Bounding surface constitutive model for soft soils considering characteristics of small-strain stiffness and its application in engineering

The construction of an excavation pit induces complex loading-unloading stress paths in the surrounding soils, while the commonly used constitutive models are insufficient to reflect the over-consolidation and small-strain stiffness characteristics during excavation. In this study, a modified bounding surface model considering the over-consolidation and strain-stiffness relationship of soft soils is proposed by modifying the shear modulus under small-strain state. Based on the user defined material subroutine (UMAT), the modified bounding surface model is developed into the finite element software ABAQUS through an accurate and efficient semi-implicit stress update algorithm, and the numerical simulation is carried out for a deep excavation of underground station. The constitutive relationship of typical Shanghai soil under small-strain state is well predicted by the proposed model. The stress-strain curve results show that the proposed model reflects the relationship between soil strain and stiffness reasonably. Since the modified bounding surface model reasonably reflect the over-consolidation and small-strain stiffness characteristics of soil in the same time, the numerical simulation results by the proposed model satisfy the field data.