This paper presents some results of an experimental study performed to characterize the hydro-mechanical behaviour of compacted marls from Abadia, Portugal. The main purpose of the study was to understand the effects of the degradation of the fragments on the global behaviour of the aggregate. The knowledge earned allowed the identification of the main factors to be considered in the definition of constitutive models for compacted soft rocks. From a general point of view, traditional unsaturated constitutive models are not adequate to reproduce the large amplitude of the deformations observed in experimental tests on samples of compacted marls. Models for hard rockfill materials are not adequate either because they are not able to reproduce the behaviour of the material after the strong degradation of the fragments. The comparison between the model requirements and those of existing models for clayey materials and rockfill is performed. Suggestions are made on possible modifications of some known existing models from the literature.