Advanced numerical modelling of the cyclic/dynamic response of offshore structures in inhomogeneous non-cohesive soils

Abstract

Currently available constitutive models for the non-linear behaviour of granular non-cohesive soils are typically only able to capture the response due to changes in shear stress ratios while assuming elastic response for induced changes to mean effective stresses. As a result, their applicability is limited to cases where shearing is the governing response aspect, such as earthquake-induced liquefaction, under practically undrained conditions. However, they do not allow for accurate simulations in cases of partial or complex drainage, such as post-liquefaction consolidation, phenomena involving large numbers of loading cycles, the use of drains, the inclusion of gravel and other permeable materials and the localised application of sand densification methods for liquefaction mitigation. To address this shortcoming, a novel constitutive formulation is proposed for capturing the non-linear elastoplastic response of sands under isotropic or constant shear stress ratio loading and unloading. This formulation can be incorporated as an extension to existing bounding surface models, which are typically based on the framework of critical state soil mechanics and capable of capturing most shear-related aspects of sand behaviour, including dilatancy, shear modulus degradation and damping increase, excess pore pressure build-up towards liquefaction and fabric evolution effects. The proposed formulation is based on the introduction of a “plastic cap” as a second plasticity mechanism, which adopts the principles of bounding surface plasticity with a vanished elastic region. In this work, this additional mechanism is incorporated into the existing NTUA-Sand model. However, most bounding surface models can be used as “base” models for this purpose, extending the applicability and potential impact of the proposed framework. Following a definition of the novel framework and the plasticity components, a step-by-step procedure is introduced to calibrate the associated model parameters. Calibration is subsequently performed for Ottawa and Erksak sands, based on available laboratory data in the literature. The developed constitutive model is subsequently implemented as a User-Defined Model (UDM) into the commercially available Finite-Difference code FLAC, allowing to perform fully coupled dynamic effective stress numerical analyses with groundwater flow, thus extending the practical applicability of the proposed framework. The associated code is then used for the simulation of a series of relevant centrifuge experiments that involve seismic liquefaction, post-liquefaction consolidation, soil inhomogeneity and complex drainage conditions. It is demonstrated that the proposed model extension does not influence the accuracy of the base model for a shear-related response. However, it significantly improves the performance when isotropic loading and unloading become dominant.

Date of Award	21 Mar 2023
Original language	English
Awarding Institution	University of Bristol
Sponsors	CONACyT
Supervisor	Dimitris K Karamitros (Supervisor) & Andrea Diambra (Supervisor)