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Going to ground

Twenty five kilometres west of the earthquake's epicentre, the town of Adapazari lies in ruins. University of Colorado professor of geophysics Roger Bilham blames the destruction on the decision to site the town on an alluvial plain.

The cyclic ground movements of the earthquake caused the soft soil to shear pushing water upwards. As the pore water pressure increased to match the downward pressure of the buildings the ground got softer. Once the effective vertical pressure equalled zero, liquefaction occurred and the ground turned to liquid. Unable to carry downward load, buildings simply sank.

'This town is an alluvial plain with the water table very close to the surface so the buildings are floating,' said Bilham. 'You can't build anything in these regions. They should relocate all these buildings on the high ground nearby.'

Allott & Lomax technical director Paul Doyle added: 'Jacking up is required on many buildings and piles should be installed down to the bedrock to stabilise the structures. Stone filled columns could be constructed to lower the water table But this work could be damaging and costly. It would be better to knock them down and start again.'

Soft ground has also been blamed for destruction of a area of west Istanbul over 100km away from the epicentre. Almost 1,000 people were killed in the Avcilar district of the city. Soft ground in that area amplified horizontal forces, producing lateral accelerations of 0.25g, horizontal force equal to 25% their mass. In the rest of Istanbul the peak horizontal acceleration was measured at 0.05g.

Soil/structure interaction is key. A site investigation near Yelova revealed a holiday camp complex was destroyed because poor clay at 17.5m depth exaggerated the effect of a soft storey ground floor.

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