CHAPTER 13.4
Sublevel Stoping Rimas T. T. Pakalnis and Paul B. Hughes
INTRODUCTION This chapter refers to the generic mining method of sublevel stoping. The most commonly used sublevel stoping mining methods are sublevel open stoping, long-hole open stoping or blasthole stoping, an d vertical crate r retreat (VCR) . Variations Variations of this method include vein mining, transverse stoping, Avoca, longitudinal, and other less-used methods such as slusher mining of uppers. The shrinkage stoping method is also a variation of sublevel stoping and is discussed in Chapter 13.3.
Sto top pe
Pillar
Sto top pe
Longit udinal P i llar St ope Span Longit udinal P illa lar r
r a l l i P b i R
St ope
70°
Definitions
W Width
Following are denitions of general terms for this mining method, also shown schematically in Figure 13.4-1.
H e i H g h t
70°
• Span: The length of the stope along the strike. • Width: The perpendicular distance between the footwall and the hanging wall. • Height: The distance along the exposed hanging wall and not the vertical height between levels. • Longitudinal pillar: A pillar aligned along the strike of the stope. • Rib pillar: A pillar aligned transverse of the stope, per pendicularr to the str ike. pendicula • Sill pillar: Horizontal pillars that separate levels or stopes. • Dilution: The reduction of ore grade due to mixing of ore with barren rock. • Internal dilution: Rock that must be mined because of the geometry of the ore body and the requirement to mine rectangular areas. The term is synonymous with planned with planned dilution. • External dilution: Dilution caused by sloughing or failure of stope walls and back and is outside the blasted stope boundary. External dilution dilution is dened as the external waste tonnage divided by the ore tonnage. The term is synonymous with unplanned dilution. dilution. Sublevel stoping, in the absence of consolidated ll, employs pillars to separate the individual stopes to reduce the potential for wall slough. S ublevel stop ing requir es a straigh t/ linear layout of stope and ore boundaries. Inside of the stope,
Figure 13.4-1 Terms used in sublevel stoping everything is ore with no chance of recovering small mineralization in the wall rock. This method requires knowledge of the ore boundaries as shown in Figure 13.4-2. Sublevel stoping with no ll is a mining method in which ore is mined and the stope is left empty. The result is a large void that requires individual pillars be placed to separate the stopes. Sublevel stoping is largely restricted to steeply dipping ore bodies (50°–90°) with a competent hanging wall (HW) and footwall (FW). Figure 13.4-3 shows the general approach to sublevel stoping whereby ring drilling is used from levels generally spaced ~20 m apart in a vertical dimension. Level spacing is largely limited by the length of the production holes, which range in diameter from 50–75 mm and maximum lengths of 25 m. This can be modied if in-the-hole (ITH) drills or top hammer tube drills are used. Characteristic with the sublevel stoping method are the intermediate levels, which largely differ from long-hole (blasthole) stoping as depicted in Figure 13.4-4 where the intermediate level has been removed. In sublevel stoping the mining is accomplished from individual levels at predetermined vertical intervals. These intervals are largely governed by
Rimas T. Pakalnis, Associate Professor, Norman B. Keevil Institute of Mining, University of British Columbia, Vancouver, British Columbia, Canada Paul B. Hughes, Research Assistant, Norman B. Keevil Institute of Mining, University of British Columbia, Vancouver, British Columbia, Canada
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Long-hole Drilling and Blasting
Planned Mining Surface
Drill Access 1
Unplanned Dilution Stope
Planned Dilution
Drill Access 2
Mineralized Zone
G e ol o gi c a l M in in g E xt r a c ti o n
Blasted Ore
Drawpoint
Undercut Fan Blasting
Loading Crosscut
Transport Drift
Source: Scoble and Moss 1994.
Source: Hamrin 2001.
Figure 13.4-2 Defining dilution
Figure 13.4-3 Sublevel open stoping
• Ore geometry, in order to minimize internal dilution by enabling the extraction of irregular ore bodies, • Rock mechanics constraints in terms of minimizing the external dilution through wall slough, and/or • Operational restrictions such as drilling equipment constraints.
Long-hole Drilling and Blasting
Sublevel and long-hole methods require blasting into a “vertical slot/free face,” whereas a VCR, shown in Figure 13.4-5, differs in terms of blasting to a horizontal free face, which is largely conned due to the muck remaining within the stope as only the swell is drawn. Variations of the sublevel method include narrow vein mining/Alimak, Avoca, longitudinal, sublevel retreat, and transverse stoping, as well as historical methods such as slusher and track mining (Haycocks and Aelick 1998).
Drill Access
Stope
Blasted Ore
Undercut
SUBLEVEL STOPING REQUIREMENTS AND CONSTRAINTS The following variables must be addressed in sublevel stope designs: • Size: The minimum width generally ranges from 3 m to 6 m; however, in isolated cases it reaches 1.5 m (Clark and Pakalnis 1997) and lower (0.8 m). The width is governed by the production blast pattern, which, with the use of 50-mm blastholes, is typically 1.2 # 1.2 m, and the stope layout is based on this spacing (Figure 13.4-6). • Shape: The shape is preferably tabular and regular in shape from level to level. • Dip: The dip is preferably greater than the angle of draw, which typically is in excess of 50 ° in practice. The concern also is that a shallow hanging wall dip will result in a less-stable HW conguration because of gravity inuences and increased wall exposures between vertical stope horizons, all resulting in increased potential for external dilution.
Drawpoint
Loading Crosscut
Transport Drift
Source: Hamrin 2001.
Figure 13.4-4 Long-hole or blasthole stoping • Geotechnical: This requires a moderate to strong ore strength and generally a competent HW-FW as these will be exposed and affect the level of external dilution. The ore will determine the potential pillar sizes, hole squeeze, and block size that affect production stope productivity (Pakalnis 2002). • Stope spans: Since this is a nonentry method, stope spans can be larger. The span should be designed to control
Sublevel Stoping
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Secondar y S tope tion No. 1 in Produc Drill Overcut
Drill Overcut Primary Stope No. 2 Undercut and Drilling Done
Crater Blasting Charges Primary Stope No. 1 in Production
Scondary Stope No. 2 Undercut and Drilling Done
Primary Stope Backfilled Loading Drawpoints Fill Barricade
A. Primary stopes mined
B. Secondary stopes mined
Source: Hamrin 2001.
Figure 13.4-5 Transverse stoping
Ore Outline Sublevels r ive Dr i ll D
Blast Pattern Dictates Maximum Stope Width Blasted
Note: Sublevel required ensuring maximum recovery and minimum dilution.
Figure 13.4-7 Geometric constraints
Proposed Rib Pillars
Proposed Drill Level
Ore Contour Dr ive Dra w
Proposed Draw Level
Figure 13.4-6 Sublevel stope layout
Figure 13.4-8 Initial stope planning
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–0 m Access Level
–10 m
Offset
–30 m –20 m
–50 m
–40 m
Ore Contour
–60 m Access Level
Stope Outline
Figure 13.4-9 Composite plan section of ore body used to design blast layout Note: Toe spacing is the same for all boreholes.
Slot Location
Figure 13.4-11 Ring section 16 15 14 1 3 1 2 11 10 9 8 7 6 5 4
x
3 2 1 Upper Drill Drive
Pillar Crosscut
E
E
0 0 2 5
0 5 2 5
Elevation 0 m
Internal Waste –10 m
Draw Level –20 m
Figure 13.4-10 Longitudinal section of blasthole layout –30 m
external dilution and avoid stope collapse and air blast. Span length is governed by HW rock mass quality and generally is in the range of 30+ m with the stope height (inclined) in excess of 30 to 60+ m. • Pillar size: The purpose of the pillars is to support the crosscuts and divide up the stopes. The size of the pillars is dependent on induced stresses, structure, rock mass, and operational constraints. • Selectivity: Selectivity is limited because waste zones can be incorporated as pillars. Changes in ore-body geometry outlines are difcult to address unless the ore body narro ws to the next pillar or sublevel whe re the drill pattern ca n be modie d (Figure 13.4-7).
DESIGN CONSIDERATIONS FOR SUBLEVEL STOPING General Design Guidelines The design of a sublevel stope starts with an engineered layout that incorporates the geometry of the stope, stope span, stope height, pillar dimensions, drill levels, and draw levels (Figure 13.4-8). This layout is then superimposed upon the ore contours (plan) as dened from the upper drill drive to the lower draw level horizon. The example shown in Figure 13.4-9 is a schematic of a 60-m long-hole stope (151-mm blastholes) with geologic contour intervals shown every 10 m in the plan. The resultant longitudinal composite is shown in Figure 13.4-10 employing a ring burden (distance between drill rings) of 3 m from pillar/stope boundary to pillar/stope boundary.
–40 m
Ore Zone
–50 m
–60 m
Figure 13.4-12 Engineered ring section, looking north (example)
HW t o l S
FW
Figure 13.4-13 Location of slot raise (plan view)
Sublevel Stoping
Top of Stope 1st Lift
2nd Lift 15 m
Footwall Drive
e r p l a o l i t S P t t n n e a c n a m j d e A R
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Hanging Wall Drive
Drill Level
31 m Slot Intermediate Level 31 m
12 m Undercut Draw Level
A. Initial Development - Drill Drives - Slot Crosscut - Slot Raise
B. Stope Silled Out
C. Slot is taken full stope width (FW-HW). Rings are blasted on either side of the slot and retreat to pillar access.
D. Production Blasting. Rings are blasted on either side of the slot and retreat to pillar access. Rings coincide on each level.
Figure 13.4-14 Sublevel open stope The ring section is shown in Figures 13.4-11 and 13.4-12 with toe spacing (distance between toes of blastholes) of 4.2 m. The ring section incorporates the geological ore outline as dened by the geologic level contours with the stope outline coinciding with the drilled-and-blasted layout.
Development Considerations Sublevel stoping uses long-hole drilling employing extension drill steels to achieve the appropriate blasthole depth. When ring drilling is used, the entire cross section of the stope is drilled with holes that radiate from the drill drive. The drilling pattern is matched to the sh ape of the ore body and loca tion of the drill drift. Parallel holes are drilled when the drill drive can be silled out from the HW-FW, but this largely is constrained by the stability of the exposed working back. Two principal drill systems exist: top hammer and in-the-hole hammer. Both require long-hole rock drills equipped with extension steel in 1.2–1.8-m-long sections. Top hammer drills are more suited for narrower ore bodies (sublevel stoping), while ITH hammers are more suited for wider ore bodies (long-hole stoping). These will be discussed in a later section. The blast layout for the individual rings will incorporate the ring number, hole number on that particular ring, the amount of explosive required (kilograms), delay interval, angle of hole to be drilled, length of hole to be drilled, and the depth of collar (stemming) to be used. A slot raise must be developed in order to accommodate the swell of the blasted muck. It generally is developed at the extremity of the stope as shown in Figure 13.4-13, and subsequently the slot raise is enlarged FW to HW to open up the area for blasting. Generally, one cannot blast rings into a narrower void, so the slot should be located in the largest area of the stope.
Ore Handling Considerations Ore handling in sublevel stoping involves removal of the ore at the bottom of the stope, and historically it involved track and/or slushers to remove the muck. This process is now
Figure 13.4-15 Sublevel open stope development conducted largely by trackless mining equipment such as scoop trams used for drawpoint loading into mine trucks and/ or orepasses as shown in Figures 13.4-3 and 13.4-4.
Sublevel Stoping Sublevel stoping design is schematically shown in Figure 13.4-14, and the sequence of development and extraction sequencing is shown in Figure 13.4-15. The dimensions noted in the gures are typical of sublevel stoping dimensions and are employed solely to assist in the description of the method and not intended for design, as the dimensions of a stope are based on the geometry of the ore body and operational constraints. This mining method employs sublevels located approximately 20–30 m apart. The distance between sublevels is largely governed by the length of hole that can be drilled with
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2nd Lift
Top of Stope 1st Lift 15 m
Drill Level
e r p l a l o i t S P t t n n e a c n a m j d e A R
46 m
12 m Undercut Draw Level
A. Initial Development - Drill Drives - Slot Crosscut - Slot Raise
B. Stope Silled Out
C. Slot is taken through to upper level and slashed for the full stope width (FW-HW).
D. Production Blasting. Production slashing is on either side of the slot retreating to the pillar.
Figure 13.4-16 Long-hole open stoping minimal drill-hole deviation (under 2%). The drill-hole diameter ranges from 50–75 mm using top hammer drills, which restricts the length of the hole to generally under 30 m with blasthole burden and toe spacing between approximately 1 # 1 m and 2 # 2 m (typically 1.2 # 1.2 m). Modern tube drills (top hammer) at 100 mm in diameter are able to drill 35–40-m-long holes. Generally, if the stope width (HW-FW) is greater than 15 m, an FW and HW development drive as shown in Figure 13.4-14 is used; otherwise, only a center drive in the middle of the stope is developed. The initial development is shown in Figure 13.4-15A, whereby the drill drives, slot x-cut, and raises are driven. The drill drives are comprised of the draw level, intermediate level, and the upper drill level. The undercut (Figure 13.4-15B) is silled out for a vertical height of approximately 12 m above the draw level. The height of undercut or void can be minimized through the use of programmable detonators, ensuring that sufcient void is created for the subsequent blast. The undercut serves the purpose as well to ensure breakthrough of the holes from the upper drill drive. A 2 # 2-m slot is bored/ blasted to the full length of the level above the upper drill level (Figure 13.4-15B), which is subsequently slashed to 3.7 # 3.7 m for the full stope height and width from FW to HW (Figure 13.4-15C) to provide sufcient void space for the subsequent rings to be mined. Production blasting (Figure 13.4- 15C) is comprised of individual rings blasting into the void for the full stope width on either side of the slot. This assumes pillar access exists on either side of the slot. Normally the production rings blasted from the intermediary level correspond to a similar set of rings on the upper drill level (Figure 13.4-15D) to ensure that a void from draw level to upper drill level exists. The geometry shown in Figure 13.4-14 employs a ring burden of 1.5 m and toe spacing of 2.1 m. The stope is normally drilled off prior to commencement of blasting, and only the holes that are scheduled for the blast are loaded. The upholes from the
Figure 13.4-17 Long-hole open stoping development/mine sequence intervening levels must ensure interleaf coverage of approximately 1 to 2 m. The example shown in Figure 13.4-15 uses 15-m-long drill holes with uppers and downholes, and 1–2-m interleaf coverage.
Long-Hole Stoping Long-hole (blasthole) stoping development is shown in Figure 13.4-16 with the sequence of development and extraction shown in Figure 13.4-17. The subsequent examples given are typical of long-hole stoping dimensions and are employed solely to assist in the description of the method and not intended for design, as these dimensions change based on the geometry of the ore body and operational constraints. Long-hole stoping largely eliminates the intermediary level with the draw and drill horizon interval governed by the length of hole that can be drilled with minimal drill-hole
Sublevel Stoping
Footwall Drive
2 T F I L
–1.5 m of Stemming (Sand)
Top of Stope 1st Lift
2nd Lift
e r p l a l o i t S P t t n n e a c n a m j d e A R
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Slot Crosscut
Hanging Wall Drive
Slot
15 m
L Charge = 6 Times Hole/Charge Diameter
31 m
–0.3 m of Cuttings 31 m Footwall Drive
Hole Diameter
Slot Crosscut
Plug
D Hanging Wall Drive 1 T F I L
e r p l a l o i t S P t t n n e a c n a m j d e A R
31 m Slot
Stope Back
31 m
Figure 13.4-18 Piggyback stope deviation (under 2%). The drill-hole diameter ranges from 75 to 150 mm using ITH hammer bits, thereby enabling the lengths to approach 30–60 m in length with blasthole burden and toe spacing approximately 3–4 m 2 (3 # 3 m). The development is as shown in Figure 13.4-17. Generally, if the stope width (HW-FW) is greater than 15 m, an FW and HW development drive is used as shown in Figure 13.4-16; otherwise, a center drive is driven in the middle of the stope. The initial development is shown in Figure 13.4-17A, whereby the drill drives, slot crosscut, and raises are driven. The drill drives are comprised of the draw level and the upper drill level as the intermediary level has been removed. The undercut (Figure 13.4-17B) is silled out for a vertical height of ~12 m above the draw level. A 3.7 # 3.7-m slot is bored/ blasted to ~12 m above the upper drill level (Figure 13.4-17B), which is subsequently slashed to 6.1 # 6.1 m for the full stope height and width from FW to HW (Figure 13.4-17C). Production blasting (Figure 13.4-17D) is comprised of individual rings blasting into the void for the full stope width on either side of the slot. This assumes pillar access exists on either side of the slot. The geometry shown in Figure 13.4-16 employs a ring burden of 3 m and toe spacing of 4.2 m with
Figure 13.4-19 Typical cross section of a VCR-charged hole 150-mm-diameter blastholes. The stope is normally drilled off prior to commencement of blasting. The example shown in Figure 13.4-17 uses 46-m-long downholes and 15-m upholes. A variation in the above sublevel and long-hole mining methods is to use nonconsolidated backll above the upper drill drive of Lift 1 and subsequently drawing out from the level that serves as the draw horizon for the level above (Lift 2) as shown in Figure 13.4-18. This negates the need for cones in ore and consequently maximizes ore recovery as shown in Figures 13.4-15 and 13.4-17. The cones can be eliminated with the use of remote mucking equipment; however, the equipment will be traversing under potentially extended, unsupported spans (see Figure 13.4-5).
Vertical Crater Retreat As shown in Figure 13.4-18, a VCR is a variation of long-hole open stoping where the “free face” is not a vertical slot but a “at back” at the base of the block to be mined. Spherical charges are used to break the ore into slabs as shown in Figure 13.4-5 and have a length/diameter (L/D) ratio of 6:1. Field testing has shown that a ratio of explosive column length (L) to hole diameter (D) of 6 or less will behave similarly to a spherical charge. Blasting is carried out in horizontal slabs with only the swell being mucked at the drawpoint. This is a form of shrinkage stoping where the broken stope muck provides passive support to the stope walls. The ore is recovered at the base of the stope through drawpoints. Similar requirements and constraints to that of sublevel stoping exist except for the need for a competent HW-FW due to the option of maintaining the stope full of muck. Development is similar to that of long-hole stoping, requiring an upper drill horizon and draw level, and it is
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Drill Drive
Drill Drive
Drill Drive
(2) Support Walls (1) Driving Alimak Raise (3) Long-Hole Drilling (4) Charging and Blasting Long Holes
v e i r D k a m i l A
v e i r D k a m i l A
v e i r D k a m i l A
v e i r D k a m i l A
Consolidated Backfill
v e i r D k a m i l A
Figure 13.4-20 Alimak raise mining generally recommended to sill out at the drill horizon to provide drill coverage for the entire block. The vertical separation between drill and draw level is largely a function of the ore regularity and drill accuracy as detailed in general for the long-hole mining method. The dimensions are similar to that of long-hole mining where ITH drills are employed with heights ranging from 30–60 m and 75–150-mm drill diameters are used. A typical loaded blasthole for VCR is shown in Figure 13.4-19 employing a single deck charge. Advantages of the VCR are the high productivity associated with this bulk mining method and the ability to mechanize. The ability to only muck the swell enables support to the stope walls. An advantage of this method over shrinkage is the nonentry and high mechanization associated with VCR. Disadvantages of this method are the extensive pre-stope planning and develo pment that is required prior to commencement of production mining, as the stope must be largely drilled off prior to bench blasting. Similar disadvantages to that of shrinkage mining exist in having the broken ore within the stope until the end of mining of the block.
VARIATIONS ON SUBLEVEL STOPING The sublevel mining method has variations that have been implemented and will be discussed in the context of its similarity with sublevel stoping.
Vein Mining Vein mining—also ter med Alimak mining —has been employed within narrow vein ore bodies as detailed in the Namew Lake mine (Canada) case study (Madsen et al. 1991). Access to the ore is gained by a bored raise/Alimak such as that shown in Figure 13.4-20. The diameter of the raise is approximately
2–3 m and extends from draw level (Alimak drive) to upper drill drive as shown in Figure 13.4-20 (item 1 in the gure) and spans the length of the ultimate stope span with similar constraints as those detailed for sublevel stoping. Support may be in the form of cable bolts (item 2) in the HW providing the nal wall support on stope extraction. The ore is drilled laterally by conventional drills, long-hole jumbos, or other methods and ranges from 5 m to 15 m in length to the adjacent stope (item 3) with blasting from the draw level vertically to the upper drill level (item 4). An intervening pillar may be left between stopes or the stope mined from one Alimak raise to the next depending on the geotechnical constraints. The major advantage is the ability to mine narrow ore bodies with minimal horizontal development. The vertical height of the Alimak is largely limited by operational and geotechnical constraints and reaches heights of 30 to 100 m. Blasthole sizes are generally 50–75 mm with burdens and spacing similar to that of sublevel stoping (1–2 m 2).
Transverse Open Stoping Variations of sublevel stoping with delayed ll are shown in Figure 13.4-5. This mining method is largely used for stope widths in excess of 20 to 30 m or as dictated by geotechnical stable back spans; otherwise, conventional longitudinal or strike mining is used. Figure 13.4-5 shows the objective is to recover the secondary pillars between the primary stoping blocks, w hich can be excavated by sublevel stoping (general) and subsequently lled with consolidated ll that can be com prised of hydraulic ll, paste, or cemented rock ll. Mining of the secondaries occurs after curing of the primaries to a strength that is able to withstand minimal dilution. Generally, the binder content ratio is 30:1 to 20:1 (ll to cement by volume). Alternatively, a permanent pillar is left behind to conne
Sublevel Stoping
Source: Hamrin 2001.
Figure 13.4-21 L ongitudinal mining without fill
Backfill Cycle
Height Backfill
Blasted Ore
45° Mucking Cycle
Source: Caceres 2005.
Figure 13.4-22 Longitudinal mining with fill (Avoca) the unconsolidated ll with only primaries excavated along the strike. With this variation the secondaries are narrow pillars left behind (approximately 3–5 m). A disadvantage of this method is its inability to follow the variations of an irregular hanging wall dip.
Longitudinal Mining Figure 13.4-21 shows sublevel extraction employing mucking along the strike (retreat). This is a variation of conventional FW drawpoints as shown in Figure 13.4-21. The stopes with no ll are as shown in Figure 13.4-21 and with delayed ll are as shown in Figure 13.4-22. The delayed ll method of longitudinal mining is also referred to as Avoca mining. Having longitudinal mucking access requires that remote load-hauldump (LHD) equipment be used. This method is also referred to as sublevel benching.
CONCLUSION Sublevel stoping accounts for more than 60% of all underground production in North America. This is largely due to the developments of extension steels, hollow tube and special
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long-hole rock drills, and ITH drilling techniques requiring less development and greater production capacities. Several variations exist; however, characteristic to this method is the development from a top drill drive and removal of muck from a draw level below for a steeply dipping stope. The variations of the method are selected to suit the ground conditions and operational requirements of the mine. An essential part of sublevel stoping is the stope extraction sequence. The extraction sequence is governed by the development, rock mechanics, tonnage requirements, and, if applicable, ll cycle. Sublevel stoping is a safer mining method because the operator is never within the stope under the unsupported back. Further, the mining method works on a retreat pattern where the equipment and operator work under a supported back. This mining method is suitable to modern hauling equipment including the use of remote LHD units where the operator is removed from any potential hazard associated with the stope. An important safety consideration with open stoping is to ensure that drawpoints remain full above the brow of the stope. Adhering to this safety standard largely eliminates the risk of potential air blast due to hanging wall collapse. The main advantage of sublevel stoping is the efciency associated with drilling, blasting, and loading operations as they can be performed independently from each other. A high potential exists for mechanization with moderate to high productivities of more than 25 t per worker-shift. The main disadvantage is the complicated and comprehensive development that is needed and the requirement for regular tabular ore geometries.
REFERENCES Caceres, C. 2005. Effect of backll on longhole open stoping. M.A.Sc thesis, University of British Columbia. Clark, L., and Pakalnis, R. 1997. An empirical design approach for estimating unplanned dilution from open stope hangingwalls and footwalls. Presented at the 99th Annual General Meeting of the Canadian Institute of Mining, Metallurgy and Petroleum, Vancouver. Hamrin, H. 2001. Underground mining methods and applications. In Underground Mining Methods: Engineering Fundamental s and International Case Studies. Edited by W.A. Hustrulid and R.L. Bullock. Littleton, CO: SME. Haycocks, C., and Aelick, R.C. 1998. Sublevel stoping. In SME Mining Engingeering Handbook . Edited by H.L. Hartman. Littleton, CO: SME. Madsen, D., Moss, A., Salamondra, B., and Etienne, D. 1991. Stope development for raise mining at the Namew Lake mine. CIM Bull . 84:33–39. Pakalnis, R. 2002. Empirical Design Methods—UBC Geomechanics. Presented at NARMS–TAC 2002, Toronto, July. Scoble, M.J., and Moss, A. 1994. Dilution in underground bulk mining: Implications for production management. In Mineral Resource Evaluation II: Methods and Case Histories. Special Publication No. 79. London: Geological Society. pp. 95–108.
