Geohazards Flashcards
geohazard def
a geological condition that is dangerous/ potentially to enviro or people who live within it
examples of geohazards
earthquakes
vol eruptions
tsunamis
landslides
subsidence
avalanche
cliff fall
what is the focus and epicentre of an earthquake
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
how is stored elastic strain energy released?
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)
amplitude def
maximum extent of displacement of an oscillation from the position of rest
what does amplitude show
greater the amp = greater energy released
what happens to amp as waves move away from focus
reduces as energy released and transferred to surrounding rock
attenuation of seismic waves
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
Mercalli scale
magnitude scale for measuring earthquakes
empirical
gives 12 categories earthquake can fall into based on destruction and how people felt it
instrumental to cataclysmic
Mercalli scale pros and cons
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
different earthquake measurement systems in time order
earliest to oldest
Mercalli- Richter- moment mag
Richter scale
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
Pros and cons of Richter scale
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
moment magnitude scale
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)
Pros of moment mag
interpret broader range of seismic frequencies- determines more accurate e from large earthquake
accounts for rock type and rigidity
moment mag equ
(given to us)
Mw= 2/3logE-6.1
different wave types
P- primary
S- secondary
L-love
R-rayleigh
features of P waves and effect on buildings
SEE BOOKLET FOR DIAGRAM
features-
travel through L and S
body waves
arrive first- fastest
longitudinal
effect on building-
least dangerous
features of S waves and effect on buildings
SEE BOOKLET FOR DIAGRAM
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
features of L waves and effect on buildings
SEE BOOKLET FOR DIAGRAM
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
features of R waves and effect on buildings
SEE BOOKLET FOR DIAGRAM
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
effect on ground movement on built environment
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
how does rock competency affect how e is transferred and effect on buildings
comp rocks allow easier propergation of seismic waves and move on
less e dissipated to rock
less movement
also moves quicker through comp rock
liquefaction and effect on buildings
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
where are developments banned
soft unconsol rock or incomp
on fault zone
nearby fault zones
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)
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
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
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
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
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
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
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
seismic risk def
possibility of death, injury or damage that may occur within a certain period of time
IT CAN CHANGE
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
what do national governments prioritise when mitigating seismic risk?
maintain H20, electric, communications e.g. defence and public services ect
what do local governments prioritise when mitigating seismic risk?
protecting public buildings, emergency planning and local transport
what do businesses prioritise when mitigating seismic risk?
protecting building to avoid loss of equipment and production
what do householders prioritise when mitigating seismic risk?
pets, house, people e.g. family
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
What may we see in a seismic risk assessment
FILL IN USING TEXT BOOK
PML
probability definition
how likely it is something will happen
can be fraction, decimal or percentage
0-1
won’t to will
return period definition
average length of time for an earthquake of a certain mag to occur again or be exceeded
(sometimes called reoccurrence period)
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
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
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
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
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
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
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
advantages of predictions
lives saved - people move out of dangerous areas
makes people think about safety measures e.g. escape routes
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
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
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
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
role of a geologist in seismically active area
provide facts, produce hazard maps and risk assessments
decide/ advise if public warning is necessary
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
what science is used to make predictions
physical properties
stress
animal behaviour
radon emissions
seismic gap model
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
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
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
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
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
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)
what can trigger tsunamis
shallow focus submarine earthquakes
release of glacial lake
landslide
submarine volc eruption
meteorite impact
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
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
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
how does clay form?
from the chemical weathering of carbonate minerals e.g. quartz and feldspar
mostly feldspar (exposed to air and H20)
why is Britain covered in clay
clay deposits post glacial retreat
kaolinite information
non-expanding and low shrink swell capacity
1:1
used in ceramic and porcelain
used for glossy high finish paper
montmorillonite information
expanding structure and high shrink swell capacity
2:1
used for:
protective linings
catalytic processes
facial powder
cat litter
vermiculite information
limited expansion and medium shrink swell
2:1
used for:
when hot expands to low density medium
gardening
cat litter
Illite information
non expanding and low shrink swell capacity
2:1
uses:
ceramics
fillers
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
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
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
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
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
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’
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
subsidence def
vertical downward movement of ground ( and building) due to the shrinking of soil, clay or clay rich soil
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
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
how does shallow mining cause subsidence
old bell pits + pillar and stall –> unstable voids underground
no records of old mines = surprise collapses
how does deep mining cause subsidence
bigger operation -> bigger voids–> large amounts of subsidence
however better recorded so less surprise
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
how does crown holes cause subsidence
anthropogenic- mad made
circular depression –> causes vertical downward movement
often due to mines
how does sink holes cause subsidence
natural circular depressions that cause vertical downward movement
common causes of subsidence
mining - deep, shallow and salt
crown and sink holes
clay shrink swell
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
how can subsidence be corrected
(FILL IN WITH TEXTBOOK)
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
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
what kind of mass movement do competent rocks undergo?
steep slope–> so transitional slides –> all moves at once breaking up into scree (talus)
what kind of mass movement do incompetent rocks undergo?
shallow slopes –> low shear strength –> rotational slide
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
how are mass movements classified
velocity and type of material
e.g.
soil creep - very slow
rock fall- fast
flow - H2O major component
what triggers mass movement
addition of H2O –> heavy rain
earthquakes
building on slope- adds mass
deforestation
sudden drop in H2O table
how does deforestation tigger mass movement
deforestation- increases H20 flow and decreases root binding
how does changes in water table tigger mass movement
sudden drop in H2O table- change pore pressure in rocks –> cause break up of rock
how does addition of H2O trigger mass movement
Adds mass and increases lubrication increasing friction
How does bullion on slopes trigger mass movement
Adds mass and can also increase slop angle
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
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
How is rock strength defined
ability to resist stress without large scale failure
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)
stress definition
force per unit area acting on rock
shear, compressive or tensional
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
strain definition
Response of a system to applied stress
when material is loaded with stress it produces strain e.g. deformation
what is rock stress measured in
Mpa or PA
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
what is brittle deformation + where is it seen
shatters
near surface and cold
when stress causes rock to fracture (possibly some elastic deformation first)
what does UCS stand for
uniaxial compressive strength
when is UCS used
before construction
to design rock crushers in mining
and determine strength of concrete
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
why is the UCS test class as unconfined
No where for stress to be transferred from rock
confining pressure def
combined lithostatic and hydrostatic pressure (at depth all principal stress is =)
how does confining pressure change way rock behaves under pressure
would be able to transfer stress to surrounding rock so would have higher USC
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
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
lithostatic pressure def
vertical pressure due to mass of overlying rock only
also called overburden pressure
density definition and units
mass found in a set volume of a material
gcm-3 or kgm-3
density calculation
density = mass/volume
p=m/v
how does rock density change with depth
increased depth, increased pressure –> particles more tightly packed = more dense
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
lithostatic pressure calculation
p x g x h
p=density
g= acceleration due to gravity
h=depth
units: likely kgm-3
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
homogenous def
same/ similar nature throughout
how are rocks not always homogenous
magmatic differentiation
different composition = different hardness
where is weakness present in rock
bedding planes
fractures
joints
discontinuities
foliation
lamination
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
asperity def
roughness of the surface of a discontinuity
how does asperity effect rock
as asperity decreases rock is more likley to fail/ easier for it to fail
residual strength def
remaining resistance to movement after rock has failed and been displaced
joint def
fracture in a competent rock where there has been no observable displacement
joint sets def
see multiple joints that often look sub-parallel (but can cross) formed as a result of regional stress e.g. folding
how do joints react to tensional stress
pull apart
producing angular discontinuity which may resist shear stress
how do joints react to shear joints
move past horizontally
smoother and less likely to resist stress
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
how does water in joints decrease rock strength
freezes - frost shattering = accelerate weathering + widen joint
hydrolysis and carbonation weather rock
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
why are unloading joints problematic
form dangerous joint sets which are unpredictable and need grouting to fill in spaces/voids
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
bedding plane def
mark in time where dep temporarily ceased
usually between rock types
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
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
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
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
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
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
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
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
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
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
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
stress equation
stress= amount of deformation in direction of applied force ÷ initial length/vol/shape
