Letter: Gone with the clay

From Philip Pells

There is no need to invoke flooding or a major earthquake to explain the
collapse of the ground beneath Herakleion and eastern Canopus
(20 October, p 42).
The failure is consistent with quick-clay landslides.

Quick clays have relatively high cohesive strength in the undisturbed state,
but when disturbed they have the consistency of thick pea soup. This makes them
very susceptible to rapid and dramatic landslides.

The famous quick-clay landslide at Rissa in Norway, which was recorded on
video, occurred in April 1978 and covered an area of 330,000 square metres. The
slide volume was about 6 million cubic metres and it was triggered by quite
minor works associated with the construction of a new barn near the shore of
Lake Botnen. Areas of land up to 4 hectares moved downhill at speeds of 30 to 40
kilometres per hour, taking intact houses along for the ride.

The article mentions that there was up to 20 metres of soft sediments under
Herakleion and eastern Canopus. It is quite possible those were quick clays and
that the catastrophes that engulfed the cities were, as at Rissa, triggered by
quite minor events, man-made or otherwise.

Eastwood, New South Wales