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Abstract For soils in soft status, with high porosity or under high loading, the finite deformation theory should be used to analyze their deformation characteristics because the calculated results based on the small deformation theory may deviate from their actual practice which is not allowed. Based on the triple-shear unified failure criterion and the methods of equivalent substitution and coordinate translation, the triple-shear failure stress ratios for saturated clays in normal consolidation are derived and combine with the modified Cambridge model to obtain the triple-shear unified yield surface equations. Two triple-shear unified elasto-plastic constitutive models with finite deformations using the methods of equivalent substitution and coordinate translation are established to reflect nonlinear and large deformation characteristics for saturated clays in normal consolidation. In order to verify the applicability of these models, the conventional triaxial CU and CD tests are conducted for Jiangxi red clay samples with three compaction degrees under different confining pressures, and the experimental data are compared with the theoretical results obtained from the two finite deformational models and the corresponding two small deformational models. Conclusions show that the calculated results deduced from the constitutive models with finite deformational theory are closer to the test results compared with the models of infinitesimal deformational theory due to deformation development, so the former can better reflect the large deformation characteristics of the clays with high porosity caused by low initial compaction and low confining pressure in the early compression stage. Although the deformation deviations calculated with the infinitesimal deformation models would decrease with the increase of initial compaction degrees or confining pressures, deformation deviations with the finite deformation models are all relatively small with different initial compaction degrees or confining pressures. The finite deformation model with the equivalent substitution method gives the minimal deviations under conditions of large initial compaction degrees or high confining pressures. True triaxial simulations with the constitutive models proposed in this paper show that the influence coefficient b of the intermediate principal stresses and the initial compaction degrees have certain influences on the strength and deformation characteristics of the clays. Results show that the principal stress differences, the pore water pressures and the volume strains are positively correlated with b; the principal stress differences are positively correlated with the initial compaction degrees, while the pore water pressures and volume strains are negatively correlated with the initial compaction degrees.
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Received: 14 September 2020
Published: 14 April 2021
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2021, Vol. 42 
