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MI 31003 Underground Metal Mining Methods Lecture Notes

K.UMAMAHESHWAR RAO

Chapter 1

Salient features of Indian Mining Industry

1. The major contributors of mineral in the country are:

Table1. Share of key mining states on India’s mineral resources (Ministry of Mines, Government of

India; Ministry of Coal, Government of India, Indian Bureau of Mines, Centre for Monitoring Indian Economy -2006)

State Coal% Iron ore% Bauxite% Manganese

%

Lead-Zinc % Chromite

%

Jharkhand 29% 14% - - - -

Orissa 24 17 51 35 - 98

Chhattisgarh 16 10 - - - -

MP 18 - - 10 - -

AP (old) 7 7 21 - 1 -

Rajasthan - - - - 90 -

Karnataka - 41 - 29 - 1

Total 84 89 72 74 91 99

2. India produces about 87 minerals that include 4 fuel minerals, 3 atomic minerals, 10

metallic minerals, 47 non-metallic minerals and 23 minor minerals (including

building & other materials). India occupies a dominant position in the production of

many minerals across the globe.

3. There are close to 3000 mines in India. As per the records of 2010-11, of 2928 mines,

573 were fuel mines, 687 were mines for metals, and 1668 mines for extraction of

non-metallic minerals. Of the total number of about 90 minerals, the three key

minerals are coal, limestone and iron ore. There are 560 Coal mines (19% of total

number), 553 limestone mines (19% of total number) and 316 iron ore mines (11 % of

total number) bauxite (189), manganese (141), dolomite (116) and Steatite (113).

India ranks 3rd in coal production, 3rd in limestone production and 4th in iron

ore production, in the world as of 2010.

Table 2 .India’s Production Rank across Key Minerals – 2010 (Ministry of Mines, Government of

India; Ministry of Coal, Government of India, Indian Bureau of Mines, Centre for Monitoring Indian Economy -2006)

Mineral Application Total

Production

(‘000 tonnes)

India’s global

rank in

production

Coal Power, steel, cement 5,37,000 3rd

Limestone Cement, iron & steel, chemical 2,40,000 3rd

Iron ore Iron and steel 2,60,000 4th

Bauxite Transport vehicles, packaging, construction materials

18,000 4th

Barite Oil and gas, paints, plastics 1,000 2nd

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Chromite Steel, dye & pigments, preservatives, refractory appli

cations

3,800 2nd

Zinc metal Iron & steel (galvanization), communication equipment –as alloys)

750 4th

Managanese Iron & steel, packaging ( as alloy with aluminium)

1,100 5th

Lead metal Paints 95 6th

Copper Electronics , architecture, alloys 161 10th

Aluminium Transport vehicles, packaging, construction 1,400 7th

4. Amongst the BRIC countries (Brazil, Russia, India and China), India is the least

developed in terms of per capita mineral consumption. As India’s per capita GDP

increases, its mineral consumption will grow at a rapid pace in line with the growth

witnessed in other emerging markets like China and Brazil.

5. Problems of sustainability of Indian mining industry:

• Regulatory challenges:

There is no guarantee of obtaining mining lease even if a successful exploration

is done by a company. The mining licenses are typically awarded on a first

come first serve basis in principle but there is no transparent system.

• Inadequacy of infrastructure: The inadequacy of infrastructure is related to

the absence of proper transportation and logistics facilities. Many of our mining

areas are in remote locations and cannot be properly developed unless the

supporting infrastructure is set up. For example, the railway connectivity in

most key mining states is poor and it has inadequate capacity for volumes to be

transported which adds to the overall supply chain cost. The government

foresees that steel production capacity in the country by the year 2025 will

increase to 300 million tonnes per annum. This would require Indian Railways

freight capacity to be around 1185 million tonnes, for only steel and its raw

material requirements.

• Environmental clearance: A large percentage of mining proposals has failed to

get environmental / forest clearance from the Ministry of Environment and

Forests, Government of India.

Over and above these regulations, the mining companies also need to take the

local communities along, to ensure that they have the support of the ‘local’ side

for their projects. As a result, several projects are impacted with challenges by

way of opposition from local communities / NGOs, difficulties in land

acquisition, denial of clearances from the governing bodies, etc. A few instances

of some of the major projects that have been impacted in recent past are as

follows:

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a) Pohang Steel Company (POSCO’s) US$ 11 billion investment plan:

strong opposition from local people over land acquisition.

b) Vedanta’s proposed US$ 1.7 billion bauxite mining project in Odisha:

opposition by local community and eventual withdrawal of the forest

clearance

c) Utkal alumina project, which was a US$ 1 billion joint venture between

M/s. Hindalco (India) and Alcan (Canada) to mine and refine bauxite:

delayed by more than a decade due to challenges in land acquisition

d) Uranium Corporation of India Ltd., UCIL’s two mining projects worth

US$ 200 million and US$ 225 million in Meghalaya and Andhra

Pradesh respectively: opposition from local communities and

organizations on the grounds of likely effects of radiations on human

health and environment

6. Non-metallic mineral: The resource base of industrial / non-metallic minerals in India is adequate except for Rock Phosphate, Magnesite and Ball Clay, for which the estimates show decreasing reserves. In fact, country is deficient in fertilizer minerals and heavily depends upon imports. Based on the industry these minerals find use in, they are grouped under four categories

A. Fertilizer Minerals

1. Rock Phosphate 3. Sulphur and Pyrites

2. Potash

B. Flux and Construction Minerals

4. Asbestos 7. Gypsum

5. Dolomite 8. Wollastonite

6. Fluorspar 9. Non-cement grade limestone

C. Ceramics and Refractory Minerals

10. Quartz and other silica minerals 15. Pyrophyllite

11. Fireclay 16. Kyanite

12. China clay and Ball clay 17. Sillimanite

13. Magnesite 18. Vermiculite

14. Graphite 19. Non-metallurgical bauxite

D. Export Potential Minerals

20. Barytes 23. Mica

21. Bentonite 24. Talc, Soapstone and Steatite

22. Fuller’s Earth

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7. Mining of granite, marble, sandstone of building material quality (Chunar sandstone),

slate, barite, etc.; are classified under small scale mining sectors in the country

Chapter 1

1.0 Formation of ore deposits/ ore genesis

1.1 Introduction

The geological environment, the earth s has been subjected to various activities and as a consequence it undergoes a cyclic change through a number of stages such as :

1. Erosion and planning (running down of mountains)

2. Weathering Stage, formation of sedimentary rocks

3. Sedimentary stage. burial in the deep crust –

4. Plutonic stage. When molten rock solidifies within pre-existing rock, it cools slowly,

forming plutonic rocks with larger crystals.(Plutonic – meaning deep underground; it

refers to the hydrothermal process where igneous rocks are formed by solidification at

considerable depths)

5. Orogenic stage –a stage characteristic of forces or events leading to large structural

deformations (folding, faulting, mountain building and igneous intrusions) of earth

lithosphere (crust & uppermost mantle) due to tectonic activity.

2. Concepts of Genesis of Ore

Ore genesis theories generally involve three components: source, transport or conduit, and

trap. The genesis of ore deposit is divided into internal (endogenic) and external (exogenesis) or surface processes. More than one mechanism may be responsible for the formation of an ore body.

• Source is required because metal must come from somewhere, and be liberated by some process

• Transport is required first to move the metal bearing fluids or solid minerals into the right position, and refers to the act of physically moving the metal, as well as chemical or physical phenomenon which encourage movement

• Trapping is required to concentrate the metal via some physical, chemical or geological mechanism into a concentration which forms mineable ore.

The various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Ore genesis theories are very dependent on the mineral Syngenetic - A deposit formed at the same time as the rocks in which it occurs.

Ex. Banded Iron Formation

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Epigenetic- A deposit introduced into the host rocks at some time after they were deposited Ex. Valley-type Deposits

GENESIS OF ORE DEPOSITS

Origin Due to Internal Processes

Magmatic Segregation

Separation of ore minerals by fractional crystallization during magmatic differentiation.

Settling out from magmas of sulfide, sulfide-oxide or oxide melts which accumulate beneath the silicates or are injected into country rocks or extruded on the surface.

Pegmatitic Deposition

Crystallization as disseminated grains or segregations in pegmatites.

Hydrothermal Deposition from hot aqueous solutions of various sources.

Lateral Secretion Diffusion of ore and gangue forming materials from the country rocks into faults and other structures.

Metamorphic Processes

Pyrometasomatic (skarn) deposits formed by replacement of wall rocks adjacent to an intrusive.

Initial or further concentration of ore elements by metamorphic processes.

Origin Due to Surface Processes

Mechanical Accumulation

Concentration of heavy minerals into placer

Sedimentary Precipitation

Precipitation of certain elements in sedimentary environments.

Residual Processes Leaching of soluble elements leaving concentrations of insoluble elements.

Secondary or Supergene Enrichment

Leaching of certain elements from the upper part of a mineral deposit and their reprecipitation at depth to produce higher concentrations.

Volcanic Exhalative Process

Exhalations of sulfide-rich magmas at the surface, usually under marine conditions.

2.1 Spatial Distribution of Ore Deposits

It is considered that in certain periods of geological time scale, the deposition of a metal or group of metals was pronounced; and also that specific regions of the world possess a notable concentration of deposits of one or more metals. Mineral deposits are not distributed uniformly through the Earth's crust. Rather, specific classes of deposit tend to be concentrated in particular areas or regions called metallogenic provinces.

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2.2 Mode of Formation

As hot (hydrothermal) fluids rise towards the surface (magma charged with water, various acids, and metals in small amounts) through fractures, faults, brecciated rocks, porous layers and other channels (i.e. like a plumbing system), they cool or react chemically with the country rock.

Some form ore deposits if the fluids are directed through a structure where the

temperature, pressure and other chemical conditions are favourable for the

precipitation and deposition of ore minerals. The fluids also react with the rocks they are passing through to produce an alteration zone with distinctive, new minerals.

2.2.1 Characteristic types of hydrothermal ore formations

Cavity Filling

The hydrothermal fluid fills in the cavities within the country rock and based on the shape of solidified ore mineral several names have been attributed to the ore body shape, such as: The cavity filling deposits are loosely termed as vein deposits Eg. gold, silver, copper and lead-zinc. Veins range in thickness from a few centimeters to 4 meters. They can be several hundreds of meters long and extend to depths in excess of 1,500 meters. The process of cavity filling has given rise to a vast number of mineral deposits of diverse forms and sizes. The Vein deposits resulting from cavity filling may be grouped as follows:

• fissure veins, ( it is a tabular ore body that occupies one or more fissures: two of its dimensions are much greater than the third)

• shear zone deposits, ( thin sheet like connecting openings of a shear zone)

• stock-works, (interlacing network of small ore bearing veinlets traversing a mass of rock.

• saddle reefs,

• ladder veins, and

• replacement veins or veinlet’s

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Fig .1 Various fissure veins: (A). Chambered vein; (B). Dilation veins; (C).Sheet veins; (D). En-echelon vein (E). Linked vein

Fig.2 (a) Stockwork

Fig.2(b). Stockwork of a sulphide ore body

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Fig 3(a) Saddle reef

Fig.3(b). Bendigo Goldfield, Victoria, Australia

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Fig Ladder vein deposit. Ladder veins are short, rather regularly spaced, roughly parallel fractures that traverse dikes (tabular bodies of igneous rock). Their width is restricted to the width of the dike, but they may extend great distances along it. Ladder veins are not as numerous or important as fissure veins.

Questions:

Q1. What are the salient features of Indian Mineral industry?

Q2. Discuss the challenges of sustainability of Indian Mineral Sector?

Q3. Describe the geological processes involved in the formation of mineral resources.

Q4. Explain the characteristics and geometry of hydrothermal ore formations?

Q5. Geometric Measures of an Ore body

• Axis of ore body: line that parallels the longest dimension of the ore body.

• Pitch (Rake) of ore body: angle between the axis and the strike of the ore body

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ORE DEPOSITS and the Tectonic Cycle

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Lecture 2: Economic analysis for the assessment of viability of a

mineral resources sector

The first step of assessment whether a mineral deposit under consideration is viable under the existing techno-economic conditions is to prepare a detailed feasibility report of the mining project

Feasibility Report

A feasibility study is an evaluation of a mineral reserve to determine whether it can be mined effectively and profitably or not. It includes the detailed study of reserve estimation, mining methods evaluation, processing technique analysis, capital and operating cost determination and the process effect on environment. The feasibility study can be considered into two stages: prefeasibility studies and detailed feasibility. Both stages are similar in term of content. The difference exist in the accuracy and time required to perform the studies.

Detailed Feasibility Report:

This is the most detailed study to evaluate whether to proceed with the project. It is the basis for capital estimation and provides budget figures for the project. It requires a significant amount of formal engineering work and accurate within 10 - 15%.

Steps for a feasibility study

1. Geology and Resource: This is the step where drilling and sampling works is performed. Various methods are available for drilling based on the soil and mineral properties. The drill samples are prepared for the assay in order to determine the minimum, maximum and average ore grade and these figures are used to make the reserves estimation.

2. Mine design and Mineable Reserve: This is the step where most economic way of mining is developed. Mine planning, model development, operation models and cost analysis are performed and thus the mineable reserve is estimated based on the economy.

The major steps for the mine development are:

• mine access (surface/underground),

• conveying system (especially in UG mines),

• backfill requirement,

• ore haulage, ventilation,

• Selection of mining equipment and justified against the performance and economy.

• disposal of tailings generated.

3. Mineral processing facility: Sampling must be carried out to ensure that the samples used in the mineral beneficiation processes are real representative of the ore body. Some major characteristics of the ore body is determined prior to the development of the plant design which includes Grinding work indices, feed size,

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settling characteristics, filtration characteristics etc. Sometimes a mineral processing tests are performed in order to determine the amenability of the given ore to different concentration technologies. The major processes that are looked at are:

• Crushing and grinding,

• Concentration (Sizing, Gravity or Flotation)

• Dewatering (Mechanical or filtering)

• Chemical extraction (especially for gold)

When these tests are completed, based on the test results the basic material flow sheet is developed. This helps in the selection of the equipment selection and the stages of processing. These data are used to estimate the amount and grade of concentrate, middling and tailings that are used to search potential customers and revenue earned.

4. Tailings disposal: Tailing disposal system plays a crucial role in order to get the

mine permit. Mostly the tailings didn't place any major challenges. But, if the tailings have hazardous or toxic materials like cyanide, mercury etc. in it, then the disposal system must be effective in order to reduce the harmful effect on the environment and society.

5. Infrastructure development: This section includes the civil and major earthworks required to start the production. The office, labs, storage units, plant buildings, mining equipment shelters etc. are included in the infrastructure.

6. Power supply: Determining the power source, power line distribution, total power required and the power cost are the major things to be looked into in this step.

7. Water: Most of the plant processes are water based, so, the estimation of water

requirement plays an important role in the feasibility studies.

8. Environmental impacts: For a project to be permitted by any government, an environmental clearance is required. In order to get the clearance, the environmental impacts need to be studied. The important aspects are acid mine drainage, cyanide management, and other toxic material controls (Arsenic, mercury, sulfur etc.)

9. Other key parameters: Support facilities, maintenance, transport cost of man and

material, labor cost, site access (road facility or construction, fly in fly out, marine etc.), social impacts are also need to be studied and the steps for social responsibility.

10. Cost estimation: Based on the entire above-mentioned steps, capital and operating

cost for each unit is estimated. It included all the costs for mine equipment, process equipment, construction costs etc.

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11. Financial Evaluation: This is the stage where the project is evaluated based on the economy. The total cost and expenses are looked against the expected revenue gained from the selling of final products and by-products. The key financial indicators examined to determine the viability of the project include Net Present Value (NPV) and the Internal Rate of Return (IRR). Annual cash flow need to be estimated over the entire life of the project, from construction to reclamation phase, based on clear and realistic capital expenditures mine and mill operating costs, employee wages and sales revenue.

12. Sensitivity Analysis: A sensitivity analysis is then carried out to determine the

impact of variation in metal price, operating cost, metal recovery, metal grade, and capital cost on the overall project NPV and IRR values.

The viability of the mine project is established by all these stages and if based on these considerations if mine is feasible, then the next stage of actual development occurs.

Design elements of Underground Metal Mine (UMM)

The following constitutes the elements of underground metal mine design

1. Mineral resources and reserves i.e. mineral inventory 2. Cut-off grade 3. Production rate and mine life 4. Price of the mineral

Classification of Mineral resources

Of all the aspects of mining operations, the ore deposit and its characteristics is the only aspect which is unalterable. Therefore the viability of a mining project is dependent on the knowledge of mineral resource. Geologists distinguish between mineral resources and reserves. The term resource refers to hypothetical and speculative, undiscovered, sub-economic mineral deposits or an undiscovered deposit of unknown economics. Reserves are concentrations of a usable mineral or energy commodity, which can be economically and legally extracted at the time of evaluation.

• Mineral resources is the name given to minerals which contain elements such as gold, silver, copper, lead, zinc, iron, aluminum, nickel, molybdenum etc., as well as fossil fuels, like oil, natural gas, and coal

• Mineral reserves are concentrations of various minerals and it is a geological term. Whether a mineral deposit is also an ore deposit depends on its economic value.

• "Ore deposit" is therefore an economic term of a mineral deposit.

Mineral inventory (stock ) is commonly considered in terms of resource and reserve.

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Fig 1 Classification of Mineral Resources

Fig.2 Losses of various types in an u/g. metal mine In terms of the mining project a mineral resource is divided into three categories as follows:

•••• Geological resource (QG)

•••• Mineable or workable reserves(QW)

•••• Commercial reserves (QC)

INFERREDINFERREDINFERREDINFERREDSUBSUBSUBSUB----ECONOMIC ECONOMIC ECONOMIC ECONOMIC

RESOURCESRESOURCESRESOURCESRESOURCES

DEMONSTRATEDDEMONSTRATEDDEMONSTRATEDDEMONSTRATEDSUBSUBSUBSUB----ECONOMICECONOMICECONOMICECONOMIC

RESOURCESRESOURCESRESOURCESRESOURCES

INFERREDINFERREDINFERREDINFERREDMARGINALMARGINALMARGINALMARGINALRESERVESRESERVESRESERVESRESERVES

MARGINALMARGINALMARGINALMARGINALRESERVESRESERVESRESERVESRESERVES

INFERREDINFERREDINFERREDINFERREDRESERVESRESERVESRESERVESRESERVES

RESERVESRESERVESRESERVESRESERVES

SPECULATSPECULATSPECULATSPECULATIVEIVEIVEIVE

HYPOTHETIHYPOTHETIHYPOTHETIHYPOTHETICALCALCALCAL

INDICATEDINDICATEDINDICATEDINDICATEDMEASUREDMEASUREDMEASUREDMEASURED

PROBABILITY RANGEPROBABILITY RANGEPROBABILITY RANGEPROBABILITY RANGE

INFERREDINFERREDINFERREDINFERRED

DEMONSTRATEDDEMONSTRATEDDEMONSTRATEDDEMONSTRATED

UNDISCOVERED UNDISCOVERED UNDISCOVERED UNDISCOVERED RESOURCESRESOURCESRESOURCESRESOURCES

IDENTIFIED RESOURCESIDENTIFIED RESOURCESIDENTIFIED RESOURCESIDENTIFIED RESOURCES

Economic

Marginally

Economic

Sub-

Economic

Eco

no

mic

Feasi

bil

ity

Certainty Of Existence

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Fig 2 . Reserve Classification

�� � �� � ��� (��� � ��� ������� � ����������������

�� � �� � �� (��= various unavoidable losses of ore reserve in pillars, etc)

Cut-Off Grade:

• Cutoff grade can be defined as the minimum grade of metal present in the mine which

could be mined economically. Cut-off Grade can be used to separate two courses of

action i.e. mine or to dump. The grade of mineralized material below cut-off grade is

classified as waste whereas the material above cutoff grade is classified as ore.

• The cut-off grade is extremely crucial with respect to economical, production and

geological parameters of the mine. Too high a grade can reduce the mineral recovered

and possibly the life of the deposit whereas too low a cut-off would reduce the

average the average grade ( and hence profit) below an acceptable level.

• Cut-off grade can be classified into two basic categories namely fixed cut-off grade

and the variable cut-off grade.

• The fixed cut-off grade assumes a static cut-off for the life of the mine while the

variable cut-off grade assumes dynamic cut-off maximizing the mine present value.

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• Professor Lane outlined three distinct stages in amine operation namely ore

generation (mining), concentration (milling), and refining.

• The various factors which are essential for assessing cut-off grade for mining

operations are the type of ore resource/reserve present, extent of mine development or

present day cost development of mine, cost of drilling, mucking and transportation,

present value of revenues to be obtained from selling the ore, net cash flows have to

be considered.

• For each of the stage as mentioned, there is grade at which cost of extracting the

recoverable metal equals the revenue from the metal. This is commonly known as

break-even grade. If the capacity of the operation of an operation is limited by one

stage only, the break-even grade for the stage will be the optimum cut-off grade.

Where an operation is constrained by more than one stage optimum cut-off grade may

not necessarily be beak-even grade. In such a case balancing the cut-off grade for

each pair of stages need to be considered as well.

Fig. Influence of cut-off grade on mining design parameters

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Fig. Optimum Mine Production rate

Categories of resources based on degree of assurance of occurrence

Identified (Mineral) Resource: Are the specific bodies of mineral-bearing material whose location, quantity, and quality are known from specific measurements or estimates from geological evidence. Identified resources include economic and sub-economic components. To reflect degrees of geological assurance, identified resources can be divided into the following categories:

Measured: Are the resources for which tonnage is computed from dimensions revealed in outcrops, trenches, workings, and drill holes, and for which the grade is computed from the results of detailed sampling. The sites for inspection, sampling, and measurement are spaced so closely, and the geological character is so well defined, that size, shape, and mineral content are well established.

Indicated: Are the resources for which tonnage and grade is computed from information similar to that used for measured resources, but the sites for inspection, sampling, and measurement are farther apart or are otherwise less adequately spaced. The degree of assurance, although lower than for resources in the measured category, is high enough to assume continuity between points of observation. Demonstrated: A collective term for the sum of measured and indicated resources.

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Inferred: Are the resources for which quantitative estimates are based largely on broad knowledge of the geological character of the deposit and for which there are few, if any, samples or measurements. The estimates are based on an assumed continuity or repetition for which there is geological evidence. This evidence may include comparison with deposits of similar type. Bodies that are completely concealed may be included if there is specific geological evidence of their presence.

Categories of resources based on economic considerations.

Economic: This term implies that, at the time of determination, profitable extraction or production under defined investment assumptions has been established, analytically demonstrated, or assumed with reasonable certainty (see guideline iii).

Sub-economic: This term refers to those resources which do not meet the criteria of economic; sub-economic resources include Para-marginal and sub-marginal categories.

Para-marginal: That part of sub-economic resources which, at the time of determination, almost satisfies the criteria for economic. The main characteristics of this category are economic uncertainty and/or failure (albeit just) to meet the criteria which define economic. Included are resources which could be produced given postulated changes in economic or technologic factors.

Sub-marginal: That part of sub-economic resources that would require a substantially higher commodity price or some major cost-reducing advance in technology, to render them economic.

Some definition related to mineral resources:

• Ore is a naturally occurring, in-place, mineral aggregate containing one or more

valuable constituents that may be recovered at a profit under the existing techno-

economic indices. In metal mines, the amount of ore is usually expressed in tons

(metric ton =1000kg),

• Grade is a measurement of the metal content of ore.

• The grade of precious metal ore is usually measured in grams per tonne. The grade of

ore bearing other metals is usually a percentage (the weight for weight proportion of

metal in the ore).

• The grade of ore from a mine changes over time. Mining of a lower grade is likely to

incur (other things being equal) a higher cost per unit weight of extracted metal.

The most important factor in the profitability of a mine is usually the price of the

metal that it produces.

• Dilution is the result of mixing low-grade material with high-grade material during

material production, generally leading to an increase in tonnage and a decrease in

mean grade relative to original expectations.

Reserves of minerals are difficult to determine as the value and costs of extraction and metallurgical treatment and transportation costs determine whether the resource are potentially economic. Because of these uncertainties, mineral, mineral exploration is a program that raises even more uncertainties.

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Lecture 3

3.0 Mine development

Opening a new mine is an expensive, time-intensive operation. Most mines must operate for

years to cover initial start-up costs, the period of capital investment for mine development

without any return on the investment is known as gestation period Mining is the process of

extracting valuable minerals from the earth. Mining involves a number of stages which occur

in a sequence. This sequence of stages is known as the mining sequence. The mining

sequence covers all aspects of mining, including: prospecting for ore bodies, analysis of the

profit potential of a proposed mine, extraction of the desired materials and, once a mine is

closed, the restoration of all lands used for mining to their original state.

3.1 Sequence of a mining enterprise

The mining sequence is divided into six stages. Each stage represents a certain period in the

life of a mineral deposit. The stages, ordered chronologically from earliest and following the

order in which they occur, include:

1. Exploration - gather data about potential mineral deposits and acquire the rights to

harvest those mineral deposits

2. Evaluation - determine which mineral deposit has the most profit potential

3. Mine Development - construction of a mine or mines

4. Production - operation of the mine or mines

5. Closure demolition of the mine or mines and rehabilitation of all lands used for

mining

Mine develop involves construction of various types of openings within the rock mass It is

therefore important to identify the importance of different types of mine openings on the

basis of their specific role in the entire term or life of the mine. Based on these criteria all the

mine openings are categorized into three types of openings, such as:

• Main access to the deposit, which connects the surface and the ore body is also the

called the primary development opening.

• Net-work of the openings like the levels, cross-cut, raise & winze, etc. – secondary

opening; which is the access to the stope

• Source of the ore (stope) also termed the tertiary opening.

The role of primary opening is to provide an access to the deposit from the surface and

therefore the life of these openings is as much as the life of the mine. The secondary

openings are next important development openings in terms of the life. The life term of a

stope, the tertiary opening, is the shortest compared to any other opening of the mine.

The primary development is creation of a main access from the surface to underground, such

as shaft, incline, decline, adit etc., and any development which generates a network of

openings connecting the main access and the main production zone (stope) are called the

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secondary developmental works. For example, levels, raises & winzes, ore pass, cross-cuts,

ore chutes, u/g electrical sub-station & mechanical workshop, first aid room, etc., are

categorized as secondary development openings. A stope, which the place of main zone of

mine production comes under tertiary development

3.2 Stages of Mine Development

3.2.1 Primary Development – access to the deposit

Access to the ore deposit is first operation, which establishes the entry to the mine. For an

underground metal mine, the modes of entry to a deposit are: adit, incline, decline, a vertical

shaft, inclined shaft. Based on the geometry, strike & dip dimensions of the ore deposit, and

depth one or more combinations of different modes of access is decided. Once the deposit is

accessed, in order to commence the mine excavation of ore, various types of constructions

within the rock mass are needed for various engineering purposes. Some of these openings

are vertical, inclined, parallel to the strike and along the dip etc. The shape and the cross

section of the excavation depend primarily on the target production, purpose of the opening

(transportation, ventilation, water outflow, etc.,), nature & stability of the rocks type, the

period of service.

Permanent access and service openings, as shown in the above figure, are expected to

meet rigorous performance specifications over a time span approaching or exceeding the

duration of mining activity for the complete orebody. For example the service shaft must be

capable of supporting high speed operation of cages and skips continuously. Ventilation

shafts and airways must conduct air to and from stope blocks and service areas. Main haulage

drives must permit the safe, high speed operation of loaders, trucks, ore trains and personnel

transport vehicles. In these cases, the excavation are designed and equipped to tolerances

comparable with those on other areas of engineering practice. The mining requirement is to

ensure that the designed performance of the permanent openings can be maintained

throughout the mine life. The magnitudes of the mining induced perturbations at any point in

the rock medium surrounding and overlying an orebody are determined, in part, by the nature

and magnitude of the displacements induced by mining in the immediate vicinity of the

orebody.

3.2.1.1 Selection of a suitable access to the deposit

The decision of selecting the suitable access to the deposit, between a vertical shaft and an

incline is based on the following factors:

• depth of ore deposit, size and shape of ore body,

• surface topography,

• geological condition of the ore and overlying rock mass ( it also includes the strength

condition of ore body as well as the surrounding rock type.

• time for development,

• method of mining (stoping)

• cost and choice of material handling system.

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Incline is not suitable for a deep seated ore body. Because with the increase in the depth of

ore body the haulage distance, at the required gradient, increases enormously and

proportionately the cost of material handling also increases. The cost of maintenance of the

inclined roadway increases. Though the rate of advance for incline/decline/drift are better

than sinking a shaft, with the advent of modern mechanized methods of shaft sinking can give

higher advance rates. Fully loaded ore trucks can travel up the incline and can travel straight

to ore dump. For shaft mine cars are to be loaded on a level via an ore pass and chute and

hauled to shaft. This system is not as flexible as trucks. However when a complete cost study

is made the use of inclines is never economical for deeper ore deposits.

22

Fig . A-E different modes of access to deposits

Fig. Cross-section of a service shaft

23

24

3.2 Secondary development

There are two categories of secondary development; first type is development in the nearest

proximity of the stope, like the stope access levels and the second type of development is

concerned to a stope or in-stope development. The in-stope development such as drill

headings and slot raises, horizontal and vertical openings for personnel access to stope, and

ore drawpoints from the stope. The life of drill headings, slot raises, draw points, sill & crown

is limited to life of the stoping. The openings, such as haulage levels and ore passes which are

developed near stress filed zone of a stope orebody rock. Their operation life approximates

that of adjacent stoping activity.

3.2.1 Levels and Level Interval

Level is an opening developed along the strike direction of an ore deposit and is driven with

zero to near zero (1 in 200) gradient. It is considered as the secondary mine development

operation of an underground metal mine, because it opens out the extent of mineralization

and thus a level offers a scope for a detailed evaluation of grade of the mineral deposit. Every

single underground mine developmental operation is a capital intensive and there is a

significant degree of risk, because any increase in the length of development openings could

augment high capital expenditures. In this respect mine development, involving levels and

their interval is an important operation. The levels also offer the service of transportation, for

men and material, from the shaft to the production site. Of the many factors influencing the

selection of a suitable level interval, the important factor is to facilitate quick disposal of

broken ore from the workings

3.2.1.1 Level intervals

Underground mining of ore deposits is necessarily worked with multiple levels. A level

interval is selected which lead to lowest overall mining cost for the mine development and

exploitation plan chosen. Number of factors affects these costs and some of them are

following:

• geological and natural conditions of the deposit and country rock

• method of mining

• development layout

• method of drivages of openings

• life of openings, mine life

• other financial considerations

The selection of optimum level interval is usually dependent on the development cost

(construction, supporting). Generally development cost increase with the number of main

levels required whereas exploitation cost as well as convenience of access for the miners

decrease with increasing number of levels. From the point of view of cost, a long interval

between levels is desirable. However in case of high grade ore deposits preclude higher level

intervals. The levels are placed at a closer interval to avoid missing high grade ore bodies.

25

Speed of stoping and character of ground are related factors. Levels interval should be such

that stopes are completed and abandoned within the time that they can be kept open without

undue maintenance cost. In order to determine optimum level interval calculations of

development and exploitation cost for different assumed level intervals are made and plotted

graphically and the lowest overall mining cost point gives the optimum point as shown in

figure below. The current trend with mechanized high production method is to have fewer

levels with large level intervals and supplemented by less cost sublevels as required by the

stoping method adopted.

Fig Determination of optimum interval between levels for a hypothetical multi-level mine

Fig. Sublevel Open Stope

26

Fig. Stope developmental openings, ore draw points, slusher drifts.

3.3 Parameters considered in the design of stopes- tertiary openings

A stope, as shown below, is the site of ore production in an orebody. The set of stopes generated during ore extraction usually constitutes the largest excavations formed during the exploitation of the deposit. The stoping operation, that is, ore mobilization form it’s in situ setting and its subsequent transportation from the mine void, forms the core of the mine production process. In order that the stoping operations are safe it is essential to assess rock performance within the orebody, and in the rock mass adjacent to the orebody. It ensures the efficient geomechanical and economic performance of the individual stopes, and of the mine as a whole. The size of stopes is large relative to all the other mine excavations. Therefore the location, design and operational performance of other excavations connecting the stope and the main access play a dominant role.

3.4 Raising Methods

3.4.1 Manual raising method

This is a simple and most common method adopted in majority of the metal mines.

The unit operations followed in the construction of a manual raise are:

• drilling and blasting

• mucking and transportation

• erection / construction of a manual platform or also known as scaffold

The workers stand on a platform or scaffold made of timber planks supported in stulls

or iron bars fitted into the footwall. The clamps used for supporting the platform are made in

standard lengths out of old rails.

Drilling & Blasting: Jackhammers / stoppers are used for drilling either wedge pattern or burn

cut pattern holes of 32 mm diameter and 1.5m deep. Before each round is blasted the

platform is dismantled. Immediately after blasting, compressed air is forced to the working

faces to remove the fumes of blasting. In longer raises sometimes a blower with a flexible air

duct is installed. Access to the faces is by a ladder way.

27

Mucking & Transportation: The muck (ore if the raise in driven within the orebody, or a

waste rock if the raise is placed in foot-wall rock) based of ore or waste rock are trammed by

a mine car to the nearest grizzly.

Construction of a scaffold: The stoppers can reach a height of 2m and it facilitates the

construction of scaffold after every two rounds of drilling and blasting. The scaffold is

advanced regularly so as to maintain necessary head room at the face. The broken rock rolls

down by gravity. The scaffold is constructed by fixing steel bars into the holes drilled in the

side walls

Limitations: A simple but a very tedious method and has a limitation of comfortable raising

operations upto 15m. Careful checking and dressing down of the loose rock by skilled

workers before allowing workers to go up is essential At Jaduguda mine of UCIL where

this method of open raising was adopted for a number of stopes, the longest raise driven

was 90 m at 450 inclination.

Fig. Manual Raising method

3.4.1.1 Two compartment method

This method of raising is adopted for vertical or very steep raises only. After initial

excavation from the lower level on the direction of the raise for 2m the raise is divided into

two compartments and the follows a conventional driving methods

Raising with shallow holes is started by cutting out a recess at the bottom level, from which

subsequent operations are performed. Work is done from stage 1. After firing a round of

holes the stage rests on two or three stulls 2 temporarily set into holes made in the walls of

the raise. It consists of wooden planks laid over the stulls. Holes 3 are drilled from the stage

28

by means of stoppers. After the drilling is completed the drilling equipment and the tools are

removed from the face and the holes are charged with explosives. Before firing, the ladder

way 4 of the raise is covered by inclined wooden planks 5 which guide the broken rock away

into rock, while standing under protection of the stage. Then the timber sets are erected and

the working stage is transferred closer to the face. As the face advances, the ladder

compartment is extended and equipped with ladders. Rope ladder 7 connects the upper

segment with the working stage.

The raising cycle comprises the following operations:

• inspection and dressing down of loose rocks,

• timbering extending the ladder way,

• construction of the working stage and drilling,

• removing the working stage,

• charging and firing of the blast holes, and

• clearing the smoke.

One of the drawbacks of the method of raising by firing shallow holes is the need for

performing a number of subsidiary tasks (like building the stages and ladder ways, their

extension, and repairs, etc.).

Fig. Fig. Two compartment method

29

3.4.2 Mechanized Raising

Raising and winzing is one of the common development operations in underground metal

mines. These are vertical or sub-vertical connections between levels and are generally driven

from a lower level upward through a process called raising. An underground vertical opening

driven from an upper level downward is called a winze.

Raises with diameters of two to five metres and lengths up to several hundred metres are

often are developed either by manual and or mechanized methods, depending upon the size

and the extent of mechanization of a mine. The openings so created may be used as ore

passes, waste passes, or ventilation openings.

Earlier raising was done by manual method which was time consuming and hazardous.

Developments of raise climbers and raise boring machines have made the process faster and

safer.

The unit operations such as drilling blasting, mucking and erecting the support and surveying

for marking the centre line of a raise are done manually. The raising is done either dividing

the available area into two-compartments or a single chamber.

• height of raising is limited specially by conventional and raise climbers ladder

climbing and making platform is hazardous in conventional method

• potential hazard of rock falling

• surveying is difficult

In mechanical raise climber most of these difficulties are avoided and the most popular to this

kind are:

1. Jora raising method

2. Alimak raise climber.

3. Raising by long hole drilling

4. Raise borers

3.4.2.1 Jora raising method

Jora raising method is suitable only for the condition when two levels are available for

connectivity by a raise. The method consists of drilling a large diameter hole at the centre of

the intended raise to get through into the lower level (Fig. below). From the upper level a

cage is suspended using a flexible steel rope that can be hoisted up and down using a winch.

There is a working cabin also known as Jora cabin. The Jora cabin is provided with a sturdy

working platform on top of it, it is from this platform that the drill operators make the drill

holes.

Drilling: Usual practice is to follow parallel hole pattern and the central hole is used as a

relief hole. A stopper is used for drilling the holes of 34 mm diameter. Before blasting the

entire jora cabin is lowered to the lower level.

30

Limitations:

1. One of the main limitations is that two levels are essential and arrangements are made

in both the levels.

2. The need to drill large diameter central hole for the hoisting rope.

3. Slow and a tedious operation.

4. Rate of advance is low.

1- Winch for rope; 2- winch skid; 3- drilling platform; 4- hoist rope;

5- Jora cabin; 6- steel rope; 7- Hole reel; 8- Drill hole for steel rope

3.4.2.2 Raising by Large Diameter Blast Holes

31

Fig. Raising by Large Diameter Blast Holes

3.4.2.3 Alimak Raising:

Alimak raising is a mechanised blind raising method. It was introduced in mines way

back in 1957 and over the time it has proved to be economical, flexible, and a safe method of

raising for as long as 900 m. It can be used for vertical and inclined raises.

The machine along with a cage runs up and down on a guide rail that incorporates

rack and pinion gear mechanism (Fig. below). The guide rails are in segments and fastened to

the rock by rock bolts. They are extended as the raise advances.

The drilling operation is carried out standing on the platform after charging the holes

the cage is taken down at to a safe place for blasting the face. After the fumes clearance the

cage goes up again and guide rail extension is done. The blasted muck is removed.

Fig. Rack-and-pinion gear mechanism

32

Alimak raising provides the safest of all entry methods involving the least risk to the

miner and can excavate safely through all types of ground conditions supporting the face after

each blast is taken ensuring the integrity of the excavation during all stages of development.

The Alimak raising system ensures fast mobilisation, minimal preparation, is flexible,

accurate, economical and very cost effective even over short distances. Even multiple raises

with directional changes in the raise of up to 90° can be carried out easily making this method

the ideal choice for ore passes, crusher chambers, split level ventilation raises or any difficult

excavation profile.

Alimak raise climbers are widely being used to drive shafts and raises in Mount Isa

mine Australia. Importantly the longest Alimak raise developed to date in these mines is

more than 1000m in length.

Fig. Preparatory work for installation of Alimak raise climber

Cycle of Operation

Step -1(Fig. a) –Drilling; Drilling is undertaken from the drill deck on top of the raise

climber, which is sized to suit the size, shape and angle of the raise. Drill machine is jack

hammer for drilling a 34 mm diameter and 2 m long blast holes. Burn-cut parallel blasting

patter in the common pattern used for raise blasting.

Step -2 (Fig b)-Loading: When drilling is completed the face is charged with explosives

along with MSD & HSD delay detonators. Of all the rounds, perimeter round is very

33

important in raise blasting, and smooth blasting techniques are followed to contain over-

break.

Step-3 (Fig c)- The Alimak climber is then lowered to the bottom of the raise and into a

station for protection before the blast is triggered from a safe location.

Step-4: Ventilation: The Alimak system provides for efficient post blast ventilation and a

powerful air/water blast effectively dislodging loose rock from the freshly blasted face

making ready for re-entry.

Figure Steps of operation in Alimak raising method.

34

This method has the following advantages:

• permits driving of long raises

• personal are well protected in a cage under the platform

• the miners work from the platform that can be easily adjusted for convenient height

• timbering is avoided and stability can be increased by rock bolting if necessary

• no danger from falling of rock pieces

However the cost and other arrangements required cannot justify this for short raises. Figure

above shows complete cycle of raising.

Special feature of Alimak raise climbers:

A. Drive Units:

The raise climber is developed with three kinds of drive units: air driven, electrically driven,

and diesel/hydraulically driven.

Of the different types of Alimak raise climbers, compressed air driven raising is very

common in the country, followed by diesel operated raise climbers are popular.

Air Driven:

In the air driven raise climbers, compressed air comes through a hose. An automatic winch or

reel winds the hose up and down as per the movement of the alimak in the raise construction.

The air motors are effective for raising up to 200m length.

Electrical drive:

Electric are not common in mines, however they have a capacity of driving about 1000m long

raises. The longest vertical raise for ventilation shaft at the Densison mines, Ontario, Canada,

in 1974 [SME-UMM Hand book].

Diesel / Hydraulic drive:

Diesel operated Alimak raises climbers are also common after the compressed air driven

machines. However there is a risk of excess air pollution due to diesel operated machines

underground. The diesel/hydraul;ic driven raise climber can drive more than 1000 m long

raises in one step.

35

The figure above gives the scope and limitation of various types of Alimak raise climbers.

B. Safety features

For the types of Alimak raise climbers the following safety features make them more

adoptable in mines;

• Over speed control system; the permitted speed limits on descent are 0.9m/s, if the

climber exceeds this speed limit the automatic braking system stops the climber to

further descend.

• The rack-and-pinion gear plates are wielded to the guide rails thus ensure a guided

manoeuvring of the climber up and down the raise.

• The cross section of a guide rail is as shown in the figure below

(a) (b)

Fig (a). Cross-section of a guide rail; (b). Rack-and-pinion mechanism

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• The air, water supply is provided through the ports within the guide rail,

approximately 25m3/min air supply is provided continuously at the face point. This

facilitates the operators with fresh air at the working face. There is a provision to

increase the air quantity as per the requirement.

• Telephone communication between the face crew and the bottom crew is provided by

an insulated wire passing through one of the ports in the rail.

• Blasting cable also runs through the port within the rail.

• A canopy is also provided for the safety of the face workers while scaling down the

loose material from the roof.

C. Initial guide rail sections

The guide rails for negotiating the curves are special made in angular sections, 80, 250, 250,

250, 80 and having a radius of 2.3 ~3 m for vertical raises. The brow point is the point where

the cross cuts terminates into a vertical raise (Fig below), is slashed at 450 to accommodate

the circular guide rail segments.

RAISE BORING METHODS

Raise-Boring

In this system, the pilot hole is drilled down to a lower level in the mine or civil project. Once

the pilot hole connects to the lower access level in the rock, the drill bit is removed and a

reamer or raise head is attached and the reamer is rotated and pulled upwards. The broken

rock falls to the lower level by gravity. This system operates with the drill string in tension

and this provides the most stable platform.

37

Figure. Raise Boring

Down-Reaming

In this system, the pilot hole is drilled downwards until it connects to a lower access level.

The drill string (all drill rods, stabilizers and cutting bits) is retrieved and then a reamer is

pushed downwards. The cuttings flow down the previously drilled pilot hole. This method

uses drill string in compression and usually stabilizers must be installed to eliminate the

potential of the drill string buckling.

Figure: Down Reaming method of raise boring

38

Box-Holing

The most difficult raise method, known as Box-Hole excavation. It is to drill a pilot hole to

any level up from the raise borer. Once the desired length is achieved the drill string is

retrieved, and a reamer attached and pushed upwards. The broken rock falls down the

enlarged hole onto a special collection chute attached to the top of the raise borer. This

technique has been largely used to replace ladder rises, which completes the box-hole using

conventional methods. Ladder rise excavation is very dangerous

.

Figure. Box-holing method of raising.

ADVANTAGES OF BORED RAISES

• Raise boring offers several advantages over the conventional drill and blast method.

The most important are safety, speed, physical characteristics of the completed hole,

labour reduction and cost reduction. The safety factor in raise drilling cannot be over

emphasized. No men are exposed to the danger of rock fall from freshly blasted

ground or to the continual use of explosives, with their fumes and inherent danger of

misfires. Raises can be safely drilled in ground that would be extremely hazardous, if

not impossible, to drive by conventional methods.

• A hole drilled by Raise Boring Machine can generally be completed in a fraction of

the time required for conventional methods. The bored raise, with its firm undisturbed

walls, is more adaptable to use as ventilation and rock passes. As conventional

methods require a relatively large opening, it has become customary to drive raises

larger than actually required for ore and rock passes, a fact that long experience has

borne out. The advantage of smooth walls in ventilation raises is well known.

39

• Raise boring will not only reduce labour requirements by achieving a higher advance

per day but, along with another technological advances, will have the tendency to

attract a higher level of skilled labour to the mining industry.

• Last, and probably most important from the long-range viewpoint, is cost reduction.

Although, it is true that the direct cost of conventional raises, especially short ones,

may currently be less in many cases, labour and material costs are continually

escalating and therefore their costs increasing. Skilled conventional miners, always in

short supply, are not required to operate a Raise Boring machine. Improved raise

drills, drilling techniques, pilot bit and cutters are lowering the cost of machine

excavated (RBM) raises. Less total manpower, less rock to handle, less construction

time and increased safety all add up to less costs and earlier projects.

Shaft Station

Underground mining operations involve deployment of different types of heavy duty rock

excavation and transportation machines. Some are electric power driven, others are diesel

operating machines. There are a few specialized openings such as bunkers, pumping station,

electric sub-station etc., at the bottom of the main shaft, and it is the horizon where the

vertical shaft intersects with horizontal openings. This is known as the shaft station.

The shaft station serves as the principal terminus of all underground and surface operations.

Those related to materials handling involve: skip loading pockets, retention bunker;

ventilation arrangements; pumping stations; electrical sub-stations; underground mechanical

shop / workshop; first aid centre & rest rooms etc.

The design considerations depend on the number of shafts within the station, type of deposit,

mode of materials handling in the mine and in the shaft, water inflow, ventilation

requirements, mining equipment, etc.

Fig. Standard shaft station layouts

40

a-with circular mine traffic; b- with shuttle traffic; c- loop like layout of shaft staion;

1- Main shaft; 2- service shaft

Shaft station is an aggregate of working located in the immediate vicinity of the shaft. These

are provided to afford connection between a shaft and the different levels in a mine. Their

primary use is to tenable men and material to be delivered at the different working horizons

and for raising the ore. The size of the station will depend on the size and amount of material

that it will be required to accommodate.

Generally the longer the life of a mine and larger the output the shaft station becomes more

complex. Some of the factors that are considered for design of shaft station are:

• Type of deposit

• Mode of material handling in the mine

• Hoisting of ore in the shaft

• Water inflow and ventilation

• Mining equipment

Shaft stations related to the material handling are skip loading pockets, retention bunkers

pump chamber, explosive storage chamber, locomotive room and sometimes primary

underground crusher. These chambers are important link in the extraction process, transport

etc. They are located near the main or auxiliary shaft because of their functions.

The first group of chambers includes explosive storage, pump house, miners’ rest room

where as locomotive repair and clearing, dispatcher rooms are related to the transport. The

construction of shaft station chamber is made by conventional drilling and blasting method

taking into consideration of ground conditions. These chambers are properly supported by

bolting, grouting etc.

Question

Explain with a neat sketch a shaft with skip hoisting system for a production level of say,

1200 tpd . Show the surge bin, loading pocket, measuring hopper excavated and installed in

the shaft station label the sketch ?

Answer

The shaft stations in hard- rock mines for material handling arrangement will have the

following:

1. Skip loading pockets,

2. Retention bunkers

3. Pump chambers

4. Main power station

5. Explosive storage chamber

6. Locomotive room

7. Mechanical & electrical workshop

8. Dump (ore/waste) chamber – with bunker & u/g crusher.

41

9. Arrangements for the type of ore/waste transport system ( eg: belt; train)

1- Access drift to waiting room; 2- basement for two-level traffic and swinging platforms;

3- Basements for pushers and barrages (blocking cars); 4- a slot for control equipment

Fig. Inset of cage shaft with three levels to step in and out for crew.

The size of the inset of a cage shaft depends on the width and number of cages being hoisted

on this level, number of decks in cages, and length of the supplies to be delivered. Depending

on the skip loading system and horizontal transportation arrangements, there could be the

following sets of openings for loading facilities:

1. For rail transport :

a. Dump(tippler) chamber or unloading ramp (for Granby cars), batchers chambers( this

for accommodating a batch or a train of mine cars), skip chamber

1-Skip chamber; 2- batcher chamber; 3.- tippler chamber; 4- basement of shifting mechanism; 5- basement of

braking system; 6- drive slot; 7- electrical equipment slot; 8- ventilation slot.

Fig. Connection of production skip shaft with the opening of loading system for rail transport system.

42

b. Dump(tippler) chamber or unloading ramp (for Granby cars), retaining bunker, loading

devices chamber, batchers chambers( this for accommodating a batch or a train of mine

cars), skip chamber

1- Skip shaft; 2- skip chamber; 3- batchers chamber; 4- switches chamber 5- loading chamber; 6- retaining bunker; 7-

distribution chamber; 8- distribution ramp; 9- drift for clearing away jams; 10- chute

Fig Connection of production skip shaft with the openings of the loading devices for horizontal rail transport.

c. For belt transport: unloading chamber, retaining bunker, loading chamber, batchers

chambers( this for accommodating a batch or a train of mine cars), skip chamber

1. Skip shaft; 2- skip chamber; 3- belt scale ; 4- retaining bunker; 5- unloading chamber.

Fig. Connection of production skip shaft with the opening of loading devices for horizontal belt

transport system.

43

Lecture 4- Stope Development

Once the economic extraction of ore body is ascertained, the step follows next is

development and preparation stope for extraction or ore. The development of an ore drift

(cross-cut) will confirm the thickness (extent of orebody) and continuity of the ore body and

enable the planners to finalize stope design.

Different development configurations and construction arrangements are possible for ore

body geometry. The stope preparation involves development of haulage level and sill-level.

This approach allows the development of draw points (figure below)

Fig Plan view of development of ore and footwall drives.

Draw points are developed at the bottom of open stopes as an inverted cone by drilling and

blasting. Their form is determined by the way in which the ore is to be loaded.

A large chute can be used to load ore from a main ore pass into a dump truck or smaller

chutes can be installed on each of several ore passes along a level to load directly into mine

cars.

Figure shows ore loading chutes. Chutes cause production holdups if they become blocked by

large pieces and to exclude the large pieces from coming to chute, ore is fed through grizzly

which has a grating made up of steel bars. Lumps which do not fall through grizzly are

broken with hammer of pneumatic pick.

Fig. Ore loading chutes

44

The figure below shows a typical draw point configuration for LHD/Shovel loading draw

point. In this configuration the draw points are usually 10m long and driven perpendicular to

the haulage-way to facilitate ore loading into mine cars. The interval of draw points is around

10m apart. The dimensions of these draw points are selected considering the ease of loading.

The draw point around the mouth or the entrance of the stope requires a lower back to

establish a brow that will prevent ore from spreading too far into the draw point.

Fig. LHD/ Rocker shovel draw points

T

Plan view of the draw point with track system of transportation

45

Fig. Cross section of a draw point configuration-track system of transportation

Another form if scram (also known as scraper) driven draw point. Ore is broken in the stope

and gravitates down into the drive. A scraper bucket is used in the drive to scrape ore and

drop it down through a grizzly down a mil hole into mine cars. Figure shows a scram driven

draw points and mill holes. Another from is to load ore from a stope by a mucking machine,

figure showing LHD draw points.

Fig. Scram drive points and ore draw points

In some mines construction of individual draw points for open stopes in not carried out. The

stope bottom is percussive drilled from the draw point level and blasted into a continuous v-

shape. Broken ore is loaded out from the bottom drive as it comes down. It is still necessary

to drive a raise to form an initial cut-off slot. Figure shows v-shaped draw point. A sill pillar

is left horizontally around and above the level drive to protect them and provide height to

develop draw points. As stopes are worked upwards to meet the level above a horizontal

crown pillar is left below the level above to stope them from collapsing.

Stope development thus includes haulage drifts cross cuts drifts, chutes and draw points,

raises. The size of the development is dependent on the equipment and winning methods to

be used. Minimum development requirements for a typical ore body include a drift from the

46

main haulage to the ore body, raising into the ore body, driving the stope sill and finally

installing draw points and chutes.

Fig Draw point

47

Fig . Mechanised ore loading methods

Ore pass system

Ore passes are underground passageways for the gravity transport of broken ore, waste rock

from one level of a mine to a lower level. Inclination of ore pass varies widely within a range

of 450-900, and most common angles are 700 and cross sections are mostly circular. Besides

transport of ore it also sometimes serves as a storage which is required for efficient mines

operation. Ore pass length range from 10 m to 200m or more

The components of ore pass system include: (1). a raise connecting two or more levels, (2).

Top-end facilities for material size and volume control such as grizzles, crusher and (3).

bottom end structures to control material flow.

Unlined ore pass may be located in country rock (FW) but some mines are lining ore-passes

with steel fibred-reinforced shotcrete. The bottom of the ore-passes at the haulage level

usually contains a loading chute equipped with pneumatic / hydraulic operated gates. The ore

is loaded in to tubs and a train of tubs then dump the ore in the main ore-pass which is usually

located at a haulage shaft.

48

Fig. Schematic of an ore-pass: tip section; discharge zones.

In mechanized stopes the ore is removed from the stope by LHD units and is dumped at the

stope ore pass for handling at the lower level from where it is transported and dumped in the

main ore pass. The main ore pass are developed within the ore body rock or within the ore

body peripheral rock. Their operational life approximates that of adjacent stoping activity and

in some cases the excavations may be consumed in the stoping process.

Proper design of ore pass requires that the broken ore, waste rock will flow when the outlet is

activated. The flow process is driven by gravity and resisted by friction and cohesion. Proper

design will see that their malfunctions of ore pass operations are to be prevented: failure to

flow resulting in hang-ups and failure to flow over the entire cross-section of the ore pass

referred to as piping. The other important design consideration is the stability of ore pass

walls.

Ore pass construction

Ore pass systems are an integral part of the materials handling system in the majority of

underground mines. Ore passes are developed using either mechanical (raise borer) or drill

and blast techniques (Alimak, conventional raising and drop raising). The conventional

manual method of raising is slowly being replaced by Alimak raising. In Quebec mines,

Alimak raising was used in 63% of driven ore passes while only 3% were raise bored. The

dominance of Alimak driven passes over raise bored passes in Quebec mines is attributable to

several causes. It ensures a reasonable degree of safety for the miners, while still allowing the

installation of support. Furthermore, the ability to drive the Alimak pass from a single access

49

(as opposed to raise boring, which requires that both the bottom and top accesses be

developed) and a strong expertise of local mining contractors are also contributing

factors.Conventional and drop raises represent 29% and 5% of the sections, respectively (Ref:

Ore pass practice in Canadian mines by J. Hadjigeorgiou, J.F. Lessard*, and F. Mercier-Langevin; The

Journal of The South African Institute of Mining and Metallurgy vol. 105 Dec. 2005). The dominance of

Alimak raising is attributed to several reasons. It ensures a reasonable degree of safety for the

miners, while still allowing the installation of support. Furthermore, the ability to drive the

Alimak in blind raises (as opposed to raise boring, which requires that both the bottom and

top accesses be developed) and it provides comfortable working environment at the face.

Table Case example of U/G mines of Lead & Zinc Quebec, Canada

(Ref: Ore pass practice in Canadian mines by J. Hadjigeorgiou, J.F. Lessard*, and F. Mercier-Langevin; The Journal of The South African

Institute of Mining and Metallurgy vol. 105 Dec. 2005).

Ore pass section length

50

There is an inherent relationship between the type of excavation method and section length.

Typically, sections excavated by drop raising or conventional rising are shorter than sections

driven by Alimak or raise borers.

There are several practical and financial considerations that influence the selection of an ore

pass length. If, for example, an operation aims to minimize its capitalized development, it

will end up driving short ore pass sections, going from one level or sub-level to the next,

concurrently as the various levels are entering into production. Quite often a mine that

experienced problems when driving and operating long sections will subsequently opt for

shorter sections when constructing new ore and waste passes. An excavation of greater length

is more likely to intersect zones of poor ground. It also has a higher potential for problems

and is harder to bypass. Longer sections may also result in higher material flow velocity in

passes operated as flow-through.

Ore pass section inclination

Ore pass inclination varies between 45° and 90°, with an average inclination of 70°. The

choice for a particular inclination is dictated by the need to facilitate material flow. Shallow

sections may restrict flow, especially if a high proportion of fine material is present, while

steeper excavations result in higher material velocities and compaction. It should be noted

that all vertical sections are shorter than 100 m. Generally steep ore passes (80º ± 8.3º) are

advantageous because it ensures continuous material flow and limit hang-up occurrences.

Ore pass section shape

The majority of excavated ore passes are square or rectangular. Circular sections are usually

associated with raise boring methods but in some instances, circular sections were excavated

using Alimak. In most cases, the main factor indicating the choice between a rectangular and

a square section is local mine experience. Circular shape was used based on anticipated

higher stress regimes. It is of interest to note that a review of ore pass surveys reveals that

under high stress, and with material flowing in an ore pass, a design circular shape is not

maintained for long (in unlined ore passes). Ore pass size is an important factor influencing

material flow. This is reflected in empirical guidelines linking the potential for hang-ups with

ore pass size and material size. A common dimension of 2.0 m is widely used, however there

are some mines where a relatively larger cross-sectional dimension of 2.5 ± 0.6 m have also

been adopted.

Finger raises

Finger raises are used to funnel material into a pass intersecting two or more production

levels. Typically, a finger raise is a square opening with a smaller cross-sectional area than

the rock pass it feeds. The most common dimensions for a finger raise are 1.5 and 1.8 m.

51

Screening of oversize material

Oversize material dumped into the passes may lead to blockages or interlocking hang-ups.

This can be avoided by either instructing the mucking crew or by installing the necessary

infrastructure to restrict the entrance of the oversize material.

The mechanical method of retaining oversized material at the mount of an ore-pass is by the

installation of a grizzly. Sometimes mucking crews can be ‘persuasive’ in trying to push the

block through the bars with the bucket. This practice damages both the bars and the scoop.

Broken and missing bars are often the result of this practice. In addition, the intrusion of a bar

in the ore pass can lead to severe obstruction further down the system. Grizzlies are the best

to keep big blocks out of the passes. Grizzlies require less maintenance than scalpers.

Reinforcement

Resin-grouted rebar constitutes the most popular reinforcement type for ore pass systems.

Nevertheless, the most recently developed excavations are reinforced by resin grouted short

cable bolts. An ore pass section is considered to have ‘failed’ if it had expanded to twice its

initial volume as recorded in the original layout.

Ore pass problems

Analysing the causes of degradation is a complex process due to the potential interaction of

several mechanisms. There is a relationship between the material unit weight and the degree

of observed degradation of the walls of the ore pass. A qualitative assessment of the dominant

degradation mechanisms include: structural failures facilitated by material flow; scaling of

walls due to high stresses; wear due to impact loading caused by material flow; wear due to

abrasion and blast damage caused by the hang-ups clearing methods.

Wall damage attributed to impact loading is most often localized at the intersection of finger

raises to the ore pass. It is most probable that the presence of structural defects in the rock

mass accentuates the influence of impact loading, resulting in more pronounced degradation.

The use of ‘rock boxes’ can reduce impact damage but in most cases impact damage is

localized on the ore pass wall facing the finger raise. Abrasion rate depends on the

abrasiveness of the material and the ore pass walls’ resistance to abrasion.

Blockages

Blockages are the most commonly encountered type of flow disruption in ore pass systems.

Flow disruption near the chute may be due to blocks wedged at the restriction caused by the

chute throat. Another source of problems is caused by the accumulation of fine or ‘sticky’

material in or near the chute, on the ore pass floor. This reduces the effective cross-sectional

area and results in further blockages.

Material flow problems

52

Some types of material flow problems are reported in every mine operating an ore pass

system. Sometimes the transfer of coarse material can result in hang-ups due to interlocking

arches, while the transfer of fine material results in hang-ups due to cohesive arches,

Hang-ups

Restoring material flow is a priority in operating mines. There are several methods to restore

the material flow in case of a material hang-up with in the ore pass and they can be classified

as those that employ water and those that rely on explosives,

Most hang-ups lower than 20 m are brought down by attaching explosive charges on wood or

aluminium poles used to push the charge up to the hang-up. As a last resort, holes drilled

toward the hang-up can be driven and explosive charges set inside the hole, near the supposed

hang-up location. If the location of the hang-up is not clearly identified, it may take more

than one attempt to restore flow.

Cohesive hang ups are difficult to dislodge using explosives. Some operations resort to

blowing compressed air through a PVC pipe raised up to the hang-up location or dumping a

predetermined amount of water from a point above the hang-up. All mines have strict

procedures about the use of water in order to avoid the risks of mud rushes.

Fig. Hang-ups in an ore pass due to (a) interlocking; (b). cohesion arching,

53

Fig. . Damage zones in an ore pass.

ORE PASS DEGRADATION DUE TO IMPACT (ref: Influence of finger configuration on degradation of ore pass walls K. Esmaieli Université Laval, Quebec City, Canada J. Hadjigeorgiou University of Toronto, Toronto, Canada; ROCKENG09: Proceedings of the 3rd CANUS Rock Mechanics Symposium, Toronto, May 2009 ; Ed: M.Diederichs and G. Grasselli)

In ore pass systems gravity movement of rock includes rolling, sliding and inter fragment

collision. The interaction of moving material and ore pass walls can result in the development

of wear and/or impact damage zones. Wear is associated with the particles rolling and sliding

along a surface resulting in the scouring of the wall surface. Damage attributed to impact

loads can be caused by single falling boulders in the ore pass, a stream of rock or a large mass

of material, Iverson et al. (2003). The mechanical properties of the rock mass along the ore

pass wall can influence the extent of damage. Stacey & Swart (1997) note that wear of ore

pass walls is greater in weak rock material and in the presence of stress scaling. If the ore

pass is located in a rock mass with structural defects the action of moving material can

initiate further wall degradation, including falls of ground. Ore pass wall damage, induced by

impact, is one of the most important mechanisms of ore pass degradation. This paper reports

on-going work, using numerical models, on the influence of material impact for several ore

pass and finger raise configurations.

Figure above illustrates a typical finger raise - ore pass configuration. Hadjigeorgiou et al.

(2005) report that, in Canadian underground mines, finger raises have cross section

dimensions of 1.5 m x 1.5 m and 1.8 m x 1.8 m. The fingers are linked to ore passes of larger

cross section dimensions. A well designed finger raise can minimize the ore pass wall

damage and maximize ore pass longevity. Current practice is often based on empirical rules

which quite general and may not always be appropriate for site specific conditions. Empirical

guidelines recommend an inclination of 60o for finger raises in order to ensure free flow of

rock fragments in the finger raise. This recommendation may not be valid for all the

54

conditions. The finger raise inclination influences the motion and interaction of rock

fragments flowing in the ore pass and the resulting load on the ore pass wall. If the finger

raises are steep this will result in higher impact velocity on the ore pass walls. On the other

hand if the finger inclination is shallow material flow is slow and can result in hang-ups. A

steeply inclined finger raise results in narrower pillars at the intersection of the ore pass and

finger raise which are more susceptible to stability problems. Consequently an operational

design will use a finger raise inclination that will minimize impact load on the ore pass wall

while maintaining material flow in the finger.

It has been demonstrated that particle impact velocity and kinetic energy increase with finger

raise inclination. The impact duration decrease with increase of finger inclination. These

observations can be used to evaluate different options of finger inclination for any particular

ore pass inclination. The analysis clearly demonstrated that the choice of intersection angle

has a significant influence on the resulting impact loads on the ore pass wall and the location

and magnitude of damage to the ore pass. The highest impact loads were reported for

intersection angles of 1400 and 1450.

Q. Explain the gravity ore transportation methods in u/g metal mines

Fig. Ore pass system in Mount Isa Copper Mines –Australia (Ref.L.J.Thomas Intro. to mining)

55

Lecture 5 Factors influencing the selection of a suitable stoping method

The following factors are considered in selecting a suitable method of stoping operation.

1. Mining excavations and their importance in terms of the life term of a mine

2. Rock mass response to stoping activity

3. Spatial distribution of the ore-body

4. Disposition and orientation

5. Size

6. Geomechanical setting

7. Ore body value and spatial distribution of value

8. Engineering environment.

1. Mining excavations and their importance in terms of the life term of a mine

The three types of openings are employed in the mine operation, these are the ore sources, or

stopes, the stope access pathways, or the levels, cross cuts; and the main mine service

openings – shafts, inclines, declines, or adits. The geomechanical performance of these

different types of openings is specific to the function of the opening. Based on their function

and the life term of these openings, they are categorized as:

• Primary openings - shafts, inclines, declines, or adits, these are the permanent

openings in comparison to the other two types

• Secondary – levels, cross cuts, raises & winzes, drifts, etc., - these are semi-

permanent openings, their life terms is relatively less compared to the primary

openings.

• Tertiary openings: stopes or the source of ore – the main production zone. The life

term of the stopes is the shortest of the three above openings.

Stopes:

A mine has a large number of stopes therefore; a set of stopes constitutes the largest

excavation underground. The stability of stopes is controlled not only by the orebody strength

condition but also on the strength of the peripheral rock (HW and FW) the principles of stope

layout and design are integrated with the set of engineering concepts (like the rock

mechanics) and physical operations (such as mine transportation of the ore and waste) which

together compose the mining method for an orebody.

It is a commonly held belief amongst underground mine planning and design engineers

that in a sub-level open stoping mine, the bigger the stopes – up to the geotechnical limits –

the greater will be the production rate and hence, the more cost efficient the mine. This paper

shows that this can be a fallacy – it is usually true for the individual stope but may not be true

for the mine when considered as a system of inter-related stopes.

In a fixed size orebody there is a limit in the production rate achievable which in turn is

related to the number of active stopes, in the sense that the stopes are in some phase of the

stope development cycle (preparation, production, filling or curing) at a given time frame.

Once this limit is reached, there are no more stopes that can be brought into production. This

is a physical constraint, which places a limit on the production rate achievable for the stoping

56

system. However, this constraint, the number of stopes, can be changed. This can be

accomplished by either altering stope size or cut-off grade.

Fig. Division of the ore body into active workable stopes based on grade value

Fig. Longitudinal section of a mine

57

58

2. Rock mass response to stoping activity

The extraction of mineral resources involves rock excavations of different shapes, sizes, and

orientation based on the purpose for which the excavation is made. And it is obvious on the

creation of an opening (stope / drive) the state of equilibrium in the surrounding rock is

disturbed and the redistribution of the induced stresses is dependent on the type of rock mass,

size of the opening and method of excavation.

The dimensions of ore bodies of mining significance typically exceed hundreds of meters in

at least two dimensions. During excavation of an orebody, the spans of the individual stope

excavations may be of the same order of magnitude as the orebody dimensions. The

performance of the host rock mass during mining activity can be easily measured in terms of

the displacements of orebody peripheral rock. It is clear from the studies of stresses around

mine openings, the zone of influence is usually taken as 3dm, where dm is the minimum

dimension of the opening. The zone of influence is considered as the near field zone and the

zone outside this is termed the far field zone.

The rock mass response to stoping operations is dependent on the inherent strength of the

rock. Therefore on the basis of its response, a rock mass can be categorised into a class of

competent (strong and self-supporting) and in-competent (weak and crushing & crumbling

type of rocks). There are many rock types which fall in between these two extremes.

Therefore there can be stoping methods which are self-supporting, and a few stoping methods

need some artificial supporting and lastly there can be some which cannot be supported, such

stopes are left to crumble and cave down.

Fig. Rock mass response to mining

• The supported methods of working can succeed only if the induced stresses are less

than the strength of the near-field rock. Caving methods can proceed where low states

Underground mining methods

Pillar supported Artificially supported Unsupported

Room &

Pillar

Sublevel

Long hole

Open

stoping

Cut-and-Fill Shrinkage VCR Sub Level

Caving

Block

caving

Magnitude of displacement in country rock

Strain Energy storage in near-field rock

59

of stress in the near field can induce discontinuous behaviour of both the orebody and

overlying country rock, by progressive displacement in the medium.

• In supported methods, since the strength of the rock mass in higher, they exhibit the

ability to store more strain energy in comparison to the caving methods.

• For caving method prevents the accumulation of strain energy by continuous

dissipation of pre-mining energy by fracturing.

• Fully supported stopes may completely depend on natural support in the initial

stoping phase, using ore body remnants as pillar elements. In the early stages of pillar

recovery, various types of artificial support may be placed in the mined voids, to

control local and regional rock mass displacements. In the final stages of pillar

recovery, pillar wrecking and ore extraction may be accompanied by complete failure

of the adjacent country rock. This change in the state from one geomechanical basis to

another can have important consequences on the performance of permanent openings

and other components of a mine structure. This indicates that the key elements of a

complete mining strategy for an orebody should be established before any significant

and irrevocable commitments are made in the pre-production development of an

orebody.

3. Spatial distribution of the ore-body

This property defines the relative dimensions and shape of an orebody. It is related to the

deposit’s geological origin. Ore bodies described as seam, placer or stratiform (strata-bound)

deposits are of sedimentary origin and always extensive in two dimensions. Veins, lenses and

lodes are also generally extensive in two dimensions, and usually formed by hydrothermal

emplacement or metamorphic processes. In massive deposits, the shape of the orebody

is more regular, with no geologically imposed major and minor dimensions. Porphyry

copper ore bodies typify this category. Both the orebody configuration and its related

geological origin influence rock mass response to mining, most obviously by direct

geometric effects. Other effects, such as depositionally associated rock structure, local

alteration of country rock, and the nature of orebody–country rock contacts, may impose

particular modes of rock mass behaviour.

4. Disposition and orientation

These issues are concerned with the purely geometric properties of an ore body, such as its

depth below ground surface, its dip and its conformation. Conformation describes orebody

shape and continuity, determined by the deposit’s post-emplacement history, such as episodes

of faulting and folding. For example, methods suitable for mining in a heavily faulted

60

environment may require a capacity for flexibility and selectivity in stoping, to accommodate

sharp changes in the spatial distribution of ore.

5. Size

Both the absolute and relative dimensions of an ore body are important in determining an

appropriate stoping method. A large, geometrically regular deposit may be suitable for

mining using a mechanized, mass-mining method, such as block caving. A small deposit of

the same ore type may require selective mining and precise ground control to establish a

profitable operation. In addition to its direct significance, there is also an interrelation

between ore body size and the other geometric properties of configuration and disposition, in

their effect on mining method.

6. Geomechanical setting

The geo-mechanical setting includes:

• Rock material properties such as strength, deformation characteristics (such as

elastic, plastic and creep properties) and weathering characteristics.

• Rock mass properties are defined by the existence, and geometric and mechanical

properties, of joint sets, faults, shear zones and other penetrative discontinuities.

• The pre-mining state of stress in the host rock is also a significant parameter.

In addition to the conventional geomechanical variables, a number of other rock material

properties may influence the mining performance of a rock mass. Adverse chemical

properties of an ore may preclude caving methods of mining, which generally require

chemical inertness. For example, a tendency to re-cement, by some chemical action, can

reduce ore mobility and promote bridging in a caving mass. Similarly, since air permeates a

caving medium, a sulphide ore subject to rapid oxidation may create difficult ventilation

conditions in working areas, in addition to being subject itself to degradation in mechanical

properties.

Other more subtle ore properties to be noted are the abrasive and comminutive properties of

the material. These determine the drillability of the rock for stoping purposes, and its particle

size degradation during caving, due to autogeneous grinding processes. A high potential for

self-comminution, with the generation of excessive fines, may influence the design of the

height of draw in a caving operation and the layout and design of transport and handling

facilities in a stoping operation.

In some cases, a particular structural geological feature or rock mass property may impose a

critical mode of response to mining, and therefore have a singular influence on the

appropriate mining method. For example, major continuous faults, transgressing an orebody

and expressed on the ground surface, may dictate the application of a specific method, layout

and mining sequence. Similar considerations apply to the existence of aquifers in the zone of

61

potential influence of mining, or shattered zones and major fractures which may provide

hydraulic connections to water sources. The local tectonic setting, particularly the level of

natural or induced seismic activity, is important. In this case, those methods of working

which rely at any stage on a large, unfilled void would be untenable, due to the possibility of

local instability around open stopes induced by a seismic event. A particular consequential

risk under these conditions is air blast, which may be generated by falling stope wall rock.

7. Orebody value and spatial distribution of value

The monetary value of an orebody, and the variation of mineral grade through the volume of

the orebody, determines both mining strategy and operating practice. The critical parameters

are average grade, given various cut-off grades, and grade distribution. The average grade

determines the size and monetary value of the deposit, since the market price for the mineral

changes with time and demand.

The significance of dilutions of the ore stream, arising, for example, from local failure of

stope wall rock and its incorporation in the extracted ore, is related to the value per unit

weight of ore. In particular, some mining methods are prone to dilution, and marginal ore

may become uneconomic if mined by these methods. Grade distribution in an orebody may

be uniform, uniformly varying (where a spatial trend in grade is observed), or irregular

(characterized by high local concentrations of minerals, in lenses, veins or nuggets). The

concern here is with the applicability of mass mining methods, such as caving or sublevel

stoping, or the need for complete and highly selective recovery of high-grade domains within

a mineralized zone. Where grade varies in some regular way in an orebody, the obvious

requirement is to devise a mining strategy which assures recovery of higher-grade domains,

and yet allows flexible exploitation of the lower-grade domains.

Engineering environment

8. Engineering Environment

A mining operation must be designed to be compatible with the external domain and to

maintain acceptable conditions in the internal mining domain. Mine interaction with the

external environment involves effects on:

• Local groundwater flow patterns, changes in the chemical composition of

groundwater,

• Possible changes in surface topography through subsidence. In general, caving

methods of mining have a more pronounced impact on subsidence than supported

methods.

• Mine gases such as methane, hydrogen sulphide, sulphur-dioxide, carbon dioxide or

radon may occur naturally in a rock mass, or be generated from the rock mass during

mining activity.

62

In fact, stope backfill generated from mill tailings is an essential component in many mining

operations. Specific mining methods and operating strategies are required to accommodate

the factors which influence the mine internal environment.

Problems

Q1. Discuss the effects of rock mass response to stoping?

Q2.Explain how rock mass movement due to stoping affect ore dilution in different

stoping operations?

Answer:

Dilution is defined as the low grade (waste or backfill) material which comes into an ore

stream, reducing its value. By-and-large, dilution control may be more difficult in the caving

methods where displacements of large magnitudes within the host rock are experienced.

Artificially supported mining methods rely on achieving close control of the performance of

the rock mass surrounding a stope. Cut and fill relies on passive support from the applied

backfill, while shrink and VCR stoping use the broken ore as a temporary support for the

stope walls. Shrinkage stopes can be susceptible to external dilution due to time dependent

failure of the exposed walls, while excessive damage (external dilution) to the stope walls can

be experienced during VCR mining, specially when used for pillar recovery.

The success of naturally supporting methods such as sublevel open stoping (for large tabular

and massive ore-bodies) relies on achieving large stable and mostly unsupported stope

boundaries. The stand-up time before backfill support is introduced as well as support

provided by cable bolting is also an important factor controlling stability.

(Source of information: Ernesto Villaescusa)

Q3.What technical information is needed for preliminary mine planning?

Answer:

Many details must go into the planning of underground mine and information must come

from several sources. Geological, structural, and mineralogical information must first be

collected and combined with data on resources and reserves. This information leads to the

preliminary selection of a potential mining method and sizing mine production.

The following information should be gathered during the exploration phase and passed on to

the mine evaluation team of the mine development team. The information is:

• Property location and access

• Description of surface features

• Description of regional, local, and mineral deposit geology

• Review of exploration activities

• Tabulation of potential ore reserves and resources

63

• Explanation of ore-reserve calculation method

• Description of company’s land position

• Description of the company’s water position

• Ownership and royalty conditions

• History of the property

• Any special studies by the exploration team

• Any social issues or environmental issues that have surfaced while exploration was

being completed.

Q4. What specific planning is required related to physical properties of the ore body

and surrounding ground?

Answer:

The physical nature of the extracted rock mass and the rock mass left behind are very

important in planning many of the characteristics of the operating mine. Four aspects of any

mining system are particularly sensitive to rock properties.

(a). the competency of the rock mass in relation to the in situ stress existing in the

rock determines open dimensions of unsupported roof unless specified by

regulations. It also determines whether additional support is needed.

(b). When small openings are required, they have a great effect on productivity,

especially in harder materials for which drill and blast cycles must be used.

(c). The hardness toughness and abrasiveness of the material determines the type

and class of equipment that can extract the material efficiently.

(d). If the mineral contains or has entrapped toxic or explosive gases, the mining

operation will be controlled by special provisions in mine regulations.

64

Chapter 5 Mining Methods

The emphasis is confined to the relations between working method, the rock mass conditions

essential to sustain the method, and the key orebody properties defining the scope for

application of the method. The mining methods commonly employed in industrial practice

are classified as shown below. Other mining methods, mostly of historical or local

significance, such as top slicing or cascade stoping, could be readily incorporated in this

categorization. The gradation of rock performance, ranging from complete support to induced

failure and granular flow, and in spatial energy change from near-field storage to far-field

dissipation, is consistent with the notions discussed earlier.

Classification of stoping methods based on the strength of the rock mass

A. Naturally supported stopes

1. Open stoping with pillar supports

a. Room-and-pillar stopes

• Room-and-pillar with regular pillars

• Room-and-pillar with irregular pillars

2. Open stopes

a. Sub-level open stoping

b. Large Diameter Blast Hole stoping (Long hole stoping)

B. Artificially supported stopes

3. Shrinkage stoping

a. With pillar (post pillar)

b. Without pillars

c. With subsequent back filling

4. Cut-and-fill stoping

a. Horizontal cut-and-fill stoping

b. Post pillar cut-and fill stoping

5. Vertical Crater Retreat – with back filling

6. Square set stoping

C. Caved stopes

7. Sub-level caving

8. Block caving

A summary of factors for each U/G mining method, including the suitable orebody

geometries, orebody grades, orebody and country rock strengths, and depths are shown in

Table 1.

65

Table 1: Summary of geotechnical factors for each underground mining method

Method

Class Method

Relative

magnitude of

displacements

in country

rock

Strain

energy

storage

in near

field rock

Suitable

orebody

geometry

Suitable

orebody

grade

Suitable

orebody,

country rock

strength

Suitable

depth

Pillar

supported Room-and-pillar Very low

Very high

Tabular,

maximum

dip 55°

High

Both strong

and

competent,

low frequency

of cross

jointing in

roof

Shallow

Pillar

supported

Sublevel open

stoping Very low Very high

Massive or

steeply

dipping

stratiform,

regular

boundary

Moderate

Must be

sufficient to

provide stable

walls, faces,

and crown for

stopes

Variable

Artificially

supported Cut-and-fill Low High

Veins,

inclined

tabular,

massive;

35-90° dip

High;

variable

with lenses

is

acceptable

Competent

orebody, can

be weaker

country rock

Shallow

or deep

Artificially

supported Bench-and-fill Low High

Narrow

vein

mining

High

Competent

orebody, can

be weaker

country rock

Shallow

or deep

Artificially

supported Shrink stoping Moderate Moderate

Narrow

extraction

blocks;

veins,

inclined;

tabular,

massive

High;

variable

with lenses

is

acceptable

Competent

orebody (and

resistant to

crushing), can

be weaker

country rock

Shallow

or deep

66

Artificially

supported VCR stoping Moderate Moderate

Minimum

3 m width

orebody;

veins,

inclined

tabular,

massive

High;

variable

with lenses

is

acceptable

Competent

orebody (and

resistant to

crushing), can

be weaker

country rock

Shallow

or deep

Unsupported Sublevel caving High Low

Steeply

dipping ore

bodies

High

enough to

sustain

dilution

(perhaps

>20%)

Reasonably

strong

orebody rock

enclosed by

weaker

overlying and

wall rocks

From

shallow

to deep

Unsupported Block caving Very high Very low Large ore

bodies

where

height

>100 m

High

enough to

sustain

dilution

Rock mass of

limited

strength, with

at least two

prominent

sub-vertical

and one sub-

horizontal

joint set

Shallow

or deep

1. Naturally Supported Method- Room-and-Pillar Mining

A mining method based on natural support seeks to control the rock mass displacements

through the zone of influence of mining, while mining proceeds. This implies maintenance of

the local stability of the rock around individual excavations and more general control of

displacements in the near-field domain. (Ref: Brady & Brown1993).

Conditions

• Ore strength: weak to moderate

• Host rock strength: moderate to strong

• Deposit shape: massive; tabular

• Deposit dip: low (< 35 degrees), preferably flat

• Deposit size: large extent – not thick

• Ore grade: moderate

67

Features

• Generally low recovery of resource as pillars needs to be left (40-60%)

• Moderately high production rate

• Recovery can be improved with pillar extraction (60-80%) but caving and

subsidence will occur

• Suitable for total mechanization, not labour intensive

• High capital cost associated with mechanization

• Versatile for variety of roof conditions

Applications

• Room and pillar mining – eg. Agnigundala Lead-Zinc mine of HZL,

Tummallapalli Uranium Mines of UCIL

• Variation: Stope and pillar mining

Stope development;

• In-stope raises – minimum two as per the regulation, so that one raise acts as a

ventilation in-take raise and the other the return. (eg.2x2 m raise dimension)

• The level interval decides the width of the stope - that is the length between

the upper and lower level.(eg. 30 – 60 m level interval)

• The length of the stope, i.e the distance between the terminal raises of a stope;

it is also known as the block size and it is usually as per the grade value of the

ore deposit. (eg. 60m – 100m)

• Ore draw point development. – Ore drawing is based on the degree of

mechanization of the mine. Eg. The ore-drawl in UCIL mines is by LHD (load

Haul Dumpers) and LPTD (Low Profile dump Trucks). The LPDTs move into

the stope and carry the material through a ramp to the main ore pass.

Fig . Low Profile Dump Truck (LPDT)

Method:

The room and pillar mining method is a type of open stoping used in near horizontal

deposits in reasonably competent rock, where the roof is supported primarily by pillars. Ore

is extracted from rectangular shaped rooms or entries in the ore body, leaving parts of the ore

between the entries as pillars to support the hanging wall or roof. The pillars are arranged in a

68

regular pattern, or grid, to simplify planning and operation. They can be any shape but are

usually square or rectangular. The dimensions of the rooms and pillars depend on many

design factors. These include the stability of the hanging wall and the strength of the ore in

the pillars, the thickness of the deposit, and the depth of mining. The objective of design is to

extract the maximum amount of ore that is compatible with safe working conditions. The ore

left in the pillars is usually regarded as irrecoverable or recoverable only with backfill. In this

case backfill costs or the potential loss of valuable resource may be a limiting factor in room

and pillar mining at greater depths.

The principal advantage of room and pillar stoping is that it is readily adaptable to

mechanized mining equipment, which results in high productivity and a relatively low cost

per ton of material extracted. For large ore bodies, a large number of working places can be

easily developed so that high daily rates of production can be counted upon. Most of the mine

development work is in ore, so waste extraction is kept to a minimum.

Figure Elements of a Room-and-Pillar stoping method

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Figure Ore handling in a Room-and-Pillar stoping method.

The main disadvantage of room and pillar mining is that a large area of roof is continuously

exposed where work activities or movement of men and supplies are carried out.

Consequently, roof condition is a primary concern for the safety of personnel and ground

support is generally a major cost, especially in rooms with high backs. Also, recirculation of

ventilating air can be difficult to minimize in room and pillar mines.

Components of a supported mine structure

A mining method based on pillar support is intended to control rock mass displacements

throughout the zone of influence of mining, while mining proceeds. This implies maintenance

of the local stability of rock around individual excavations and more general control of

displacements in the mine near-field domain. As a first approximation, stope local stability

and near-field ground control might be considered as separate design issues. Near-field

ground control is achieved by the development of load-bearing elements, or pillars, between

the production excavations. Effective performance of a pillar support system can be expected

to be related to both the dimensions of the individual pillars and their geometric location in

the orebody. These factors are related intuitively to the load capacity of pillars and the

loads imposed on them by the interacting rock mass.

Room-and-Pillar stoping method

70

Fig Plan view of a Room-and-Pillar stope

Fig. Samsung limestone mines – South Korea

71

Analysis of Pillar support system in Room-and –Pillar stoping

A mining method based on pillar support is intended to control rock mass displacements

throughout the zone of influence of mining, while mining proceeds. This implies maintenance

of the local stability of rock around individual excavations and more general control of

displacements in the mine near-field domain. Near-field ground control is achieved by the

development of load-bearing elements such as pillars, between the production excavations.

Effective performance of a pillar support system is related to:

1). the properties of the material,

2). geological structures,

3). absolute and relative dimension of the pillar,

4). the nature of surface constraints applied by the country rock,

5). geometric location of the pillars in the orebody.

These factors are related to the load capacity of pillars and the loads imposed on them by the

interacting rock mass. Since a lot of ore remains locked-up in the pillars, an economic design

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suggests that ore locked-up in pillars be a minimum, while fulfilling the essential

requirement of assuring the global stability of the mine structure. Therefore, detailed

understanding of the properties and performance of pillars and pillar systems is

essential in mining practice, to achieve the maximum, safe economic potential of an orebody.

Figure. Schematic illustration of problems of mine near-field stability and stope local

stability, affected by different aspects of mine design.

In a classical Room-and-Pillar stoping method, pillars in flat-lying, stratiform ore-bodies are

frequently isolated on four sides, providing a uniaxial loading condition from the

hang-wall rock mass. Interaction between the pillar ends and the country rock results in

heterogeneous, triaxial states of stress in the body of the pillar, even though it is uniaxially

loaded by the abutting rock.

The figure below illustrates the types of pillars in an ideal room-and-pillar stope.

Fig. Layout of barrier pillars in a room-and-pillar stope(Ref. Rock Mech. for u/g mining Brady & Brown)

In order to restrict the stope instability limited to a single room-and-pillar stope, the adjacent

stopes are separated by a barrier pillar, similar to the division of panels in a coal mine. The

barrier pillars are designed such that each stope (panel) performs as an isolated mining

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domain. The maximum extent of any collapse is then restricted to that stope pillars itself. The

stope stability is therefore controlled by the response of stope pillars in a room-and-pillar

stope. A set of uniaxially loaded pillars is illustrated in the Figure below.

Fig. Room-and-Pillar stope, pillar configuration

Fig. Room-and-Pillar layout showing load carried by a single pillar assuming total load to

be uniformly distributed over all pillars(Ref. Hoek & Brown Underground excavation in rock)

Fig Average pillar stress in room-and-pillar stope

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Figure Redistribution of stress in the axial direction of a pillar.

Modes of Pillar failure

Stoping activity in an orebody causes stress redistribution and an increase in pillar loading,

illustrated conceptually in Figure above. For states of stress in a pillar less than the in situ

rock mass strength, the pillar remains intact and responds elastically to the increased state of

stress. Mining interest is usually concentrated on the peak load-bearing capacity of a pillar.

Subsequent interest may then focus on the post-peak, or ultimate load-displacement

behaviour, of the pillar. The structural response of a pillar to mining-induced load is

determined by the rock material properties, the geological structure, the absolute and relative

dimensions of the pillar and the nature of surface constraints applied to the pillar by the

country rock. Three main modes of pillar behaviour under stresses approaching the rock mass

strength have been recognized, which may be reproduced qualitatively by laboratory tests on

model pillars.

Different modes of failure as seen in the ffield observations are:

1. Fretting or or necking of the pillar: Fretting occurs in relatively massive rock with

moderately strong H/W, F/W, and ore body. One of the main causes for necking is the

development of tri-axial stress condition at the wall contacts (H/W and F/W), which

result in the development of shear stresses at the contact zones and the failure is

localised in the central part of the pillar. The failure is due to tensile stress

concentration. The most obvious sign of pillar stressing involves spalling from the

pillar surfaces, which consequently leads into the development of hour-glass shaped

pillar.

Fig. Fretting (Samsung Limestone Mines – South Korea)

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2. Shear failure: The effect of pillar relative dimensions on failure mode is illustrated in

the second most common failure- which is shear failure along a shear plane. For

regularly jointed orebody rock, a high pillar height/width ratio may favour the

formation of inclined shear fractures dividing the pillar across plane of weakness.

There are clearly kinematic factors promoting the development of penetrative,

localized shear zones of this type. Their occurrence has been reproduced in model

tests by Brown (1970-Ref. Rock Mech. Brady & Brown ), under the geometric

conditions prescribed above.

Fig. Failure along a shear plane (Samsung Limestone Mines – South Korea)

3. Axial Splitting (Bulging or barrelling): The third major mode of pillar response is

seen in an ore body which is relatively strong in comparison to the wall rocks and

hang-wall rocks form highly deformable plane of weakness at the contact plane of the

pillars. The relative deformation of the pillar and the hang-wall rocks generates

transverse tractions over the pillar end surfaces and promotes internal axial splitting of

the pillar. This may be observed physically as lateral bulging or barrelling of the pillar

surfaces. Geomechanical conditions favoring this mode of response may occur in

stratiform orebody, where soft bedding plane partings define the foot wall and

hanging wall for the ore-body. The failure condition is illustrated in Figure 13.5c.

Fig. Splitting of pillars ( Barrelling/ bulging)

4. Structural failure: This mode of failure is commonly seen in layered ore bodies,

such as limestone or banded hematite quartzite (BHQ). The response of the failure to

the super incumbent load is related directly to the structural geological features of the

pillar. A pillar with a set of natural fractures or bedding planes forms the weak planes

for the fracture initiation along these planes of weakness. The failure is similar to the

shear failure, where in slip takes place when the shearing stress on these planes is

more than the frictional resistance.

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Fig. Structural failure of the pillar.

5. Buckling of Pillars: This is common in slender pillars, where width/height ratio of

the pillars is very less (0.4 -0.5).A slender pillar with well-developed foliation or

schistosity parallel to the principal axis of loading will fail in buckling mode, as

shown in the figure below.

Figure Buckling mode of deformation of pillars

Figure. Mode of fracture and failure in mine pillar

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Measures to control the Pillar failure

Table Rock mass classification of Pillars in limestone mines (ref. Pillar stability issues based on a survey of

pillar performance in underground limestone mines; 25th international conference on ground control in mines, Gabriel S. Esterhuizen etal)

Some of the common methods of preventing the pillar failure in room-and-pillar stoping are:

1. Back filling the stope, the fill material surrounding a pillar may act as a confining

material and hence prevents the failure of the pillars.

Figure Plan view of a room-and-pillar stope

2. Rock bolting or lacing the pillar.

78

79

Pillar stress estimation by tributary area method

The term tributary area method is used for estimating the average state of axial stress in the

pillar. The area extraction ratio, R, defined as the ratio of area mined to total area of ore body.

Considering the representative element of the ore body illustrated in the figure above, the

area extraction ration is also defined by

Figure below shows a cross section through a flat-lying orebody, of uniform thickness,

being mined using long rooms and rib pillars. Room spans and pillar spans are

Wo and Wp respectively.

Figure. Tributary area method to calculate the average pillar stress (ref. Brady & Brown)

Considering the requirement for equilibrium of any component of the structure under the

internal forces and unit thickness in the anti-plane direction, the free body shown in the figure

below yields the following equation

On considering equilibrium,

���� � !!(�# +���

Or

�% � !!(�# +�%�/�%

In this expression, is the average axial pillar stress and is the vertical normal component of

the pre-mining stress field. The width (of the representative free body of the pillar structure is

often described as the area which is tributary to the representative pillar. The term tributary

area method is therefore used to describe this procedure for estimating the average state of

axial stress in the pillar. The area extraction ratio, r, defined as the ratio of area mined to total

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area of ore body. Considering the representative element of the ore body illustrated in the

figure above, the area extraction ration is also defined by

� � �#/(�# +���

So that

1 � � ���

�# +��

Insertion of this expression in the above equation, yields:

�� � !![1

1 � �]

The mining layout shown in the following figure, involving pillars of plan dimensions a and

b, and rooms of span c, may be treated in an analogous way.

The area tributary to a representative pillar is of plan dimensions (a+c), (b+c), so that

satisfaction of the equation for static equilibrium in the vertical direction requires

��� � !!( + *�(� + *�

Or

�� � !!( + *�(� + *�

�

The area extraction ratio is defined by

� �[( + *�(� + *� � �]

( + *�(� + *�

With some simple rearrangement the above equation yields the following:

�� � !![1

1 � �]

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For a square pillar, of plan dimension WpxWp, are separated by rooms of dimension Wo, the

equation is

�� � !![(�# +���/��]+

The pillar stress expression given above helps in a rough estimation of the pillar stresses.

Fig. Relationship between the pillar stresses and the area extraction ratio

The relationship between the pillar stress and the area extraction ratio is illustrated in the

above figure. The main observations from the above graph are that:

1. The average pillar stress is directly proportional to the area extraction ratio and the

relationship is non-linear.

2. There are two distinct zones in the above relationship, where in the increment in the

pillar stress until r = 0.75, is near linear and the slope is mild, whereas the nonlinear

exponential increment is seen beyond a point where r > 0.75.

3. In the second zone, a very small increase in the extraction ratio is developing a high

increment in the pillar stress.

4. It is therefore inferred that for keeping the stope stable, it is imperative that the

extraction ratio needs to be within the limits of tolerable stress concentration levels, in

the pillar (Factor of safety of the pillar is > 1).

Limitations of Tributary area method

1. The stress estimated by this method represents an average stress within the pillar, and

it is purely a convenient way of representing the state of loading of a pillar in a

direction parallel to the principal stress.

2. Tributary area analysis restricts attention to the pre-mining normal stress (in-situ

stress) component directed parallel to the main axis of the pillar support system.

3. It is assumed that the effect of other stresses in other direction have no effect, which

in reality is not always true.

4. The stress coming on the pillar is the induced stress.

5. Strength of the pillar is related to its volume and geometric shape.

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6. Tributary area method provides a simple method of determining the average state of

axial stress in a pillar. The size of the pillars is bigger in the mines, say 4x4 or 4x6, or

6x8 and so on. Increasing the volume of the pillar increases the number

discontinuities but the shape of the pillar may give rise to the effect of confinement to

the core pillar.

Fig. Distribution of vertical stresses in a pillar (Ref. – Brady & Brown Rock Mech- Wagner-1980)

The measurement of the load distribution in a pillar at various states of loading is shown in

the above figure. It is seen from the above figure that the failure commences from the

boundaries of a pillar and migrates towards the centre (core pillar). It may so happen that the

structural failure of the pillar has occurred but the core pillar has not reached its full load-

bearing potential.