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

Laboratory ExamplesLaboratory Examples

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