Advanced constitutive modelling for deformation prediction of deep excavations in structured soft clay

Abstract

Deep excavations in urban areas underlain by structured soft clay are critical for transportation infrastructure development, yet their deformation behavior is strongly influenced by natural soil structure and stress-path-dependent degradation. Conventional constitutive models (e.g., MCC and HS/HSS), which neglect soil structure and intermediate principal stress, often lead to unconservative deformation predictions. A modified constitutive model for structured soft clay is developed within the critical state framework, incorporating the superloading surface concept to represent soil structure. The model considers three key mechanisms governing excavation-induced deformation: (i) the intermediate principal stress effect through the SMP yield criterion, (ii) the initial soil structure described by a structural parameter R* , and (iii) strain-path-dependent structural degradation. A modified evolution law is introduced, in which a parameter A is used to distinguish the relative contributions of plastic volumetric and deviatoric strains to structural degradation. The model is implemented in finite element analyses and validated against centrifuge model tests. Parametric analyses are conducted to investigate the effects of intermediate principal stress, initial soil structure, and the plastic strain ratio parameter. The results show that neglecting intermediate principal stress underestimates wall deflection and ground surface settlement, while accounting for soil structure increases predicted deformation. This effect is moderated when strain-path-dependent structural degradation is considered, leading to improved agreement with experimental observations. Overall, the proposed approach provides a more realistic framework for deformation prediction of deep excavations in structured soft clay, with direct relevance to underground transportation infrastructure design.