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Earthquake induced Earthquake induced LIQUEFACTIONLIQUEFACTION
byby
Jimmy McLauchlanJimmy McLauchlan
Peat NicholasPeat Nicholas
Case study: Mexico City, 1985Case study: Mexico City, 1985
IntroductionIntroduction
Development of the Mexico City BasinDevelopment of the Mexico City Basin The 1985 earthquakeThe 1985 earthquake LiquefactionLiquefaction The damaging affects of liquefaction The damaging affects of liquefaction
triggered by earthquakestriggered by earthquakes Mitigation against liquefactionMitigation against liquefaction SummarySummary
Development of Mexico City Development of Mexico City BasinBasin
Mexico City itself lies in a broad basinMexico City itself lies in a broad basin Formed approximately 30 million years agoFormed approximately 30 million years ago
Volcanic activity closed the basin and resulted in the Volcanic activity closed the basin and resulted in the formation of Lake Texcocoformation of Lake Texcoco
The Aztecs chose an island in this lake as an easily The Aztecs chose an island in this lake as an easily defendable location for their capitoldefendable location for their capitol
Mexico City Basin
Mexico City EarthquakeMexico City Earthquake
September 19September 19thth, 1985, 1985 Subduction zone of the Subduction zone of the
Cocos PlateCocos Plate Richter magnitude 8.1 Richter magnitude 8.1 The epicentre occurred The epicentre occurred
on the Pacific coast 350 on the Pacific coast 350 km km
Damage was Damage was concentrated in a 25 concentrated in a 25 kmkm2 2 area of Mexico Cityarea of Mexico City
Population of 18 millionPopulation of 18 million
Overall Damage estimationOverall Damage estimation
10,000 people 10,000 people were killedwere killed
50,000 were 50,000 were injuredinjured
250,000 people 250,000 people lost their homeslost their homes
Property damage Property damage $5 billion$5 billion
Over 800 buildings Over 800 buildings collapsedcollapsed
Liquefaction?Liquefaction?
General definitionGeneral definition Liquefaction is a phenomenon in which Liquefaction is a phenomenon in which
water-saturated sediment temporarily water-saturated sediment temporarily loses its strength and stiffness and loses its strength and stiffness and acts as a fluidacts as a fluid
Liquefaction explainedLiquefaction explained
A soil deposit A soil deposit consists of an consists of an assemblage of assemblage of individual soil individual soil particlesparticles
Each particle is in Each particle is in contact with a contact with a number of number of neighbouring neighbouring particlesparticles
Gravity forces the Gravity forces the particles together particles together and gives the soil its and gives the soil its strengthstrength
In extreme cases:In extreme cases: Pore water pressure Pore water pressure
loosens the soil loosens the soil particlesparticles
Soil particles lose Soil particles lose contact with each contact with each otherother
Strength diminishesStrength diminishes Soil behaves more Soil behaves more
like a liquid than a like a liquid than a solidsolid
““LiquefactionLiquefaction""
Liquefaction occurs when:Liquefaction occurs when: Soil structure breaks down Soil structure breaks down Loosely-arranged particles Loosely-arranged particles
try to consolidatetry to consolidate The speed of an earthquake The speed of an earthquake
reduces the time for water reduces the time for water to be squeezed outto be squeezed out
The water becomes The water becomes "trapped“"trapped“
Increase in water pressure Increase in water pressure Soil particles are prevented Soil particles are prevented
from compactingfrom compacting Reduced contact forces Reduced contact forces
between particlesbetween particles
Ground conditions in Mexico CityGround conditions in Mexico City
Built on a drained lake bed in a Built on a drained lake bed in a basin ringed by mountainsbasin ringed by mountains
‘‘Montmorillonite Clay’Montmorillonite Clay’
YoungYoung PorousPorous Highly compressibleHighly compressible Possible water content of Possible water content of
around 300%around 300%
Mexico City clays contain rock-Mexico City clays contain rock-flour siltsflour silts Very fine weathered mineral Very fine weathered mineral
fragments of the predominant fragments of the predominant rocks in the arearocks in the area
Significance for Mexico CitySignificance for Mexico City For geotechnical engineers, Mexico City’s underlying For geotechnical engineers, Mexico City’s underlying
clay poses extraordinary complex problems:clay poses extraordinary complex problems: Clays between 10 to 30% silt by weight have less Clays between 10 to 30% silt by weight have less
resistance to liquefaction than 100% claysresistance to liquefaction than 100% clays Low shear strengthLow shear strength Extremely high compressibilityExtremely high compressibility Increases chances of liquefactionIncreases chances of liquefaction
Compared to rock, soft soils amplify the waves created Compared to rock, soft soils amplify the waves created by an earthquakeby an earthquake Damage can be up to three times greaterDamage can be up to three times greater
Soft sediments “liquefy” allowing the Soft sediments “liquefy” allowing the seismic waves to bounce back and forthseismic waves to bounce back and forth Increasing the duration of the earthquakeIncreasing the duration of the earthquake Increased fatigue of buildingsIncreased fatigue of buildings
CFD model of Mexico CityCFD model of Mexico City Snapshot depicts:Snapshot depicts:
Horizontal ground motionHorizontal ground motion Green:Green:
Movement of rock ≈ 80 cmMovement of rock ≈ 80 cm Varying BLUE to RED Varying BLUE to RED
shades shades (standardised to rock (standardised to rock movement)movement):: Area movements within Area movements within
Mexico CityMexico City Red-orange region in the Red-orange region in the
basin's centre:basin's centre: Moved a distance 25 times Moved a distance 25 times
greater than the of the greater than the of the surrounding formationsurrounding formation
Possible Ground Possible Ground DeformationsDeformations
Liquefaction Liquefaction associated ground associated ground deformations:deformations: Lateral spreading Lateral spreading
and ground fissures and ground fissures Flow failures and Flow failures and
subsidencesubsidence Sand boilsSand boils Slope failuresSlope failures
MitigationMitigation
Avoid construction on Avoid construction on liquefaction Susceptible Soilsliquefaction Susceptible Soils
Easy to identify if ground Easy to identify if ground conditions are known, ‘Zoning’conditions are known, ‘Zoning’
Build Liquefaction Resistant Build Liquefaction Resistant StructuresStructures
Special foundations can be Special foundations can be build to reduce the effects of build to reduce the effects of liquefaction liquefaction
Improve the SoilImprove the Soil Improving the strength, Improving the strength,
density and drainage density and drainage characteristics of the soilcharacteristics of the soil
Three main possibilitiesThree main possibilities
SummarySummary
Liquefaction augments the effects of Liquefaction augments the effects of an earth quakean earth quake
Created in saturated unconsolidated Created in saturated unconsolidated soil such as those found in the soil such as those found in the Mexico City BasinMexico City Basin
Possible to mitigate against however Possible to mitigate against however it is time consuming and expensiveit is time consuming and expensive