Tim Rushton, Phoebe Martin and Amy Fitzgerald

History  Faulting of an uplifted plateau, between the ‘North American Plate’ and the ‘Cocos Plate’.  Volcanic activity 30 million years ago caused the river, that filled the valley, to dam and a lake was formed – Lake Texacoco.  Mexico City was originally built on an island in the lake. As the lake was drained the city grew outwards onto the unconsolidated land.

 Because of this Mexico City lies in a broad basin of unconsolidated lake bed sediment.  The island was susceptible to flooding so the decision was made to build a drainage system.  In1967 a network of channels and underground tunnels built to control water levels – arid surrounding area.  Inhabitants needed to pump water from the ground causing sinking of the unconsolidated subsurface soil - cm/yr.

The Earthquake  8.1 magnitude earthquake on 19 th September 1985 with smaller aftershocks.  Epicentre was 240 miles away, off the coast of Michoacán.  Very active subduction zone – Cocos plate pushes under North American plate.

 Multiple breaks in the fault line caused several minutes of shaking.  Ground conditions worsened the effect of the earthquake, particularly for taller buildings due to the effects of resonance.  Approximately 400 buildings collapsed and 3,000+ damaged.  Around 10,000 people died and many more injured.

Geology of Mexico  Mexico is made up of micro-continents that were accreted to the North American Continent during the late Palaeozoic up until the early Cenozoic.  A subduction zone was formed in the Pacific Ocean, where the Cocos Plate is forced under the North American Plate.  The resulting tectonic activity produced four mountain ranges:  Sierra Madre Occidental  Late Eocene to Early Miocene Extrusions and Oligocene Ignimbrites  Sierra Madre Oriental  Sierra Madre del Sur  Trans-Mexican Volcanic Belt (TMVB)  Mesozoic and Cenozoic

Geology of Mexico City  Lies within the Mexico Basin in the TMVB  Partially separated by lower mountains  Deposition in pre-existing lakes created lacustrine clay layers  up to 100m deep  low permeability  Separated by sections of silt and sand  Harder and 10 – 40m deep  Alluvial fill containing strata of Pleistocene and recent basalt deposits  100 – 500m thick  Stratified Volcanic Deposits held together Pliocene lacustrine clay deposits.  100 – 600m thick  500 – 1000m deep Lacustrine Clay Deposits Quaternary Chichinautzin Volcanic Series

Clay Layer Sand and Silt Clay Layer Gravel, Silt and Compact clay

Effect of Geology during Earthquake  Felt 745 miles North in Houston, Texas and 621 miles South in Guatemala City.  Seismic waves travelled quickly from epicentre to Basin of Mexico through plutonic, metamorphic, and continental and marine rocks.  Mexico City was the worst affected even though 240 miles away.  Due to soft, poorly consolidated lake-bed sediment which reduced the velocity of the seismic waves and increased their amplitude. Types of Seismic Waves Present in Mexico City  Surface waves such as Rayleigh and Love waves were dominant.  Sub soil was more rigid due to historic engineering such as aqueducts and pre-Hispanic buildings.  S-waves also present but lower amplitude.  Characterized by shaking and travel at lower velocities through solids.

Liquefaction  Soil had very low pore water pressure due to historical draining of pre- existing lakes.  Vibrations from earthquake increased water content.  Soil became fully saturated.  Water pressure exerted a force on the soil particles and allowed particles to move amongst each other.  Reduction in soil strength. Building has sunk into the ground as a result of liquefaction.

Possible Solutions Prior to construction of structure  GSA zoning – site specific geotechnical surveys.  Densification of the liquefiable sand.  Deep drains installed to prevent build up of excess pore pressure.  Cement grouting.  Subsurface barriers.

During construction  A stiff mat foundation over locally liquefied zones.  A ductile cap connection to allow some rotation to occur without failure.  Large diameter piles to resist moments.

After construction  Dynamic compaction.  Vibro-compaction.  Vibro-replacement.  Compaction piles.

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