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					School Process, Environmental and
School ofof something
FACULTY OF OTHER
Materials Engineering
Mining, Quarry & Mineral Engineering




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.
School Process, Environmental and
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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                      Coal carried away
         supports.                          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.)
School Process, Environmental and
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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
•       One example is given here.                 circuit
    •     Longwall mining            Clean air
    •     Two-entry retreat face
    •     Horizontal boreholes
    •     Some methane drained from seam
                                                                    Methane
    •     Will not remove all.                                    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/    Toppling   Rockfall
                   circular
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               Overtopping                Failure Of Decant Tower
                                                                      (Widespread)
                                        Water or Fines
               Slip Circle
  Wedge              Liquefaction




             Piping                   Erosion                         Washaways
           (Common)                 (Widespread)                       (Common)
                                                      Wind And
                      Coarse                            Rain
                       Wall

                                        Toe Erosion
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.

				
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