Economic and engineering geology

Question 1

tunnelling techniques in crystalline igneous and metamorphic rocks (hard rock)
and pros/cons

Answer 1

drilling and blasting

cons-
slow + expensive

at depth there is high confining pressure –> increased risk of rock bursts

pros-
doesn’t often require support

Question 2

tunnelling techniques in soft rocks e.g. chalk, limestone and sandstone
and pros/cons

Answer 2

ideal
pros- cheap and easy to tunnel –> could tunnel 30m in a day

cons- require lining with concrete or steel ribs

Question 3

tunnelling techniques in weak rocks e.g. shale, clay or unconsolidated material
and pros/cons

Answer 3

No pros listed

cons- prone to collapse –> require most support e.g. shotcrete/steel ribs

prone to H20 leaks –> require dewatering

Question 4

best and worst material for tunnelling

Answer 4

best- soft rock
crystalline igneous and metamorphic rocks (hard rock)
worst- weak rock

Question 5

why do changes in rock type cause problems when tunneling

Answer 5

require different equipment and techniques

every bedding plane = site of weakness

permeability differences= different dewatering techniques

some areas require support some don’t

Question 6

types of support

Answer 6

shotcrete

steel ribs

grouting

steel bolts

Question 7

are dipped or horizontal beds better for tunnel building

Answer 7

horizontal

Question 8

why are dipped beds dangerous/worse in tunnels compared to horizontal

Answer 8

more changes in rock types- require diff equip and techniques and support
whereas horizontal = all same

dipped beds are subject to slippage, landslides, collapse especially is permeable/ h20 present

easier to swap beds based on what is easier/ better to tunnel in horizontal beds

Question 9

what do we considerer before tunnelling

Answer 9

rock type

changes in rock type

dipped or horizontal bed

geological structures e.g. faults, joints, bedding planes and folds

Question 10

how do faults pose problems for tunnelling

Answer 10

zone of weakness- fault breccia + fault gouge clay

zones of permeability- flood tunnel

different rock types either side of fault

earthquake/ slippage may cause tunnel to collapse

Question 11

how do joints pose problems for tunnelling

Answer 11

zone of weakness + permeability - flooding and slippage

often more closely spaced than faults–> loose blocks of rock between joints
–> may fall from tunnel roof

Question 12

how do bedding planes pose problems for tunnelling

Answer 12

bedding planes in sed rocks + foliation in met rocks

planes of weakness may allow slippage or leakage of H20

Question 13

how do folds pose problems for tunnelling

Answer 13

folded rock sequence may have changed angles of dip + slippage may occur on fold limbs

gentle syncline
–>tunnel can follow dip of fold and stay in 1 bed

Question 14

how are tunnels below H20 table effected by water table

Answer 14

will become flooded and require dewatering / depumping

increased hydrostatic pressure

lubricate bedding planes/joints causes slippages

may flow along joints in limestone, through unconsolidated material

or sandstone developing high pore fluid pressure

clays shrink swell

Question 15

how is spoil from tunnelling recycled/ reused

Answer 15

millions of tons extracted from tunnels

sometimes used to build embankments
somewhere else e.g. Crossrail made new nature reserve on Essex coast

chalk from channel tunnel landscaped bottom of Shakespeare cliff

Question 16

how to prevent flooding in tunnels

Answer 16

grout surrounding rocks
use rock drainage
pump water out

Question 17

how to prevent tunnel collapse

Answer 17

line with concrete segments e.g. shotcrete or steel ribs

using rock bolts to secure lose

Question 18

channel tunnel facts

Answer 18

50.5 km long

through chalk marl- gentle syncline followed same level

took longer on French side as more faults and folds

conveyers carries material out

lined with concrete and cement grouted

many water inflows on French side

Question 19

Seiken tunnel

Answer 19

volcanic rock and sedimentary rock in a syncline

intrusions and faults caused many issues

used boreholes to analyse

originally used tunnel boring machine but switched to dynamite blasting

had to pump out lots of H20 and had cave ins

steel and concrete reinforcements

Question 20

general good conditions for dams

Answer 20

impermeable rock

valley (steep)- only have to build 1 side

geologically stable - no seismic activity + faulting + folds

stable valley sides

river with low sed load- prevent build up

no toxic metals if used for drinking or agriculture

Question 21

underlying rock type suitable for dam

Answer 21

competent + strong rock - high load bearing strength e.g. interlocking crystals and impermeable I and M rock + well cemented s rock

avoid clay and shale

impermeable- prevent leakage + no joints prevent leakage

rock types- uniform so no differential leading to unstable

old mines- collapse and prevent leakage

Question 22

what strata is good/ bad for dams

Answer 22

horizontal and dipping upstream = good

dipping downstream = bad
because leakage and slippage along bedding plane–> collapse dam

Question 23

should you build a dam on an anticline or syncline

Answer 23

syncline = good
because stable
but H20 can pass under dam through permeable layers (cut off curtain)

anticline = bad
because slippage can occur along bedding planes on fold limbs and tension joints on crest = leakage

faults + joints = permeable
reopening of old faults increase sesmic activity

Question 24

Type of dams

Answer 24

Arch dam

gravity dam

arch-gravity

embankment or earth dam

Question 25

Arch dam

Answer 25

curves upstream - pushes the hydrostatic pressure from the H20 and dam to the valley side--> strengthens it thinner than other types--> less construction suitable for narrow gorges with steep sides

Question 26

Gravity dam

Answer 26

Held in place by gravity due to immense mass of concrete best type for impermeable and high load bearing sometimes they are hollow as it is more economical may be supported by inflexible buttresses downstream

Question 27

arch-gravity dam

Answer 27

combines strength of arch and force of gravity basically thick arch dam doesn't have to be massive good for high flow low material for building- economic

Question 28

embankment or earth dam

Answer 28

sort of like a mound have an impermeable clay or concrete held in place by piles of rock, earth, sand and clay with an impervious covering the material binds itself together by friction-cement unnecessary built in broad shallow valleys- mass of dam spread over large area so foundations don't need to be a strong require very large quantities of filler material

Question 29

what are Ground improvement methods for dams?

Answer 29

grouting- drill holes unto rock + fill with liquid cement fills joints and porespaces ect. reduce permeability + increase strength clay/plastic lining- prior to filling line with impermeable material prevent leakage cut of curtain- impermeable barrier (extension below dam) e.g. concrete prevents leakage on synclines + slippage + strengthens foundations

Question 30

environmental impacts of dam

Answer 30

Habitat loss- upstream- lose grassland downstream- lose fresh water creates microclimate- High Specific heat capacity- absorbs heat - prevents extreme temps nutrient + mineral block- nutrients settle out a dam - less downstream migration blocker- aquatic organisms and people in boats increased evap and precip- effect water cycle build up of pollution-e.g. heavy metals or pathogens - leak into ground H20 resivour induced sesmicity

Question 31

Social impacts of dams

Answer 31

increased energy security- cheaper displacement of people recreational use microclimate increased seismic activity- damage to buildings possible increased water security better flood control

Question 32

What is reservoir induced sesmicity?

Answer 32

earthquakes/sesmic activity caused by the overlying mass + pressure of the dam and reservoir on underlying rock

Question 33

Examples of suspected resiviour induced seismic activity

Answer 33

Hoover dam- 600 earthquakes in 10 years tibet- 3 gorges dam - 6.8mag oroville-6.1 mag Ulley,Rotheram 2007- Cracks appeared in the dam - emergency services pumped out millions of gallons to ease pressure

Question 34

What indicates reservoir induced sesmicity?

Answer 34

tremors while/ soon after filling when previously not sesmically active

Question 35

why is it difficult to link dam construction and seismicity

Answer 35

little reasearch funded historic sesmic records = inadequate each dam has different geological characteristics = not comparable some earthquakes occur years after construction- can't prove due to dam

Question 36

What recommendations are in place to reduce RIS

Answer 36

More research + monitoring necessary- in view of disastrous consequences model changes in pore pressure + rock mechanics build no new high dams- until research complete design to account for RIS monitor seismic activity as reservoir filled- stop if increases possible seismic activity could occur in previously void of activity - buildings not prepped

Question 37

HEP case study

Answer 37

Hydroelectric power Chinas 3 gorges dam + largest HEP dam contain 32 generators H20 turns turbines which turns generator producing energy

Question 38

Causes of land contamination

Answer 38

extraction of geological resources- Acid mine drainage heavy industry accidental spills or naturally occurring pollution salt water encroachment oil and gas extraction- fracking- fracking fluid toxic can contaminate pores in rock spaces leachate from spoil heaps agriculture-organic waste + hazardous chem + inorganic waste disposal of domestic + commercial --> leachate brown field sites contain pollutants

Question 39

why is bioavailability of a pollutant important than amount

Answer 39

bioavailability- ability of toxins to enter living things potential for bioaccumulation if not taken up then not an issue

Question 40

explain how metal ions become mobile

Answer 40

when they enter solution- more easy in acidic conditions changes in H2O make more mobile + exposed to 02 become oxidised and go into solution

Question 41

what causes resivoir induced sesmicity

Answer 41

- due to H20 + mass of dam creating stress in rocks and increased hydrostatic pressure --> H20 infiltrates pores and fissures increasing pore pressure (leads to fractures) + lubrication --> slippage