Influence of destructuration of soft clay on time-dependant settlements

The time-dependant settlement of soft soils following application of surface loading may be modelled using elastic visco-plastic constitutive models to describe the soil behaviour. For applied loadings that increase the stresses to around the in-situ yield stress, the predicted behaviour is strongly influenced by the associated breakdown of clay structure and the way in which this is modelled. The paper describes some predictions for a hypothetical case prediction exercise recently organised by the Norwegian Geotechnical Institute (NGI), to compare different calculation methods used in settlement analyses of soft soil around the world. The results presented here were made using a one-dimensional coupled consolidation analysis implemented in the spreadsheet-based software Briscon. Parameters were obtained from an oedometer test and the results were extrapolated over the full soil profile. Various plausible assumptions about the shape of the isotaches around yield were explored, and it is shown that the predicted long-term settlement may vary by factors of two or more depending on the assumptions made.