Soil leaves Chch vulnerable

Press, 13 September 1988, Page 4

Soil leaves Chch vulnerable

The type of soil on which much of Christchurch is built leaves the city vulnerable to a phenomenon that would cause tremendous property damage in an earthquake but would pose little immediate danger to people, says a geological engineer.

Dr Don Elder, a specialist in liquefaction, says large parts of Christchurch are built on the wet, sandy soil in which the phenomenon commonly occurs during earthquakes. Buildings often sink a metre or more into the ground and utility lines are often disrupted because of liquefaction, which American research has shown is one of the

leading causes of property damage during earthquakes, Dr Elder says. However, liquefaction would cause few injuries compared to the other earthquake effects, Dr Elder says. “A well-engineered building would hold together, so there would be little injury to the people inside, although some of the desks might get moved around.”

.Geological evidence suggests a serious earthquake has occurred on the alpine fault about once every 500 years for the past 2000 years, Dr Elder says. The last earthquake appears to have been about 500 years ago.

The vibrations from an

earthquake causes some types of soil to lose the ability to support itself, Dr Elder says. The shaking forces denser soil to sink while lighter soil remains on the surface. The water that separates from the soil in the earthquake prevents the dense soil from sinking so that soil "floats” on the water until the water bubbles to the surface.

The weight of a building on top of soil as it undergoes liquefaction forces the water to rush out from under it and the building sinks, Dr Elder says.

Between 30 and 40 per cent of Christchurch is built on soil that is vulnerable to liquefaction, Dr Elder says.