An earthfill tailings retention embankment across the Ok Ma valley in a remote tropical rainforest area of western Papua New Guinea was abandoned during construction in early 1984 because of a very large landslide that occurred in the eastern abutment of the dam. Ensuing litigation between the mine owners and their insurers brought together highly qualified and experienced geotechnical experts. The experts developed their own views on the form, nature and mechanism of the landslide which occurred in limestone colluvium and Miocene mudstone bedrock.

After preliminary studies and site investigations, work for the mining company on the tailings dam was carried out as a design-and-construct package by a major consortium. Sub-sequent design investigations used most of the available techniques, including aerial photograph interpretation, seismic traverses, pitting, drilling and in situ and laboratory testing, all carried out by experienced specialist companies. Although concern was expressed on various aspects of temporary and permanent slope stability, the large size of the landslide came as an unforeseen event.

This paper presents the authors' interpretation of six different models proposed for the landslide by various authorities who worked on the case. This has been done to illustrate the difficulties in attempting a precise interpretation of a large landslide and as a commentary on the state of the art on the subject.

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