Earthquake induced LIQUEFACTION by Jimmy McLauchlan Peat Nicholas Case study: Mexico City, 1985

Introduction Development of the Mexico City Basin Development of the Mexico City Basin The 1985 earthquake The 1985 earthquake Liquefaction Liquefaction The damaging affects of liquefaction triggered by earthquakes The damaging affects of liquefaction triggered by earthquakes Mitigation against liquefaction Mitigation against liquefaction Summary Summary

Development of Mexico City Basin Mexico City itself lies in a broad basin Mexico City itself lies in a broad basin Formed approximately 30 million years ago Formed approximately 30 million years ago Volcanic activity closed the basin and resulted in the formation of Lake Texcoco Volcanic activity closed the basin and resulted in the formation of Lake Texcoco The Aztecs chose an island in this lake as an easily defendable location for their capitol The Aztecs chose an island in this lake as an easily defendable location for their capitol Mexico City Basin

Mexico City Earthquake September 19 th, 1985 September 19 th, 1985 Subduction zone of the Cocos Plate Subduction zone of the Cocos Plate Richter magnitude 8.1 Richter magnitude 8.1 The epicentre occurred on the Pacific coast 350 km The epicentre occurred on the Pacific coast 350 km Damage was concentrated in a 25 km 2 area of Mexico City Damage was concentrated in a 25 km 2 area of Mexico City Population of 18 million Population of 18 million

Overall Damage estimation 10,000 people were killed 10,000 people were killed 50,000 were injured 50,000 were injured 250,000 people lost their homes 250,000 people lost their homes Property damage $5 billion Property damage $5 billion Over 800 buildings collapsed Over 800 buildings collapsed

Liquefaction? General definition General definition Liquefaction is a phenomenon in which water-saturated sediment temporarily loses its strength and stiffness and acts as a fluid Liquefaction is a phenomenon in which water-saturated sediment temporarily loses its strength and stiffness and acts as a fluid

Liquefaction explained A soil deposit consists of an assemblage of individual soil particles A soil deposit consists of an assemblage of individual soil particles Each particle is in contact with a number of neighbouring particles Each particle is in contact with a number of neighbouring particles Gravity forces the particles together and gives the soil its strength Gravity forces the particles together and gives the soil its strength In extreme cases: In extreme cases: Pore water pressure loosens the soil particles Pore water pressure loosens the soil particles Soil particles lose contact with each other Soil particles lose contact with each other Strength diminishes Strength diminishes Soil behaves more like a liquid than a solid Soil behaves more like a liquid than a solid “Liquefaction" “Liquefaction" Liquefaction occurs when: Liquefaction occurs when: Soil structure breaks down Soil structure breaks down Loosely-arranged particles try to consolidate Loosely-arranged particles try to consolidate The speed of an earthquake reduces the time for water to be squeezed out The speed of an earthquake reduces the time for water to be squeezed out The water becomes "trapped“ The water becomes "trapped“ Increase in water pressure Increase in water pressure Soil particles are prevented from compacting Soil particles are prevented from compacting Reduced contact forces between particles Reduced contact forces between particles

Laboratory Examples

Ground conditions in Mexico City Built on a drained lake bed in a basin ringed by mountains Built on a drained lake bed in a basin ringed by mountains ‘Montmorillonite Clay’ ‘Montmorillonite Clay’ Young Porous Highly compressible Possible water content of around 300% Mexico City clays contain rock-flour silts Mexico City clays contain rock-flour silts Very fine weathered mineral fragments of the predominant rocks in the area

Significance for Mexico City For geotechnical engineers, Mexico City’s underlying clay poses extraordinary complex problems: For geotechnical engineers, Mexico City’s underlying clay poses extraordinary complex problems: Clays between 10 to 30% silt by weight have less resistance to liquefaction than 100% clays Clays between 10 to 30% silt by weight have less resistance to liquefaction than 100% clays Low shear strength Low shear strength Extremely high compressibility Extremely high compressibility Increases chances of liquefaction Increases chances of liquefaction Compared to rock, soft soils amplify the waves created by an earthquake Compared to rock, soft soils amplify the waves created by an earthquake Damage can be up to three times greater Damage can be up to three times greater Soft sediments “liquefy” allowing the seismic waves to bounce back and forth Soft sediments “liquefy” allowing the seismic waves to bounce back and forth Increasing the duration of the earthquake Increasing the duration of the earthquake Increased fatigue of buildings Increased fatigue of buildings

CFD model of Mexico City Snapshot depicts: Snapshot depicts: Horizontal ground motion Horizontal ground motion Green: Green: Movement of rock ≈ 80 cm Movement of rock ≈ 80 cm Varying BLUE to RED shades (standardised to rock movement) : Varying BLUE to RED shades (standardised to rock movement) : Area movements within Mexico City Area movements within Mexico City Red-orange region in the basin's centre: Red-orange region in the basin's centre: Moved a distance 25 times greater than the of the surrounding formation Moved a distance 25 times greater than the of the surrounding formation

Possible Ground Deformations Liquefaction associated ground deformations: Liquefaction associated ground deformations: Lateral spreading and ground fissures Lateral spreading and ground fissures Flow failures and subsidence Flow failures and subsidence Sand boils Sand boils Slope failures Slope failures

Mitigation Avoid construction on liquefaction Susceptible Soils Avoid construction on liquefaction Susceptible Soils Easy to identify if ground conditions are known, ‘Zoning’ Easy to identify if ground conditions are known, ‘Zoning’ Build Liquefaction Resistant Structures Build Liquefaction Resistant Structures Special foundations can be build to reduce the effects of liquefaction Special foundations can be build to reduce the effects of liquefaction Improve the Soil Improve the Soil Improving the strength, density and drainage characteristics of the soil Improving the strength, density and drainage characteristics of the soil Three main possibilities Three main possibilities

Summary Liquefaction augments the effects of an earth quake Liquefaction augments the effects of an earth quake Created in saturated unconsolidated soil such as those found in the Mexico City Basin Created in saturated unconsolidated soil such as those found in the Mexico City Basin Possible to mitigate against however it is time consuming and expensive Possible to mitigate against however it is time consuming and expensive