ABSTRACT
Practical boundary value problems with large deformations often involve stress paths which deviate from proportional loading. This leads to plastic instabilities inducing shear bands or diffusive modes of failure. Traditional coaxial plasticity theories are not sufficient to predict the inelastic stretching caused by principal axis rotations which occur during the formation of shear bands. This paper highlights the influence of the non-coaxial plasticity on granular soil followed by its influence on the pipe-soil interaction under large deformations. The tangential plasticity concept proposed by Hashiguchi (1993) is integrated into the three invariant Nor-Sand constitutive model which is enriched with critical state and state parameter theories. This model is numerically implemented into the finite element software ABAQUS and is validated for simple shear simulations of drained sand.
The peak mobilised force and the corresponding displacement are main design parameters of the pipe-soil interaction problems. The predictions of these design parameters for the upheaval buckling of buried pipelines using the non-coaxial model are compared with those of conventional coaxial models. The effect of soil friction and dilation along with the pipe depth to diameter ratio are discussed. It is observed that the non-coaxial model predicts a softer response leading to higher mobilised displacements at the peak uplift force. Ultimate pulling capacities of the non-coaxial model are close to those of the coaxial model. The discrepancy between coaxial and non-coaxial models increases with the greater rotation of principal stress axis inside the shear bands formed around the pipe. This study sheds light on the fundamental understanding of the phenomenon of principle axis rotation during strain localisation and its effect on the soil-pipe interaction.
