RIVISTA ITALIANA DI GEOTECNICA 2/2008 Finite element analysis of model piles axially loaded in sands Vincenzo De Gennaro,* Roger Frank,* Imen Said** 1. Introduction When considering the interaction of two media in contact with highly-distinct deformability charac- teristics failure is often accompanied by the forma- tion within the more deformable medium of a rather thin zone oriented in the direction of the contact surface. This zone, called the soil-structure in- terface, or simply interface, experiences intense strain localization and plays the role of a kinematic discon- tinuity characterized by extremely high strain gradi- ents. Quite a large number of civil engineering struc- tures lie in contact with soils. Such is the case, for ex- ample, in soil-retaining walls, soil-anchorage rods, soil-piles or micropiles, or soil-reinforcements (e.g. “terre armée”, nailed soils). Failure in these struc- tures occurs mainly due to progressive shearing and is often observed at the interface, in the softer me- dium (i.e. the soil mass), where stresses and strains are transmitted. The description of the mechanical behaviour, mainly in terms of mobilized friction be- tween the structural element and the soil, must con- sequently entail constitutive modelling of this heav- ily 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 soil- pile interface exerts significant influence in defin- ing structural stability conditions. The paper will focus first on the general frame- work of the soil-pile interaction modelling using the finite element method (FEM); a description will be provided of how contact problems have been tack- led 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 pur- pose for describing the behaviour of the interface between a granular soil and a rigid structure within the framework of hardening plasticity in two-di- mensional or axisymmetric conditions, will be briefly described. The role of the surrounding soil mass will also be investigated; elasticity, ideal plas- ticity 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 CESAR- LCPC 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 in- terface. In the majority of the numerical applica- tions piles are “wished in place”, assuming that in- stallation effects, if any, have only a limited impact on their mechanical behaviour. Given the obstacles in generating accurate simulations of pile installa- tion 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 is- sue. 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]. 2. Axisymmetric FEM modelling of soil-pile interface When solving problems with boundary condi- tions that integrate interfaces, the use of FEM for modelling contact with standard elements is often cumbersome, since the relative movement of solids in contact is not, at first inspection, compatible with the displacement continuity condition required at the nodes of adjacent elements (e.g. POTTS and ZDRAVKOVIC, 2001). The soil-pile interaction provides a typical example of an interface problem. So-called "conventional methods" for piles computing are based on empirical correlations that lead to deter- mining the limit load-bearing capacity. Somewhat less attention is usually paid to determining the deformation characteristics corresponding with the * Université Paris-Est, Navier (ENPC- CERMES), Paris, France ** Université Paris-Est, Navier (ENPC- CERMES), Paris, France, ENIT, Tunis, formerly URIG
Finite element analysis of model piles axially loaded in sands
Jun 04, 2023
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