Mechanical Behavior Of Soft Clay And Its Elastoplastic Modelling

The stress history and soil structure,as two important factors,have significant effect on the mechanical behavior of soft clays.Some research results show that the stress-strain curves of overconsolidated clay is obviously different from that of normally consolidated clay,and the strength and deformation characteristics of undisturbed clays are also different from those of reconstituted clays.In order to further study the effect of structure and stress history on the mechanical behaviors of soft clays,especially effect of the structure on the mechanical behavior of clay,a series of compression and triaxial shear tests are carried out on undisturbed and reconstituted samples of eight different soft clays.Finally,based on the further understanding of the mechanical behavior of clays,the corresponding constitutive models are improved for the stress-strain relation elasto-plastic simulation of structural clays.The detailed research results are as follow.1.Compression indices C_c,swelling indices C_s of undisturbed sample and corresponding reconstituted sample of soft clays,obtained from different district of Shanghai and Jiangsu province,are measured using different test methods including isotropic compression-swelling tests and one-dimensional compression-swelling tests.The relations between the one-dimensional compression index C_c and void ratio e₁₀ of reconstituted samples and undisturbed samples with different degrees of disturbance are set up respectively.Furthermore,Based on the comparion of compression indices C_c obtained form oedometer and isotropic compression tests,the relations set up based on the oedometer tests can be used for the results obtained from the isotropic compression tests.2.Consolidated-drain triaxial tests on overconsolidated reconstituted samples of clay obtained from Pudong,Shanghai,are carried out.The tests are grouped by two different stress paths,constant confining pressure and constant mean effective stress,with different confining pressures and over-consolidated ratios.The stress ratio-strain curves obtained form the tests show that over-consolidated ratio and stress path affect the stress ratio-stress curves of clay;however,the confining pressure has no effect on the stress ratio-stress curves of clay.3.A series of compression and shear tests are carried out on the undisturbed samples and reconstituted samples of eight different soft clays obtained with the different sampling methods.By the comparison of compression curves and stress ratio-strain curves of undisturbed samples and corresponding reconstituted samples,the effect of soil structure on the strength and deformation characterists of soft clays are confirmed.The shear test results show that the strength of undisturbed samples is lower than the corresponding reconstituted samples at the same confining pressure,which is different from some test results obtained by some researchers.The reason is the difference in void raito before shear.If they have the same void ratio,the strength of undisturbed samples will be higher than that of the corresponding reconstituted samples.4.A reference void ratio e₁₀^* is introduced as a fabric index.The test results further dealt with the reference void ratio e₁₀^* show that the compression indices C_c have a unique relationship with the reference void ratio e₁₀^* for undisturbed and reconstituted samples of different tested soft clays.Also,the strength of different undisturbed and reconstituted clays has the unique relation with a ratio of void ratio at failure to the reference void ratio e₁₀^*.Based on the relation between compression index and shear strength with reference void ratio e₁₀^*,it can be speculated that if the stress is larger than the structure yield stress,most of bonding is destructured and the differences in compression and strength characterists between undisturbed and reconstituted samples are caused by the fabric,which can be expressed by the reference void ratio e₁₀^*.5.A constitutive model for overconsolidated clays proposed by Yao et al.and the corresponding model parameters obtained from the test results are used to simulate the mechanical behavior of overconsolidated reconstituted samples of clay obtained from Pudong,Shanghai.Based on the above constitutive model,an elastoplastic model for structural clays,in which the bonding exists,is introduced and the Hvorslev surface of overconsolidated clay is revised.The corresponding simulation is also carried out on the undisturebed samples of different four clays.The predicted results show that the constitutive models can capture the mechanical behavior of undisturbed clay samples well.