transverse-mined stopes
Total dilution, mean value (%)
Total measured hanging-wall and footwall DDcms (m)
Hanging-wall DDcms (m)
4 Comparison of Zone 3-1 and Block 5 Hanging-Wall Overbreak
Differences between the two zones can be attributed, in part, to differences in hanging-wall dimension. On the basis of empirical and parametric trends, the 10 m × 30 m stopes of Zone 3-1 were anticipated to be more stable that the larger 15 m × 30 m Block 5 stopes. As a result, less overbreak is anticipated from the smaller Zone 3-1 stopes.
With Block 5 stopes, a minority of primary and secondary stopes, 29% and 30% respectively, had hanging-wall Dilution Density of one meter or less. About 55% of primary stopes experienced hanging-wall Dilution Densities exceeding two meters, of which 11% exceeded three meters. Similarly, 44% of secondary stopes generated hanging-wall Dilution Densities exceeding two meters, including 16% of stopes which exceeded three meters.
Relationships between the individual Zone 3-1 stope types appear consistent with those identified in the previous numerical modelling study, although the actual DDcms values for the 10 m wide stopes were greater than the parametric estimates in that previous study. Numerical modelling (Henning and Mitri 2007) suggested that due to increased unsupported span along strike, leading to enhanced stress relief, secondary stopes experienced greater hanging-wall overbreak that primary stopes. S2-type stopes, being an extreme from primary stopes, experienced the greatest dilution, as illustrated in Fig. 3. Differences in the magnitude between actual and parametric values can be attributed to contributions from a variety of factors in addition to geometry, such as stope setting and construction.
With Block 5, the distinction between primary and secondary stopes is less apparent. The DDcms of primary and secondary P1 and P3 stopes were 30% and 19% lower, respectively, than S2 stopes. However, DDcms of P2 stopes exceeded that of S2 stopes by 11%. When considering all primary stopes against S2-type stopes, dilution from S2-type stopes exceeds that of the average Block 5 primary stope by 7%. (DDcms = 2.02 m vs. DDcms = 2.18 m).
Relationships of Block 5 stope hanging-wall dilution values differed from that of Zone 3-1 stopes, as average values for primary and secondary stopes tended to be similar. As with Zone 3-1, differences between the magnitude of actual and parametric values can be attributed to contributions from a variety of factors.
Apart from stope dimensions, another major difference between Zone 3-1 and Block 5 stopes is associated with hanging-wall reinforcement methodology. Stope hanging-wall reinforcement approaches for the two orezones are discussed in the following section.
4.1 Stope Hanging-Wall Reinforcement
In Block 5, hanging-walls are supported with modified geometry cablebolts installed on two meter centers, drilled in a fanned pattern, from cable drifts developed in the hanging-wall at 60 m vertical intervals. Hanging-wall cablebolt rings provide cablebolt reinforcement to stope blocks positioned above and below the cable drift elevation. Cablebolt orientation with respect to the stope hanging-wall ranges from approximately 85° (near-perpendicular to the hanging-wall) to 40°.
4.1.1 Influence of Hanging-Wall Cablebolting on Overbreak
To examine relationships between Dilution Density and Block 5 hanging-wall cablebolting, average DDcms values from all primary stopes were compared against that of S2-type secondary stopes. As mentioned previously, S2-type stopes represent an extreme from primary stopes, with mined and backfilled primary stopes on both side walls. The data was further processed to isolate DDcms values according to the orientation of the hanging-wall cablebolting; cable fan-up and cable fan-down.
Taking the relative effectiveness of cablebolt reinforcement into account, the DDcms values from stopes supported by upward fanning cablebolts are more representative of actual overbreak severity that would be generated with non-reinforced Block 5 stopes.
4.2 Relationship Between Hanging-Wall Overbreak and Extraction Sequence
4.3 Relationship Between Stope Cycle Time and Hanging-Wall Overbreak
The open stope cycle time exposure time is defined as the number of days between the first blast and the date at which the CMS scan was conducted. At Bousquet #2 mine, CMS scans were performed immediately upon the completion of mucking, and prior to the start of backfilling.
For Block 5 stopes, the stope exposure time ranged from as little as 17 days to as many as 127 days. Fifty percent of Block 5 stopes had a hanging-wall exposure time less than 37 days. As anticipated, the smaller Zone 3-1 stopes typically had a shorter stope exposure time ranged from as little as 12 days to as many as 127 days. Fifty percent of Zone 3-1 stopes had a hanging-wall exposure time less than 24 days.
5 Summary of Hanging-Wall Dilution Database
The severity of hanging-wall overbreak, as measured by CMS scans, was assessed for two discrete Bousquet orezone blocks, referred to as: Zone 3-1 and Block 5. To aid in the comparisons, measured Dilution Density (DDcms) was a term introduced.
Trends Identified from the Data Included:
Differences of overbreak associated between the differing stope types of Zone 3-1 and Block 5 were examined. With Zone 3-1, average hanging-wall overbreak associated with secondary stopes was found to exceed that of primary stope types by 62%.
In Block 5, hanging-wall overbreak associated with secondary stopes was found to exceed that of primary stope types by 7%. However, it was shown that the magnitude of Block 5 hanging-wall dilution was influenced by the presence of cablebolt reinforcement. When considering only those stopes supported by cablebolts installed in an unfavourable orientation (cablebolts fanning upwards), average hanging-wall overbreak associated with secondary stopes was found to exceed that of primary stope types by 16%.
Although at greater depth, Zone 3-1 stopes, with smaller hanging-wall dimensions, generated significantly less hanging-wall overbreak that did Block 5 stopes. Mean DDcms values for Zone 3-1 stopes were 0.94 m, values for Block 5 were DDcms = 1.96 m.
The Zone 3-1 stopes generated less total overbreak (hanging-wall and footwall overbreak combined) than did Block 5 stopes. Mean total DDcms values for Zone 3-1 stopes were 1.69 m, values for Block 5 were DDcms = 2.45 m.
The distribution of overbreak varied between the two orezones. In Block 5, 80% of the total overbreak was associated with the hanging-wall, while 60% of the total Zone 3-1 overbreak was associated with the hanging-wall.
The stopes of Zone 3-1 and Block 5 had numerous comparable features, such as hanging-wall dip, mining method, stope construction, mucking rate and workforce.
The authors wish to thank Agnico-Eagle Mines Limited for granting permission to use the data compiled from Bousquet #2 Mine.