The cyclic behaviour of sand, from the Prevost model to offshore geotechnics

The main objective of this thesis is the studying of the cyclic behaviour of dense sand. Its centre of gravity is the implementation of a convenient constitutive law in the finite element code LAGAMINE. The first step consists in summarising the salient features of the cyclic behaviour of sand observed in laboratory tests. Undrained monotonic and cyclic experiments are addressed. The phase transformation line is a key parameter in the description of its behaviour. The plasticity effects in both loading and unloading, the progressive pore pressure accumulation coupled with the degradation of the stiffness of the soil are of uttermost importance in the cyclic behaviour. The Prevost model is adopted for its conceptual simplicity and its physically related parameters. The basic equations of the model are described and its variants are illustrated. Calibration of the required parameters is carried out by means of simplified routines implemented in Matlab. A unique set of parameters is determined for each soil at a given relative density. Monotonic experiments are well reproduced. The model also replicates satisfactorily the trend of cyclic experiments. An implicit scheme is embraced in order to implement the model in the finite element code LAGAMINE. The implicit Prager translation rule is adopted for that purpose. It is ensured the discrete formulation reproduces exactly the analytical model. Accuracy, efficiency and robustness of the model are addressed throughout triaxial and multi-axial numerical examples. A hydro-mechanical interface finite element is developed. It consists in a three-node 1D isoparametric element. It is able to reproduce fluid flows across and along the interface. The unsticking of both walls of the interface is coupled with a suction effect due to the filling of the vacuum created. The behaviour of the element is illustrated by simple 1D examples of transient consolidation of a soil column. A final application consists in the modelling of a suction caisson, part of a tripod structure for wind turbines. An axisymmetric representation of this foundation is carried out. It is assumed embedded into dense Lund sand described by the Prevost model. Monotonic and cyclic simulations are performed in both drained and partially drained conditions. The salient features of the resistance of such foundations are highlighted. Their partially drained behaviour strongly increases their transient resistance. A storm including an extreme event is employed to simulate a cyclic loading. A pseudo-random short signal and its sinusoidal equivalent representation finally lead to an identical vertical settlement. This is confirmed by a long duration storm. Finally, a cyclic diagram summarising the final settlement attained for combinations of average and cyclic vertical loads is elaborated. Keyword