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On dangerous ground: understanding earthquake induced soil liquefaction

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Earthquakes cause tremendous damage to man-made structures. Soil liquefaction and the associated phenomena are the main cause for extensive damage to infrastructure. Solid ground that normally offers good support to the structures can turn into a semi-liquid state temporarily under the action of earthquake loading. This results in structures sinking into the ground and/or rotating severely. Underground structures such as tunnels and pipelines can float towards ground surface.

In this lecture examples of damage to structures from soil liquefaction observed in the recent earthquakes such as the New Zealand earthquakes of 2011, Haiti earthquake of 2010 and other such events will be presented. The basic mechanisms at play while the granular matter turns into a semi-liquid state will be discussed. Modelling of liquefaction problems is very interesting. Mathematically it involves solving of coupled differential equations if one follows the finite element method. Physically, liquefaction modelling requires the use of a high gravity centrifuge in conjunction with powerful earthquake actuators that fly on the centrifuge. The development of earthquake actuators at Cambridge from simple mechanical Stored Angular Momentum (SAM) actuator to the sophisticated Servo-hydraulic actuators will be highlighted. Some example problems will be considered which were modelled using both mathematical and physical modelling tools. Some of the boundary value problems will highlight how physical modelling can clarify mechanisms of failure for the cases shallow and deep foundations.

The lecture will end by raising some philosophical questions relating to earthquake induced soil liquefaction. Do individual grains lose contact during liquefaction? Will this lead to opening of gaps/cracks in an intact body of soil? What happens to the soil in that state if it is subjected to shearing? Can the soil below buildings really turn into ‘dangerous ground’?

This talk is part of the Cambridge Philosophical Society series.

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