Geohazards Flashcards

1
Q

geohazard def

A

a geological condition that is dangerous/ potentially to enviro or people who live within it

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2
Q

examples of geohazards

A

earthquakes
vol eruptions
tsunamis
landslides
subsidence
avalanche
cliff fall

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3
Q

what is the focus and epicentre of an earthquake

A

focus-where movement occurred

point within the earth at which the earthquake og as movement occurs along fault plane

sesmic waves radiate away from it in all directions

epicentre point on surface above focus

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4
Q

how is stored elastic strain energy released?

A

stress = forces
strain = reaction

relative movement of rock on either side of fault applies stress

rock undergoes strain

this process transfers energy and increases elastic strain stored in rock

energy releases and elastic energy decreases

energy released due to movement

released as heat and seismic waves (greater amp greater energy)

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5
Q

amplitude def

A

maximum extent of displacement of an oscillation from the position of rest

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6
Q

what does amplitude show

A

greater the amp = greater energy released

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7
Q

what happens to amp as waves move away from focus

A

reduces as energy released and transferred to surrounding rock

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8
Q

attenuation of seismic waves

A

loss of energy experienced by a wave shown as reduction in amp as it propagates through material

waves faster through rigid as transfer is easier so attenuation is reduced in more rigid rocks

amp decreases with distance from focus

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9
Q

Mercalli scale

A

magnitude scale for measuring earthquakes

empirical

gives 12 categories earthquake can fall into based on destruction and how people felt it
instrumental to cataclysmic

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10
Q

Mercalli scale pros and cons

A

pros-
don’t need specific equation
don’t need formal training to understand

cons-
empirical (based on observation/experience )

subjective

not comparable - e.g. all observation relate to building but they are diff in each country

may not remember experience

intensity decreases from focus but mag doesn’t

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11
Q

different earthquake measurement systems in time order

earliest to oldest

A

Mercalli- Richter- moment mag

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12
Q

Richter scale

A

measures energy output of the earthquake

1-10

use seismogram:
lag time - time diff between P and S waves tells us distance from focus
height of greatest S wave

plot on richter scale graph tells mag

should give same value no matter where

log so 1= 10x amp

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13
Q

Pros and cons of Richter scale

A

Pros:
Maths equ so less subjective
but also has desc for public

Cons:
assume same rock type

larger is underrepresented and less accurate the larger it gets (seismologists only measured at certain frequencies)

outdated

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14
Q

moment magnitude scale

A

What is used now

when waves arrive at seismometer measure:
lag time - tells us distance from focus
amplitude and waves
actual displacement of rock at quake site (new, not in richter)

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15
Q

Pros of moment mag

A

interpret broader range of seismic frequencies- determines more accurate e from large earthquake

accounts for rock type and rigidity

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16
Q

moment mag equ
(given to us)

A

Mw= 2/3logE-6.1

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17
Q

different wave types

A

P- primary
S- secondary
L-love
R-rayleigh

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18
Q

features of P waves and effect on buildings

SEE BOOKLET FOR DIAGRAM

A

features-
travel through L and S

body waves

arrive first- fastest

longitudinal

effect on building-
least dangerous

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19
Q

features of S waves and effect on buildings

SEE BOOKLET FOR DIAGRAM

A

features-
can travel through S

body waves

arrive second- second fastest

shear waves - transverse

effect on buildings-
S waves more destructive than P
as greater amp

horizontal worse than vertical

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20
Q

features of L waves and effect on buildings

SEE BOOKLET FOR DIAGRAM

A

Features-
Surface waves do the most damage

shear wave where shearing motion is confined to horizontal plane at earths surface (oscillate)

effect on buildings-
responsible for most damage

greatest effect- sideways movement is more effective

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21
Q

features of R waves and effect on buildings

SEE BOOKLET FOR DIAGRAM

A

features- surface waves are most destructive

slowest

earths surface moves in a vertical elliptical path parallel to wave movement

effect on building-
worse than P and S

most movement with R- up and down so less destructive

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22
Q

effect on ground movement on built environment

A

L waves cause most damage to buildings

lose objects thrown upwards

buildings fracture
pancake

freeways collapse

brick and stonework seperate along motar

sects of utility pipes separate

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23
Q

how does rock competency affect how e is transferred and effect on buildings

A

comp rocks allow easier propergation of seismic waves and move on

less e dissipated to rock

less movement

also moves quicker through comp rock

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24
Q

liquefaction and effect on buildings

A

saturated or partially saturated unconsolidated material losing strength and rigidity in response to applied stress

e.g. Earthquake

unconsolidated and incompetent rock holds more H2O in pore spaces

when rock compressed due to seismic waves, pore spaces reduce and H2O comes up and out

loss of strength makes buildings tip and sink

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25
where are developments banned
soft unconsol rock or incomp on fault zone nearby fault zones
26
earthquake engineering ideas
reenforce concrete with steel base isolation tuned mass damper- giant pendulum near top of building brings back to centre by swing in opp direction (Taipei 101) Crossbracing- X shaped brackets transfer seismic forces back to ground reducing lateral movement seismic damper- absorb energy emergency shut off switch rubber/flexi pipes metal plates around building- easier to move through than building as more rigid reduces impact on building)
27
Taipei 101 case study
tuned mass damper- giant pendulum near top of building brings back to centre by swing in opp direction used for earthquakes and hurricanes
28
what building design is used to reduce vertical + horizontal stress?
height irregular + assym design is more susceptible to shaking in sym buildings stress is dissipated equally avoid ornamentation think about materials- wood = more flexible = absorbs stress brick more likely to fracture reinforced/ deeper foundations
29
what are ground/ base isolation systems? + purpose?
structure is separate from base so moves independently of foundations protects against seismic activity e.g, rollers, rubber pads, springs, lead rubber bearings and sliders Buildings can be retrofitted
30
what building designs are used to resist shear forces?
encase building in steel framework using diagonal steel cables reduces movement in building very ridged so doesn't save building but does stop pancaking and increases time for evacuation
31
what building designs are used to absorb sway?
use mass dampers use hydraulic systems e.g. shock absorber in car but on larger scale flexible connections between pipes / parts of buildings controlled rocking systems - allow to rock and be pulled back to centre after
32
how do we protect building services e.g. water pipes ?
gas pipes = main concern as cause fires also worried about - water pipes, elec wiring ect use flexible pipes or joints to prevent fracturing
33
how does the natural frequency of buildings effect them in earthquakes? and how do we change it?
all structures vibrate at own seismic frequency is similar to the frequency of the ground there will be more / amplified movement so we should insure they are different to reduce movement Natural frequency can be found/ changed using stiffness and mass greater stiffness = higher frequency greater mass= lower frequency
34
seismic risk def
possibility of death, injury or damage that may occur within a certain period of time IT CAN CHANGE
35
what impacts seismic risk?
history of activity frequency of seismic activity nature of hazard areas geology earthquake resistant buildings- lower risk no of people/ buildings type of building
36
what do national governments prioritise when mitigating seismic risk?
maintain H20, electric, communications e.g. defence and public services ect
37
what do local governments prioritise when mitigating seismic risk?
protecting public buildings, emergency planning and local transport
38
what do businesses prioritise when mitigating seismic risk?
protecting building to avoid loss of equipment and production
39
what do householders prioritise when mitigating seismic risk?
pets, house, people e.g. family
40
what is a seismic risk assessment?
assessment of probable max loss e.g. extent of damage to buildings in terms of finances more linked to finance than people
41
What may we see in a seismic risk assessment FILL IN USING TEXT BOOK
PML building an sites stability hazard map rock types and geological features of area + groundwater historical data
42
probability definition
how likely it is something will happen can be fraction, decimal or percentage 0-1 won't to will
43
return period definition
average length of time for an earthquake of a certain mag to occur again or be exceeded (sometimes called reoccurrence period)
44
return period calculation NOT GIVEN IN EXAM
T = (n+1)/m T= return period n= no of years on record m=number of recorded earthquakes
45
Case study : earthquake dills in Japanese schools
children get under desks- head first and hold legs of desk wear padded head covers lead out of building by teachers in drills they sometimes use earthquake seismicity device ,makes special room shake
46
Case study: San Francisco resilience program
plan to make San Francisco more resilient to immediate and long term climate change and natural hazards split into adaptation and mitigation adaptation include better fire safety as well as planning for rising oceans mitigation includes sustainable travel and renewable energy
47
Forecast definition and examples
calculations and data used to make a statement of probable occurrence of an event e.g. return period calculation forecast high probability of an earthquake on a plate boundary
48
prediction definition and examples
a statement about what you think will happen in the future- normally based on experience e.g. predicting your grades predict when, where and how
49
What data is used for forecasting?
historical data - produces hazard maps that can be used to forecast likelihood of earthquake in area/ where is most at risk also use probability calculations and monitor seismic activity to do this earthquake forecasting is a probabilistic assessment of hazard- using frequency and magnitude of damage we can also create Venn diagram using area affected and area liable to be effected - more overlap more certain to occur
50
how do we make predictions? and why aren't they certain?
ask useful questions: what is worst case where is nearest active fault largest earthquake that could occur there estimate return period assume epicentre closest to site calc potential ground movement model max + use for engineering Not currently possible to make deterministic prediction of when/ where/ how strong earthquake will be can't identify- seismic, biological, physical or chem changes which would indicate earthquake will happen only say what will happen if it does occur
51
advantages of predictions
lives saved - people move out of dangerous areas makes people think about safety measures e.g. escape routes
52
disadvantages of predictions
panic + mass movement of people leads to injury may not happen - people stop listening sometimes the time period is so short it may be counter productive e.g. imminent
53
what is the maximum considered earthquake? + its use?
earthquake that is expected to occur in given area once every 2500yrs (2% prob every 50yrs) not necessary largest possible - insure building code is accurate
54
what is a building code?
set of rules that specify standards for construction of objects e.g. buildings (can also include non buildings e.g. bridges) different codes in diff areas
55
examples of building codes in earthquake areas
buildings must have steel reinforcements buildings with more occupancy designed to withstand more seismic activity non- structural components e.g. non load bearing walls must also be designed to withstand earthquake old buildings must be retrofitted
56
role of a geologist in seismically active area
provide facts, produce hazard maps and risk assessments decide/ advise if public warning is necessary
57
steps necessary for old buildings in seismically active areas
retrofitted with modern protection e.g. ground base isolation systems plan evacuation routes to insure they don't pass old buildings
58
what science is used to make predictions
physical properties stress animal behaviour radon emissions seismic gap model
59
how are physical properties used to predict earthquakes
large earthquakes are proceeded by smaller ones increased frequency before event- displace strain energy P wave velocity decreases then increases before as rock expands slightly close to fracture point coloured lights in the sky- changes in electron properties
60
how is stress used to predict earthquakes
many minute cracks develop under stress -> volume increases, H20 and gas peculates in so increases elec conductivity of ground resistivity decreases as P wave vel decreases then increases ground may tilt due to deformation + swell slightly H20 percolates into cracks lower water levels in wells
61
how are animal behaviours used to predict earthquakes
animals shows distressed behaviour just before earthquakes e.g. pig squeal may feel waves before higher amplitude s waves or slight change in magnetic field
62
how are Radon emissions used to predict earthquakes
Radon = decay product of uranium easy to detect + short half life so it is sensitive to short term flux it is a heavy gas an accumulates in H2O wells increased radon suggests microcracks that allow it to escape so earthquake is imminent
63
how are seismic gap models used to predict earthquakes- more used for forecasting
2 plates slip past each other every section of fault slips but not at same time different section are in different stages of cycle next big earthquake is expected in gap between recent slipped sections as strain has built
64
what is a tsunami? + how do they work?
large wave caused by the displacement of water, normally due to shallow focus earthquakes (submarine) as the approach land and slow the amplitude increases (especially is in narrow area)
65
what can trigger tsunamis
shallow focus submarine earthquakes release of glacial lake landslide submarine volc eruption meteorite impact
66
Early warning systems for tsunamis?
used to save lives + protect: phone + television alerts siren/radio/loudspeaker signs showing evac routes educate about signs prohibit building on coast + maintain coral reefs + costal trees --> reduce wave + impact seismologist monitor- earthquakes focus and mag + type of crustal movement, tsunamis associated with high mag - evac
67
tsunami case studies
2011 - Tohoku 20000 deaths 9-9.1 mag earthquake several waves over 12 hr period uk: Storegga slide- caused tsunami off coast Lisbon earthquake- caused tsunami
68
What is clay? + examples
very fine grained sedimentary rock made up of flat platy particle- malleable and plastic e.g. Kaolinite, montmorillonite, vermiculite and illite
69
how does clay form?
from the chemical weathering of carbonate minerals e.g. quartz and feldspar mostly feldspar (exposed to air and H20)
70
why is Britain covered in clay
clay deposits post glacial retreat
71
kaolinite information
non-expanding and low shrink swell capacity 1:1 used in ceramic and porcelain used for glossy high finish paper
72
montmorillonite information
expanding structure and high shrink swell capacity 2:1 used for: protective linings catalytic processes facial powder cat litter
73
vermiculite information
limited expansion and medium shrink swell 2:1 used for: when hot expands to low density medium gardening cat litter
74
Illite information
non expanding and low shrink swell capacity 2:1 uses: ceramics fillers
75
1:1 clays chemistry
1:1 clays are made up of alternating Al octahedra and Si tetrahedra sheets these are joined by Hydrogen bonding = very strong this prevents hydration and therefor shrink swell
76
2:1 clays chemistry
2:1 clays are made up of layers each layer is made of 2 silica tetrahedra surrounding 1 Al octahedra (like a sandwich) these are joined by van der walls forces allowing hydration and shrink swell between the layers e.g. smectite
77
silica tetrahedra in clays
many silica tetrahedra join together to partially satisfy charge via bonding oxygens and form a sheet the left over change is used to bond to the next sheet by bonding to charged ions between sheets shown as a trapezium
78
Aluminium octahedra in clays
many octahedra share 2 o atomso partially satisfy charge via bonding oxygens and form a sheet the left over change is used to bond to the next sheet by bonding to charged ions between sheets shown as a rectangle
79
problems with building on clay
shrink swell- likely to be vertical movement which effects foundations leads to subsidence (shrinking, downward) then hardens and crack pipes swells and rises causing tilt doesn't necessarily be pure clay can be high clay soil
80
How can engineers mitigate problems with building on clay
understand water table fluctuation - monitor + maintain WT by pumping out or in H2O - useful drainage system build super deep foundations - pass through clay (pile foundations) add/ remove trees - add/ remove H20 reinforced raft foundations (concrete slabs + steel mesh)--> larger than building --> spreads weight and 'floats'
81
How can we change the properties of clay
smectite 2:1 can shrink swell 1500%/15x vol often contains Na+ forms ionic bonds that hold sheets together add lime --> Ca+ ion --> displaces Na+ --> only shrink swells 100%/2x vol OR add bacteria which changes Fe3+ to Fe2+ --> reduces shrink swell
82
subsidence def
vertical downward movement of ground ( and building) due to the shrinking of soil, clay or clay rich soil
83
why does subsidence occur?
lots of clay - shrink swell mineshafts below damaged drain-> water escapes -> softens ground non cohesive soil e.g. sand = washed away soil decomposition e.g. plant matter trees - add/remove H2O
84
how does longwall mining cause subsidence
traditional+ current coal mining method move forward allowing material behind to collapse into cavity - subsidence not all the subsidence occurs at once 10,50,100 yrs later 10m of mining doesn't mean 10m subsidence necessary fault and fractures in rock weaken it, can happen yrs later
85
how does shallow mining cause subsidence
old bell pits + pillar and stall --> unstable voids underground no records of old mines = surprise collapses
86
how does deep mining cause subsidence
bigger operation -> bigger voids--> large amounts of subsidence however better recorded so less surprise
87
how does salt mining cause subsidence
drill and pump in hot H2O -> leaves large caverns e.g. under Cheshire often pump water down to hold them up but if water escapes pressure drops leading to subsidence
88
how does crown holes cause subsidence
anthropogenic- mad made circular depression --> causes vertical downward movement often due to mines
89
how does sink holes cause subsidence
natural circular depressions that cause vertical downward movement
90
common causes of subsidence
mining - deep, shallow and salt crown and sink holes clay shrink swell
91
how can we avoid subsidence
avoid karst terrain- limestone (dissolves) reinforced raft formations - isolate foundations from movement monitor subsidence using ground surveys or satellite radar (expensive) can use concrete pining
92
how can subsidence be corrected (FILL IN WITH TEXTBOOK)
93
what are the causes of mass movement basics?
increasing mass - H2O or building increased slope angle-due to e.g. river erosion, building roads or tectonic uplift decreasing friction- lubrication with H2O
94
which types of dipping beds are best and worst for causing mass movement
dipping in direction of slop is most likely to cause mass movement - worst horizontal beds are okay dipping away from slope = best
95
what kind of mass movement do competent rocks undergo?
steep slope--> so transitional slides --> all moves at once breaking up into scree (talus)
96
what kind of mass movement do incompetent rocks undergo?
shallow slopes --> low shear strength --> rotational slide
97
types of mass movement
flow - H2O major component mudslide- H2O and clay soil creep - very slow landslide- slow mud/debris flow- medium rock fall- fast
98
how are mass movements classified
velocity and type of material e.g. soil creep - very slow rock fall- fast flow - H2O major component
99
what triggers mass movement
addition of H2O --> heavy rain earthquakes building on slope- adds mass deforestation sudden drop in H2O table
100
how does deforestation tigger mass movement
deforestation- increases H20 flow and decreases root binding
101
how does changes in water table tigger mass movement
sudden drop in H2O table- change pore pressure in rocks --> cause break up of rock
102
how does addition of H2O trigger mass movement
Adds mass and increases lubrication increasing friction
103
How does bullion on slopes trigger mass movement
Adds mass and can also increase slop angle
104
How do we stabilize material to prevent mass movements
slope modification - decrease angle build retaining wall of concrete use gabbons or mash wire - steel cages of rock use rock bolts - drill into rock and cement rock drains- drain H2O decreasing mass and lubrication shotcrete- sprayed at high pressure and reinforces add plants - root binding rocks in place
105
Mass movement case studies
blubberhouses landslide - used mesh wire and rock bolts to stabilize originally caused by heavy rain youngay landslide- peru killed 20,000 people triggered by ancash earthquake
106
How is rock strength defined
ability to resist stress without large scale failure
107
what does rock strength depend on?
rock type- competence composition- e.g. quartz = hard texture and Fabric e.g. crystalline or clastic foliation crystal/grain shape cracks or fractures (diff section of small rock can have diff strength)
108
stress definition
force per unit area acting on rock shear, compressive or tensional
109
Fabric definition
spacial and geometric configuration of all the components in a rock e.g crystal and void shapes,sizes and orientations also know as texture
110
strain definition
Response of a system to applied stress when material is loaded with stress it produces strain e.g. deformation
111
what is rock stress measured in
Mpa or PA
112
what is ductile deformation + where is it seen
malleable- changes shape occurs at deep depths and high temps when rock suffers large strain without large scale fracturing
113
what is brittle deformation + where is it seen
shatters near surface and cold when stress causes rock to fracture (possibly some elastic deformation first)
114
what does UCS stand for
uniaxial compressive strength
115
when is UCS used
before construction to design rock crushers in mining and determine strength of concrete
116
UCS test method
1) sample of rock extracted 2)sample prepped- flat,smooth and cylindrical 3)sample placed in test machine 4)gradual compressive load is applied 5)load increased till sample fails 6)max compressive load is recorded as UCS
117
why is the UCS test class as unconfined
No where for stress to be transferred from rock
118
confining pressure def
combined lithostatic and hydrostatic pressure (at depth all principal stress is =)
119
how does confining pressure change way rock behaves under pressure
would be able to transfer stress to surrounding rock so would have higher USC
120
how does mineral composition effect overall strength of rock
Different mineral have different strengths for example quartz is very strong as it has a hardness of 7 whereas micas only have a hardness of 3 and have a strong cleavage that makes them weak
121
how does cementation effect overall rock strength
rocks with the same composition can have different cementations that make them stronger or weaker for example sandstone is weaker than metaquartzite as it inly has a weak cement whereas metaquartzite has interlocking crystals
122
lithostatic pressure def
vertical pressure due to mass of overlying rock only also called overburden pressure
123
density definition and units
mass found in a set volume of a material gcm-3 or kgm-3
124
density calculation
density = mass/volume p=m/v
125
how does rock density change with depth
increased depth, increased pressure --> particles more tightly packed = more dense
126
why is it important to know the lithostatic pressure
if high it can be dangerous and lead to rock burst (rock explodes inward) at depth important for mining and tunnelling at depth (below 1km) it = hydrostatic pressure
127
lithostatic pressure calculation
p x g x h p=density g= acceleration due to gravity h=depth units: likely kgm-3
128
what are the disadvantages with testing rock strength in a lab
unconfined - not in situ may be stronger when it can transfer stress rock sample- large vol of rock may not be homogenous so may not be representative doesn't account for temp increase with depth also doesn't account for faults or fractures in other parts of rock
129
homogenous def
same/ similar nature throughout
130
how are rocks not always homogenous
magmatic differentiation different composition = different hardness
131
where is weakness present in rock
bedding planes fractures joints discontinuities foliation lamination
132
what happens when stress is applied to a discontinuity
rock is liable to fracture as weakest here stress builds at discontinuity, then will reach peak strength and fail causing movement and strain --> then returns to residual strength
133
asperity def
roughness of the surface of a discontinuity
134
how does asperity effect rock
as asperity decreases rock is more likley to fail/ easier for it to fail
135
residual strength def
remaining resistance to movement after rock has failed and been displaced
136
joint def
fracture in a competent rock where there has been no observable displacement
137
joint sets def
see multiple joints that often look sub-parallel (but can cross) formed as a result of regional stress e.g. folding
138
how do joints react to tensional stress
pull apart producing angular discontinuity which may resist shear stress
139
how do joints react to shear joints
move past horizontally smoother and less likely to resist stress
140
how to jointed rocks become stronger again
compressive forces=close fractures/voids intrusion into void secondary precipitation of mineral into voids narrow and tight joints = stronger fewer joints are also stronger
141
how does water in joints decrease rock strength
freezes - frost shattering = accelerate weathering + widen joint hydrolysis and carbonation weather rock
142
unloading joints def
rock is compressed by overlying mass--> mass removed--> competent rock expands in direction of pressure release --> causes fractures perpendicular to direction of release
143
why are unloading joints problematic
form dangerous joint sets which are unpredictable and need grouting to fill in spaces/voids
144
how do faults reduce rock strength
rock is ground producing fault gouge (incompetent and contains clay) which may shrink swell and reduces rock strength especially is saturated
145
bedding plane def
mark in time where dep temporarily ceased usually between rock types
146
why are bedding planes points of weakness
clay rich material settles out --> weak can lead to unexpected failure sudden change in permeability = water percolates down and accumulates
147
Malpassat dam case study
built on gneiss with foliation -> 30-50 degree angle a huge block of the rock was lifted and released all the water presence of fault and force of water compressed the gneiss which increased pressure which dislodged block leading to failure
148
Tebay case study
lots of folding + joint sets + changes in rock type + bedding planes used extensive rock bolting to hold contorted cutting sides together rock bolting = drilling steel rods into rock
149
stages of geological site investigation
Desk study site surface mapping geophysical surveys site subsurface mapping Rock and soil property measurements geohazard mapping integration of data
150
desk study
BGS mapping of UK show drift, bedrock and structures - available to consult + previous borehole data is available-shows geology changing with depth aerial and satellite photos look how water table changes
151
Site surface mapping
large scale + cleared sites all changes in earth material are mapped not just about rock type and weathering but also subsoil e.g. may contain clay also maps structures e.g. faults + frequency + openness + other discontinuity
152
geophysical surveys on site
small sites - include ground penetrating radar to detect near surface anomalies larger areas- use resistivity survey seismic surveys required for deeper changes simple data e.g. depth of weathering lager use seismic refraction surveys
153
site subsurface mapping
test pits dug to sample all critical areas cheaper to excavate a shear sided pit large enough to climb in to extract samples some soil mech properties can be tested in site --> deeper than 1m may collapse (health and safety) cored metals drilled in samples
154
rock and soil property measurements
samples of rocks and soil analysed for strength and composition soils: simple penetration test --> test strength rocks: carry out compressive and shear strength testing --> established how rocks react test very important on weathered rock permeability and porosity assesses to see effect on strength
155
Geohazard mapping
gradient mapping --> slope stability and how this would effect work and what would need to be done to stabilise use contour maps and aerial photos and ground surveys ---> tell-tale signs of landslides heavy duty programs do calculations ground surveys necessary if can be affected alert of hazard
156
integration of data
analysis of all data GIS enabled to look at all information needed to plan the work in map form with data represented as a layer
157
stress equation
stress= amount of deformation in direction of applied force ÷ initial length/vol/shape