In this work and in order to consider theoretically the effect of gas hydrates on the mechanical properties of their host sediments, hydrate fraction was introduced as a state variable in the framework of a Critical State model. For the proposed model, the process of de-structuration and softening of natural clay was considered as a proxy for the gas-hydrate-bearing-sediments behaviour. The developed constitutive model seemed to reproduce the main key behaviours of gas-hydrate-bearing sands observed by Masui and co-authors: i) the elastic properties (Young Modulus, shear modulus) and the strength of the sediment-hydrate medium increase with the hydrate fraction ii) the mechanical softening of the gas-hydrate bearing sediment is amplified with the hydrate fraction and iii) the dilation angle increases with the hydrate fraction.