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The Southern African Institute of Mining and Metallurgy
Platinum 2012
101
S.M. Rupprecht
MINE DEVELOPMENT – ACCESS TO DEPOSIT
S.M. Rupprecht University of Johannesburg
Abstract
A deposit to be mined by underground methods can be accessed by a number of
methods:
• Adit
• Decline or ramp
• Inclined shaft
• Vertical shaft.
Adits are an economical approach when the orebody is above the general floor
elevation i.e. suitable in hilly or mountainous terrain. Incline shafts are limited to
relatively shallow deposits, and because they are developed on an incline,
development lengths for a given depth are the three to five times longer than for a
vertical shaft. Vertical shafts are the preferred method for deposits deeper than 300
m but the development rate is slow and construction costs are very high. Declines or
ramps offer early access to shallow deposits, which develops the ore body
expediently, but are generally developed at a gradient of approximately 12 per cent.
Decline haulages have become an attractive alternative to shaft hoisting, and over
recent years the role of decline access has become more widespread throughout
South Africa. Traditionally, South Africa has enjoyed the use of shaft systems, largely
due to the large knowledge base of mining the Witwatersrand Basin, where vertical
and inclined shafts were the norm. South Africa has also had the advantage of cheap
electricity, giving shafts a definite economic advantage. However, in recent years
the national power utility ESKOM has undergone an expansion programme that has
led to tariff increases of nearly 100% over a three-year period.
Based on the changes in electricity tariffs and technological improvements to
underground haulage trucks, the economic inputs to access development have
changed. This paper reviews mine access for shallow deposits as currently applied in
South Africa. Based on current economic inputs, the paper investigates at what
point a vertical shaft would be more economical than a decline system utilizing
typical South African mining equipment.
The Southern African Institute of Mining and Metallurgy
Platinum 2012
102
Introduction
The question of which access method is applicable to exploit an underground
deposit is one that mine engineers and planners are faced with when investigating
the viability of most shallow deposits. Basically there are four approaches to gain
access to an orebody; namely, adits, incline shafts, vertical shafts, and declines or
ramps.
The four methods are briefly discussed in this paper for the sake of continuity, but
the details are not included. Wilson et al. (2004) provide a comprehensive discussion
of access methodologies between vertical, incline, and decline shafts and it is not the
intention of the author to repeat the detail of this discussion.
However, with the increased use of mechanized mining methods in the narrow-reef
environment of South Africa, the question of when to convert from decline truck
haulage to vertical shaft hoisting is pertinent to most shallow greenfield projects in
the Bushveld Complex. The economics of vertical shafts versus decline ramps is
further complicated with the electricity tariff increases since 2010, and simply
applying the ’old rule of thumb’ to establish the changeover depth may not apply
any more, especially as trucks are becoming larger, more powerful, and fuel-
efficient. This paper looks at the economics of a shaft versus decline system and
when it becomes more economically attractive to utilize a vertical shaft rather than a
ramp decline system for a shallow deposit.
Initial considerations
Many factor influence the decision of selecting a shaft or decline/ramp to access an
underground mine. Some of these factors include the depth of the deposit,
geotechnical aspects, production rate, dimensions, availability of capital, and
operating costs.
A key consideration is that it is extremely expensive to convert from a ramp to a
shaft system, so the mine engineer/planner must consider the entire mineral
resource or potential to increase the resource at depth. Figure 1 depicts a typical
access strategy for platinum mine where the initial orebody is exploited by means of
an incline or decline shaft system, and later accessed by vertical shafts.
A
A
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The Southern African Institute of Mining and Metallurgy
Platinum 2012
106
Given the above background regarding methods of access to exploit underground
deposits, the question of selecting the appropriate changeover depth between a
decline ramp and vertical shaft is explored.
Background
As early as 1973, Northcote and Barnes investigated the optimum changeover depth
for Australian conditions and recommended changeover depths of the order of 350
m, a depth still often quoted in South African operations.
McCarthy and Livingstone (1993) suggested that the transition depth from decline to
shaft in Western Australian practice had increased from 300 m to 500 m or more,
with potential to increase this depth to 1000 m. McCarthy and Livingstone noted
that every mine has its own peculiar circumstances, which would influence the
determination of the changeover depth. Some factors that they identified and
which still hold true today include:
• Funding or capital available for project development
• Mining method and ground conditions
• Requirements for service access via a decline
• Requirement for lateral and vertical ramp coverage of the orebody and the
lateral extent of the orebody
• Depth from decline portal to top of orebody
• The planned rate of vertical advance and its relation to the ore distribution
and hence production rate
• The ore reserve and development schedule and thus the planned mine life
• The existence of exploration shafts suitable for conversion to production
hoisting
• Whether the decline can be advanced sufficiently ahead of current mining
areas to enable raisebored hoisting shafts
• The discount rate used in the analysis
• Life of mine
• Haulage distance to shaft.
McCarthy (1999) expanded further on shaft hoisting versus decline trucking, focusing
on the impact that the production rate and depth had on the ultimate changeover
depth. McCarthy commented that advances in trucking technology would challenge
current changeover limits. McCarthy highlighted the fact that 50 t capacity diesel
trucks had become the benchmark in Australian mines, operating at 1 in 7 gradients
at speeds of approximately 9 km/h.
Future trucking improvements would include greater payloads (60 t to 80 t) with
more powerful fuel-efficient diesel engines. Thus, future operations should see
greater haulage speeds, better availability, and improved ergonomics.
The Southern African Institute of Mining and Metallurgy
Platinum 2012
107
Wilson (2004) documented the issue of shafts versus declines for the South African
platinum industry. The situation prevailing in South Africa, in contrast to the
Australian experience, indicated that decline systems were advocated between 350
m to 500 m and enabled early project start-up. Wilson highlighted that increasing
operating costs detracted from the decline option, and thus as orebodies progressed
deeper shafts became more economically sensible, offering reduce operating costs
but higher capital requirements and a longer project development schedule.
However, this work related to an economic environment where electricity was still
very cheap in South Africa.
Tatiya (2005) in a mining textbook describes the modes of accessing a deposit,
shown in Table I. Tatiya recommend declines not exceeding 250 m and further
describes the general attributes for the various options.
Matunhire (2007) compared vertical, decline, and incline shafts (Table II), citing that
vertical shafts should be considered when the orebody is steeply dipping or deep,
being most economic at depths exceeding 500 m. Decline shafts were seen to be
advantageous for shallow flat-dipping orebodies requiring low initial capital. Incline
shafts were also found to be suitable for shallow flat-dipping deposits but had
several disadvantages, namely derailments, shaft spillage and maintenance, and
limited hoisting capacity.
Decline ramp versus vertical shaft – a South Africa reality check
Based on the argument in the previous section, between 250 m and 500 m appears
to be the recommended limit to decline ramp systems, although Australia is
exploring the use of deeper declines. In the current South African economic climate
of increased electrical tariffs, fuel prices, and labour increases one must question if
the previous findings are still valid.
Since 2004, specifically with the power shortages associated with 2007 and 2008,
there has been a dramatic shift in the South African electricity tariff. In 2010, South
African electricity costs increased dramatically and will continue to increase in the
order of 30 per cent per annum for the next two years, thus changing the economic
dynamics. The following describes the findings of the analysis conducted based on a
medium-sized operation applying mechanized trackless mining methods and
operating to a depth of 800 m.
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Table I-Modes of accessing a deposit (after Tatiya, 2005)
Parameters Decline/ramp Incline shaft Vertical shaft
Opening
inclination
limit
Up to 8° Up to 20° >20° degrees to
vertical
Depth
limitations Not exceeding 250 m Not exceeding 150 m
Depth exceeding
~100 m
Usual rock
type through
which an
entry driven
Mostly in waste rock
or black rock
Mostly in waste rock
or in orebody
Mostly in waste rock
or black rock
Principal
purpose
Early access to the
shallow deposit to
develop and produce
ore at the earliest
using trackless
equipment
Early access to the
shallow deposit to
develop and produce
ore at the earliest.
Also equipped with
mine services and
serves as personnel
access
Access to any deposit
and produce ore on a
regular basis. Usually
serve as permanent
mine entry
Position w.r.t.
deposit Preferably in F/W
side of deposit
Along deposit or in
F/W side in waste
rock.
For flat deposits in
overlying strata but
for steep deposits in
F/W
Driving rate Fast Faster Slow
Construction
cost High Low Highest
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Table II: Shaft comparison (After Matunhire, 2007)
Shaft Selection criteria Advantages Disadvantages
Vertical
Quick access to deep ore bodies High skilled labour required
Steeply dipping or
body High labour costs
Efficient at depths exceeding 500m High initial capital costs
Deep orebody
High maintenance costs
Cheaper per meter as depth increases Requires headgear
Limited hoisting capacity
Early return on investment
Requires constant power
supply
Can be mined in the strike or dip
direction
Longer distance to ore body
Easy access to shallow ore body Only economical to 500m
Flat-dipping
orebody Low initial capital costs
Excessive travelling time to ore
body
Decline
Low operating costs
Trackless hauling is slow and
congested
Construction skills and equipment
readily available
Heat pickup from rock over
length
Shallow ore body
High hoisting capacity with conveyor
belts
Slower return on capital
invested
Water handling can be
problematic
Flat dipping ore
body
Limited development to ore body Derailments
Inclined
Shaft maintenance and repair
time consuming
Shallow ore body
Short ore pass system required
Spillage cleaning is time
consuming
Limited hoisting capacity
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In South Africa, the question of accessing an orebody by decline or vertical shaft remains a
topical subject. The evaluation of alternative methods of accessing the orebody is one of the
first steps of developing a mine plan. The selection of the proper size, configuration,
arrangement and type of opening required to develop a new underground orebody or expand
an existing mine is a complex and often difficult engineering problem. Each deposit has it own
characteristics and requirements and requires an accurate evaluation of all factors that may
affect the mine design to access the orebody. The basic design parameters that should be
considered are as follows:
• Lowest capital expenditure
• Lowest operating cost
• Safe and reliable operating system
• Flexible and efficient system
• Supports the mine planning
• Provides fast access to the ore body to promote early cash flow.
Some of the design criteria that need to be considered are:
• Geology and mineral resources
• Hydrology
• Depth of orebody
• Flexibility for changes to mine plan, mining method, or expansion of project
• Production tonnage requirements
• Geotechnical inputs
• Ventilation requirements
• Capital and operating costs
• Schedule completion i.e. commencement of cash flow
• Availability of skills and labour requirements
• Safety
• Productivity and management of system.
The design of a mine’s access is an important aspect of the overall mine design. Each individual
deposit must be carefully reviewed. The selection of decline or shaft access may not be
straightforward as the economics of the access options change with depth and tonnage, and
often the decision is influenced by mitigating factors such as the availability of capital or the
ability of the project to become cash positive as soon as possible. If all the design criteria are
not considered in the initial phase of the project then the mine’s access can potentially become
a bottleneck. For example, the opening must be of sufficient size to handle ventilation and
planned equipment. Therefore, it is advisable to design for a certain amount of flexibility in the
mine’s access as insurance against unexpected changes in the design. It may become
impractical to increase production throughput due to the size of the shaft or decline.
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Decline access is attractive for shallow orebodies or for continuation of operations from open
pit to underground, whereby access is gained through a decline portal situation within the pit.
However as mining progresses deeper and tonnage requirements increase, shaft hoisting
becomes more appealing. The following access options have been considered for the purpose
of this paper and notable exclude incline shafts and conveyor declines, as well as the
consideration of capital expenditure to develop the various shaft systems.
• Trackless declines utilizing trackless mechanized (diesel) equipment. This mining
method is well proven worldwide and is often used for shallow orebodies.
• Vertical shafts servicing standard track haulages. This is a common access method for
many South African mines.
Decline haulage
A decline system from surface has been assumed consisting of a spiral ramps or declines
inclined at 8° to 9° (1:7) and developed to a height of 5.0 m and 5.0 m width. Vertical distances
of 100 m to 800 m have been considered in the comparison between decline and vertical shaft
costs. Operating costs are based on a production rate of 80 000 t/month and are based on a
deposit located near surface (50 m) extending 800 m below surface.
Operating costs for the haulage are based on initially estimating the speed of haulage
equipment over the various segments of the haul route. Based on equipment manufacturers’
recommendations and approximate speeds used for other South African operations utilizing
truck hauling, the following speeds and operational times were used in this study:
• Up a 14% gradient loaded 6.0 km/h
• Level loaded 12 km/h
• Level empty 15 km/h
• Down a 14% gradient empty 15 km/h
• Loading of truck 11 minutes
• Spot and manoeuvre 3 minutes
• Tip 1 minute
It is important to note that the above times are used as a guide and can vary widely between
operations. Of interest is the gradient of the decline and the condition of the haul road.
Operationally, 1:7 (14 per cent or 8 degrees) is now the norm, which provides for the steepest
practical gradient while still including curves and allowing for safe stoppage of machines on the
down slope.
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Construction and maintenance of the haul road is also important to enable the above haulage
speeds. A screened road base material is required to maintain a good working road surface,
which is to be applied to at least 300 mm in thickness, with a cross-fall to allow for adequate
drainage.
Services can also affect the smooth operation of the decline haulage if placed in apposition,
where they may foul with the loaded truck (see Figure 4). Drains should be established on the
service side of the decline, opposite to any muck bays and on the inside of all curves. All
services installed in the main declines are to be located on the shoulder above truck tray height
and above the drain. This is critical for the positioning of any dewatering or water lines that are
to be installed in the declines.
Figure 4-Truck profiles for various haulage sizes
Three haulage trucks were considered in the evaluation, namely 30 t, 40 t, and 50 t trucks.
Operating costs are based on actual costs for a 30 t haulage truck operating in a South African
mine. Operating cost for the 40 t and 50t trucks are based on manufacturers’ databases with
adjustments made to maintenance costs to reflect actual on-mine costs.
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Table III reflects the operating and cost parameters for a 30 t haulage truck. Table IV and Table
V depict the general operating parameters for all three types of haulage truck.
Table III-Operating parameters and cost For 30 t haulage truck
Description Criteria Cost per hour (Rand)
Life expectancy 20000 h
Average tons per shift 1160
Average hours per year 4000
Service items and labour 175
Tyres 2050 hours per tyre 94
Fuel R12 per litre 366
Lubricant 20% of fuel 72
Major repairs 516
Insurance 28
Labour 135
Total 1414
Operating costs for the various sizes of haulage trucks were derived based on the following
cycle times as shown in Tables VI–VIII.
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Table V-Operating parameters - haulage truck
Description Unit
Equipment utilization 83%
Equipment availability 72%
Payload 27/36/45
Days per month 23
Days per year 276
Hours per year 6667
Operating hours 4000
Table VI-30 t haulage trucks
Depth, m Number of trucks Operating cost (R/t)
100 3 26
200 4 36
300 5 46
400 6 57
500 7 67
600 8 77
700 9 88
800 10 98
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Table VII-40 t haulage trucks
Depth, m Number of trucks Operating cost (R/t)
100 2 22
200 3 31
300 4 40
400 4 49
500 5 58
600 6 67
700 7 76
800 8 85
Table VIII-50 t haulage truck
Depth, m Number of trucks Operating cost (R/t)
100 2 21
200 2 29
300 3 37
400 3 45
500 4 54
600 5 62
700 5 70
800 6 78
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Figure 5 reflects the production capacity per working shift for 30 t, 40 t, and 50t capacity trucks
operating from various depths ranging from 100 m to 800 m. Production per truck reaches over
700 t per shift while using a 50 t haulage truck, while the more commonly used 30 t truck
approaches 450 t per shift for a depth of 100 m. As the depth increases productivity between
the various sized trucks narrows, ranging from 193 t per shift to as little as 116 t per shift at a
depth of 800 m.
Figure 5-Truck production capacity per shift versus depth
Shaft system
Shaft operating costs (Table IX) are based on rates provided by a South African shaft-sinking
company based a production rate of 80 000 t/month. The size, speed, and cycle time of the
skip based on a 20 t skip travelling at 15 m/s was used as a basis for estimating the shaft
operating costs. Table IX indicates shaft costs based on various depths and accounts for
electricity, rope costs, maintenance and labour, shaft steelwork, and general contingency. As
can be seen, operating cost are decreased some 10 per cent when the overall tonnage profile is
increased to 120 000 t/month. The reader should note that the outcome of this study is based
a production profile of 80 000 t/month.
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Table IX-ShaftcCosts (R/t)
Depth
(m)
Elect-
ricity Rope
Level
Maint-
enance
Mainte-
nance
accessories
Guides
and
buntons
LabourContin-
gency
Total
at 80
kt/m
Total at
120 kt/m
150 0.87 1.01 0.66 1.80 1.01 30.14 2.28 37.77 34.12
300 1.75 1.01 1.33 3.55 1.30 31.05 2.58 42.57 38.46
450 2.62 1.01 2.53 5.31 1.58 31.95 2.90 47.90 43.28
600 3.50 1.01 4.52 7.07 1.87 32.86 3.28 54.11 48.89
750 4.37 1.01 6.38 8.05 2.16 33.17 3.64 58.75 53.99
900 5.25 1.01 7.31 10.61 2.45 34.67 3.95 65.25 58.96
Results
Figures 6–8 indicate the various breakeven points for various size trucks for a production rate of
80 000 t/ month. An additional graph (Figure 9) illustrating a 50 t haul truck at a production
rate of 120 000 t/ month is displayed for comparison purposes.
The changeover point, as shown in Figure 6, for a 30 t haul truck and 80 kt/month shaft is just
under 200 m. This indicates that trucks are a cheaper option up to 200 m, while the shaft
option is economically viable beyond 200 m.
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Figure 6-Break-even analysis for 30 t trucks and 80 kt/month shaft
Figure 7 indicates that the changeover point between a 40 t haul truck and a 80 kt/month shaft
is just under 360 m. Trucks are a cheaper option up to 360 m while the shaft option is
economically viable beyond 360 m.
Figure 7-Break-even analysis for 40 t trucks and 80 kt/month shaft
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The changeover point between a 50 t haul truck and vertical shaft producing 80 kt/month, as
shown in Figure 8, is just under 450 m, trucks are a cheaper option up to 450 m while the shaft
option is economically viable beyond 450 m.
Figure 8-Break-even Analysis for 50 t trucks and 80 kt/month shaft
For comparison purposes, Figure 9 shows the changeover point between a 50 t haul truck and
120 kt/month shaft decreases from just approximately 450 m to 400 m, indicating that as
tonnage is increased the shaft operating cost will decrease, in this example, by some 10 per
cent.
Figure 9-Break-even analysis for 50 t trucks and 120 kt/month shaft
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Results
This exercise indicates that the old rule of thumb in South Africa, that the economic changeover
point between truck haulage and vertical shafts is 300 m to 350 m, remains valid for the smaller
haul trucks, but the changeover depth increases to 450 m for the larger 50 t haul trucks.
Noticeably, as tonnage and depth increase, the shaft hoisting systems becomes more attractive.
As with any new project it is advisable for the mine engineer to validate the changeover depth
for their own specific project as operating costs, will vary from operation to operation.
In conclusion, the decline system offers an alternative to vertical shafts from 200 m to 450 m,
depending upon the size of the haul truck and the tonnage profile. This is especially true when
there are capital constraints to developing the project, when an early cash flow is required or
mineral resources are limited to a depth of 450 m.
References
Anooraq Resource Corporation. 2010. Company fact sheet, December 2010.
Lonmin Platinum/AngloAmerican. 2001. Pandora Joint Venture Analyst Pack
Matunhire, I. 2007. Design of Mine Shafts. Department of Miing Engineering, University of
Pretoria, Pretoria, South Africa
http://www.infomine.com/publications/docs/Matunhire2007.pdf
McCarthy, P.L. and Livingstone, R. 1993. Shaft or decline? An economic comparison. Open Pit to
Underground: Making the Transition. AIG Bulletin, vol. 14. pp. 45-56.
McCarthy, P.L. 1999. Selection of shaft hoisting or decline trucking for underground mines.
Driving down haulage costs, Kalgoorlie, Western Australia.
Northcote G.G. and Barnes ELS: Comparison of the Economics of Truck Haulage and Shaft
Hoisting of Ore from Mining Operations; The AusIMM Sydney Branch, Transportation
Symposium, October 1973.
Pittuck M. and Smith A. Preliminary Assessment, Namoya Gold Project, NI 43-101 Technical
Report, August 2007
Tatiya, R.R. 2005. Surface and Underground Excavations. A.A. Balkema, Rotterdam.
pp. 318-319.
Wilson, R.B., Willis, R.P.H., and Du Plessis, A.G. 2004. Considerations in the choice of primary
access and transportation options in platinum mines. First International Platinum Conference
‘Platinum Adding Value’, Sun City, South Africa, 3-7 October 2004. Symposium Series S38. The
South African Institute of Mining and Metallurgy, Johannesburg. pp. 269–274.
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The Author
Steven Rupprecht, Senior Lecturer, University Of Johannesburg
Steven Rupprecht graduated from the University of Nevada, Reno in 1986 with a BSc. in Mining
Engineering. In 1987, Steven immigrated to South Africa to work with Gold Fields of SA where
held various positions on the gold mines before transferring to Head Office as Group Mining
Engineer. In 1998, Steven joined CSIR Miningtek where, as Research Area Manager he
investigated mining to 5000m and the evaluation of new technologies for the SA mining
industry. In 2003, Steven received his PhD in Mechanical Engineering for Underground
Logistics. Between 2003 and 2007, Steven was Principal Mining Engineer for RSG Global, an
Australian based mining consultancy. In 2007 joined Keaton Energy as Technical Director. In
2010, Steven joined the University of Johannesburg and is a private consultant to the SA Mining
industry. Steven is a Fellow of the SAIMM, and a Professional Registered Engineer.
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