BSc Environmental Studies Non-Renewable Resources Impacts of Resource Extraction EV3903 Non...

BSc Environmental Studies

Non-Renewable Resources

Impacts of Resource Extraction

EV3903 Non Renewable Resources

Impacts of Resource Extraction - 1

11

Introduction - 1

• mining activity has major effect on stability of rocks at depth

• probably most intrusive industrial activity in terms of penetration of lithosphere

• mine workings can reach depths of hundreds or even thousands of metres

• mining changes stress conditions within rocks (Fig. 1) - hence their geotechnical behaviour

• mining also affects surface stability

• also leads to landscape changes through extensive storage of:

Fig. 1 EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources

• soil

• rock

• ore processing waste


11

Introduction - 2

• requirement for large-scale drainage – mainly in case of subsurface mining

• 4 main impacts of mining:

• instability of land surface (Fig. 2) and its effects on land use (subsurface mining)

• creation of enormous quantities of waste materials (Fig. 3)

• contamination of air, soil and water

Fig. 2

EV3903 Non-Renewable ResourcesEV3903 Non-Renewable ResourcesFig. 3


EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources33

Introduction - 3

• number of different metals contained in deposit - determines degree of risk of emissions from mine area

• characteristics of rock and overburden underlying mine area – controls:

• extent of environmental damage and cost of mitigating it site specific

• influenced by local geology, geography and climate

• chemistry of deposit and thus its pollution potential may also vary considerably

• specific controls are:

• degree of seepage from unlined mine dumps and tailings ponds



Introduction - 4

• degree of neutralisation of acidified water emanating from mine wastes

• geographical location of deposit relative to urban centres

• topographic location of deposit relative to water table (near surface, height above OD)

• climate – specifically:

• prevailing winds and shelter or exposure of the mine area

• total precipitation - weathering and generation of acid mine drainage

• aridity - dust


Surface Mining - 1

• associated with both surface and subsurface mining, but main effects related to surface mining (Figs. 4 & 5)

Blasting - 1

Fig. 555 EV3903 Non-Renewable ResourcesEV3903 Non-Renewable ResourcesFig. 4


Surface Mining - 2

• damage to buildings related to size of charge and distance from point of detonation (Figs. 6 & 7)

EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources

Blasting - 2

Fig 6. Probability of damage versus charge and distance

66

Fig. 5


77

Surface Mining - 3

• damage to buildings classified into three categories :

• Threshold Damage

Blasting - 3

• Minor Damage – does not affect strength of structure – includes:


• dislodgement of loose objects

• broken windows,

• loosened or fallen plaster

• hairline cracks in masonry

Fig. 8

• widening of old cracks and formation of new ones in plaster


88

Surface Mining - 4

Blasting - 4


• Major Damage seriously weakens structure – includes (Fig 8):

• large cracks

• shifting of foundations and bearing walls

• distortion of superstructure caused by settlement

• walls out of plumb

• blasting vibrations (Fig. 9) related to

• amplitude

• particle velocity

• acceleration Fig. 9 Record of typical blasting vibrations


99

Surface Mining - 5

Blasting - 5

• peak particle velocities of up to 50 mm sec-1 regarded as safe as far as structural damage concerned

• 50-100 mm sec-1 requires caution

• above 100 mm sec-1 - high probability of damage occurrence

• particle velocity most closely related to damage in frequency range of typical blasting operations (Figs. 10 & 11)


Fig. 10



Surface Mining - 6

Blasting - 6

• other effects include human discomfort and sensitivity (Fig. 12), noise, dust, etc.

Fig. 11 Particle velocity and damage in basement walls1010

Fig. 12


Surface Mining - 7

Sand and Gravel Pits

• visual impact - scar on landscape (Fig. 13) - generally don’t re-vegetate easily

• slopes unstable - slumping and sliding possible, but not significant hazard

• abandoned gravel pits commonly used as dumps (Fig. 14) - gravel overburden highly permeable - leachate percolates rapidly down to water table - little attenuation – groundwater pollution

1111 EV3903 Non-Renewable ResourcesEV3903 Non-Renewable ResourcesFig. 13 Fig. 14


Surface Mining - 8

Quarries - 1

• visual impact - scar on landscape - bare rock (Fig. 15)

• slopes generally vertical - very dangerous (Fig. 16)

Fig. 15Fig. 121212 Fig. 16 EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources


Surface Mining - 9

Quarries - 2

• slopes generally relatively stable, but danger of toppling failure (Fig 17)

• quarrying involves blasting - extremely dangerous - nuisance effect

• abandoned quarries often become filled with water - also major safety hazard (Fig. 18)

Fig. 181313 Fig. 17 EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources


• abandoned quarries commonly used for dumps (landfills) (Fig. 19) - very dubious - all overburden stripped off - so no attenuation of leachate

• rocks highly fractured due to blasting, - open pathways for leachate to percolate down to water table - groundwater pollution likely

• even if not used as landfills, infiltrating rainwater not purified due to absence of overburden

Surface Mining - 10

Quarries - 3

1414 Fig. 19

Fig. 14



Surface Mining - 11

Placer Mining - 1

• removal of material from streambed changes stream dynamics

• may lead to severe erosion immediately downstream from dredging operation

• enhances flood potential of stream (Figs. 20 & 21)

1515 Fig. 20 Fig. 21 EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources



Surface Mining - 12

Placer Mining - 2

• stream pollution likely due to use of heavy equipment, oil etc. during dredging operations (Figs. 20 & 21)

• processing of immense amounts of gravel, sand and mud, results in the severe siltation of streams and lakes

Fig. 221616

• particularly damaging in countries such as the Philippines, Indonesia, Brazil, etc.

• in one river in Guyana, water undrinkable for 65 km downstream (Fig. 22)



Surface Mining - 13

Placer Mining - 3

• pollution damages fish stocks –also destroys fish habitats and alters migratory patterns (Fig. 23)

Fig. 231717


Surface Mining - 14

Solution Mining - 1

Brining - 1• controlled brining produces stable cavities that cause ground subsidence only if

allowed to coalesce (Figs. 24 & 25)

Fig. 24 Fig. 25 EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources1818



Surface Mining - 15

Solution Mining - 2

Brining - 2

Fig. 261919

27

• wild brining (Fig. 26) less predictable - produced large subsidence zones in Cheshire saltfield - often elongated over subsurface ‘brine streams’ (Fig. 27)


Surface Mining - 16

Solution Mining - 3

Brining

• room and pillar mining with excessive extraction ratios - even more damaging method - eventually banned around 1930

Fig. 24 EV3903 Non-Renewable ResourcesEV3903 Non-Renewable ResourcesFig. 29

• bastard brining - resulted in catastrophic formation of sinkholes up to 100m wide and 10m deep as remaining pillars dissolved and collapsed (Fig. 28)– led to major property damage (Fig. 29)

2020 Fig. 28


2121

Surface Mining - 17

Solution Mining - 4

• mercury pollution due to extraction of gold with mercury during placer gold mining (Fig. 30)

• estimated that gold mining introduces ~100 tons of mercury into Amazon ecosystem in Brazil every year

• numerous other streams similarly affected

• mercury accumulates in plants and animals - biomagnifies as it rises through food chain (Figs. 31 & 32)

Mercury Separation - 1



Surface Mining - 17

Solution Mining - 4



• causes severe neurological diseases and birth defects in both animals and humans

Fig. 312222


Surface Mining - 18

Solution Mining - 5

• mercury poisoning insidious - often occurs years after person exposed to metal

• mercury levels in fish in several Amazon tributaries and other South American streams now exceeds safe levels for human consumption (Fig. 33)

• mercury poisoning begun to appear amongst native and other people living in Amazon riverside villages, where fish major food source


EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources2323 Fig. 33


Surface Mining - 13

Solution Mining - 3

• permits allowing use of highly toxic cyanide for gold treatment readily granted

• in well-constructed and well-managed heap leach operations, cyanide can be looped through a closed system so that none is lost (Figs. 34 -36)

Heap Leaching - 1

EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources1515 Fig. 34 Fig. 35


1515

Surface Mining - 13

Solution Mining - 3

• in practice cyanide solutions commonly escape - enter surface and groundwater

• numerous accidental spills have occurred in US, including:

Heap Leaching - 1

• failure of dam on leaching pond -resulted in 10,000 gallons of cyanide pouring into nearby river

• major fish and bird kills due to cyanide leaks

• Summitville Mine disaster



1515

Surface Mining - 13

Solution Mining - 3

Heap Leaching - 1

• Summitville Mine, Colorado – elevation 3800 m – headwaters of Rio Grande (Fig. 37)

• high snowfall - 7-11 m per year – creates landslides and avalanches


• mining began 1985 – leach pads 73 acres in area - one heap > 60 m high - $3 m bond posted

• HDPE liner damaged by avalanches during construction – not repaired

• 1991 –very high snowfall - release of excess water from snowmelt contaminated with cyanide and heavy metals into Alamosa R.

Fig. 37


1515

Surface Mining - 13

Solution Mining - 3

Heap Leaching - 1


forfeiting $3 m bond – don’t even lock doors

• 1992 - EPA take over Summitville - 200-m gals cyanide-laced water in leach pit

• cost to date of cleanup $150 m and still rising

• Clinton signed bill to increase size of environmental bonds for mining activities – but Bush administration has reduced size of bonds

• all aquatic life for 17 miles downstream exterminated

• report on fish kills estimates $20 m clean up costs

• 3 days later mine owners walk awayFig. 38



Surface Mining - 14

Solution Mining - 3

• sodium cyanide solutions chemically unstable

• cyanide quickly decomposes in surface waters where oxygen is plentiful and acidic conditions prevail

• cyanide can persist at toxic levels for much longer periods in groundwater

• so poses long term threat to water wells used for human consumption, livestock and irrigation

• cleanup costs immense - at one major gold mine in US, operation running at no profit, only break even situation

• decision to keep mine in operation based on fact that cheaper to keep it running than closing mine down and starting to pay for cleanup

Heap Leaching - 2


99

Surface Mining - 15

Open Pit Mines - 1

• visual impact – huge hole and enormous surface spoil heaps - major scars on landscape (Fig. 39)

• slopes generally very steep- very dangerous

• collapse

• sliding failure

• mudflows


• particularly prone if discontinuities (bedding, cleavage, joints, faults) dip towards open pit

• slopes designed for stability - but danger of oversteepening slopes – leads to:Fig. 39


Surface Mining - 15

Open Pit Mines - 2

• dewatering of mine area - creates cone of depression around mine (Fig. 40)

• modifies the hydrogeological regime for mine area and perhaps larger region (more later)

• abandoned open pits commonly become filled with water - also major hazard (Fig. 41)

EV3903 Non-Renewable ResourcesEV3903 Non-Renewable Resources99 Fig. 40Fig. 41


99

Surface Mining - 15

Open Pit Mines

• extraction of the ore deposit, exposes sulphide minerals to oxygen and water

• results in weathering and oxidation (Fig. 42)

• leads to acidification of surface and groundwater and dissolution of heavy metals = acid mine drainage (AMD - more later)

• contaminates groundwater and standing surface water within the open pit – e.g. Berkeley Open Pit – pH = 2.5 (Fig. 43)


Fig. 42

Fig. 43



Surface Mining - 10

Strip Mining

• major environmental degradation

• topography altered - land rarely rehabilitated in past - now requirement (Fig. 44)

• abandoned mine area subject to severe soil erosion - sediment eroded from spoil banks etc. can silt up streams - increases potential flood risk

• coal mining wastes highly toxic – sulphur, zinc, lead, arsenic

• leads to contamination of drainage (both surface and groundwater) by base metals and sulphuric acid - AMD

• visual impact - spoil banks unsightly and highly toxic, so vegetation wont re-establish, even after several decades

Fig. 44



Subsurface Mining - 1

Mine Drainage Operations - 1

• mine dewatering mainly necessitated for underground mining (Fig. 45)

• has objective of protecting shafts and adits from flooding

• also required for deep open-pit mining if water table relatively close to surface

• pumped water dumped at surface, usually into surface streams





• leads to changes in hydrogeological conditions - have following impacts:

• changes groundwater flow dynamics, e.g. flow rates and direction - due to creation of artificial discharge zone

• change in groundwater recharge - due to fluctuation in water exchange rate above water table

• change in groundwater discharge - affects recharge of surface waters





• changes in groundwater regime increase extent of interconnection between:

• possible deterioration in groundwater quality – common when water from various sources mix – e.g. if dewatered mine area near coast may lead to intrusion of highly saline water into aquifer

• may lead to reduction in river discharge - since groundwater in high latitude parts of globe feed rivers

• different aquifers

• ground and surface water

• two major consequences:




• rivers may become sources of recharge for groundwater, reversing the hydrogeological regime completely - in the case of small rivers, this can lead to them drying out completely and so generating intermittent flow

• other impacts of dewatering and changes in the hydrogeological regime are:


• reduction in soil moisture due to dewatering, may affect vegetation

• productivity of agricultural crops may decrease

• drainage of bogs and marshes

• vegetation degradation - will affect the whole ecosystem, and the diversity of fish, birds, animals and other fauna within the dewatered area may be substantially reduced. - thus ecosystem degradation from large scale dewatering is additional to the impacts on ecosystems of chemical contamination from mining activity

• introduction of air into previously saturated rocks - triggers or accelerates mineral oxidation - accentuates acid mine drainage (AMD)





• removal of water from rock pore spaces increases the potential for deformation of rock strata (consolidation) - pore water pressure (PWP) reduction changes the physico-mechanical characteristics of rock - stress previously accommodated by PWP is redistributed to the adjacent rock grains - reduces stability of the rocks - impacts on construction stability and also agricultural activity in area affected by dewatering

• leads to the intensification of karstification and suffusion - rate of flow of groundwater in the drawdown cone is increased, so:

• water exchange is accelerated, leading to an increased potential for solution and karstification, where the bedrock is limestone, e.g. Silvermines

• washes out of fines from unconsolidated sands and gravels (suffusion = internal erosion)

• openings created by karstification and suffusion affect stability of overlying rocks

• local water supplies in dewatered areas affected - wells go dry




• groundwater extracted during dewatering is discharged into surface streams - affects stream dynamics - can also affect natural balance of ecosystems by changes in river velocity, river depth and even the amount of oxygen in the discharged groundwater

• chemical composition of groundwater differs from that of surface water, so this may also have impacts on aquatic flora and fauna

• groundwater may be contaminated with acid mine drainage

• finaly, degree of impact of dewatering depends on:

• natural (local) hydrogeological conditions

• size of ore body

• depth of ore deposit





Effects of Water Table Recovery - 1

Mine Drainage Operations -5

• abandonment of mining activity and cessation of water extraction leads to recovery of the water table and progressive flooding of underground workings

• rate of recovery depends on permeability of the dewatered zone and the size of the depression cone (depth and radius)

• original hydrogeological conditions may be restored over a period of time – but numerous environmental problems may be associated with flooding of old mine workings:

• pollution of water entering old workings, with resulting potential pollution of groundwater aquifers, possibly used for groundwater supplies - also potentially surface waters through springs and streams.

• reduction in stability of the mine area, due to the influence of the rewatering on:

• the mechanical properties of mined out rocks or rocks surrounding mine area

• the stability of fine-grained unconsolidated backfill deposits




• addition of water reduces compressive strength, as pore water acts oppositely to normal stresses, and reduces the angle of friction on joint surfaces:

Mine Drainage Operations -5

Effects of Water Table Recovery - 2

= S0 + ( - p) tan

• estimated reduction in strength of order of 10% may result

• clay-rich rocks may undergo a reduction in physico-mechanical characteristics of up to 50-70% due to water saturation - become plastic and begin to creep, destabilising strata above them

• some materials, e.g. fine-grained backfill or clays may undergo liquefaction, and become displaced towards voids at the bottom of the mine, destabilising empty shafts.

• although flooding threatens the stability of mine areas already at the limit of their stability, if failure does not occur during the flooding stage or immediately afterwards, long-term stability of area should be enhanced




Land Use and Ground Stability (Subsidence) - 1

• stability of rock subject to mining a function of the geotechnical properties of the rock material, which are dependent on:

• pre-existing stress conditions within the rock mass• rock strength• rock deformation parameters (i.e. elastic moduli)• water content

• changes in geotechnical properties of rock due to mining considerable - strongly dependent on extraction technique.

• in subsurface mining, creation of mine openings changes pre-stress conditions within rock mass - leads to collapse of rock material into mine workings, and displacement of floor, roof and walls into shaft space –

• process leads to deformation of adjacent rock, extent of which dependent on size of mined out space, and parameters listed above





Sag subsidence (left), the most common type of mine subsidence, appears as a gentle depression in the ground and can spread over an area as large as several acres. Collapse of pillars supporting the mine roof is a typical cause.

Pit subsidence (right) forms a bell-shaped hole 6-8 feet deep and from 2-40 feet across, and occurs when a shallow mine roof collapses




• cave-ins give rise to three major zones of rock deformation within overlying strata (Fig. 13):


• zone of collapse – blocks of rock cave in on mine workings (thickness can exceed the thickness of the mined area by 3-4 times)

• zone of fractures – within which transverse (layer perpendicular) and longitudinal (layer parallel) fissures form.

• zone of subsidence - - strata are deformed, but undergo no fracturing

13

• all rock above the mined area undergoes deformation - commonly this may reach surface, giving rise to extensive subsidence (more later)





• extent and character of rock deformation depends on geological and technical factors - e.g.:

• ore body location

• ore body size

• ore body depth

• presence of weak strata

• geological structure – particularly presence of faults and fractures

• presence of saturated rock, i.e. within the saturated zone

• extraction technique

• strength of backfilling material, where a backfill technology is employed

• ground stability ultimately depends on style of mining - generally dictated by shape, size, depth and value of ore or extractable rock.




• old mine shafts a widespread hazard in many countries - thousands in UK

• small old mines had far more shafts than large modern mines - records of old shafts very incomplete

• old abandoned shafts abound - mainly 1-5 m in diameter and 10-300 m deep

• may be lined with brick, concrete or dry stone or may be completely unlined

• loose or uncompacted waste may completely or partially fill shafts, or shafts may be empty

• shaft mouths may be closed up with timber, steel or concrete or may be left open

• may be overgrown by vegetation, or may be properly sealed and capped

Old Abandoned Mine Hazards





Stoping

• creates large open underground stopes

• subsidence threat localised, but may locally sterilise ground directly above mine

Room and Pillar (Pillar and Stall) - 1

• better controlled modern mines have no surface effects

• old mines commonly undergo roof span failure and progressive breakdown of beds causing upwards stoping (migration of cavities) - may reach the surface to create a crown hole by sudden collapse (Fig. 14)

• older mines often over-extracted – create long-term subsidence threat

Fig. 14





• stoping may be stopped by:


• beam action of a strong bed • formation of a stable arch in thinner beds • support of the roof due to accumulation of debris

• crown holes rare from adits deeper than 30 m or 10 times thickness of extracted seam • mine pillars fail where:

• they are left too slim, • are subsequently overloaded• are subject to weathering and erosion

• multiple domino-style failures may affect large areas, and were common in the past due to over-extraction and pillar-robbing (Fig. 15)





• collapse of old mines can be delayed for in excess of 100 years

• modern threat of ground failure is minimal where:

• mine is > 50m deep

• any imposed structural load is slight in proportion to existing rock overburden

• pillar erosion decreases with depth

Bell Pits

• rarely more than 10m deep - only present a localised subsidence hazard

• generally occur in dense groups - must be filled or excavated if development over them can’t be avoided






• total extraction of all coal and removal of roof support brings about roof collapse and inevitable subsidence displaying well-defined pattern (Fig. 16)

• roof failure behind longwall face propagates upwards and outwards through overlying rock

Longwall Mining - 1

Fig. 16


• geometry function of angle of draw

• varies with rock strength -roughly 30-35

• increases slightly in weaker rocks

3232


3232


Longwall Mining - 1

• depth of working (h)

• width of the mined panel (w)

• extracted thickness of coal (t)

• end result of roof failure is subsidence bowl at ground surface

• extends 0.7 h outside the panel

• not clearly defined as tapers to zero (Fig. 17)

• other critical parameters, which control subsidence movements are:


Fig. 16





• maximum depth of subsidence bowl always less than seam thickness

• due to volume increase as cracks open up within subsiding rocks

• can accumulate to several metres over time if multiple seams worked

Longwall Mining - 2


3333

• subsidence wave has length of ~ 1.4 h (Fig 18)

• mid-point of maximum tilt and neutral strain close to vertically above coal face

• migrates with the advancing face

• also develops to a similar shape over panel sides Fig. 18




• ground tilt as subsidence wave passes damaging to built structures

• also related cycle of surface extension and shortening

Longwall Mining - 2


• strain and subsidence profiles shown in Fig. 19

• strain profiles show an outer zone of extension and inner zone of compression

• line of neutral strain roughly above panel edge

• subsidence and strain most severe over shallow wide panels in thick seams

Fig. 17


3434


• pattern and timing of subsidence over longwall faces is predictable

• so structures at risk can be strengthened before mining begins

Longwall Mining - 3

• also complicated by geological factors (faults, strong rocks, steep dips) and multiple workings

• subsidence effects more severe with older shallow mining than during modern mining of deeper seams






• approximate predictions of maximum values of subsidence, strain and tilt with respect to h, w, and t estimated using graph (Fig. 19)

• typical values shown

• better predictions can be made with graphs for specific coalfields, based on coalfield records and local rock characteristics

Longwall Mining - 3

Fig. 19





Example of Calculations

Site factors (from mine plans) Thickness (t) = 1.2 m

Panel Width (w) = 160 m

Depth (h) = 400 m

Ratios w/h = 160/400 = 0.4

t/h = 1.2/400 = 0.003

Reading off graph for value of w/h = 0.4

Longwall Mining - 4





Subsidence Factor (direct from graph) = s/t = 0.3

Subsidence (s) = 0.3 x t = 0.3 x 1.2 = 0.36 m = 360 mm

Extension (E) = 0.28 (from graph) x t/h = 0.28 x 0.003 = 0.00084

Compression (C) = 0.62 (from graph) x t/h = 0.62 x 0.003 = 0.00186

Strain = E + C = 0.00084 + 0.00186 = 0.0027 = 2.7 mm/m

Tilt = 1.4 (from graph) x t/h = 1.4 x 0.003 = 0.0042 = 1 in 238

Longwall Mining - 4





• scale of subsidence problem illustrated by extent of surface depression due to subsidence in coal mining regions of former USSR

• areas > 200 km2 affected in Donbass Basin, Ukraine and Chelibensk Province

• instability and subsidence problems in underground mining eased by choice of appropriate mining method - so deformational stress brought about by ore extraction does not exceed strength of the rocks

• backfilling technology can increase stability of many underground mines - perhaps combined with room and pillar approach.


Longwall Mining - 5




Wastes Storage and Landscape Degradation - 1

• wastes include:

• overburden from surface mining

• broken and discarded rock dumped in spoil heaps

• tailings emplaced in dumps or ponds

• slags from smelters

• mine wastes represent the highest proportion of waste produced by any industrial activity - billions of tonnes produced annually





• due to its high volumes, mine wastes historically has been disposed of:

• at lowest possible cost

• without regard to safety

• with considerable environmental impact

• with extreme landscape degradation





• surface mines produce per ton of ore ~ 8 x waste of subsurface mines

• grade of ore determines quantity of waste produced (Fig. 20)

• at Cu ore grades of 0.9%

• to produce 9 million tonnes of Cu

• 990 million tonnes of ore must be extracted

Fig. 20





• gold mining requires processing of even greater quantities of material to obtain very small quantities of metal ~ 325,000 tonnes of Au ore for only 50 kg of Au

• 50 billion tonnes of mining waste in US alone

• create mountains of spoil heaps covering extensive areas of land, withdrawing them from agricultural and forestry activities

• in Poland, surface mining has resulted in destruction of agricultural land by:

• agricultural land - 32,000 ha

• forestry land - 9,000 ha

• in Germany, by 1985, coal mining had led to a reduction in:

1975 1980

25,000 ha 56,000 ha





• type of waste rock disposal facility depends on topography and drainage of site and volume of waste

• in terms of coarse mine waste - disposal can be classified as:

• valley fills

• side-hill dumps

• open piles





• valley fills normally commence at upstream end of valley and progress downstream

• side-hill dumps constructed by placement of waste along hillsides or valley slopes - avoid natural drainage courses

• open piles tend to be constructed in relatively flat lying areas – due to their upstanding nature, subject to intense erosion - visually highly intrusive





• spoil heaps also highly toxic - contain significant contents of pyrite and other heavy metal-bearing sulphide minerals

• also highly permeable - so drain relatively rapidly

• don’t vegetate easily due to their toxic nature and low moisture content

• dry out readily - so highly susceptible to wind erosion

• spread toxic dust and contaminate surrounding land for miles around, e.g. Silvermines 1983

• acid mine drainage (AMD) from spoil heaps another major environmental problem





• instability arose from poor siting of a series of tips over natural springs on the valley slopes (Figs. 21 & 22)

• lubricated base of tips

• important factor in construction of spoil heaps is their long-term stability

• tip failure at Aberfan, Wales in 1966 an example of many similar colliery tip failures

• had tragic consequences - buried village school killing 112 children

Fig. 21





• several previous failures in these tips had same cause

• on this occasion, during wet weather, tip 7 underwent rotational slip

Fig. 21

Fig. 22

• unable to drain due to saturated nature - result of fine-grained impermeable nature of spoil

• degenerated into flow slide and finally mudflow

• travelled almost 1 km down valley side and into village





• tailings - fine-grained slurries

• formed from crushed rock from which ore separated - or produced by washings from coal mines

• deposited as slurry generally in specially constructed tailings dams - usually confined by embankment dam

• contain high proportions of pyrite, heavy metals and other toxic chemicals

• source of AMD if seepage occurs - from their base, if unlined - through dam wall

• failure of tailings dams another potential catastrophe - occurred after heavy rains at Buffalo Creek, West Virginia, 1972 - over 1500 houses destroyed - 118 lives lost





• resulting dereliction of land and overall environmental degradation due to such disasters more difficult to assess

• derelict land defined as land so damaged by human activity as to need remedial treatment before further use

• in England, mineral extraction responsible for more derelict land than any other single activity (Fig. 23a)

23





• extraction-related derelict land decreasing steadily - from:

• in 1988 derelict areas made up of:

• spoil heaps - 30%

• excavations -15%

• mining subsidence - 2.5%

• estimated 25,000 ha (64% of total) in 1969

• 19,000 ha (47.5%) in 1988

• so derelict land being reclaimed faster than it’s being produced by closure of pits, mines and quarries (Fig. 23b)





• net annual reclamation only small proportion of total derelict areas

• order of 50 years or more before all land reusable

• new extraction permits subject to more stringent restoration conditions than old licences

• inadequate reclamation conditions still apply to > ⅓ of 96,000 ha permitted surface workings in England - mostly for construction materials (Fig. 24) 24





• these sites will add to stock of derelict land when present working finishes

• underground mining permits affect at least 8 x area of surface licences

• licences require compensation for subsidence damage.

• but water pollution from AMD not covered

• rapid closure of coal mines in Britain likely to exacerbate this problem

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