The paper advances a number of ideas towards developing a constitutive framework for unsaturated soils that offers advantages over previous models. These ideas are substantiated by published experimental evidence from laboratory tests on normally consolidated soils. In the proposed model the only stress variable is the "average soil skeleton stress" (i.e. the Bishop stress (Bishop, A.W. (1959) Teknisk Ukeblad 106(39): 859-863) with the chi parameter equal to the degree of saturation). However, a further scalar parameter, combining suction and degree of saturation, is needed to account for the bonding effect exerted by meniscus water at the contacts between soil particles. Remarkably, for both sets of experimental data considered, the variation ofvoid ratio during virgin loading can be described by combining the saturated model expressed in terms of the average soil skeleton stress with an additional relationship accounting for the bonding effect due to meniscus water. These concepts provide a useful platform for developing a full elasto-plastic model.
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