Long history of landslips

Press, 11 October 1979, Page 11

Long history of landslips

Landslips were happening at Abbotsford 9000 years ago. the head of Otago University's geology department. Professor D. S. Coombs, has said. Professor Coombs was addressing a joint meeting of the Geological Society and the Geomechanical Society at the University of Canterbury. He told the meeting that it would be improper for him to anticipate the findings of the Commission of Inquiry set up to study the disaster. He would, however. touch upon some aspects of what had happened.

An overburden of loess and other material rested upon a layer of loose sand at Abbotsford, which in turn rested on mudstone. Professor Coombs said. The sand when wet acted “rather like ball-bearings. There is a good possibility that the sliding took place on the interface between sand and mudstone,” he said.

Landslides had long been known in the Abbotsford district. Professor Coombs describing two slides relatively close to the most recent one. These others had probably occurred about 9000 years ago.

This .'.ear's slide had first been noticed on May 31, when a ivater main m an Abbotsford street burst. February. Mai ch and April had been quite dry months, said Professor Coombs, but May had bee n 00 per cent wetter than average. Movement might have begun years before, howe\ er. A movement of 00 mm had been noted in a period of about 10 years between two surveys. When the movement vias actually recognised in May this year, however, the earth was moving at a speed of Smm a day. A few days before the final slide in August it was moving at more than 200 mm a day. Eventually the entire mass had slipped about 50 metres south-east, at an angle of about 7deg. Professor Coombs said a creek to the east of the slide area might have gained access to the layer of sand. Where it ran through the housing area it had been piped underground. Although there was no evidence that anything was wrong with these pipes it was one of the points being discussed