The modelling and simulation of cyclic sand ratcheting is tackled by means of a plasticity model formulated within the well-known critical state, bounding surface SANISAND framework. For this purpose, a third locus – termed the ‘memory surface’ – is cast into the constitutive formulation, so as to phenomenologically capture micro-mechanical, fabric-related processes directly relevant to the cyclic response. The predictive capability of the model under numerous loading cycles (‘high-cyclic’ loading) is explored with focus on drained loading conditions, and validated against experimental test results from the literature – including triaxial, simple shear and cyclic loading by oedometer test. The model proves capable of reproducing the transition from ratcheting to shakedown response, in combination with a single set of soil parameters for different initial, boundary and loading conditions. This work contributes to the analysis of soil–structure interaction under high-cyclic loading events, such as those induced by environmental and/or traffic loads.

This work presents a critical state plasticity model for predicting the response of sands to cyclic loading. The well-known bounding surface SANISAND framework by Dafalias and Manzari is enhanced with a memory surface to capture micromechanical, fabric-related processes directly affecting cyclic sand behavior. The resulting model, SANISAND-MS, was recently proposed by Liu et al. and successfully applied to the simulation of drained sand ratcheting under thousands of loading cycles. Herein, novel ingredients are embedded into Liu et al.’s formulation to better capture the effects of fabric evolution history on sand stiffness and dilatancy. The new features enable remarkable accuracy in simulating undrained pore pressure buildup and cyclic mobility behavior in medium-dense to dense sand. The performance of the upgraded SANISAND-MS is validated against experimental test results from the literature—including undrained cyclic triaxial tests at varying cyclic loading conditions and precyclic consolidation histories. The proposed modeling platform will positively impact the study of relevant cyclic and dynamic problems, for instance, in the fields of earthquake and offshore geotechnics.

Serviceability criteria for offshore monopiles include the estimation of long-term, permanent tilt under repeated operational loads. In the lack of well-established analysis methods, experimental and numerical research has been carried out in the last decade to support the fundamental understanding of monopile–soil interaction mechanisms, and the conception of engineering methods for monopile tilt predictions. With a focus on the case of monopiles in sand, this work shows how step-by-step/implicit, three-dimensional (3D) finite-element (FE) modelling can be fruitfully applied to the analysis of cyclic monopile–soil interaction and related soil deformation mechanisms. To achieve adequate simulation of cyclic sand ratcheting and densification around the pile, the recently proposed SANISAND-MS model is adopted. The link between local soil behaviour and global monopile response to cyclic loading is discussed through detailed analysis of model prediction. Overall, the results of numerical parametric studies confirm that the proposed 3D FE modelling framework can reproduce relevant experimental evidence about monopile–soil interaction, and support future improvement of engineering design methods.

Optimised design is essential to reduce the cost of monopiles for offshore wind turbines. For this purpose, an in-depth understanding of the behaviour of monopile–soil interaction is required. As more wind farms are planned in seismically active areas, the undrained behaviour of sandy soils (and the possibility of soil liquefaction) and these soils’ effects on monopile cyclic response need critical evaluation. Considering the lack of well-established test programs, implicit three-dimensional (3D) finite-element (FE) methods stand out as a robust tool to identify and highlight the governing geo-mechanisms in monopile design. In this work, an implicit 3D FE implementation of SANISAND-MS for undrained soil behaviour, termed SANISAND-MSu, is deployed in OpenSees to serve these objectives. The role of pore-water pressure on monopile performance is comprehensively investigated by comparisons between drained and undrained soil behaviour. Local soil responses are studied in detail in relation to parameters in laboratory soil testing and application to monopile geotechnical design. The results of simulations are also used to evaluate numerical p–y curves as a function of the number of load cycles on the pile. The conclusions in this work contribute to ongoing research on monopile–soil interaction and support the development of lifetime analysis for monopile–soil systems.