BSc Environmental Studies
Non-Renewable Resources
Impacts of Resource Extraction
EV3903 Non Renewable Resources
Impacts of Resource Extraction - 1
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:
1
• soil
• rock
• ore processing waste
Fig. 1
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 1
Introduction - 2
• 4 main impacts of mining:
• requirement for large-scale drainage
– mainly in case of subsurface
mining
• instability of land surface (Fig. 2)
and its effects on land use
(subsurface mining)
Fig. 2
• creation of enormous quantities of
waste materials (Fig. 3)
• contamination of air, soil and water
1
Fig. 3
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 3
Introduction - 3
• 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:
• 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:
• degree of seepage from unlined mine dumps and
tailings ponds
3
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 4
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
4
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 5
Surface Mining - 1
Blasting - 1
• associated with both surface and subsurface mining, but main effects related to
surface mining (Figs. 4 & 5)
5
Fig. 4
Fig. 5
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 6
Surface Mining - 2
Blasting - 2
• damage to buildings related to size of charge and
distance from point of detonation (Figs. 6 & 7)
Fig. 5
Fig 6. Probability of damage versus charge and distance
6
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 7
Surface Mining - 3
Blasting - 3
• damage to buildings classified into three categories :
• Threshold Damage
• widening of old cracks and formation of new ones in plaster
• dislodgement of loose objects
• Minor Damage – does not affect
strength of structure – includes:
• broken windows,
• loosened or fallen plaster
• hairline cracks in masonry
7
Fig. 8
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 8
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
8
Fig. 9 Record of typical blasting vibrations
Fig. 8
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 9
Surface Mining - 5
Blasting - 5
• particle velocity most closely related to damage in frequency range of typical
blasting operations (Figs. 10 & 11)
• 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
Fig. 10
9
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 10
Surface Mining - 6
Blasting - 6
• other effects include human discomfort and sensitivity (Fig. 12), noise, dust, etc.
Fig. 12
10
Fig. 11 Particle velocity and damage in basement walls
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 11
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
11
Fig. 13
Fig. 14
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 12
Surface Mining - 8
Quarries - 1
• visual impact - scar on landscape - bare rock (Fig. 15)
• slopes generally vertical - very dangerous (Fig. 16)
12
Fig. 12
Fig. 15
Fig. 16
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 13
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)
13
Fig. 17
Fig. 18
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 14
Surface Mining - 10
Quarries - 3
• 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
Fig. 14
14
Fig. 19
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 15
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)
15
Fig. 20
Fig. 21
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 16
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
• particularly damaging in countries
such as the Philippines, Indonesia,
Brazil, etc.
• in one river in Guyana, water
undrinkable for 65 km downstream
(Fig. 22)
16
Fig. 22
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 17
Surface Mining - 13
Placer Mining - 3
• pollution damages fish stocks –also destroys fish habitats and alters migratory
patterns (Fig. 23)
17
Fig. 23
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 18
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)
18
Fig. 24
Fig. 25
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 19
Surface Mining - 15
Solution Mining - 2
Brining - 2
• wild brining (Fig. 26) less predictable produced large subsidence zones in Cheshire
saltfield - often elongated over subsurface
‘brine streams’ (Fig. 27)
27
19
Fig. 26
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 20
Surface Mining - 16
Solution Mining - 3
Brining
• room and pillar mining with excessive extraction ratios - even more damaging
method - eventually banned around 1930
• 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)
20
Fig.
Fig.
28 24
Fig. 29
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 21
Surface Mining - 17
Solution Mining - 4
Mercury Separation - 1
• 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)
21
Fig. 30
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 22
Surface Mining - 17
Solution Mining - 4
Mercury Separation - 1
• causes severe neurological diseases
and birth defects in both animals and
humans
22
Fig. 31
Fig. 32
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 23
Surface Mining - 18
Solution Mining - 5
Mercury Separation - 2
• 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
23
Fig. 33
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 15
Surface Mining - 13
Solution Mining - 3
Heap Leaching - 1
• 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)
15
Fig. 34
Fig. 35
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 15
Surface Mining - 13
Solution Mining - 3
Heap Leaching - 1
• in practice cyanide solutions commonly escape - enter surface and groundwater
• numerous accidental spills have occurred in US, including:
• 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
15
Fig. 36
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 15
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
Fig. 37
• 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
15
EV3903 Non-Renewable Resources
with cyanide and heavy metals into Alamosa R.
Impacts of Resource Extraction - 15
Surface Mining - 13
Solution Mining - 3
Heap Leaching - 1
• all aquatic life for 17 miles
downstream exterminated
• report on fish kills estimates $20 m
clean up costs
Fig. 38
• 3 days later mine owners walk away
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 –
15
EV3903 Non-Renewable Resources
but Bush administration has reduced size of bonds
Impacts of Resource Extraction - 16
Surface Mining - 14
Solution Mining - 3
Heap Leaching - 2
• 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 EV3903 Non-Renewable Resources
14
Impacts of Resource Extraction - 17
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
Fig. 39
• slopes designed for stability - but danger of oversteepening slopes – leads to:
• collapse
• sliding failure
• mudflows
9
• particularly prone if discontinuities (bedding, cleavage, joints, faults) dip towards
open pit
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 17
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)
9
Fig. 41
Fig. 40
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 17
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)
9
Fig. 42
Fig. 43
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 18
Surface Mining - 10
Strip Mining
• major environmental
degradation
• topography altered - land
rarely rehabilitated in past now requirement (Fig. 44)
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 re18
EV3903 Non-Renewable Resources
establish, even after several decades
Impacts of Resource Extraction - 19
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
19
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 19
Subsurface Mining - 1
Mine Drainage Operations - 1
• 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
19
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 19
Subsurface Mining - 1
Mine Drainage Operations - 1
• changes in groundwater regime increase extent of interconnection between:
• different aquifers
• ground and surface water
• two major consequences:
• 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
19
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 20
Subsurface Mining - 2
Mine Drainage Operations - 2
• 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)
20
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 21
Subsurface Mining - 3
Mine Drainage Operations - 3
• removal of water from rock pore spaces increases the potential for deformation of rock
strata (consolidation) - pore water pressure (PWP) reduction changes the physicomechanical 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
21
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 22
Subsurface Mining - 4
Mine Drainage Operations - 4
• 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
22
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 23
Subsurface Mining - 5
Mine Drainage Operations -5
Effects of Water Table Recovery - 1
• 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
23
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 24
Subsurface Mining - 6
Mine Drainage Operations -5
Effects of Water Table Recovery - 2
• addition of water reduces compressive strength, as pore water acts oppositely to normal
stresses, and reduces the angle of friction on joint surfaces:
 = 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 5070% 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
24
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 25
Subsurface Mining - 7
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
25
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 25
Subsurface Mining - 7
Land Use and Ground Stability (Subsidence) - 1
25
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
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 26
Subsurface Mining - 8
Land Use and Ground Stability (Subsidence) - 2
• 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)
26
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 27
Subsurface Mining - 9
Land Use and Ground Stability (Subsidence) - 3
• 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.
27
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 28
Subsurface Mining - 10
Land Use and Ground Stability (Subsidence) - 4
Old Abandoned Mine Hazards
• 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
36
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 29
Subsurface Mining - 11
Land Use and Ground Stability (Subsidence) - 5
Stoping
• creates large open underground stopes
• subsidence threat localised, but may locally sterilise ground directly above mine
Room and Pillar (Pillar and Stall) - 1
• older mines often over-extracted – create long-term subsidence threat
• 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)
29
Fig. 14
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 30
Subsurface Mining - 12
Land Use and Ground Stability (Subsidence) - 6
Room and Pillar (Pillar and Stall) - 2
• 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)
30
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 31
Subsurface Mining - 13
Land Use and Ground Stability (Subsidence) - 7
Room and Pillar (Pillar and Stall) - 3
• 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
31
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 32
Subsurface Mining - 14
Land Use and Ground Stability (Subsidence) - 8
Longwall Mining - 1
• 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
• geometry function of
angle of draw
• varies with rock strength roughly 30-35
• increases
slightly
weaker rocks
32
Fig. 16
in
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 32
Subsurface Mining - 14
Land Use and Ground Stability (Subsidence) - 8
Longwall Mining - 1
• other critical parameters, which control subsidence movements are:
• 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
•
32
not clearly defined as tapers to
zero (Fig. 17)
Fig. 16
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 33
Subsurface Mining - 15
Land Use and Ground Stability (Subsidence) - 9
Longwall Mining - 2
• 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
• 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
33
Fig. 18
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 33
Subsurface Mining - 15
Land Use and Ground Stability (Subsidence) - 9
Longwall Mining - 2
• ground tilt as subsidence wave
passes damaging to built
structures
• also related cycle of surface
extension and shortening
• strain and subsidence profiles shown in Fig. 19
Fig. 17
• 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
33
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 34
Subsurface Mining - 16
Land Use and Ground Stability (Subsidence) - 10
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
• pattern and timing of subsidence over
longwall faces is predictable
• so structures at risk can be strengthened
before mining begins
34
Fig. 19
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 34
Subsurface Mining - 16
Land Use and Ground Stability (Subsidence) - 10
Longwall Mining - 3
• 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
34
Fig. 19
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 35
Subsurface Mining - 17
Land Use and Ground Stability (Subsidence) - 11
Longwall Mining - 4
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
35
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 35
Subsurface Mining - 17
Land Use and Ground Stability (Subsidence) - 11
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
35
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 36
Subsurface Mining - 18
Land Use and Ground Stability (Subsidence) - 12
Longwall Mining - 5
• 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.
36
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 37
Subsurface Mining - 19
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
37
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 37
Subsurface Mining - 19
Wastes Storage and Landscape Degradation - 1
• 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
37
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 38
Subsurface Mining - 20
Wastes Storage and Landscape Degradation - 2
Fig. 20
• 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
38
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 38
Subsurface Mining - 20
Wastes Storage and Landscape Degradation - 2
• 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:
1975
1980
25,000 ha
56,000 ha
• in Germany, by 1985, coal mining had led to a reduction in:
38
• agricultural land - 32,000 ha
• forestry land - 9,000 ha
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 39
Subsurface Mining - 21
Wastes Storage and Landscape Degradation - 3
• 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
39
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 39
Subsurface Mining - 21
Wastes Storage and Landscape Degradation - 3
• 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
39
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 40
Subsurface Mining - 22
Wastes Storage and Landscape Degradation - 4
• 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
40
• acid mine drainage (AMD) from spoil heaps another major environmental
problem
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 41
Subsurface Mining - 23
Wastes Storage and Landscape Degradation - 5
• 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
• instability arose from poor siting of a
series of tips over natural springs on the
valley slopes (Figs. 21 & 22)
• lubricated base of tips
41
Fig. 21
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 41
Fig. 21
Subsurface Mining - 23
Wastes Storage and Landscape Degradation - 5
• several previous failures in these tips had same cause
• on this occasion, during wet weather, tip 7 underwent rotational slip
• 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
41
Fig. 22
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 42
Subsurface Mining - 24
Wastes Storage and Landscape Degradation - 6
• 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
42
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 43
Subsurface Mining - 25
Wastes Storage and Landscape Degradation - 7
• 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
43
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 43
Subsurface Mining - 25
Wastes Storage and Landscape Degradation - 7
• in 1988 derelict areas made up of:
• spoil heaps - 30%
• excavations -15%
• mining subsidence - 2.5%
• extraction-related derelict land decreasing steadily - from:
• 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)
43
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 44
Subsurface Mining - 26
Wastes Storage and Landscape Degradation - 8
• 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)
44
24
EV3903 Non-Renewable Resources
Impacts of Resource Extraction - 44
Subsurface Mining - 26
Wastes Storage and Landscape Degradation - 8
• 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
44
EV3903 Non-Renewable Resources