The influence of destructuration of soft clay on time-dependent settlements – a comparison of some elastic visco-plastic models.

The time-dependent 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 towards and beyond 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. Four elastic visco-plastic models are compared and it is shown that despite presentational differences, all calculate creep rate in fundamentally the same manner. The paper describes some predictions for a hypothetical case prediction exercise recently organised by the Norwegian Geotechnical Institute, to compare different calculation methods used in settlement analyses of soft soil. The results presented here were made using a one-dimensional coupled consolidation analysis implemented in the spreadsheet-based software Briscon and using the finite element program Plaxis. 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.