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School of somethingFACULTY OF OTHERSchool of Process, Environmental and Materials EngineeringMining, 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 MiningIntroduction

• 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.


Subsidence

SubsidenceIntroduction

• 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.

SubsidenceRoom & Pillar (Pillar& Stall) workings in coal

SubsidenceUnderground Longwall Mining - Shearing

Coal carried away on conveyor

Roof collapses (goaf) behind

supports.

Roof supports

Coal

Coal shearer

SubsidenceLongwall – two different methods in plan

Retreat

Face

Main intake road

Main return road

Advance

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.

SubsidenceCoal Mining

Further examples of subsidence.

SubsidenceDuration 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.)


Acid Mine Drainage

Acid Mine DrainageIntroduction

• 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 DrainageConsequences 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 DrainageWheal 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 DrainageTypical red deposits

Acid Mine DrainagePreventing 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


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.

MethaneHow 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.

MethaneHow 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.

MethaneControl 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.

• One example is given here.

• Longwall mining

• Two-entry retreat face

• Horizontal boreholes

• Some methane drained from seam

• Will not remove all.

Methane from coal

Ventilation circuit

Clean air

Methane carried away

Horiz. boreholes


Instability

InstabilityIntroduction

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.

InstabilityRockbursts

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.

InstabilityCollapse 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.

InstabilityCollapse of a face on a quarry

Slope & Face Failure

Translational Rotational/ circular

Toppling Rockfall


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.


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).

InstabilityCollapse of a slope

• There are a number of different ways in which a slope can fail. Click on the buttons below.


Planar/Slab failure

• Planar instability along bedding with loss of quarry crest and access to lower benches.


Wedge failure

• Wedge failure in a magnesian limestone quarry along two well defined joint planes.


Rotational failure

• This rotational failure took place in the tailings dam of the Crixas gold mine in South Africa.


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).


Tailings Dam Failures

Tailings Dam FailuresIntroduction

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 FailuresIntroduction

Tailings

• Tailings settling pond at Lisheen mine in Ireland (below left)

• Lead froth flotation at Lisheen mine in Ireland (below right)

Tailings Dam FailuresDams

• Tailings material is usually highly toxic.

• Dam types:

1. Centreline

2. Upstream

3. Downstream

Tailings Dam FailuresTypes of Failures

The diagram below shows the different ways in which tailings dams can fail.

Slip CircleLiquefactionWedge

Water or Fines

Coarse Wall

Wind And Rain

Toe Erosion

Slope Failure Overtopping Failure Of Decant Tower

Piping Erosion Washaways

(Widespread)

(Common) (Widespread) (Common)

Tailings Dam FailuresTypes 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 FailuresTypes of Failures - Rotational failure

This rotational failure took place in the tailings dam of the Crixas gold mine in South Africa.

Tailings Dam FailuresTypes of Failures - Rotational failure

Crixas after rehabilitation.

Tailings Dam FailuresTypes 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 FailuresTypes 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 FailuresTypes of Failures - Decant failure

The decant pipework collapsed and tailings escaped through the outflow.

Tailings Dam FailuresTypes 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 FailuresTypes of Failures - Erosion

The photo shows repairs made at the Crown Mine tailings dam by buttressing the toe with concrete.

Tailings Dam FailuresTypes of Failures - Collapse

Collapse - Weakened Foundations?

• Los Frailes lead-zinc mine in southern Spain


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.


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 FailuresNot all bad news

A good tailings facility!


Tip Failure

Tip FailureAberfan 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 FailureAberfan 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.


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.


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, [email protected].

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