Bishop coefficients in elastic and plastic regimes of unsaturated soils

The definition of a suitable effective stress to model the behaviour of unsaturated soils has been questioned for decades. This issue is still a matter of debate in the community. Recently, Alonso et al. (2010) have shown that this coefficient might depend on the microstructure of the soil and that fine plastic soils are characterised by a Bishop coefficient tending to deviate from the commonly used assumption according to which it is equal to the degree of saturation. On the other hand, Coussy et al. (2010) have shown that this coefficient deviates from the latter assumption if the isodeformation of all pores is not satisfied. They also showed that the Bishop coefficient might be different in elastic and plastic regimes, respectively.

In this work, we take advantage of experimental data available in the literature covering, for each soil, both elastic and plastic regimes at various saturation states and including the water retention curve together with microstructure data. We conclude that the Bishop coefficient depends on the deformation regime and that, in particular, distinct values might be used depending whether the plastic regime is active or not.

References:

Alonso, E. E., Pereira, J. M., Vaunat, J., & Olivella, S. (2010). A microstructurally based effective stress for unsaturated soils. Géotechnique, 60(12), 913–925. https://doi.org/10.1680/geot.8.P.002

Coussy, O., Pereira, J. M., & Vaunat, J. (2010). Revisiting the thermodynamics of hardening plasticity for unsaturated soils. Computers and Geotechnics, 37(1–2), 207–215. https://doi.org/10.1016/j.compgeo.2009.09.003