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Underground mining/Stoping 2015

Underground mining/stoping methods
Stopes are the blocks of hidden wealth surround by an environment that is risky to

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men, machines and equipment
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INTRODUCTION
In underground mining situation selection of a mining method is very vital, as it has direct
impact on safety, productivity, cost and recovery
...
There are
cases where due to selecting improper mining methods, mines had to close
...
Basically stopes can be classified into three major groups: open or self
supported, supported by artificial means and caving stopes
...

In these stopes heavy-duty equipment can be deployed to deplete the reserves faster
...
Weak and cavable ores
instead of posing problems can prove to be helpful if stopes are properly designed
...
Isometrical orebodies
are of two types:
Stock and Nests: These are usually irregular in shape but having almost equal dimensions in
all directions
...

Stocks can have dimensions even in kilometers, whereas nests are limited in size, within
several meters
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As the name suggests columnar deposits
are like a column
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These
are also known as pipe deposits, which are almost vertical and thin, and extending in depth
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The sheet deposits are having almost a constant
thickness
...
But the veins usually do
not have uniform thickness
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Lenses are considered to
be a change from first to third group, having irregular shape and unequal dimension in all
the three directions
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2
...
2
...
Usually the following classification holds good, with
regard to dip of a deposit:
Flat dipping: 0° to below 20°
Inclined dipping: from 20° to below 50°
Steeply inclined dipping: exceeding 50°

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In addition, there are numbers of other shapes, including saddle dome shaped, in which
orebodies may occur
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Many a times they merge or separate out from each other
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Deposit’s contact with country rock: This aspect is also important as sharp and distinct
contact of the deposit with country rocks makes the mining process easier
...

In other cases, such as impregnation ores (ore rocks with small ore minerals) have no sharp
contact with country rocks
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Regular shape of orebody favors any of the mining systems (methods) but irregular and
intricate shapes preclude uses of some systems or lower their efficiency
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Thickness of deposit
Orebody thickness is a normal distance between footwall and hangingwall (sec
...
5
...
2
...
If distance is measured along the normal direction, then it is termed as true
thickness
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Vertical thickness is measured with flatly dipping
orebodies, and the horizontal one with the steeply dipping
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7 m
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7–2 m
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Thick deposit: thickness range 5–20 m
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Points to note:
● In very thin to thin deposits (up to 2 m thickness) in order to provide working space for
the man and machines, floor stripping in the country rocks becomes essential
...

● Steeply dipping thick deposits can be mined along strike direction i
...
longitudinal
stoping is possible
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Underground mining/Stoping 2015

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Underground mining/Stoping 2015
The dip of the deposit has the decisive influence on the selection of a mining method and
positioning of a stoping face
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Following are some of the important aspects regarding dip:
●Roof pressure decrease with increasing dip
...
Low dip provides
firmfoothold (figs
...
1(c) and (d)), further increase in dip does not give a firm foothold and
needs a safety belt
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For steep dip in overhand stoping operations need an immediate filling of the worked out material, which may
be sand, waste rock, mill tailings or accumulated broken ore to provide firm footing to the
workers
...
16
...

● Influence of dip during rock fragmentation: The process of rock fragmentation depends
upon hardness, coherence and dip of the deposit
...
e
...

● While mining vertical and steeply inclined deposits gravity together with weight of the
rock helps the rock breaking process, where as, this influence get diminished with decrease
in dip (figs 16
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●In steeply inclined deposits the filling (with some foreign material) of worked out space is
complete and settles well comparing the same with horizontal and low dip deposits (figs
16
...

●In steeply dipping weak strata working by underhand stoping i
...
from upper level
towards the lower level becomes essential
...
1(a), (b))
...
1(a), (b))
...
16
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●All these factors indicate that to a considerable extent stoping methods change with dip
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The mechanical strength is measured as compressive, tensile, bending and shear strength
...
), labor productivity and cost of extraction
In underground mining the stability of ore and country rock is equally important as the rock
strength
...
This is a very vital characteristic, as based on this;
the underground mining methods are classified
...
Finally, there are rocks, which do not allow their exposure
at all and must be supported immediately after undermining or exposure
...

With regard to stability of ores and country rocks, no index is available which determines
the permissible value of the exposure – time and span
...

● Unstable ores and rocks: In this type of strata the roof/back needs a strong support
immediately after its exposure
...

● Stable ore and rocks: In these strata roof and side exposures can be allowed for a
considerable time without support
...

● Very stable ores and rocks: This type of strata can allow exposure for sufficient time
and space without causing caving
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Occasionally with time it develops a tendency to swell and bulge
...

Roof Pressure: Roof pressure over a worked out space depends upon the texture of rock
constituting the roof/back, its coherence, dip of the deposit, span, and rate of mining and
duration of its exposure
...
The roof pressure can be
counter-acted using supports, filling the worked out space or by the controlled caving
...
In such cases induced caving becomes essential to avoid large
pressure building in and around such excavations
...
e
...


Underground mining/Stoping 2015
Geological disturbance means presence of any one of them or combination of more than
one of structures such as fault, folds, joints, fissures, dykes etc
...
making the
mining process sometimes more tedious and slow
...
Given a choice to select a geologically
disturbed area with the one with minimum disturbances, one should prefer the later one
...
The fissures running parallel to strike can
be mined by overhand or underhand stoping
...
If the transverse fissures, which run almost parallel to the dip, penetrate
deposit, it should be mined adopting breast stoping
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1(a) and (b) illustrate that nature of deposit to give way is more pronounced in
steeply dipping deposits than the flat once
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The working faces need support in such cases
...

In figure 16
...
If mining
carried out as shown in figures marked a, it is incorrect in relation to cleats’ orientation
...

This may be noted that dip, thickness and strata stability (ore and country rock) are the
main geological and mining factors without which it might be impossible to select a safe and
efficient mining system/method
...
dia
...

● Degree-2 mechanization means use of jumbos, trackless equipment such as LHDs (1 cu
...
capacity or more) low profile dumpers and small capacity trucks
...

● Degree-3 mechanization has the same set of machines as in degree-2, except that the
drilling (for stoping) is by the down-the-hole drills capable of drilling holes of 150–200 mm
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of _40 m length
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Higher output warrants selection of bulk mining methods for which use of
equipment of higher capacity and heavy duty becomes essential
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Degree of mechanization and output required
Mechanization means performing the underground operations using machines
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Therefore, it is advantageous to select the largest
units possible taking into account the aspects of flexibility, excavation and access size
...
The types of the equipment that are available
in mines can be grouped in the following manner:

Underground mining/Stoping 2015
But when utilized properly, the cost of mining is substantially reduced comparing the same
when small sized low capacity and conventional equipment are used
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The reason is fast growing
demand of minerals of all kinds due to rapid industrial growth and development in
infrastructure sectors
Higher output means quick depletion of a deposit and thereby a quick return of the
investment made
...
Large sized
equipment also require more specious drives and excavations which means more money
should be spent on the development operations (i
...
more amortization fund)
...
For medium sized mines and output,
degree-2 mechanization could prove beneficial, and whereas, for large sized mines and
output, high degree of mechanization with bulk mining methods becomes a usual choice
...
For low output use of conventional machines,
equipment and methods can be used but large output mines require bulk-mining methods
deploying large sized fast moving equipment
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Ore grade and its distribution, and value of the product

Ore grade plays a vital role in selecting a mining method, as low-grade deposit can be mined
out profitably if bulk-mining methods are applied
...
Also if grade Distribution is not
uniform it will be costly to mine out a deposit comparing the same, which is having a
uniform grade distribution
...
As a
matter of fact advent of equipment together with technology has brought about a drastic
change in method design and its selection
...
But
high degree mechanization, if not effectively utilized, results into high capital costs and
creates problem of unemployment, whereas use of the primitive tools and machines lower
the productivity
...
It is no good to copy what is being
done in the highly industrialized countries
...
In fact, if we make rock fragmentation and its disposal efficient, the
entire mining becomes efficient
...
1, presents proportion of ore in a mineral inventory as a function of homogeneity
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A gold deposit can be considered to be more difficult to evaluate
and win than a coal deposit
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As the cost of mining remains practically the same for all types of rocks, which means a
copper, lead, zinc or gold ore can be mined at almost the same cost, whereas the selling
price of them differ significantly
...
This is the reason the precious metals
and stones such as gold, platinum, silver, diamond etc
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Depth of the deposit
Deepening of workings below 600–800 m is often accompanied by a considerable rise in
rock pressure, thus impeding the use of some systems
...
This warrants change in stoping methods
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With the
increase in frequency and severity of rock burst occurrence use of granite masonry pack
walls in cut and fill stopes was made
...
But relief from the occurrence of rock bursts were felt when the filling material was
replaced by concrete fill and changing stoping method to stope drive (sec
...
3
...
Thus,
with the change of depth stoping methods are altered
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Ore value is the focal point while selecting a mining method
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The cutoff grade
further decides ore reserves, as mineral reserves in any deposit below cutoff grade are
considered waste and the one at and above cutoff grade as ore reserves
...
Based on this logic, one
will find that most of the gold deposits being mined are currently, are at great depths

Underground mining/Stoping 2015
Apart from this, cost of ventilation, support, drainage, hoisting, transportation also
increases
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Presence of water
Ores may be wet, dry or damp
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Ore’s moisture promotes caking and
freezing during winter
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When make of water during driving and stoping is abnormal, it adversely affects safety and
productivity of the mine
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Acidic water adversely affects health and
safety of the workers
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A proper drainage system means improvement in the haulage
productivity and better life of mine track and roads
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Other sources of methane in metal (also known as metalliferous) mines could be the old
workings
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Presence of these gases adds to ventilation costs and needs extra care towards
safety
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Oxidation: Ore containing more than 40% sulfur in the form of pyrite or pyrhotite, etc
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Fire may become intensive if ore fines and dust get
mixed with some timber
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Apart from sulfide ores, any other ore after its fragmentation, if stored for a prolonged
period, it may get oxidized and can cause problems during ore beneficiation/ concentration
...
The facility it gives for rock fragmentation: This may be examined by:
● Ease it can impart during drilling operation
● Effectiveness of the blast

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Ore & Country rocks’ susceptibility to caking and oxidation
Caking: Some ore deposits are associated with small fractions of clays or some sticky
material causes problems after their fragmentation, as these clay or sticky materials after
getting wet during their mining may result into cakes which are large immovable hard-toloose solids
...

Blasting becomes more effective, if use of earth’s gravity can be made while sequencing the
blast
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Output: Concentrating over few working faces with the application of the resources in
terms of man, machine, equipment, services and supervision can yield better productivity i
...

output/man/shift
...

3
...
A fast rate of progress can reduce expenses on supports and
mine services
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4
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Dilution should be minimum
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16
...
2–3)
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5
...
Clean mining is possible if the
method selected allows selective mining
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6
...


8
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9
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A minimum void in
the worked out space after filling should be aimed at
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Ore winning postures and danger from roof fall/caving and muck’s rolling: The ore can
be extracted either by underhand, breasting or overhand stoping
...
1 (a) &

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7
...

Another consumable item is energy/unit of output, which is required to carryout different unit
operations and services such as ventilation, pumping, illumination etc
...
In underhand stoping working below an exposed roof involves danger of roof fall or
caving
...

Some methods involve danger by rolling of the boulder of the broken muck (figs 16
...
This danger enhances if the floor is steep, smooth and fragments are round in shape
...
But with steeper dips
overhand stoping becomes more difficult and dangerous
...

In overhand stoping in steep and thick deposit there is a danger of whole mass running into
workings
...
Sometimes roof pressure becomes high,
requiring efficient support system
...
In underhand
stoping loading and transportation of broken muck is difficult

Typical considerations to be weighted in selecting mining methods:

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Maximize safety
Minimize cost (bulk mining methods have lower operating cost than selective)
Minimize the schedule required to achieve full production
Optimize recovery (80% or more of the geological reserves)
Minimize dilution (20% or less of waste rock that may or may not contain economic
minerals)
Minimize stope turn around (cycle time for various unit operations)
Maximize mechanization
Maximize automation (employment remote controlled equipment)
Minimize pre-production development (top down versus bottom up mining)
Minimize stope development
Maximize gravity assistance (underhand versus overhand)
Maximize natural supports
Minimize retention period (open stoping versus shrinkage)
Maximize flexibility and adaptability:
– Based on size, shape, and distribution of target mining areas
– Based on distribution and variability of ore grade size, shape, and distribution of
target mining areas
– To sustain the mining rate for the mine life
– Based on access requirement
– Based on opening stability, ground support requirements, hydrology (ground water
and surface runoffs), and surface subsidence

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Underground mining/Stoping 2015

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Underground mining/Stoping 2015

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