The magic of sands — The 20th Bjerrum Lecture presented in Oslo, 25 November 2005This paper represents the written version of the 20th Bjerrum Lecture. While it has been edited for the present publication, it retains the general structure of the original lecture, which was intended for a general geotechnical audience. It deliberately points out some of the still-open questions concerning soil modelling. The Bjerrum Lecture is presented in Oslo in alternate years by the Norwegian Geotechnical Society with the support of the Bjerrum Memorial Fund (Laurits Bjerrums Minnefond).

Following the discovery of sinkholes in the W.A.C. Bennett Dam, British Columbia, in 1996, investigations showed that there had apparently been movement of fine material out of the core of the dam. To be able to predict the mechanical consequences of such movement of material, a class of soil model that is able to accommodate changes in both density and grading of the soil is required. An outline of features of the Severn–Trent sand model — which incorporates effects of density variation — is presented and a suggestion is made for ways this model might be extended to include effects of changing particle size distribution.