Keywords: elastoplastic model, clay, sand, bearing capacity, foundation.

An isotropic hardening elastoplastic model for soils is presented, which can describe typical deformation and strength behaviour of normally and over consolidated soil under general stress conditions. This model can take into consideration the influence of intermediate principal stress on the deformation and strength of soil upon using the concept of modified stress t_ij. The influence of stress path on the direction of plastic flow is considered by dividing the plastic strain increment into two components. After inclusion of the parameter '' and subloading surface concept, this model can express the influence of density and confining pressure on the soil behaviour. Only one material parameter a to represent the influence of density is added to the parameters of the previous model, which are fundamentally the same as those of the Cam-clay model. The model is then applied to the analyses of bearing capacity problems of the foundation under various loading conditions.

For the appropriate prediction of the deformation and failure of the soil, we have to carry out numerical analysis using a simple and generalized constitutive model for soils. The well-known Cam clay model is is certainly very simple, - i.e., the number of material parameters is few, and the meaning of each parameter is clear. However, the basic Cam clay model [1] has problems to describe the soil behaviour in general stress systems. We also have developed simple constitutive models for clay and sand - namely the t_ij-clay model [2] and t_ij-sand model [3]. In these models, the influence of intermediate principal stress on the deformation and strength of soil and the stress path dependency of plastic flow are particularly taken into consideration. In the present study, the above models for clay and sand are extended to a simple model that can take into account the influence of the density and/or confining pressure on the deformation and strength of soil, as well as the influence of intermediate principal stress on the deformation and strength and the influence of stress path on the plastic flow [4].

This model is applied to the analyses of bearing capacity problems of shallow foundations and piled raft foundations under various loading conditions - i.e., concentric loading, eccentric loading and inclined loading. The computed results are compared with the corresponding results of model tests. There is good agreement between the computed results and the test results not only qualitatively but also quantitatively.

1

Schofield, A.N., Wroth, C.P., "Critical State Soil Mechanics", McGraw-Hill, London, 1968.

2

Nakai, T., Matsuoka, H., "A generalized elastoplastic constitutive model for clay in three-dimensional stresses", Soils and Foundations, 26(3), 81-98, 1986.

3

Nakai, T., "An isotropic hardening elastoplastic model for sand considering the stress path dependency in three-dimensional stresses", Soils and Foundations, 29(1), 119-137, 1989.

4

Nakai, T., Hinokio, T., "A simple elastoplastic model for normally and over consolidated soils with unified material parameters", Soils and foundations, 44(2), 53-70, 2004.

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