Blasting triggered quick clay slide

Road construction work in Kattmarka near the city of Namsos in mid-Norway, triggered a sudden quick clay slide on Friday 13th March 2009. The small community with ten dwellings was completely destroyed by the slide. NGI participated in the mitigation work and in the investigation committee appointed by the federal authorities.

The 700 m long county road was to be widened with with both foot and bicycle paths, and the construction work included rock blasting. Vibrations from blasting normally does not trigger quick clay slides, but a series of coincidental and unfortunate circumstances made that the blasting actually triggered the slide.

Investigations revealed that the blasting was the direct cause of the clay slide: the quick clay was lying directly onto the rock which was blasted out. The large slide volume was due to the high sensitivity of the clay. Quick clays lose their strength and become fluid when overloaded. The report from the investigating committee concluded that the entire area had a poor safety factor against instability. The soil investigations should have been much more thorough, and would have revealed this instability problem. The investigation committee was set by the Norwegian Ministry of Transport and Communications. Professor Steinar Nordal of NTNU led the work. Dr. Christian Madshus, an expert on geodynamics and vibrations from NGI, was one of the five appointed members of the committee.

NGI was engaged by several parties in connection with the Kattmarka slide. Just a few hours after the slide, experts from NGI on landslide assisted local authorities to handle the emergency situation on site. Thereafter NGI did the detailed design of mitigation measures for the areas near the slide. The county and the Public Roads Administration later asked NGI for advice for the construction of the new road through the area.

The landslide in Kattmarka displaced a volume of 300 000 - 500 000 m3 over an area 300 m by 100 m.