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A large landslide in Tuve (Gothenburg, Sweden 1977) initiated the development of a model for slope stability analysis taking the deformation-softening of soft sensitive clays into consideration. The model studies triggering agents and five phases in progressive slope failure are identified: (1) in-situ, (2) disturbance, (3) unstable 'dynamic', (4) transitory (or permanent) equilibrium, and (5) 'global' failure. The clay resistance in these phases may differ widely; mostly due to different rates of loading.Two time dependent failure criteria are defined: (i) the triggering load condition in the disturbance Phase (2), and (ii) the transitory equilibrium in Phase (4), indicating whether minor downhill displacements or a veritable landslide catastrophe will occur. The analysis explains why downhill landslides tend to spread over vast areas of almost horizontal ground further down-slope. The model has been applied to landslides in Scandinavia and Canada.Three case studies are briefly discussed.The model is a finite difference approach, where local downhill deformations caused by normal forces is maintained compatible with deviatory shear deformations above the potential (or the established) failure surface. Software and an easy-to-use spreadsheet are introduced as well as recent developments. KeywordsLandslides in long natural slopes, Progressive failure in different phases, Triggering agents, Effects of time and rate of loading, Two main failure criteria, Invalidity of one singular static load condition, Massive spread over level ground, Finite difference method of analysis.

A. Gylland et al. ABSTRACT 25A detailed characterisation of the quick clay underlying the NTNU research site at Tiller, 26 Trondheim is presented. The objective of the work is to provide guidance on quick clay 27 parameters to engineers and researchers working with similar clays in Scandinavia and 28North America especially on landslide hazard assessment. The material is lightly 29 overconsolidated and is characterised by its high degree of structure and very high 30 sensitivity (quick clay). Clay and water contents are both about 40%. The plasticity index 31 is low (5%). This relates to the low active minerals of the clay and silt fractions (illite / 32 chlorite and quartz / feldspars respectively).

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