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.
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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
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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.
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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)