1. Topic 2: Mining
From a series of 5 lectures on
Metals, minerals, mining and (some of) its problems
prepared for London Mining Network
by
Mark Muller
mmuller.earthsci@gmail.com
24 April 2009

2. Outline of Topic 2:
• Surface mining methods
• Open-pit mines
Slope failure in open-pit mines
• Open-cast mines
• Underground mining methods:
Room-and-pillar mining
Longwall mining
 Rockburst hazard in deep longwall mining
 Surface subsidence above shallow longwall mining
Block-cave mining
• Mining using in-situ leaching
• Other mining methods: hydraulic mining, dredging

3. Anatomy of a mine:
Grasberg, West Papua
0.2 million tons of
tailings are dumped
into the Ajkwa river
system every day,
causing massive
sedimentation on
coastal floodplains
(Lottermoser, 2007)
No smelter and refinery.
This project delivers
mineral-concentrate.
Figure from Spitz and Trudinger, 2009.

4. Choice of mining and processing methods:
“The simple aim in selecting and implementing a particular mine plan is
always to mine a mineral deposit so that profit is maximised given
the unique characteristics of the deposit and its location, current market
prices for the mined mineral, and the limits imposed by safety,
economy, environment” (Text book definition: Spitz and Trudinger,
2009, my italics) (Social “limits” are not mentioned specifically!)

5. Mineral extraction: from mining to metal
Mining
Mineral
concentrate
METAL EXTRACTION
Metal
Figure from Spitz and Trudinger, 2009.

6. Schematic of common mining methods
Simple in concept, highly engineered for efficiency.
Very high waste rock volume.
Better safety record.
Used for laterally extensive deposits.
Overburden cast directly back into mined out panels.
Rehabilitation keeps pace with mining.
Reduced waste rock production.
Poor safety record.
Used for soluble ores: uranium, salt, potash.
Minimal waste production: only water wastes, no solids.
Figure from Spitz and Trudinger, 2009.

7. Choice of mining method:
The choice of mining method depends on many factors, including:
(i) Shape of the orebody: tabular, cylindrical, spherical.
(ii) Orientation of the orebody: sub-horizontal, sub-vertical.
(iii) Continuity of the orebody.
(iv) Ore-grade: high-grade, low-grade.
(v) Distribution of ore-bearing minerals within the orebody: massive or
disseminated (with a cut-off grade).
(vi) Depth to the orebody.
(vii) Strength of the orebody and overburden/host-rocks rocks.
(viii) Area of land available for waste disposal – open-pit mines cover a
larger surface area and generate a greater volume of wastes.
(ix) Impacts on surface: environmental, surface drainage and sub-surface
aquifers, land-use changes, social.
(x) Rehabilitation concerns.
(xi) Projected production rates.
(xii) Capital costs, rate of (financial recovery), cash-flow.
(xiii) Safety concerns – surface mining methods have a better safety
record.

8. Mining methods:
Surface mining
Open-pit mining
Strip or open-cast mining
includes superficial deposit mining: nickel laterite,
bauxite, mineral sands, alluvial diamonds
Underground mining
Block-caving
Sub-level block-caving
Longwall
Room-and-pillar (Bord-and-pillar), Stope-and-pillar
Longwall Top Coal Caving (LTCC) (China).
In-situ leaching
Dredging from floating vessels: alluvial deposits, mineral sands.
Hydraulic mining: often associated with placer deposits and tailings
reprocessing.

9. Surface mining:
Surface mining is the predominant exploitation method worldwide.
In the USA, surface mining contributes about 85% of all minerals
exploitation (excluding petroleum and natural gas). Almost all
metallic ore (98%) and non-metallic ore (97%), and 61% of the coal is
mined using surface methods in the USA (Hartman and Mutmansky,
2002).
Surface mining requires large capital investment (primarily expensive
transportation equipment), but generally results in:
- High productivity (i.e., high output rate of ore)
- Low operating costs
- Safer working conditions and a better safety record than
underground mining

10. Comparison of waste production for surface and underground mining:
Data are for USA in 1997 (from Hartman and Mutmansky, 2002), in million tons.
Surface mining Underground mining
Waste = 73% of total rock tonnage extracted Waste = 7% of total rock tonnage extracted
266% of ore tonnage extracted 9% of ore tonnage extracted
Pit excavation initially generates huge volumes of waste rock that must be
removed to allow access the orebody, and to allow stable pit slopes to be
developed.

11. Various open-pit and orebody configurations
(i) Flat lying seam or bed, flat terrain. Example
platinum reefs, coal.
(ii) Massive deposit, flat terrain. Example iron-
ore or sulphide deposits.
(iii) Dipping seam or bed, flat terrain. Example
anthracite.
(iv) Massive deposit, high relief. Example
copper sulphide.
(v) Thick bedded deposits, little overburden, flat
terrain. Example iron ore, coal.
Figure from Hartman and Mutmansky, 2002.

12. Open-pit mine: Chuquicamata copper mine, Región de Antofagasta, Chile
http://en.wikipedia.org/wiki/File:Chuquicamata_panorama.jpg
Benches
Access ramps
Photo credit:Till Niermann
11 September 2008. Dust
Slope failure
Locality: Región de Antofagasta, Chile.
Pit dimensions: 4.3 km long x 3 km wide x 850 m deep.
Mining dates: 1915 - present
Total production: 29 million tons of copper to the end of 2007 (excluding Radomiro Tomić production).
For many years it was the mine with the largest annual production in the world, but was recently
overtaken by Minera Escondida (Chile). It remains the mine with the largest total cumulative production.
Production 2007: 896,308 fine metric tons of copper (Codelco, 2007).
Mining cost in 2007: 48.5 US¢ per kg (2006), 73.0 US¢ per kg (2007) (Codelco, 2007).
Employees: 8,420 as of 31st 2007 (Codelco, 2007).
Pre-tax profits: US$ 9.215 billion (2006), US$ 8,451 billion (2007) (Codelco, 2007).

13. Pit slope versus rock strength Pit depth versus pit diameter
Greater rock strength can support greater A greater final pit depth requires a larger
bench heights – resulting in a steeper pit, a diameter pit (assuming rock strength and pit
lower stripping ratio and less waste rock. slope remains unchanged)
– resulting in a higher stripping ratio and
more waste rock.
Figure from Spitz and Trudinger, 2009.

14. Open-pit slope failure – case study – groundwater problems
A slope failure occurred at the Cleo
Open Pit (Sunrise Dam Gold Mine,
Western Australia) in December 2000.
At the time of failure the pit-floor was at Water table
100 m depth below surface.
100 m
Two critical factors played a role in
the failure:
• The top of the water table is at a very
high level: only 30 m below surface
• A strong layer of younger clay
sediments overlies weaker weathered
bedrock.
Seepage and mineral
precipitation
The failure is thought to be due to very Top of water table
high pore fluid pressures in the
weathered bedrock that created an Original configuration
instability at the interface between the Mud pile
Stiff clay
bedrock and the overlying clays, allowing
a slippage to occur (Speight, 2002).
Weathered bedrock
high pore pressures
Plane of failure located at boundary
between bedrock and clay
Distance in meters
Figures modified from Speight, 2002.

15. Open-pit slope failure – structural problems
Pre-mining geological structures, particularly fault planes, represent zones of potential
weakness in the rock mass, and are therefore zones of potential slope failure, and should be
taken into account when designing the mine.
Fault planes dipping towards the pit (as shown in the figure) present a greater risk than faults
dipping away from the pit.
Faults planes often provide passage-ways for water movement, and these waters, through the
process of weathering and chemical alteration of minerals, may reduce the strength of the rocks
on either side of the fault plane, and reduce the “coefficient of friction” along it.
The coefficient of friction (the “traction” or
“grip”) along the fault will determine
whether failure and slippage of rock down
the fault plane is likely.
The coefficient of friction may change with
time:
• as water-flow patterns are affected by
mining
• as faults are exposed by the removal of
rock, opening fluid pathways into faults
• by the reduction of the mass of the rock
located above the fault plane. Computer model of a potential failure plane in an
open-pit mine (From Little, 2006)

16. Schematic of open-cast coal mine
OVERBU
RDEN
Dragline gathers overburden
DIRE and “casts” it back onto spoil
CTIO
NO F AD banks located behind the
VANC
E current working face
• Significant “permanent” waste dumps are not needed.
• Mine rehabilitation can be carried out progressively at the same rate as mining.
Figure from Hartman and Mutmansky, 2002.

17. Open-cast or strip mining:
Used for near-surface, laterally continuous, bedded deposits such as
coal, stratified ores such as iron ore, and surficial deposits (nickel
laterite or bauxite).
The pits are shallower that open-pit mines, and the overburden is “hind-
cast” directly into adjacent mined out panels.
It is a very low-cost, high-productivity method of mining.

18. Open-cast coal mining, Rhine Westphalia Germany
Simulated natural-color satellite
(ASTER) image of the Garzweiler
open-cast lignite (brown coal) mine
in North Rhine Westphalia, Germany.
The mine is named after the town
Garzweiler, which was located at the
center of the area being mined.
AD
V
AN
8.5 km
CE
Active mining
Photo credit: NASA/GSFC/METI/ERSDAC/JAROS,
and US/Japan ASTER Science Team, August 26, 2000.
http://asterweb.jpl.nasa.gov/gallery-detail.asp?name=Coal

19. Underground mining:
Generally underground mining is adopted when the orebody is too deep and
it’s not economically or technically feasible to use an open-pit:
Deepest “hard-rock” open pits are over 700 m deep (e.g., Palabora in South
Africa and Chuquicamata in Chile).
It is increasingly common to progress from open-pit to underground mining
of the same orebody.
Used where surface land use prohibits surface disruption (e.g., towns,
agricultural land, lakes, near-surface aquifers). Not always prioritised by
miners!
The major distinction between the different underground mining
methods is whether the mined out areas remain supported after
mining, or if they are allowed to collapse.

20. General anatomy of a deep underground mine
Both ore-rock and water are
allowed to feed to the bottom of the
mine under the force of gravity, and
from there are transported or
pumped to the surface.
Note the use of “backfill” in mined-
out areas to provide support for the
overlying rock. Backfill allows ore
recovery to be maximised, because
ore is not left in-place as support
pillars.
Backfill is generally a mixture of
cement with waste rock, sand or
tailings.
Figure from Spitz and Trudinger, 2009.

21. Supported underground mining: room-and-pillar layout
Pillars have been mined-out
in this area
Note the control of ventilation, i.e.,
the separation of contaminated
(used) and uncontaminated (fresh)
air using a variety of devices.
Figure from Hartman and Mutmansky, 2002.

22. Supported underground mining – Room-and-Pillar method:
The mining cavity is supported (kept open) by the strength of remnants
(pillars) of the orebody that are left un-mined.
Room-and-pillar mining method has a low recovery rate (a large
percentage of ore remains in place underground).
Used for tabular orebodies, with moderate dip: for example, coal and
evaporite (salt and potash) deposits.
It is an advantageous mining method for shallow orebodies – as a means
of preventing surface subsidence. Historic, ultra-shallow underground
coal mines (< 30 m) nevertheless are characterised by surface
subsidence in the areas between pillars (e.g., Witbank coal field,
South Africa).
Pillars are sometimes mined on retreat from a working area, inducing
closure and caving of these working panels, and raising the risk of
surface subsidence.

23. Underground mining: room-and-pillar mining of thick seams – “benching”
ll
wa
g ing
n
Ha
ll
t wa
Foo
Different approaches allow either the top or
bottom part of the seam to be mined out first.
Note the “hangingwall” is above the mining
cavity, and the “footwall” is below it.
Figures from Hartman and Mutmansky, 2002.

24. Unsupported underground mining – longwall mining method:
Longwall mining is suitable for tabular orebodies, with moderate dip (e.g.,
coal and stratiform hard-rock ores).
In “unsupported” mining, the mine-workings are supported temporarily
only for as long as needed to keep the active face open to mining.
After mining, the support (e.g. hydraulic props or wood packs) is
removed (or becomes crushed), and the mining cavities close up
under the pressure of the overburden material. The cavity closure
is either partial, for shallow mining, or complete, for deep level
mining.
While unsupported mining is advantageous in that it maximises ore
recovery (as little ore as possible is left behind) the method comes
with significant problems:
- Surface subsidence in the case of shallow mines
- Rockbursts underground, causing injury and death in deep level
mines.

25. Underground mining: longwall mining
http://en.wikipedia.org/wiki/File:SL500_
SCHEMATIC OF LONGWALL PANEL 01.jpg
(HANGINGWALL STRIPPED AWAY FOR
ILLUSTRATIVE PURPOSES)
Protective screen
E
ANC
ADV
~1
50
m
Mechanised cutting machine on a
longwall coal-mining face.
In hard-rock minerals mining
“Permanent” support, a “scraper” is pulled down
often timber packs, will the length of the stope face
remain in place after after drilling and blasting, to
mining. With time, these collect the fragmented ore
become deformed or rock.
completely crushed – as
Temporary support near In coal mining, a mechanised
part of the “controlled”
the working face: often cutting device is run along
closure of the panel.
hydraulic props. the length of the coal face.
Figure from Hartman and Mutmansky, 2002.

26. Unsupported underground mining
σ v = ρgh
Cartoon showing the driving Depth
σv h Virgin stress situation
below
at depth h:
mechanisms of “mining- surface
induced seismicity”. σ v = ρgh
h
The creation of a cavity
underground significantly
alters the virgin subsurface
stress (pressure) regime. σ v = ρgh
Depth
σv h
below
surface
Mining process:
Mined out blast & remove
h material at the
stopes, tunnels …
σ v = ρgh
Removal of rock causes
Depth
σv h stresses to redistribute,
stope closure & fracturing
below
surface
Slip on new fractures and
Slip! pre-existing geological
k!
h
h
ra c
features results in seismicity
C

27. The effect of mining depth on stope (cavity) closure
SURFACE
Shallow mining: partial closure
of cavity, and surface
subsidence above the mining.
“Beam width”
Deep mining: complete closure
of cavity, extensive fracturing
around the cavity, with
associated rockbursts
(explosive release of seismic
energy – effectively
earthquakes).

28. Deep level gold mining, South Africa
1.5 m
Stope face with temporary support
Stopes (yellow):
STOPE
STOPE
on-reef
excavations
where the reef
(orebody) is
mined.
http://www.bullion.org.za/MiningEducation/Images/images/
Slide 28
CrossSectMine.jpg © CSIR 2006 www.csir.co.za

29. Aftermath of a rockburst in a deep-level tunnel showing complete tunnel closure. The
energy released by this event is equivalent to magnitude M = 3.4 earthquake.

30. Longwall mining with strike stabilising-pillars at South African gold mine
Plan view of tabular orebody showing mined and un-mined areas Sub-shaft support pillar
Sub-shaft pillar boundary
SUB-SHAFT
N
Dip
Mined-out
stopes
1km
Mined out
Geological structures: 1.0 km
0 0.5 km Strike Pillars
faults and dykes Strike pillars (unmined ore rock)
providing support for hangingwall

31. Mining strategy can make a difference to rockburst activity:
Underground seismicity (and rockburst rate) can be influenced by:
- The rate of advance of a stope face (slower advance is more favourable)
- The number of adjacent panels being mined simultaneously (smaller
number is more favourable)
- The angle at which the advancing face approaches geological
structures (preferably not parallel)
The last point highlights the need for detailed advanced knowledge of
geological structures. Miners are not always prepared to invest in high resolution
surveys (e.g., geophysical surveys) to achieve the level of detail required for safer mining.
Plan view of mining panels
Seismicity and
ADVANCE ADVANCE
rockbursts
Mined out Mined out
Fault Fault
SUPPORT PILLAR SUPPORT PILLAR
Unfavourable stoping layout and progression of More favourable stoping layout and
mining. progression of mining.

32. Underground longwall coal mining at Barapukuria Mine, NW Bangladesh
Coal production started in
October 2005.
Mining depth is approximately
400m.
To date 6 panels have been
mined, with a 3 m slice height.
Total production to date has not
exceeded 3 Mt coal (a small
proportion of the total 34 Mt
recovery planned over 30 years).
Plan view Barapukuria Coal Mine
(from levels 260 m to 420 m).
Figure from Islam et al., 2008.

33. Underground longwall coal mining at Barapukuria Mine, NW Bangladesh
SURFACE SUBSIDENCE
(NOT SHOWN TO SCALE)
SURFACE
110 m
WATER
WATER
400 m
zo ctu e
Ri ctur
fra bsid
n e re
fra e
COAL SEAM
bs
CAVITY
zon
Ri
ide e
IV
440m
Schematic cross-section showing subsurface response to longwall mining cavity, and
subsidence at surface. Arrows show fluid-pathways downwards from the Dupi Tila acquifer
along mining induced fractures, into the mining panel.
Figure modified after Islam et al., 2008.

34. Underground longwall coal mining at Barapukuria Mine, NW
Bangladesh:
The impact on surface at Barapukuria is typical of many areas around the
world which have been undermined by longwall coal mining. Subsidence
problems are particularly associated with the sedimentary basins in
which coal is found (also evaporite NaCl and KCl deposits) as the
overburden is weaker than crystalline rocks. Underground coal mining
also tends to take place at shallower depths than many hard-rock mineral
mines.
Considering how little of the resource has been mined to date, the impact on the surface above
the mine at Barapukuria has been devastating:
- Land subsidence of between 0.6 – 0.9 m has been reported over an area of about 1.2 km 2.
- The water-table has dropped, leaving commonly used water reservoirs dry in 15 villages.
- At least 81 houses have developed cracks in 5 villages.
- Untreated mine water (acknowledged by the mine to contain phosphorous, arsenic and
magnesium) is passing through canals in farming areas.
- The scale of the problem has the Bangladesh government currently considering the
establishment of a new “coal city” near Barapukuria that would provide housing and
(potential) employment to people whose livelihoods are at risk in 15 villages around the mine.
Barapukuria Mine plans to increase the total panel height mined to 18 m (it is currently
only 3 m) – it is not clear whether this plan is going to be practically viable!!

35. Underground mining – Block caving:
Block-caving method is employed generally for steeply dipping ores, and
thick sub-horizontal seams of ore. The method has application, for
example in sulphide deposits and underground kimberlite (diamond)
mining.
An undercut tunnel is driven under the
orebody, with "drawbells" excavated
SURFACE above. Caving rock falls into the
TOP OF OREBODY drawbells. The orebody is drilled and
blasted above the undercut to initiate the
“caving” process. As ore is continuously
removed from the drawbells, the
orebody continues to cave
spontaneously, providing a steady
stream of ore. If spontaneous caving
stops, and removal of ore from the
drawbells continues, a large void may
form, resulting in the potential for a
Figure from Hartman and Mutmansky, 2002. sudden and massive collapse and a
potentially catastrophic windblast
throughout the mine (e.g., the
Northparks Mine disaster, Australia).

36. Block-cave mining: Mud-rushes – an under-reported hazard
Mud-rushes are sudden inflows of mud from ore drawpoints (or other underground openings), in
block-cave mines that are open to the surface. Considerable violence, in the form of an
airblast, is often associated with mud-rushes. Mud-rushes are (under-reported) hazardous
occurrences that have occurred frequently in mines in South Africa, as well as in Chile and Western
Australia, and have caused fatalities (Butcher et al., 2005).
Mud is produced by the
breakdown of rock in the
near-surface muckpile in the
presence of rainwater.
Kimberlite rock on diamond
MUD mines is particularly
susceptible to weathering by
rainwater.
SCHEMATIC CUT-AWAY VIEW OF SUB-
LEVEL BLOCK-CAVE MINE
Figure from Hartman and Mutmansky, 2002.

37. In-situ leaching (ISL)/ solution mining:
Used most commonly on evaporite (e.g. salt and potash) and sediment-
hosted uranium deposits, and also to a far lesser extent to recover
copper from low-grade oxidised ore.
The dissolving solution is pumped into the orebody from a series of
injection wells, and is then pumped out, together with salts dissolved
from the orebody from a series of extraction (production) wells.
Sodium cyanide: NaCN
Metals and minerals commonly mined by solution mining methods.
Sulphuric acid: H2SO4
Dissolving agent specified in each case. (From Hartman and
Mutmansky, 2002, and references therein). Hydrochloric acid: HCl
Ammonium carbonate
(alkali): (NH4)2CO3
Aside: The same reagents
are often used for
processing mined ores in
hydrometallurgical plants

38. In-situ leaching (ISL)/ solution mining:
Uranium deposits
Uranium minerals are soluble in acidic or alkaline solutions.
The production (“pregnant”) fluid consisting of the water soluble uranyl
oxyanion (UO22+) is subject to further processing on surface to precipitate
the concentrated mineral product U3O8 or UO3 (yellowcake).
Acid leaching fluid
sulphuric acid + oxidant
(nitric acid, hydrogen
peroxide or dissolved
oxygen)
or UO22+
Alkali leaching fluid
ammonia, ammonium
carbonate/bicarbonate,
or sodium
carbonate/bicarbonate
The hydrology of the acquifer is
irreversibly changed: its porosity, UO2
permeability and water quality. It is
regarded as being easier to “restore”
an acquifer after alkali leaching.
Figure from Hartman and Mutmansky, 2002.

39. In-situ leaching (ISL)/ solution mining:
Evaporite deposits
Has been used for many decades to extract soluble evaporite salts such as
halite (NaCl), trona (3Na2O ∙ 4CO2), nahcolite (NaHCO3), epsomite
(MgSO4 ∙ 7H2O), carnallite (KMgCl3 ∙ 6H2O), borax (Na2B4O7 ∙ 10H2O)
from buried evaporite deposits in UK, Russia, Germany, Turkey,
Thailand and USA).
A low salinity fluid, either heated or not, is injected underground directly into
the evaporite layer; the “pregnant” solutions (brines) are withdrawn
from recovery boreholes and are pumped into
evaporation ponds, to allow the salts to
crystallise out as the water evaporates.
Old underground mines, consisting typically of
room-and-pillar workings, are often further
mined using solutions to recover what remains
of the deposit, i.e., the pillars (with associated
surface subsidence risk).
Evaporation ponds, Arizona
Figure from Spitz and Trudinger, 2009.

40. In-situ leaching (ISL)/ solution mining:
Advantages:
No solid wastes.
Liquid wastes (low concentration brines with no market value) can be re-
injected into the stratum being leached. Also reported that wastes are
sometimes injected into a separate acquifer (not good practice).
Problems:
Little control of the solution underground and difficulty in ensuring the process
solutions do not migrate away from the immediate area of leaching.
Main impact of evaporite ISL is derived from surface or shallow groundwater
contamination in the vicinity of evaporation ponds. Pregnant solutions can
be highly corrosive and pyhto-toxic, and can react with the soil materials
used in pond construction, and may migrate to surrounding areas through
seepage, overflow (both bad practice), and windblown spray.
Surface subsidence and the development of sink-holes may also occur after
prolonged solution mining if inadequate un-mined material is left to
support the overburden (bad practice).

41. Hydraulic mining:
Generally used for weakly cemented near-surface ore deposits.
Note:
Riffle box
uses mercury
for gold
recovery
The “Stang Intelligiant” monitor
Hydraulic mining of a placer gold deposit. (operator controlled high
pressure water discharge point)
mounted on a skid
Figures from Hartman and Mutmansky, 2002.

42. Hydraulic mining – tailings dam reprocessing
Kaltails project, Kalgoorlie, Western Australia
The project was established to reprocess and relocate tailings dams from the Boulder and
Lakewood areas of Kalgoorlie. The operations ceased after a decade of work in September
1999. The tailings dumps were hydraulically mined, reprocessed and stored in another
engineered impoundment located 10 km south east of Kalgoorlie. Recovery was by Carbon-in-
Circuit (CIC) and Carbon-in-Pulp (CIP) leach and absorption circuits.
Sixty million tons of
tailings were mined in
an area of 333
hectares (3.33 km2),
producing 695,000
ounces (19.7 tons) of
gold at an average ore-
grade of 0.33 g/ton.
Remaining part of a waste dump being hydraulically mined (Newmont Mining).
From TAILSAFE, 2004.

43. Dredging:
Used most often for mineral-sands and some near-shore alluvial diamond
mining operations.
Typical bucket-line dredge
www.tmd.ihcholland.com
Figure from Hartman and Mutmansky, 2002.

44. Oil sand mining:
Blurs the boundary between hydrocarbon and mineral extraction.
Canada’s oil sands are the second single largest oil deposit on
Earth, second only to the large reserves in Saudi Arabia.
Resource has not been exploited significantly to date because of
the much higher costs associated with extracting the oil compared
to conventional borehole extraction in normal oil deposits.
The sands are either strip mined or the oil rich sands are heated
underground (steam) so that oil migrates to recovery boreholes.
In the case of mining, bitumen is recovered by washing the sands
in hot water, and is subsequently upgraded to a final “crude” in two
successive process: hydro-cracking and hydro-treating.
“Ore-grade”: approximately 2 tons of sand needed to produce 1 barrel
of oil.

45. Coal-bed methane and Underground Coal Gasification:
Please see my review included with course material:
“It’s not only about coal mining: Coal-bed methane (CBM) and
underground coal gasification (UCG) potential In Bangladesh”.
For a summary of these methods, their advantages and disadvantages,
and an quantitative examination of the energy return provided by these
surface based, non-invasive alternatives to coal mining.