Finite element analysis of model piles axially loaded in sands

When considering the interaction of two media in contact with highly-distinct deformability characteristics failure is often accompanied by the formation within the more deformable medium of a rather thin zone oriented in the direction of the contact surface. This zone, called the soil-structure interface, or simply interface, experiences intense strain localization and plays the role of a kinematic discontinuity characterized by extremely high strain gradients. Quite a large number of civil engineering structures lie in contact with soils. Such is the case, for example, in soil-retaining walls, soil-anchorage rods, soil-piles or micropiles, or soil-reinforcements (e.g. “terre armee”, nailed soils). Failure in these structures occurs mainly due to progressive shearing and is often observed at the interface, in the softer medium (i.e. the soil mass), where stresses and strains are transmitted. The description of the mechanical behaviour, mainly in terms of mobilized friction between the structural element and the soil, must consequently entail constitutive modelling of this heavily loaded region. In this work emphasis is given on the behaviour of deep foundations and, more particularly, of the contact between a granular soil and a pile. In this typical soil-structure interaction problem available analyses of the mechanical behaviour of single piles submitted to axial loads have shown that the soilpile interface exerts significant influence in defining structural stability conditions. The paper will focus first on the general framework of the soil-pile interaction modelling using the finite element method (FEM); a description will be provided of how contact problems have been tackled using FEM. The constitutive modelling of the interface and the soils mass will be then presented. The main features of the interface model MEPI-2D [DE GENNARO and FRANK, 2002], formulated on purpose for describing the behaviour of the interface between a granular soil and a rigid structure within the framework of hardening plasticity in two-dimensional or axisymmetric conditions, will be briefly described. The role of the surrounding soil mass will also be investigated; elasticity, ideal plasticity and strain hardening plasticity will be used to model the sand mass. Finally, comparative analyses of pile tests using FEM will be provided by means of the CESARLCPC finite element code [HUMBERT, 1989]. Note that one important issue in the case of piles is the definition of the initial state, following installation and prior to loading, both in the soil and at the interface. In the majority of the numerical applications piles are “wished in place”, assuming that installation effects, if any, have only a limited impact on their mechanical behaviour. Given the obstacles in generating accurate simulations of pile installation via FEM (e.g. simulation of driving, boring, etc.) this problem is still far from being resolved. A new numerical strategy is outlined to tackle this issue. The proposed numerical investigations will be validated against the experimental results obtained during model pile loading tests using a calibration chamber [DE GENNARO, 1999; DE GENNARO and FRANK, 2005] and on a real site [CHOW, 1997; JARDINE et al., 1998 and 2005].