On the modelling of coupled hydro-mechanical behaviour of interfaces for offshore foundations

Complexity and cost of offshore foundations are increasing. Small scale and large scale modelling alone, although essential, are not sufficient to elaborate accurate design methods. As a complement to physical modelling, numerical methods provide more systematic and numerous results. Sensitivity analyses and many configurations may be tested for a lower cost. Therefore numerical modelling, such as finite element method, has become an essential tool for engineers in the last decades. The derivation of the ultimate bearing capacity of the foundations is not sufficient anymore and serviceability becomes an important concern. The corollary effect is the need of modelling transient behaviour, especially displacement and rotation evolution, of the foundation system. If the investigated loading duration is sufficiently long, pore water pressure may be generated within the soil but also partly dissipated. Therefore the modelling of purely drained or undrained conditions are not relevant. The partially drained behaviour must be reproduced and coupled hydromechanical simulations must be carried out. Capturing the behaviour of interfaces is essential in order to well reproduce the complex behaviour of the soil foundation interaction. Indeed, shearing along the shaft of piles and suction caissons is the main mechanism of resistance upon traction. Losses of contact are frequent upon traction for suction caissons or upon lateral loading for piles in clay, reducing the surface for shear mobilisation. A zero-thickness hydro-mechanically coupled finite element is proposed to satisfy the need of interface modelling in partially drained conditions. The element couples a large displacement formulation of the mechanical contact with a transversal three-node formulation for fluid flow. The field of water pressure is discretised on each side of the interface but also between them. Two transversal and one longitudinal fluid flows are modelled. The hydromechanical coupling arises from the definition of an effective stress, the filling of an opening gap and the variation of the longitudinal permeability with the gap opening. This chapter provides the basic formulation of a coupled finite element of interface. It is followed by two examples of applications related to the vertical (uplift) or lateral loading of a suction caisson. In both cases, the role of interfaces is highlighted by purely and partially drained simulations. Coupled phenomena and their implications on practical results such as the elaboration of p-y curves or the ultimate bearing capacity computation are highlighted.