Geotechnical and Geological Engineering

, Volume 26, Issue 4, pp 349–366

Assessment and Control of Ore Dilution in Long Hole Mining: Case Studies

Authors

    • Goldcorp Canada Inc., Porcupine Gold Mines
  • Hani S. Mitri
    • Department of Mining, Metals and Materials EngineeringMcGill University
Original Paper

DOI: 10.1007/s10706-008-9172-9

Cite this article as:
Henning, J.G. & Mitri, H.S. Geotech Geol Eng (2008) 26: 349. doi:10.1007/s10706-008-9172-9

Abstract

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.

Keywords

Parametric numerical modelingOre dilutionRock mechanicsUnderground miningHanging-wall reinforcementStope typeStope case study

1 Introduction

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

The term ‘dilution’ refers to any waste material within the mining block, including barren and subgrade rock and backfill. Dilution can refer to either a measure of external waste (unplanned dilution) that has sloughed from the stope wall, or to material that is of lower grade than cut-off, but which is included in the mineral deposit, reserve or stope outline and extracted with the mining of ore (planned dilution). See Fig. 1. Unplanned dilution, the focus of this study, refers to additional non-ore material derived from rock or backfill outside the stope boundaries due to blast induced overbreak, sloughage of unstable wall rock, or sloughing of backfill (Scoble and Moss 1994). The term ‘overbreak’ is synonymous with unplanned wall dilution.
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Fig. 1

Schematic illustration of planned and unplanned dilution, plan view

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

Production control has traditionally been based on weight of rock hoisted, rather than the weight of metal. A value of dilution is recorded by most mines, (Canadian Mining Journal 2002), although not in an identical manner. A survey of mines throughout Canada by Pakalnis (1986) identified several variations on a definition of dilution. These include:
$$ {\text{Dilution}} = ({\text{Tons}}\,{\text{waste mined}})/({\text{Tons}}\,{\text{ore mined}}) $$
(1)
$$ {\text{Dilution}} = ({\text{Tons}}\,{\text{waste}}\,{\text{mined}})/({\text{Tons}}\,{\text{ore}}\,{\text{mined}} + {\text{tons}}\,{\text{waste}}\,{\text{mined}}) $$
(2)
$$ \text{Dilution = (Tonnage mucked} - \text{tonnage blasted) / (Tonnage blasted)} $$
(3)
$$ {\text{Dilution}} = {\text{Difference}}\,{\text{between}}\,{\text{backfill}}\,{\text{tonnage}}\,{\text{actually}}\,{\text{placed}}\,{\text{and}}\,{\text{theoretically}}\,{\text{required}}\,{\text{to}}\,{\text{fill}}\,{\text{void}} $$
(4)
$$ {\text{Dilution}} = {\left({{\hbox{``x''}}\;{\text{amount}}\;{\text{of}}\;{\text{meters}}\;{\text{of}}\;{\text{footwall}}\;{\text{slough+}}{\text{``y''}}\;{\text{amount}}\;{\text{of}}\;{\text{hanging}}\;{\text{wall}}\;{\text{slough}}}\right)}/{\left( {{\text{ore}}\;{\text{width}}} \right)} $$
(5)

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.

An important, sometimes overlooked, parameter affecting unplanned dilution is the local stope setting within the mining sequence. Depending on its placement within the planned mining sequence, a stope may be bound by rock on both side walls and above (a primary stope), or it may have backfill on one or both side walls (a secondary stope). Five stope categories were identified, based on their setting within the orezone mining sequence, see Fig. 2. The stope categories consisted of three primary (Type P1, P2 and P3) and two secondary-type stopes (Type S1 and S2). Type P1 stope refers to an isolated primary stope, with rock on both side walls. Type P2 stope refers to a primary stope, located above a backfilled P1-type stope, with rock on both side walls. A rock pillar of height equal to two stopes occurs on at least one side wall of the stope. Lastly, Type P3 stope refers to a primary stope, located above a backfilled P2-type stope, with rock on both side walls. A rock pillar of height equal to three stopes occurs on at least one side wall of the stope.
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Fig. 2

Stope categories within mining block

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.

Results from the parametric simulation of hanging-wall overbreak associated with a 30-m high stope with strike lengths ranging from 10 m to 30 m are provided in Fig. 3. Compared against the P1 stope, extraction of the P2 and P3 stopes is associated with greater values of overbreak. Compared against the P2 stope, extraction of the P3 stopes resulted in a slight increase in overbreak. Overbreak values for the secondary S1 and S2 stopes are significantly greater than that associated with P1 stopes. The greatest potential overbreak was associated with S2 stopes, which are bound on both sides by previously mined and backfilled stopes.
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Fig. 3

Influence of stope type on modelled hanging-wall Dilution Density (Henning and Mitri 2006)

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

The Bousquet property is situated in the Abitibi region of northwest Quebec, 50 km west of the city of Val d’Or, (Fig. 4). The principal gold-bearing lenses of the Bousquet #2 mine are steeply dipping and lenticular shaped, ranging in thickness from 4 to 19 m, with lateral and vertical dimensions of 300 m and 1500 m respectively. Mining Block #5 located at 1045 m to 1195 m depth, was advanced upwards towards the mined sill on Level 8 (1045 m depth). Zone 3-1, located approximately 1000 m west of the Bousquet #2 orebody was mined at depths of 1340 to 1580 m below surface. Zone 3-1 was accessed by a haulage drift from the base of the Bousquet #2 development. A generalized location map for the two orezones is provided in Fig. 5.
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Fig. 4

Location of Bousquet property

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Fig. 5

Longitudinal view showing locations of Block 5 and Zone 3-1

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.

The Block 5 and Zone 3-1 orezones were divided into transverse primary and secondary stopes with sublevels located at 30 m vertical intervals. Typical design dimensions of Block 5 and Zone 3-1 stopes are provided in Table 1. The long hole mining method with delayed backfill was used to take advantage of steeply dipping tabular orebody geometry, and to optimize production rates and recovery.
Table 1

Typical transverse stope dimensions, Bousquet #2 mine

 

Block 5

Zone 3-1

Primary

Secondary

Primary

Secondary

Vertical height (m)

30

30

30

30

Strike length (m)

15

15

10

10

Stope tonnage

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.

Bousquet rockmass conditions are controlled extensively by the geological setting, with the dominant schistose fabric controlling the behaviour of wall rocks in all underground excavations. Rock parameters are summarized in Table 2. Major and intermediate principal compressive stresses at Bousquet #2 mine were horizontal (Arjang 1988). The minor principal compressive stresses were orientated vertically. Stress magnitudes and orientations were comparable with other regional stress data, (Arjang 1996).
Table 2

Rock properties for Block 5 and Zone 3-1, Bousquet #2 mine (Henning 2007)

Parameter

Block 5

Zone 3-1

Host rock

Ore

Host rock and orezone

Rock type

Foliated rhyolite tuff

Massive sulphide

Foliated rhyolite tuff

Rock Quality Designation, RQD (Deere 1968)

35%

85%

50%

Rock Mass Rating, RMR (Bieniawski 1984)

50

75

48

Tunnelling quality index, Q (Barton et al. 1974)

0.47

7

1.4

Uniaxial compressive strength, MPa

112

211

59

3.2 Empirical Database Description

A comprehensive database was established for this research, incorporating information related to the design, construction, excavation and CMS cavity surveys of 172 sequentially mined long hole stope case histories from Block 5 and Zone 3-1. Information pertaining to stope setting, stope geometry, stope type, stope construction and stope recovery was compiled. Figure 6 shows the overall database structure.
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Fig. 6

Database structure

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.

Block 5 stopes have greater strike length than Zone 3-1 (see Fig. 7), which affects hydraulic radius values (5 m vs. 3.9 m) of the stope hanging-wall. Block 5 also has larger stope volume, reflecting its greater thickness. An average stope of 12300 tonnes corresponds to average thickness of 7 m. For Zone 3-1, an average stope of 4840 tonnes corresponds to average thickness of 4.7 m.
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Fig. 7

Stope strike length, Zone 3-1 and Block 5 stopes

The stability of the hanging-walls of individual Block 5 and Zone 3-1 stopes was assessed using the Stability Graph method. The Stability Graph method for open stope design (Mathews et al. 1981; Potvin 1988), utilizes information related to rockmass strength and structure, stress around the opening, and the size, shape and orientation of the opening to predict stope stability. Results, plotted on Fig. 8 indicate that the hanging-wall of typical stopes within Block 5 are potentially unstable. Hanging-wall stability decreases with rockmass quality. Hanging-walls within low-range rockmasses are strongly unstable. Compared to Block 5, Zone 3-1 stopes benefit from both a slightly more competent rockmass, and reduced hanging-wall dimensions. According to the Stability Graph approach, the hanging-wall of typical stopes within Zone 3-1 are potentially stable. Hanging-wall stability decreases with rockmass quality. Hanging-walls within low-range rockmasses are potentially unstable.
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Fig. 8

Stability graph plot of Block 5 and Zone 3-1 stopes

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

As described previously, the Block 5 and Zone 3-1 orezones were subdivided into 97 and 75 stopes, respectively. Within each orezone, stopes were mined in a sequence that resulted in the creation of a variety of primary (P1, P2, and P3) and secondary (S1, S2) stope types. The positions of the five stope types within Block 5 the zone are shown in Fig. 9. Stope type positions within Zone 3-1 are shown in Fig. 10.
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Fig. 9

Longitudinal plan of Block 5 showing stope types

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Fig. 10

Longitudinal plan of Zone 3-1 showing stope types

The total number of stope types found in each of the two orezones is expressed as a percentage of total number of stopes in Fig. 11. Similarities occur in both orezones with respect to the percentage of total stopes of each stope type. In terms of total primary and secondary stopes, Zone 3-1 consisted of 45% primary and 55% secondary stopes, including 35% S2-type stopes. With Block 5, 42% of the stopes mined were primary, while 58% were secondary stopes, including 37% S2-type stopes.
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Fig. 11

Stope types in Block 5 and Zone 3-1

3.5 Dilution

As provided in Eqs. 15, there are a variety of dilution definitions adopted by mines. At Bousquet #2 mine, dilution was calculated in the manner of Eq. 1, according to the following definition (Gauthier 2001):
$$ {\text{Dilution}}\,(\% ) = \frac{{{\text{Tonnes}}\;{\text{waste}}\;{\text{rock}}\;{\text{mined}}}} {{{\text{Tonnes}}\;{\text{ore}}\;{\text{mined}}}} \times 100 $$
(6)
where Waste = Wall rock outside of the planned stope boundary, Ore = Rock planned, drilled and blasted within the stope boundary.
A CMS was employed systematically at Bousquet #2 on mined stopes to obtain a detailed picture of the stope boundary, from which dilution values are determined. CMS surveys were conducted on the majority of stopes mined in Block 5 (98%) and Zone 3-1 (97%). Results from CMS scans were processed by mine geologists who compared CMS and planned stope sections to determine overbreak volumes and to identify the source of the dilution, an example of which is provided in Fig. 12.
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Fig. 12

Example of measured overbreak plotted against designed stope

For overbreak data measured by cavity survey (CMS) from mined stopes, measured Dilution Density (DDcms) is a term introduced in this study. It is the volume of overbreak on an exposed surface, and is expressed as:
$$ DD_{{{\text{cms}}}} {\text{ = }}\frac{{{\text{Overbreak}}\,{\text{volume}}\,{\text{(m}}^{{\text{3}}} {\text{)}}}} {{{\text{Surface}}\,{\text{area}}\,{\text{exposed}}\,{\text{(m}}^{{\text{2}}} {\text{)}}}} $$
(7)

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.

Measurements of overbreak volume and excavated stope dimensions were compiled for each individual stope, from which Dilution Density values were determined. As mentioned previously, a negative aspect of reporting dilution as a percentage is that the value is heavily influenced by the orezone width, and therefore, stope volume. The application of Dilution Density is used to quantify total overbreak as well as hanging-wall specific unplanned dilution. Average DDcms values for total stope overbreak from both the hanging-wall and footwall, and for hanging-wall specific overbreak are presented for each stope type in Table 3. Data trends indicate that for each stope type, combined hanging-wall and footwall overbreak is greater in Block 5. Much of Block 5 overbreak is associated with the hanging-wall. This trend is less pronounced in Zone 3-1.
Table 3

Summary of surveyed stope dilution from transverse-mined stopes

 

Stope type

Total dilution, mean value (%)

Total measured hanging-wall and footwall DDcms (m)

Hanging-wall DDcms (m)

Block 5

P1

34.0

1.73

1.67 

P2

33.1

2.92

2.44 

P3

19.7

2.54 

1.83 

S1

30.9

2.20 

1.89 

S2

29.0

2.86 

2.18 

Zone 3-1

P1

33.0

1.27 

0.66 

P2

24.3

1.33 

0.57 

P3

34.3

1.63 

0.81 

S1

46.7

1.71 

0.81 

S2

30.9

1.89 

1.14 

4 Comparison of Zone 3-1 and Block 5 Hanging-Wall Overbreak

The severity of hanging-wall overbreak, associated with Zone 3-1 and Block 5 stopes is summarized in Fig. 13. Overbreak was calculated as equivalent Dilution Density (DDcms) for each stope. Zone 3-1 experienced significantly less hanging-wall overbreak than did Block 5. The majority of Zone 3-1 stopes (61%) had generated hanging-wall DDcms of 1 m or less. Relatively few (7%) Zone 3-1 stopes experienced hanging-wall DDcms exceeding 2 m. Conversely, only 29% of the Block 5 stopes generated hanging-wall DDcms of 1 m or less, while a significant number of Block 5 stopes (46%) experienced hanging-wall DDcms exceeding 2 m.
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Fig. 13

Hanging-wall DDcms, Block 5 and Zone 3-1 stopes

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.

The distribution of overbreak severity associated with the five stope types within Zone 3-1 and Block 5, using Dilution Density calculations, is presented in Figs. 14 and 15, respectively. With Zone 3-1, the majority of stopes of all types were associated with overbreak values of one meter or less. Only one primary stope had a Dilution Density exceeding two meters. Four secondary stopes, or 11% of the total secondary stope population, had Dilution Density exceeding two meters.
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Fig. 14

Hanging-wall DDcms, Zone 3-1 stopes (Percentage of stopes per stope-type)

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Fig. 15

Hanging-wall DDcms, Block 5 stopes (Percentage of stopes per stope-type)

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.

Average depth of Dilution Density for a specific stope type is presented in Fig. 16. With Zone 3-1, hanging-wall overbreak associated with each of the primary stope types (P1, P2 and P3) is less than that of the secondary stopes. S2-type stopes, representing an extreme from primary stopes, with mined and backfilled stopes at both side walls, had the most severe overbreak. Hanging-wall Dilution Density of secondary S2-type stopes exceeded that of Primary stope by 40–100%. When considering all Zone 3-1 primary stopes together, dilution from S2-type stopes exceeds that of the average Zone3-1 primary stope by 62%. (DDcms = 0.7 m vs. DDcms = 1.14 m).
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Fig. 16

Average hanging-wall DDcms versus stope type, transverse stopes

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

Differences of overbreak associated between the various stope types of Zone 3-1 and Block 5 can be found in examining the presence and apparent effectiveness of hanging-wall cablebolt reinforcement. Unlike the hanging-wall of Zone 3-1, which was only reinforced by sill cablebolting (see Fig. 17), installed on two meter centers, the hanging-wall of the majority of Block 5 stopes were reinforced with cablebolts installed from a hanging-wall cable drift (see Fig. 18).
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Fig. 17

Stope sill cablebolt installation via stope sill, Zone 3-1

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Fig. 18

Hanging-wall cablebolt installation pattern, Block 5

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°.

Stopes at the lower-most level of Block 5, and stopes accessed longitudinally were not reinforced by hanging-wall cablebolts. To minimize the initiation of unravelling from the base of the stope hanging-wall in these stopes, cablebolts were installed from the sill of these stopes in a fanning pattern upwards into the hanging-wall, as illustrated in Fig. 17. Table 4 presents a summary of the variety of hanging-wall reinforcement patterns employed in Block 5 and Zone 3-1.
Table 4

Distribution of stope hanging-wall cablebolt installation pattern

 

Hanging-wall cablebolt pattern

Upward fanning cablebolts (Fig. 18)

Downward fanning cablebolts (Fig. 18)

Cablebolts installed via stope sill (Fig. 17)

Block 5

34 stopes

34 stopes

29 stopes

Zone 3-1

Nil

Nil

76 stopes

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.

The values plotted in Fig. 19, show differences in Dilution Density severity associated with the orientation of the cablebolts. There was minimal difference (DDcms = 0.1 m) between primary stopes reinforced by either upward or downward fanning cablebolts. For stopes reinforced by downward fanning cablebolts, overbreak in primary stopes exceeded that of secondary stopes by DDcms = 0.55 m. However, for stopes reinforced by upward fanning cablebolts, overbreak in secondary stopes surpassed that of primary stopes by DDcms = 0.4 m. Overall, overbreak in secondary stopes, reinforced by upward fanning cablebolts exceeded that of secondary stopes reinforced by downward fanning cablebolts by DDcms = 0.8 m.
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Fig. 19

DDcms versus hanging-wall cablebolt reinforcement, Block 5 stopes

The orientation of the cablebolts with respect to the anticipated direction of hanging-wall sloughage plays a role in the effectiveness of the reinforcement. Cablebolt capacity is mobilized under tensile load (Hyett et al. 1992; Kaiser et al. 1992; Maloney et al. 1992). The development of tensile load is possible for cablebolts orientated in a downward fan, as illustrated schematically in Fig. 20a. Under shear loading conditions, the cablebolt functions passively, providing localized reinforcement between unravelling blocks. Cablebolts orientated in an upward fan, illustrated in Fig. 20b, are subjected to downward shear leading to tensile loading.
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Fig. 20

Hanging-wall cablebolt orientation with respect to stope sequence

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

Relationships between stope type and orezone sequencing at 25%, 50%, 75% and 100% extraction were examined. Results, summarized in Fig. 21, show that the majority of stopes mined in both Zone 3-1 and Block 5 were primary-type stopes, up to 50% orezone extraction. Between 50% and 75% extraction (at approximately 57% extraction), the primary and secondary stope trend lines cross for both orezones. Beyond approximately 57% extraction, the percentage of secondary stopes mined is increasingly greater than that of the primary stopes.
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Fig. 21

Stope type mined as a function of orezone extraction

Relationships between the average hanging-wall overbreak and orezone sequencing at 25%, 50%, 75% and 100% extraction are presented in Fig. 22. Commonly, with each mining stage, average DDcms associated with secondary stopes exceeds that of primary stopes.
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Fig. 22

Average hanging-wall DDcms as a function of orezone extraction

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.

Comparison of stope cycle time against stope volume, shown in Fig. 23, suggests a linear relationship for the rates at which the stopes were mucked, with a typical rate in the range of 370 tonnes per day. Upper and lower bounds correspond to 625 tonnes per day and 115 tonnes per day, respectively. No distinct difference between the mucking rates for Block 5 and Zone 3-1 stopes are apparent.
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Fig. 23

Comparison of rate of stope mucking against stope volume

Conversely, a comparison of stope cycle time against the measured hanging-wall Dilution Density, shown in Fig. 24, does not show a direct relationship. In order to maintain scheduled mine-wide sequencing, stopes were promptly mucked and backfilled, thus hindering the potential for progressive, time-dependant degradation of the exposed stope walls.
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Fig. 24

Comparison of rate of stope mucking against hanging-wall overbreak (m)

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.

Acknowledgements

The authors wish to thank Agnico-Eagle Mines Limited for granting permission to use the data compiled from Bousquet #2 Mine.

Copyright information

© Springer Science+Business Media B.V. 2008