mine development – access to · pdf filethe southern african institute of mining and...

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.


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.

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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.


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


or in orebody


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


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


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.



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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.



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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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