Open pit mining is a method for extracting near-surface deposits, utilizing horizontal benches to remove overburden and ore. Key design aspects include ultimate pit depth, bench height, and slope stability, all influenced by economic and geological factors. Quarries, a specific type of open pit, involve stages from prospecting to reclamation, utilizing various methods for extraction and requiring careful planning for environmental restoration.

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OPEN PIT MINING
by Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
UNIVERSITY OF MYSORE
MYSORE-6

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Introduction
Open pit mining is the process of mining a near
surface deposit by means of a surface pit excavated
using one or more horizontal benches.
The term open pit mining is usually used for metallic
or non-metallic deposits and sparingly used for
bedded deposits like coal.
A quarry is a type of open pit mine used to mine
building materials (construction aggregate, riprap,
sand and gravel) and dimension stones usually at
shallower depths.

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The term quarry has traditionally been used to mine
stones.
Open pit mining
The ores in an open pit mine are covered by
overburden. Both the ores and overburden are
removed in benches ranging from height 9 m to 30 m.
A thin deposit may require one or few benches but a
thick deposit needs more number of benches and the
pit in its production stage resembles like an inverted
cone.

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Design Aspects of Open Pit Mining:
1. Ultimate pit depth:
This is the maximum depth of the mine that it will
reach at the end of its life. This is decided by
economic reasons and a breakeven point is decided
beyond which it is not economic to continue
production. As this depends on economic conditions,
a mine closed in 1980s can become economic now, as
happened in case of Copper Mountain Project.

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2. Bench Height:
The bench height is usually decided by the economic
reach of the mining equipment used in the mine. As a
thumb rule, a bench height is equal to the economic
bucket height of an excavator (like shovel or loader)
plus 3 m.
The thickness of the ore body and its dip and the
thickness overburden also play a deciding role in the
bench height.

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As mentioned in Coal Trading, the bench height
depends up on:
a) Deposit character and geology: selectivity
b) Production strategy: ore/waste ratios, blending
requirements, no. of working faces, operating/capital
costs, etc.
c) Slope stability considerations
d) Equipment set / equipment specific optimum
geometry

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3. Bench Slope:
Bench Slope is decided by the geo-technical
conditions of the overburden and the orebody.
4. Overall pit slope:
This is also decided by the geo-technical stability of
the rocks in the mine, bench height and bench slope.
Overall pit slope is always less than the bench slope.

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5. Bench Width:
This is usually decided based on the space required
for the operation of the equipment on it.
6. Haul Road Width & Slope:
This depends on the operating parameters of the
trucks and other hauling equipment used in the mine.
It is always wider than the bench width.

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7. Bench Length:
The length of the bench depends on the production
rate. A bench will be longer for higher production
rate. Usually, blasting at one bench is done not more
than once a week. The extent of the orebody also has
a deciding role here.
A simple diagram showing different design
parameters is shown here:

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A typical set of bench height, bench with and bench
slope are given below:

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Quarries
Most quarries have smaller and vertical benches.
Some of the quarries do not have any bench while
some even have a highwall up to 300m.
The kinds of materials that are extracted from quarries
comprise of the following:
 Cinder
 Chalk,China Clay,Clay
 Coal
 Construction aggregate (sand and gravel)
 Limestones

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 Granite, Gritstone
 Gypsum, Phosphate rock
 Ores
 Sandstone, Slate and Marble
Sequence of Quarry development:
A quarry usually goes through the following stages
before reaching the production stage:

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1. Prospecting and Exploration:
The following properties of the rock are studied in the
prospecting programs: physical & mineralogical
properties, color and appearance, uniformity, strength,
flaws, cracks, etc
2. Development:
Along with the removal of overburden, development
stage also involves developing the processing and
other construction facility near the quarry.

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The main role of the processing facility is to cut and
polish the rock.
Large amount of wastes are generated in processing,
so it is required to plan for some storage/dump space
for the cuttings.
3. Production:
The stones/materials in a quarry are excavated with
any of these methods:

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a) Drill and Blast:
Depending upon the quality and fineness of the
material desired, the drilling and blasting
requirements are set.
The more uniformity is required in the rock
excavated, the more drilling and controlled blasting
will be required.
Light gunpowder is used as explosive to avoid any
shattering effect which can bring a significant
destruction of the stone shape and quality.

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Two types of blasting methods are used for quarrying:
(i) Wedges method
(ii)Plug-and-feather method
b) Compressed Air/Water and Explosives:
A small charge of dynamite is used to create cracks in
the stone, which is then cut with compressed air or
water to widen the cracks and break the stone.

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c) Channelling and Use of Saw:
Channelling involves cutting long and narrow
channels into the rock to free up a slice from the large
rocks. Three techniques are used:
(i) Large amount of single-line drilling with a small
shattering
(ii) Single line drilling with use of expansion mortars
(non-explosives) like Dexpan and Rockfrac
(iii) Single-line drilling with use of non-explosives
like NonexTM
and Magnum BusterTM
, which uses non-
detonating chemicals to break rocks.

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(iv) Use of a saw to produce vertical or horizontal
cuts in the rock itself.
Three types of saw are mainly used: Disc/crosscut
Saw, Chain Saw and Diamond Wire Saw
Disc/Crosscut Saw

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4. Reclamation of quarries:
The main tasks involved in the reclamation of
quarries are:
(i) Stability Monitoring - to analyse the long-term
stability of areas or a particular landform in and
around the quarry
(ii) Grassland Establishment - to restore on grasslands
waste pile

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(iii) Tree Establishment - to restore the quarry sites
through trees plantation. Trees restoration in a quarry
site is a long term process and usually does not
provide any significant visual impact in 10 years.
Selection of tree species is also important
(iv) Landscape Assessment for future application of
the landforms for future application and habitat
creation.
Reclamation of Limestone Quarries by Landform
Simulation gives a very descriptive detail on the
reclamation of a quarry.

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Equipment used in quarries
Here are some of the equipment used in quarries:
 Conveyors
 Crushers
 Forklifts
 Front end loaders
 Hammer drills
 Saws
 Trucks
 Wheel loaders

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Advantages and disadvantages of Quarrying:
Here are the advantages and disadvantages of quarry
mining.
Advantages:
 Low capital cost and low mechanization
 Suited to some small deposits
 Easily accessible; hoisting may complicate moving
stones, supplies and workers
 Stable wall and benches; generally no bank support
required

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 High selectivity; can discard low quality stones
 Good safety; little chance of slope failures
Disadvantages:
 Somewhat limited by depth; usually less than 90m,
can be up to 300m
 Low productivity; high labor costs
 Highest mining cost because of low productivity
 Low production rate
 Relatively skilled labor is required
 Inflexible; cannot easily change the mining plan at
depth

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 Mechanisation is limited by the nature of the
method
 Complicated and costly rock breakage method
because of inability to use the maximum power of
explosives
 Waste can be 60% to 90%

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Diamond wire saw.
Open-pit mines
An open-pit mine is a mine that resides at the
ground’s surface from which ore can be extracted.
Although the mine is an open pit, some excavation is
required.
Open-pit mines are used to extract ores such as coal,
copper, gold, iron, and aluminum.
When an open-pit mine is developed, there are a
number of environmental, safety, and cost-related
factors to consider, including noise and air quality and
animal habitats.

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Other names for an open-pit mine include open-cast,
open-cut, and strip mine.
Mining Process in Open pits:
The mine operation is developed by first establishing
a bench, also known as a ledge, which enables miners
to strip away layers.
Benches are typically 13 to 197 feet (4 - 60 m) in
depth. Stripping shovels are required to strip away the
overburden that blankets the ore, which normally
takes place on an angle derived from the bench.

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Each layer of the bench begins to resemble a step,
continuing as far down as ore can be extracted.
Overburden generally consists of thin rocks that are
unsuitable for tunnels, where the top layers can easily
be removed to access the valuable ore.
Other shovels and excavators are used to dig into
successive layers and break the ore into manageable
bits so that it can be loaded into dump or haul trucks
and taken to a processing site, usually on-site.

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Some rocks may be too hard to be excavated in the
traditional manner, so blasting and drilling methods
are relied upon to complete the job. Underground
mines are built when minerals cannot be accessed
from the surface.
These types of mines usually require the construction
of tunnels.
Open-pit mines are enlarged until the resources are
exhausted or until the cost of operating them exceeds
the profit.

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When this occurs, open-pit mines are abandoned, and
will either become the site of a landfill or a pond.
Sometimes, if an open-pit mine is deep enough,
mining is taken underground; machinery is sent
underground via vertical mine shaft.