The swelling characteristics of bentonite play a very important role inensuring the long-term stability and isolation of high-level radioactive waste(HLW)repository.This paper presents an experimental testing program to investigate the swelling behavior,especially on the swelling equilibrium limit(SEL)relationship under various boundary conditions of GMZ bentonite.Aself-developed test apparatus,capable of controlling a range of boundary conditions,was designed,including constant mean stress(CMS),constant volume(CV)and an intermediate flexible boundary condition termed as constant stiffness(CS),applies increased in stress as a specified function of the volume increase.The soil samples were prepared with various initial dry densities,ranging from 1.5 to 1.7 g/cm3 and were applied with different vertical stress.During wetting,they were subjected to different boundary conditions before the swelling strain or swelling pressure reached equilibrium.It is found that boundary conditions are significant factors influencing the swelling strainand swelling pressure.More specifically,the relationship between the range of boundary conditions and the corresponding swelling pressure is:CV>CS>CMS,while the relationship between the range of boundary conditions and the corresponding swelling strain is CMS>CS>CV.In addition,the evolutions of SEL curves are governed by the initial dry density and vertical stress and the effect of dry density is more significant.Based on the results,several SEL curves are established which give evidence on the dominance of boundary conditions on soil swelling potential.The SEL also shows the potential in providing a framework to predict the final stress and volume states of GMZ bentonite during infiltration under known boundary condition.