Abstract: An experimental study was carried out on compacted samples of Gaomiaozi sodium bentonite.Complex coupled hydro-mechanical-chemical behaviour of bentonite was observed.The influence of dry density,vertical stress and soaking liquid on swelling of beutouite was studied,and the swelling deformation was obviously effected by all these factors.There was large difference of swelling deformation obtained under zero stress between the conditions of soaking with water and exposuring to saturated vapor.An investigation of the osmotic deformation induced by changing soaking liquid was also attempted.The samples dilated when the soaking liquid changed from the sodium chloride solution to the distilled water,while they shrunk when the soaking liquid changed from the distilled water to the sodium chloride solution.However,an initial collapse,after changing the soaking liquid to the distilled water,was observed for a sample with the low dry density and ever soaked in high salinity solution.The swelling deformation obtained for the samples soaked directly in the distilled water was larger than that immersed first in the sodium chloride solution and then in the distilled water.After swelling deformation or osmotic deformation reached the equilibrium,the samples were subjected to cyclic stress,and both plastic compression and plastic expansion were induced.This phenomenon would be explained with Barcelona Expansive Model.