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Based on the BBM model of unsaturated cohesive soils proposed by Alonso,an elasto-plastic constitutive model for unsaturated cohesive soils with a unifed yield surface was proposed in this investingation.The proposed model uses a single mathematical formula to express the loading-collapse yield surface（LC yield surface） and the suction increase yield surface（SI yield surface）,which are however presented in BBM model with two mathematical formulas.The proposed model can simplify the analysis of the coupling effect of LC yield and SI yield surfaces and has no corner on its yield surface which brings the inconvenience of numerical simulation.Then an elastoplastic constitutive model was established on the basis of the yield surface.It can also simulate the test paths that both net stress and suction change at the same time,which extends the scope of application of the BBM model.The model proposed in this thesis contains 14 model parameters,in which there are 12 parameters associated with the physical mechanical properties and yield characteristics of unsaturated cohesive soils.They areλ（0）,κ,β,r,pc,λs,κs, G,M,k,m and sc.Two parameters,P0* and s0,are associated with the harding law.The model parameters can,in genernal,be determined using some suction-controlled tests.These tests include the isotropic compression test at several constant suction values,the test with drying-wetting cycle at a given net mean stress and triaxial drained test at different suction values.The proposed model can simulate some basic mechanical properties and responses of unsaturated cohesive soils,such as the dry-shrinkage characteristic with the change of suction,the strain-harding trend with the increase in pressure,the phenomena that net stress has a more significant effect on deformation features than suction,and the suction harding effects.In order to verify the proposed constitutive model,the model formulation is programmed at first using the Fortran 95 computer language.Then,the model simulation and prediction are compared with experimental results obtained by Alonso, Chen Zhenghan,and also by this investigation.It is shown that the simulations accord with these experimental results well.The proposed constitutive model is thus quite reasonable and of certain practical value.
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