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Hazards Associated with Mining
Covering material required for WJEC AS-Level Geology:
GL3: Key Idea 3(c) Mining.
Prepared by Toby White, University of Leeds, Leeds, LS2 9JT
Hazards Associated with Mining
Introduction
• Most of the hazards associated with mining are well known and are
associated with the following:
• the strength or weakness of the rock being dug,
• the nature of the rock on which structures are built or material
placed,
• the influence of water at the surface or underground,
• or a combination of these.
• Many potential problems can be “designed out” at the planning stage,
provided that sufficient information is gathered.
• Putting it very crudely, a scientist can tell you what the problem is,
while an engineer can tell you how to fix it, or how to avoid the
problem in the first place!
Geology can do funny things!
The effect on mining varies in significance.
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Subsidence
Subsidence
Introduction
• Subsidence only occurs where the ground material can be displaced
into some sort of underground void. Only occur in certain types of
rocks.
• Collapse of material overlying the void where the roof is cannot
support the mass of material above.
• The voids can be of two types:
• Macrovoids
• Large cavities such as solution caves in limestones and
occasionally salt, or mined cavities in rocks of economic value
(coal, salt, metal mines, etc.)
• Microvoids
• In very porous deformable rocks (clay, peat, silts & sands)
• We will only cover subsidence due to mining.
Subsidence
Room & Pillar (Pillar& Stall) workings in coal
Subsidence
Room & Pillar (Pillar& Stall) workings in coal
Subsidence
Underground Longwall Mining - Shearing
Roof supports
Coal shearer
Coal
Roof collapses
(goaf) behind
supports.
Coal carried away
on conveyor
Subsidence
Longwall – two different methods in plan
Main intake road
Main return road
Advance
Face
Retreat
Face
What is the operational advantage of the retreat face method?
The roads driven out will reveal the geology to be worked back through.
Subsidence
Subsidence
Coal Mining
• This pub in the Black Country has been severely affected by mining
subsidence.
Subsidence
Coal Mining
Further examples of subsidence.
Subsidence
Duration of Subsidence after Coal Mining
• Long-wall
• Subsidence occurs mostly within a few days of working with small
amounts of residual subsidence over a few months but occasionally
5 years.
• Room & Pillar
• Little subsidence at time of working but significant subsidence
can occur slowly over time or suddenly after many years (50+) if
pillars collapse.
With near surface old workings, it is not
unusual to find that roof collapse and floor heave has resulted in
no surface subsidence. (Broken rock takes up more space than
solid rock.)
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Acid Mine Drainage
Acid Mine Drainage
Introduction
• Acid mine drainage (AMD) is polluted water that normally contains
high levels of iron, aluminum and sulphuric acid.
• The contaminated water is often an orange or yellow-orange colour,
indicating high levels of iron and has the smell of rotten eggs
(sulphate smell).
• AMD comes from pyrite (iron sulfide, fools gold, FeS2), a mineral
associated with coal and metal mining. When pyrite is disturbed, as
it is during mining or highway construction, it weathers and reacts
with oxygen and water to cause high levels of iron, aluminum, and
sulphate in run-off water.
Acid Mine Drainage
Consequences of AMD
• The acid lowers the pH of the water. Difficult to support aquatic
life and treat for drinking water.
• The iron oxide forms an orange/red coating on the river/stream
bed. Difficult to support aquatic life.
AMD is becoming a big problem with abandoned
metaliferous mines.
• The Environment Agency deal with AMD from abandoned mines.
They attempt to get the former operator to deal with the
consequences of abandoning the mine. Where this cannot be
achieved the EA step in to protect the environment.
• The EA are heavily involved with the clean-up of Cornish tin mines
where the owners can’t be traced.
Acid Mine Drainage
Wheal Jane
AMD disaster (1991/92)
• Wheal Jane mine (Cornwall)
principally produced cassiterite, the
main ore mineral for tin, but older
interconnecting workings also
produced pyrite and arsenopyrite.
• Similar problems occur with coal
where pyrite content is high.
Acid Mine Drainage
Typical red deposits
Acid Mine Drainage
Preventing AMD
The best practice for preventing AMD is to concentrate on
the source.
• The cheapest and most common method of preventing AMD is to
flood the tailings, dump or source of pyrite with water.
• Some other options available are:
• Sulphide removal from dump
• Alkalinity enhancement of groundwater
• Control of water infiltration
• Control of oxygen to dump
• Reduction of bacterial populations
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Methane
Methane
Methane in mines:
• is the naturally occurring gas that most commonly appears in mines,
• has the formula CH4 and is associated with carbonaceous material,
so is a particular problem in coal mines,
• has been the cause of more explosions and related loss of life than
any other cause throughout the recorded history of mining,
• forms a highly explosive mixture with air (fire-damp),
• 0.55 times the weight of air and rises,
• can provide a fuel source in its own right through drainage
technology.
Methane
How is methane held?
• Coal is not a single material, but a complex mix of fossilised organic
compounds and minerals. Its form varies depending on its history.
• There are a large number of microscopic pores, not all
interconnected, and a lot of small discontinuities (fractures).
• Methane can exist in the pore space in a:
• “free state” - able to move within the pores, or in an
• “adsorbed state” – adhering to the coal surface.
Methane
How is methane released?
• When the coal is disturbed by mining, a pressure gradient is created
which will result in flow through natural or stress-induced fractures.
• The reduced gas pressure will promote “desorption”, which will
release more methane into the system.
• As coal is removed, methane will be released quickly at the face, as
well as the roof and floor.
• This means there will be most methane where there is greatest
risk of sparks.
• But methane will continue to escape;
• into collapsed (gob) areas behind longwall face,
• into coal mass being transported.
• Sudden gas outbursts can also occur and can be very destructive.
Methane
Control of methane
•
The best way of controlling methane is by extracting it from the
carbonaceous strata in an organised way.
•
There is no single preferred method, but the choice depends on:
•
The permeability of the source seams,
•
The reason for drainage (as a fuel, or simply to protect mine)
•
The method of mining.
•
Ventilation
circuit
One example is given here.
•
Longwall mining
•
Two-entry retreat face
•
Horizontal boreholes
•
Some methane drained from seam
•
Will not remove all.
Clean air
Methane
carried away
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Instability
Instability
Introduction
Failures of the rock mass can take place in a number of
different ways, all of which are potentially dangerous.
• Rockbursts in an underground mine
• Collapse of a roof in an underground mine
• Collapse of a face on a quarry
• Rockfall and toppling
• Collapse of a slope
• Original ground, overburden dump, spoil heaps, mine tailings
(waste) dams.
Instability
Rockbursts
Rockbursts
• These are the sudden and violent detachment of rock mass from
the sides of an excavation, sometimes as much as hundreds of
tonnes.
• This is a particular problem in very deep mines in hard, brittle and
very strong rock, e.g. some gold and zinc mines.
• They are cause by the sudden release of excessive rock stress
that has built up over time.
Instability
Collapse of mine roof
Roof failure
• The problem of roof collapse varies depending on the type of
mine. A common solution is the use of wire netting fixed with rook
bolts (lower left – note the unusually well defined band of copper)
• However, they don’t always work (lower right). This is a
particularly severe fall.
Instability
Collapse of a face on a quarry
Slope & Face Failure
Translational
Rotational/
circular
Toppling
Rockfall
Instability
Collapse of a face on a quarry
Rockfall
• There is a very real danger of
rockfall from the material at the
top of this face.
• Faces are now generally kept to
less than 15m high.
• The face could be “cleaned” by
an excavator
• A buffer zone could be built a
safe distance around the bas of
the face.
Instability
Collapse of a face on a quarry
Toppling Failure
• This differs to rockfall, in that a part of the face collapses
rather than a few stones or boulders. Note the “cleanness” of the
new face in the lower right, suggesting a weak plane (joint or small
fault).
Instability
Collapse of a slope
• There are a number of different ways in which a
slope can fail. Click on the buttons below.
Instability
Collapse of a slope
Planar/Slab failure
• Planar instability along bedding
with loss of quarry crest and
access to lower benches.
Instability
Collapse of a slope
Wedge failure
• Wedge failure in a magnesian
limestone quarry along two well
defined joint planes.
Instability
Collapse of a slope
Rotational failure
• This rotational failure took place in the tailings dam of the Crixas
gold mine in South Africa.
Instability
Collapse of a slope
Bi/Multi-planar failure
• This failure took place at St Aidans OCCS on at least two planes;
the back plane (fault zone) and at least one basal plane (weak
seatearths below the coal seams).
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Tailings Dam Failures
Tailings Dam Failures
Introduction
Tailings
• Tailings are waste material from the mining industry.
• Valuable mineral is surrounded by gangue (uneconomic material),
which needs to be separated in a concentrating process.
• Crushing and grinding methods are used to reduce the mined
ore to sand and silt sizes, before concentration.
• The most common technique used today is 'flotation' which has
been used to separate minerals since the early 1920's.
• The process treats the ground ore in a bubbling mixture of
water and chemicals which the metallic minerals stick to and
rise to the surface of the flotation tank.
• This is the concentrate which is scraped or poured off for
either further processing or drying. The remaining ground ore
is the waste, known as tailings.
Tailings Dam Failures
Introduction
Tailings
• Tailings settling pond at Lisheen mine in Ireland (below left)
• Lead froth flotation at Lisheen mine in Ireland (below right)
Tailings Dam Failures
Dams
•
Tailings material is
usually highly toxic.
•
Dam types:
1.
Centreline
2.
Upstream
3.
Downstream
Tailings Dam Failures
Types of Failures
The diagram below shows the different ways in which
tailings dams can fail.
Slope Failure
Wedge
Slip Circle
Liquefaction
Piping
(Common)
Overtopping
Failure Of Decant Tower
Water or Fines
Washaways
(Common)
Erosion
(Widespread)
Wind And
Rain
Coarse
Wall
Toe Erosion
(Widespread)
Tailings Dam Failures
Types of Failures - Overtopping & erosion
Gold mine, Merriespruit, South Africa
• Water overtopped the during a storm, causing rapid erosion.
• 17 people died and toxic waste did widespread damage.
Tailings Dam Failures
Types of Failures - Rotational failure
This rotational failure took place in the tailings dam of the
Crixas gold mine in South Africa.
Tailings Dam Failures
Types of Failures - Rotational failure
Crixas after rehabilitation.
Tailings Dam Failures
Types of Failures - Piping failure
Omai gold mine, Guyana.
• No-one was killed, but 18 people
needed treatment for the effects of
cyanide poisoning.
Tailings Dam Failures
Types of Failures - Piping failure
Omai gold mine, Guyana.
• On August 19th 1995, the tailings dam at Guyana's Omai Gold
Mine cracked allowing 3.2 billion litres of cyanide-laced (28ppm)
sludge to flow into the heart of the rainforest.
• The current judgement is that the failure of the dam was
caused by massive loss of core integrity resulting from internal
erosion of the dam fill (piping).
Tailings Dam Failures
Types of Failures - Decant failure
The decant pipework collapsed and tailings escaped
through the outflow.
Tailings Dam Failures
Types of Failures - Erosion
The slumping at the toe of the
dam has been caused by the
water table intersecting the
dam on the slope, rather than
intersecting the solid ground
behind the toe.
Tailings Dam Failures
Types of Failures - Erosion
The photo shows repairs made at the Crown Mine tailings
dam by buttressing the toe with concrete.
Tailings Dam Failures
Types of Failures - Collapse
Collapse - Weakened Foundations?
• Los Frailes lead-zinc mine in southern Spain
Tailings Dam Failures
Types of Failures - Collapse
Reasons for Los-Frailes dam failure
• The reasons are still uncertain
• A report described the weak point of the dam two years in
advance of the failure.
• Geotechnical experts suggest that the foundation failure was
caused by chemical attack of the acidic tailings on the alkali marl
in the dam foundation material.
• Other experts suggest that the dam was built on clays which
deformed with wetting and drying.
• Acidic seepage, combined with continued blasting in the nearby
open pit mine would have contributed to the dam failure.
• Other experts suggest that the dam was designed with
insufficient beach width and that the dam was being filled at a
rate higher that it was designed for.
Tailings Dam Failures
Types of Failures - Collapse
Collapse – underground workings
• Far West Rand (S. Africa) sink holes caused by collapse of tailings
pond into old workings. A number of people in the mine were killed.
Tailings Dam Failures
Not all bad news
A good tailings facility!
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Tip Failure
Tip Failure
Aberfan Disaster
Aberfan Disaster (1966), S.Wales
• 144 people died, including 112 children, when a colliery spoil tip
collapsed and flowed down into the village.
Tip Failure
Aberfan Disaster
Causes
• One of the tips was in a very unsuitable position, overlying a
natural spring from a sandstone.
• Rotational slip took place which was made larger due to the
saturation of debris by the spring beneath.
• 3 previous failures were over springs. There were no records of
failures of dry tips.
• Initial movement was slow.
• A flowslide (material behaving as a liquid) then formed in
saturated debris that collected within the rotational slip.
• Flow of 110,000m3 of debris moved 610m on 130 slope.
• Rapid consolidation took place when the flow stopped.
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Careers in Mining
The University of Leeds offers a degree in Mining and Mineral
Engineering. Within this you can choose to specialise, either in
Mining (surface and underground mining) or in Mineral Processing
(more chemistry based).
Both of these subjects are currently in high demand globally,
with excellent salaries and great opportunities to travel. There
are also many possibilities within the UK quarry industry.
If you enjoy geology, but are also interested in the engineering
design or operational aspects of extracting or processing
resources, then this could be the career for you.
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Mining at Leeds
The Mining and Mineral Engineering Programme is taught within
the School of Process , Environmental and Materials Engineering
(otherwise known as SPEME), as part of the Process Engineering
group of subjects.
Most UK students are eligible for a bursary of £1,000 in
their 1st year.
For further information visit: www.canyoudigthis.com
Or go direct to: www.engineering.leeds.ac.uk
Contact: Toby White, 0113 343 2784, T.J.White@leeds.ac.uk.