MODULE 2833/1 (C) ECONOMIC AND ENVIRONMENTAL GEOLOGY
REVISION NOTES
3.1
Water Supply
Candidates should be able to:
(a)
Define and explain the following terms: porosity, permeability, hydrostatic
pressure, hydraulic gradient, aquifers and the water table.
Porosity – the amount of pore space, usually expressed as a % of total rock volume. The
porosity is only effective if the pore spaces are interconnecting. E.g. Clay has a porosity of
50% but the effective porosity is virtually zero; Sandstone may typically have a porosity of
15% but most of the pores interconnect.
Percentage porosity = total volume of pore space
x 100
total volume of rock/sediment
Permeability – the ability of the rock to transmit fluids, expressed as a rate of flow. The
permeability may be the result of fracture/joint voids rather than interconnecting pores.
Permeability = distance water has travelled
time taken
Surface water – Water on the surface in lakes, rivers and streams.
Groundwater – Water held in pore space of rocks, below the water table. The Earth’s largest
accessible store of fresh water – a very important water resource especially in dry, arid areas.
Water table – Surface separating saturated rock below from unsaturated rock above; above
the water table pores hold air and water, below water table water only. It generally follows
surface topography, but with less relief, intersecting the ground surface at lakes and most
rivers.
Aquifer – Body of porous and permeable rock capable of storing and yielding significant
quantities of water, which makes it suitable for groundwater abstraction, e.g. aeolian
sandstone, limestone, chalk.
Types of Aquifer:
 Unconfined – Open to the atmosphere, not covered by impermeable strata, under
atmospheric pressure.
 Confined – Sandwiched between impermeable strata above and below, under hydrostatic
pressure.
 Perched – Above the regional water table.
The aquifer may also be:
 Live – Currently being recharged by rain water from the surface = renewable water
resource.
 Fossil – Not being recharged – a relic of a wetter climate = non-renewable water
resource.
Aquiclude – Impermeable rock that will not transmit water, e.g. clay, shale.
Hydrostatic pressure – Water pressure at any one point in a body of water due to the weight
of overlying column of water.
Hydrostatic head – Height of overlying column of water.
Water will always flow down the hydraulic/pressure gradient from high pressure to low
pressure – important for predicting the movement of water in aquifers.
Hydraulic gradient – The difference in hydrostatic pressure or hydrostatic head between two
points. This pressure difference causes water to flow through an aquifer.
Hydraulic gradient = difference between hydrostatic pressure or hydrostatic head
(quoted as a ratio)
the distance between two points
Potentiometric surface – Height to which water under hydrostatic pressure will rise.
(b)
Describe and explain the geological conditions leading to the formation of
springs as a result of lithology, faults and unconformities.
Springs of ground water occur where the water table intersects the surface – usually where
permeable and impermeable rocks are in contact.
Types of Springs:
 Lithological (rock type) springs occur where a porous and permeable rock overlies an
impermeable rock, e.g. chalk overlying clay in south-eastern England.
1


Faults may produce springs if they have brought porous/permeable rocks into
juxtaposition with impermeable rocks. Springs associated with joints may occur in
limestones – the water flows along the joints and the spring’s location is influenced by the
outcrop of joints.
Unconformities and igneous intrusions may also influence the location of springs if they
juxtapose porous/permeable rock and impermeable rock.
(c)
Explain the geological conditions necessary for artesian basins and water
supply from wells.







Artesian basin – Large down-folded (syncline/basin), confined aquifer under hydrostatic
pressure. Has a recharge area at the edges of the syncline/basin. A borehole/well sunk in
the basin will be artesian. E.g. London Basin – synclinal structure, chalk sandwiched
between clay above and below.
Artesian well – Normally water has to be pumped out of a well. If a well is sunk in an
artesian basin (confined aquifer, under hydrostatic pressure), the water will rise under its
own pressure to the potentiometric/peizometric surface. The pressure will be highest in
centre of the artesian basin – saves on pumping costs.
Water supply – Water is abstracted from the ground by sinking wells/boreholes and
pumping it to the surface. If water extraction is well managed the supply can be renewable
and sustainable.
Cone of depression – Formed as the water is pumped out because the water table falls in
the vicinity of the well.
Draw down – Difference between water level in the well and the water table.
Ground water mining/overdraft conditions – If extraction is greater than recharge.
Problems of groundwater abstraction – If cones of depression overlap, shallow wells
become dry as the water table is lowered; Subsidence on the surface; Salt water
encroachment in coastal areas – seawater is drawn into the aquifer, water becomes
saline (brackish).
(d)
Describe water supply in relation to river, reservoir and underground sources.
Understand the advantages and disadvantages of surface and underground supply.
Understand that water resources are both renewable and sustainable if carefully
developed.
Advantages & Disadvantages of Groundwater
Advantages
Disadvantages
Rock acts as natural filter purifying water.
Requires sedimentary rocks and presence of
No loss of water through evaporation.
aquifers.
No requirement for expensive/unsightly
Problem of subsidence at surface.
dams.
Problem of salt water encroachment at coast.
If artesian – cheap pumping costs.
Pollutants have a long residence time.
Sustainable providing recharge = extraction.
Pumping costs.
Groundwater is not always suitable for
drinking.
Unsustainable if groundwater mining.
Advantages & Disadvantages of Surface Water
Advantages
Disadvantages
Readily available – no pumping costs.
Water will need treatment.
May be used for H.E.P. generation.
Requires construction of expensive dam.
Reservoir may be used for recreation and
Requires flooding of area for reservoir.
other purposes.
Seasonal/water loss through evaporation.
Reservoir will silt up.
May trigger earthquakes.
Requires sufficient rainfall and large
catchment – no backup in drought conditions.

British water supply – A useful diagonal dividing line can be drawn from the mouth of the
River Tees (Middlesborough) to the mouth of the River Exe (Exeter). To the north-west,
the area is underlain by older, impermeable mainly igneous and metamorphic strata,
2

3.2
whilst to the south-east, the area is largely composed of younger porous and permeable
sedimentary rocks separated by impermeable clays.
Rainfall is much greater in the north-west compared to the south-east. The result is that in
the north-west (SW England, Wales, Isle of Man, Northern England, Ireland and Scotland)
water supply is dominated by abstraction of water from rivers and storage of water in
surface reservoirs. In the south-east of England the water supply pattern is dominated by
the abstraction of water from underground sources, either springs or wells.
Energy Resources
Candidates should be able to:
(a)
Describe and explain the origin of oil and natural gas and migration from
source-rock to reservoir-rock under a cap rock. Define and recognise the trap
structures: anticline, fault, salt dome, unconformity and lithological.
Requirements for the formation and accumulation of petroleum:
 Source rock – An organic-rich mudstone/shale formed in shallow marine conditions, with
abundant zooplankton whose remains accumulated in low energy, anoxic conditions on
the seafloor.
 Maturation – The source rock is subjected to burial, compaction and temperatures of
between 50 and 200ºC (the oil window) to generate fluid hydrocarbons. Below these
temperatures biogenic gas forms but is usually lost. Above these temperatures the oil
denatures and is destroyed.
 Migration – The oil and gas migrate down the temperature and pressure gradients from
the source rock to a reservoir rock. This requires permeable rocks. The oil and gas will
migrate upwards until they encounter an impermeable layer or they reach the surface and
are lost.
 Reservoir rock – A porous and permeable rock, with good interconnected pore space, so
petroleum can accumulate and be extracted easily, e.g. desert sandstone, limestone. A
cap rock overlies the reservoir rock.
 Cap rock – An impermeable rock – either crystalline sedimentary (e.g. evaporites) or very
fine-grained (e.g. clay, shale) – which prevents the petroleum rising further and escaping.
 Trap – A structure that concentrates the petroleum in one place. This may be structural
(anticline, fault, salt dome) or stratigraphic (unconformity, lithological, e.g. reef, wedgeedge). All require the juxtaposition of a porous/permeable reservoir rock under an
impermeable cap rock. You must be able to recognise and draw fully annotated diagrams
of each of the trap types. Oil may be lost from a trap by erosion of the overlying cap rocks
or it may escape upwards through a fault. Oil may be destroyed by the heat from igneous
intrusions/volcanic activity or during metamorphism.
(b)
Describe the methods of primary and secondary recovery of oil and natural gas
from suitable reservoirs and the environmental and technological problems of oil and
natural gas extraction.
The trap is drilled and an oil well established. Extraction may be by primary or secondary
recovery.
 Primary recovery consists of drilling production wells to exploit the reservoir. In most
cases the production wells are in lower parts of the reservoir so that gas pressure forces
the oil to the surface – a gusher. But 75% of the oil is left in the reservoir.
 Secondary recovery involves the injection of water below or steam/carbon dioxide above
the oil to encourage more to rise towards the surface by increasing the pressure and
breaking down surface tension, which holds the oil onto grains in the reservoir rock. Oil
may need to be pumped to the surface if there is insufficient pressure within the reservoir.
20% of the oil is still unrecoverable.
 Problems – blow outs, explosions, fires, pollution.
3
(c)
Describe the occurrence of oil and natural gas in and around the British Isles
and its social and environmental implications. Understand that oil and natural gas are
examples of non-renewable energy resources.
Most oil and gas deposits are offshore, particularly in the North Sea, which is a shallow
continental shelf sea. Each geological combination of reservoir, cap and trap is called a
hydrocarbon play.
 Northern North Sea has a variety of oil and gas fields – Source rock = Kimmeridge Clay in
almost all cases; Reservoir rock = sandstones (e.g. Brent) or fractured chalk (e.g.
Ekofisk).
 Southern North Sea has extensive natural gas deposits – Source rock = Carboniferous
Coal Measures; Reservoir rock = Permian desert sandstone; Cap rock = Permian
evaporites. The traps are mainly anticlines – associated with salt doming.
 Other offshore gas fields occur in the Irish Sea and there may be extensive deposits to
the west of Scotland, but current technologies do not allow them to be fully exploited.
 Onshore there a number of small oilfields – mostly in Mesozoic rocks – e.g. Wytches
Farm, Dorset, and a number in Lincolnshire, e.g. Scampton and Gainsborough.



The social implications of oil/gas exploitation are mainly that we are very dependent on
fossil fuels for energy, plastics, etc., and the industry provides many jobs.
The environmental consequences of oil/gas exploitation are mainly the risks of pollution
and fires from oil spills, blow outs, etc.
Oil and natural gas are examples of non-renewable energy resources because they are
fossil fuels that formed millions of years ago and once they are burnt they are lost as
gases such as carbon dioxide and cannot be recycled in human timescales.
(d)
Describe and explain the origin of coal and coal seams as part of a cyclothem.
Describe the development of rank and the physical properties of lignite, bituminous
coal and anthracite.
Coal forms from the partial decay and preservation of plant material in anoxic conditions,
associated with swamps and deltas in a hot, tropical climate where there is luxuriant
vegetation. In deltaic environments (cyclothems) the rapid sedimentation and subsidence
ensure the peat is preserved. The peat deposits are gradually changed to coal seams by heat
and pressure - coalification. As the peat undergoes compaction, the volatile constituents are
driven off and the thickness of the deposit is reduced.
 Rank of a coal = The percentage of carbon in the coal – this increases as the degree of
burial/compaction increases.
 Coal series = The sequence of increasing rank:
peat
lignite
bituminous coal
anthracite



brown coal,
dull appearance,
plant matter visible
black coal,
dull & shiny layers
formed by different types
of plant matter
black, hard, iridescent,
shiny coal
Increasing temperature and pressure ------------------------------------------------------------------------>
Increasing carbon content, calorific value and density --------------------------------------------------->
Decreasing water, volatiles (H, O, N) and ash ------------------------------------------------------------->
(e)
Describe the methods of extracting economic deposits of coal by opencast and
underground mining. Describe the geological problems that can make coal mining
uneconomic.
Open cast mining is used in exposed coalfields to depths of 200m. Dragline excavators are
used and a stripping ratio of overburden:coal of up to 20:1 is profitable. Advantages –
cheaper and safer.
Underground mining is used in concealed coalfields.
Longwall retreat mining is the underground coal mining method used in Britain – it is highly
mechanised. Two tunnels/roadways are driven out to the furthest point from the shaft. A coal
face is established and the coal is mined as the face is moved back towards the shaft. This
allows the geological conditions to be determined in advance of mining. The coal face is up to
100m in length and is only economic if the seam is thick (about 2m). The coal is cut by a
rotating shearer and then falls onto a conveyor belt. The roof is held up by hydraulic chocks
4
and as the coal is mined the area behind is allowed to collapse (causes subsidence on the
surface).
Geological problems in underground coal mining include:
 Faults – offset the seam; juxtaposes different rock types, including hard sandstone; zone
of permeability – may cause flooding.
 Folds – machinery cannot cope with steep dips.
 Methane gas may be present – highly explosive.
 Porous and permeable strata may be present in deltaic sequences – problems of
flooding.
 Washouts – result of channel switching on the delta top – a distributary channel changes
course and erodes away the peat, depositing river channel gravels and sands in its place.
 Seam splits – where one thick seam splits into several thinner unworkable seams – the
result of differential subsidence of the delta.
(f)
Describe the broad structure and distribution of coalfields in the British Isles.
Describe the effects of mining and understand the environmental consequences of
mining operations. Understand that coal is an example of a non-renewable energy
resource.
Coal in the British Isles is almost all of Carboniferous age – Britain was at the Equator and
had a hot tropical climate. Most coal deposits are located onshore in deltaic sedimentary
sequences (cyclothems) that formed at this time.
There are two types of coalfield:
 Exposed coalfield – where the coal-bearing strata outcrops at the surface.
 Concealed coalfield – where the coal-bearing strata is below younger sedimentary “cover”
rocks.
The current location of British coalfields is the result of folding during the Variscan/ Hercynian
Orogeny (mountain building) at the end of the Carboniferous Period.
 South Wales Coalfield – Synclinal structure, closest to the mountain building event to the
south, suffered deformation and deeper burial → anthracite. Folding and faulting has
made this coalfield difficult to mine.
 Yorks, Notts, and Derbys – East Pennine Coalfield – Formed in the Pennine sedimentary
basin. The Variscan Orogeny resulted in the formation of the Pennine anticline and
erosion of the coal seams in this area of the original basin, leaving the Lancashire and
East Pennine coalfields on either side.
 Coal is an example of a non-renewable energy resource because it is a fossil fuel that
formed millions of years ago and once it is burnt it is lost as gases such as carbon dioxide
and cannot be recycled in human timescales.
(g)
Describe geothermal energy extraction from volcanic sources around the world
and potential 'hot' rock sources in the British Isles. Understand that geothermal energy
is an example of a renewable energy resource.
Three main types:
 Volcanic sources – High enthalpy (T and P). Mainly located at plate margins, e.g. Iceland
(constructive), New Zealand (destructive), not applicable to the British Isles. Groundwater
is converted to steam and this can be used to drive turbines to generate electricity.
 Geothermal aquifers in sedimentary basins – Low enthalpy. Similar requirements to oil –
needs a reservoir (aquifer) covered with an impermeable cap rock. Insufficient heat for
driving turbines, but can be used for ‘space’ heating, e.g. homes, greenhouses, etc.
 Hot dry rock sources – Granites – The heat source is the decay of radioactive elements.
The rock will need to be artificially fractured by explosives and cold water could be
pumped in and hot water extracted. However, the possibility of obtaining energy from ‘hot
dry rock sources’ in the granites of SW England has now been abandoned as
uneconomic.
 Advantages – renewable; cheap if in the right location; doesn't produce carbon dioxide
emissions.
 Disadvantages – wells only have a lifespan of ~30 years; water may contain
toxic/corrosive elements/salts; geographically restricted to volcanically active areas
(mainly plate margins).
5

3.3
Geothermal energy is an example of a renewable energy resource because there is a
constant supply of heat/magma from the Earth’s interior produced by the decay of
radioactive heat-producing elements.
Metal Deposits
Candidates should be able to:
(a)
Show an understanding of concentration factors to produce economic deposits
from low crustal abundances of metallic minerals; calculate concentration factors.

Resource – A naturally occurring material or substance in the Earth’s crust that is useful
and/or valuable to man. Can be divided into energy and mineral resources.
 Reserves – Amount of the resource that can be economically exploited using existing
technology. These may change due to: exploration; technological advances; World
demand and prices; politics, war and stock-piling.
 Ore – Rock which contains metal(s) of interest that can be mined at a profit. A mixture of
ore minerals and gangue minerals.
 Ore mineral – Mineral containing the metal of interest, usually oxides or sulphides. E.g.
Haematite = iron oxide
Galena = lead sulphide
Cassiterite = tin oxide
Magnetite = iron oxide
Sphalerite = zinc sulphide
Siderite = iron carbonate
Bauxite = hydrated
Chalcopyrite = copper-iron
Ilmenite = iron-titanium oxide
aluminium oxides
sulphide
 Gangue mineral – Worthless mineral mixed in with the ore mineral(s). E.g. Quartz,
Calcite, Pyrite (iron sulphide) - “fool’s gold”.
 Grade – Percentage/amount of metal in the ore deposit.
 Tonnage – Weight of ore (volume x density), i.e. the size of the deposit.
E.g. 100,000 tonnes of copper ore, grading 3.2% copper.
 Average crustal abundance – Amount of the metal in average continental crust.
 Concentration/enrichment factor – Amount by which the metal in an ore deposit is
concentrated above its average crustal abundance.
 Cut-off grade – Minimum grade required for an economic ore deposit.
Concentration factor = Concentration of metal in ore (or cut-off grade)
Average crustal abundance
E.g.
Metal
Iron
Tin
Lead
Gold
Average Crustal Abundance
5
0.0002
0.001
0.0000004
Cut off grade
25
0.5
4
0.0001
Concentration Factor
5
2500
4000
250
(b)
Describe the concentration of magnetite by gravity-settling in igneous
intrusions.
 Magmatic segregation is the result of gravity settling of magnetite and other dense ore
minerals in fluid basic/ultrabasic igneous intrusions. Magnetite is an early-formed, high
temperature, dense mineral – it sinks to the bottom of the magma chamber to form a
cumulate layer, thus concentrating the ore. Example: Bushveld Igneous Complex, South
Africa.
(c)
Describe the secondary enrichment of chalcopyrite copper deposits.
 The main ore mineral of copper is chalcopyrite. Many primary copper deposits are high
tonnage, but low grade.
 Secondary enrichment is a process by which low grade copper deposits, which may not
be economic, can concentrated by chemical weathering. Groundwater percolating
downwards from the surface leaches soluble copper minerals from the pre-existing veins
in the zone above the water table (oxidising). The copper is carried downwards in solution
and copper minerals such as chalcopyrite and bornite are re-precipitated just below the
water table, where the chemical conditions change (reducing). The ore is concentrated
6
into a smaller volume and the enriched deposit may now be economic. Example:
Secondary enrichment of porphyry copper deposits such as Bingham Canyon, Utah.
(d)
Describe the hydrothermal processes associated with igneous intrusions
forming veins of galena, sphalerite and cassiterite.
 Hydrothermal processes require a source of heat, aqueous fluid and metal ions.
 Hydrothermal fluid – A hot, aqueous fluid capable of transporting significant quantities of
metals in solution.
 Hydrothermal ore deposits may be associated with:
1. Igneous intrusions, e.g. tin, lead and zinc deposits in Cornwall. The source of heat,
water and metal is the igneous intrusion. Associated with acid magmas – rich in water
and volatiles. At the end of crystallisation the water and “incompatible” metal
elements are concentrated at the top of the intrusion. The hydrothermal fluid then
moves out from the intrusion.
 Veins/disseminations of cassiterite, galena and sphalerite are formed. The veins
are formed by ore minerals precipitating in fractures in the host/country rock, as
the fluids cool or as a result of chemical changes. Requires porous and permeable
(+/- chemically reactive) country rock, e.g. limestone, sandstone. Bedding and joint
planes often control the distribution of the mineral veins in the country rocks.
 The mineral deposits often show a zonation with higher temperature minerals (e.g.
cassiterite > 400ºC) close to the heat source and lower temperature minerals (e.g.
galena and sphalerite < 300ºC) further away. They are precipitated in order of
solubility/temperature – least soluble first at highest temperatures.
2. Sedimentary basins, e.g. lead and zinc deposits in the Peak District. The source of
heat is the geothermal gradient, the source of water is the pore fluid in the
sedimentary basin, the source of metal is from the pre-existing sedimentary rocks.
3. Mid ocean ridges – black smokers, e.g. copper and nickel deposits in the Troodos
Mountains ophiolite complex, Cyprus. The source of heat is the igneous activity, the
source of water is seawater, the source of metal is from the pre-existing pillow lavas.
(e)
Describe how residual deposits of bauxite are formed as the insoluble product
of chemical weathering.
 Residual deposits of bauxite form when rocks containing aluminium are subjected to
extreme chemical weathering in hot, tropical/equatorial climates. The source rock is often
limestone or granite. The aluminium minerals are broken down to insoluble aluminium
hydroxides. These can be smelted to obtain the aluminium – this is not possible with the
original minerals. The aluminium ore represents the insoluble residue from the chemical
weathering process. Example: Jamaica.
(f)
Describe the formation of placer deposits of cassiterite and gold in rivers and
beaches and the characteristics of these minerals which make them suitable.
 Placer deposits of cassiterite, gold and other minerals form when grains of the ore
minerals are separated by weathering of pre-existing veins, transported by water or wind
and are then preferentially deposited when the current velocity drops. This may occur on
the inside of meander bends, downstream of confluences, in plunge pools and potholes,
on beaches, etc. The ore minerals must be dense, chemically resistant and usually resist
abrasion (N.B. gold is the exception in this instance). Ancient placers are rare – most are
recent deposits. Usually small operations. Examples: Tin deposits in the Kinta Valley,
Malaysia, Gold deposits in the Witswatersrand palaeo-conglomerate, South Africa.
(g)
Describe the effects of mining and understand the environmental
consequences of mining operations. Understand that metal mining is an example of
unsustainable resource exploitation.
 Environmental consequences of mining operations include pollution (including
contaminated land), noise, dust, creation of spoil heaps, acid mine water drainage,
subsidence.
 Economic consequences of mining operations include employment opportunities, mining
operations will depend on the current price of the metal, mining operations may be short
term, and significant capital investment is required before mining operations can begin.
Many developing countries are heavily dependent on mining operations for generating
foreign income.
7
2.4
Applied Geology
Candidates should be able to:
(a)
Describe the geological factors affecting major construction or engineering
projects: (i) road cuttings and embankments: (ii) tunnels through both hard rock and
unconsolidated material; (iii) waste disposal in quarries; (iv) dams and reservoirs.
Describe methods which can be used to stabilise rocks for these projects: grouting,
gabions, rock drains, rock bolts, retaining wall.
This is a very important aspect of Geology as it impacts on many Human activities including
building and civil/structural engineering, motorway construction, slope stability, tunnelling,
quarrying, mining, land restoration, waste disposal, and dam construction. Please refer to
your handout on Applied/Engineering Geology for more specific detail on each application.
Main Geological Considerations in any construction or engineering project:
 General considerations - Best if aseismic – no likelihood of earthquakes to cause
collapse.
 Rock type - Mechanical strength is very important (high load-bearing strength) –
competent rocks are best. Texture – grain/crystal size, homogeneity, etc. – fine-grained
best. Mineralogy – hardness; chemical stability; calcite and swelling clay minerals are
problematic. Porosity and permeability – problems of leakage (dams); problems of
flooding/lubrication (tunnels, slope stability); pollution of groundwater (waste disposal) –
impermeable best.
 Condition – Weathering (weakens rock). Degree of consolidation/lithification,
compaction/cementation of sedimentary rocks – if variable this is problematic. Attitude –
flat-lying or dipping? Flat-lying best. Dipping – may be unstable, water/fluids can migrate
down dip.
 Structure – Jointing – increases permeability, lines of weakness. Faulting – zones of
weakness, increases permeability, may juxtapose contrasting rock types. Folding –
changes in dip, slippage along bedding planes, etc. Presence of structures are generally
problematic!
 Groundwater - Position of water table – may vary if aquifer. High water table – risk of
flooding in tunnels; easier for pollution (leachate) to reach groundwater in waste disposal,
etc. Water adds weight and lubrication to rocks/slopes – can lead to failure. Limestone –
caves (karst) are problematic, may cause collapse.
 Availability of construction materials – Bulk commodities, expensive to transport, best if
locally available.
 Ground stabilisation/improvement techniques can be used if necessary.
Main Techniques for Ground Stabilisation/Improvement:
 Profile/slope modification – Regrading/recontouring slopes to lower angle – increases
stability (e.g. strong, competent rocks – vertical cuttings are possible; incompetent rocks
such as clay – cutting angle must be less than 10°!). Berm ledges – level steps in slopes
~5m wide, 10m high – increases stability. Lateral toe support (at bottom). Cut and fill – to
reduce slope angle.
 Drainage – Pore fluid pressure is critical in providing weight and lubrication, promoting
slips, etc. Drains, relief wells, pipes and tunnels, etc., can be constructed.
 Support – Concrete retaining walls. Gabion walls and boxes (wire mesh boxes filled with
rocks). Sprayed liquid concrete (shotcrete) – increases strength, reduces permeability,
protects against weathering. Wire meshing – prevents rock falls.
 Rock bolts – Pin loose rock to sound rock behind. Only useful for competent rocks.
 Ground improvement – Vegetation – fixes soil, prevents erosion and slippage. Geotextiles
– artificial materials used to increase stability. Weathering protection – shotcrete.
 Grouting – Holes are drilled into the rock and liquid cement is pumped in to reduce
permeability and increase strength.
 Lining – Clay, or geomembrane (plastic) if hazardous waste, used to reduce permeability.
 Cut off curtain – Specifically used for dams – an impermeable barrier, usually made of
concrete, is constructed below the dam to prevent leakage.
8
Geological factors affecting the construction and siting of dams and reservoirs:
 General conditions
Requires a lack of seismic activity and mass movement – may have landslides into reservoir.
 Catchment considerations
Requires a large catchment with sufficient rainfall; with a lack of sediments in feeder streams
– possibility of silting up of the reservoir; and impermeable rocks to promote surface runoff.
No mineral veins containing toxic elements, e.g. lead, zinc.
 Rock type for foundations
Dams require sound/strong foundations to withstand the stresses from the mass of water
dammed behind them. Dams made of concrete usually require strong/impermeable rock
foundations, which may have to be improved by grouting. Earth fill/embankment dams are
more ‘flexible’ and spread their load over a much greater area. There may need to be a ‘cutoff curtain’ to prevent leakage from the dam – this may be clay or concrete, depending on the
type of dam. Rock should be homogeneous – if sited on two rock types differential
subsidence of the dam may occur. Depth of weathered material is important – weakens rock,
increases permeability. The construction of the dam may cause problems with the river
channel downstream, which now has less water in it. The construction of the reservoir may
cause micro-earthquakes to occur as the crust adjusts to the increased load.
 Attitude/Structures
Land slipping may occur if strata/beds in valley dip towards reservoir. Horizontal strata is
stable, strata dipping upstream is stable, strata dipping downstream is unstable – potential for
slippage and collapse of dam.
Faults are zones of permeability and weakness and old faults may be reactivated by added
pressure/lubrication by water. Joints can cause leaks. Synclines may permit leakage as the
water bypasses dam through permeable strata. Anticlines are unstable as slippage may occur
on the limbs and tension joints may be present on the crest.
 Ground improvement strategies can be employed – Grouting; Clay lining; Cut off curtain –
all designed to reduce permeability, prevent leakage of water.
There have been some notable dam/reservoir ‘failures’ resulting form poor siting of the
reservoir or from poor design of the dam – ‘Classics’ include the Vaiont dam/reservoir in Italy;
Dol-y-Gaer dam in Wales – built on Carboniferous Limestone – it leaked!; The Teton Dam in
Idaho failed because the clay core of the embankment dam was eroded away by seepage.
(b)
Describe the geological factors that cause landslips and slumping hazards.
 Geomorphological hazards are usually related to mass movement of material under the
force of gravity. They are seldom attributable to one cause – a series of events lead to
failure.
 The main geological factors involved in slope processes and failure are: rock type
(strength, porosity and permeability); presence of structures; slope angle; and the
presence of water (rainfall, climate). Addition of water is often a key component in slope
failure – adds weight and lubricates the rocks – failures often happen after heavy rain or
snow melt.
 Human contributions include: adding weight (building at top of slope); poor drainage –
leaking water mains, sewage disposal and cess pits; removal of material at base of slope,
e.g. cuttings for roads; removal of natural vegetation that intercepts water and binds soil
together; and the creation of impermeable surfaces that promote surface runoff.
(c)
Describe how geophysical exploration techniques are used for finding
hydrocarbons, coal and metals: seismic reflection and refraction, gravity surveys,
magnetic survey using proton magnetometer, electrical resistivity and down hole
logging surveys.
 Seismic survey – Both seismic reflection and seismic refraction techniques are used.
Seismic reflection is the most widely used. An artificial source (explosion/compressed air
gun/vibrations) generates seismic waves that are transmitted through the ground and the
reflected/refracted waves are detected by geophones/hydrophones. The technique is very
effective for showing subsurface layering and finding traps. Used for oil exploration.
 Gravity survey – Gravimetric surveys are useful for detecting areas of unusually high (e.g.
metallic minerals) or low density (e.g. salt domes) rocks. A gravimeter is used and the
data is plotted on gravity anomaly maps. Used for both oil and metal exploration.
9




Magnetic survey – Useful for detecting metal deposits which are rich in magnetic
minerals. A proton magnetometer is used to detect variations/anomalies in the Earth’s
magnetic field. Data can be plotted on magnetic anomaly maps
Resistivity – Used to measure the electrical resistance of the rocks. Two electrodes are
placed in the ground and an electric current passed between them. Can show the
presence of conductors (metals, water). Used for metal exploration, but also in down-hole
logging.
Gamma Radiation Spectrometry – Used in metal, particularly uranium, exploration.
Wire-line/down-hole logging techniques – Instruments are mounted on a sonde which is
passed down the drill hole – measures resistivity, magnetic susceptibility, gamma ray
count, pH, etc. – used to characterise and identify rock types in oil exploration.
(d)
Describe how the geochemical exploration methods: soil sampling and stream
sampling are used for metal exploration.
Geochemical exploration methods include stream sampling and soil sampling. Samples are
analysed to detect anomalous concentrations of metal ions. Used exclusively for metallic
mineral exploration.
 Geochemical anomaly – A concentration of a metal above its normal background value.
 Stream sediment sampling – Used for regional surveys. Sediments from rivers are
collected and analysed to allow anomalies/targets to be identified and located. Stream
sediments downstream of the target will have anomalous concentrations. Stream
sediments upstream will have background levels. The size of the anomaly will decrease
downstream of the target. Where tributaries meet catastrophic dilution will occur. The best
sampling strategy is to sample each tributary immediately upstream of each confluence.
 Soil sampling – Used for local surveys, once a target area has been identified. Soil
samples down slope of the target will have anomalous concentrations. Soil samples up
slope will have background levels.
© D. Armstrong, 2004.
10