In this work, five tests were carried out on synthetic marine sediments in order to identify the gas effect on the hydro-mechanical behaviour of unsaturated marine sediments. Based on the experimental results obtained from this study, on previous constitutive models [Alonso, E.E., Gens, A., Josa A., 1990. A constitutive model for partially saturated soils. Géotechnique 40(3), 405–430; Gens, A., Alonso, E.E., 1992. A framework for the behaviour of unsaturated expansive clays. Can. Geotech. J. 29, 1013–1032] and on the relationship of Van Genuchten [Van Genuchten, M.Th., 1980.