The quick-clay slide at Baastad, Norway, 1974

Abstract

The Baastad landslide occurred within a large area of marine clay deposits where the clay had been transformed into quick clay from leaching. The landscape in the area is highly dissected from stream erosion and numerous slope failures and slides. The slide scar covers about 80,000 m 2 of farmland. Three buildings were destroyed in the slide. According to observations and investigations, the whole area slid out almost simultaneously, i.e., the slide was of the so-called flake type. For all known documented slides of this category, conventional stability analysis based on ultimate effective stress parameters φ′ and c′, give values of the factor of safety which are too high. It is an unsafe procedure. The behaviour of quick-clays has been studied extensively at the Norwegian Geotechnical Institute over the last 20 years. These studies show that the strength properties of these clays should be based on undrained shear strength values rather than the parameters φ′ and c′. Back calculations of the Baastad slide based on undrained shear strength values have been carried out and were in good agreement with the theoretical value of the factor of safety.