A contact problem aplication for the local behaviour of soil pile interaction

In geotechnical engineering, the main parameter for the performance of structures such as reinforced walls or deep foundations is often the shaft bearing capacity. In numerical analysis, important advancements have been made on studying the behavior of the soil and the retaining structures separately. The performance of many geotechnical foundation systems depends on the shear behavior at the soil structure interface. For deep foundations, the main component that affects friction is the horizontal earth pressure. When a pile is getting axially loaded, the soil grain network at the interface, starts to move and rearrange. In conditions of axial cyclic loading a contractive behavior of soil can generally be observed as in [1] and [2]. This can be explained by the progressive densification and relaxation of the soil under cyclic shear at the soil pile interface, as well as the local refinement of the grain distribution by grain breakage and rearrangements. As the soil contracts and decreases in volume, the normal stress around the pile surface decreases and the soil pile friction degrades. This can lead to failure of the whole geotechnical foundation system. The purpose of the work presented in this paper is to analyze locally (at the element level) the contact behavior of a soil-pile contact problem. Therefore, a 2D shear test is modeled using the Finite Element Method. The formulation of a 4 nodded zero-thickness interface element of Beer [3] is chosen with a linear interpolation function. Four constitutive contact models adapted for contact problems have been implemented. The simple Mohr-Coulomb [4] and Clough and Duncan [5] models were chosen initially, due to the ease of implementation and few number of parameters needed. After, more complicated models in the framework of elasto-plasticity such as: Lashkari [6] and Mortara [7] were implemented for the first time into the finite element code of the shear test problem. They include other phenomena such as: relative density of soil, the stress level and sand dilatancy. From the results the relation between shear displacement and shear stress has been deduced. Finally, a discussion of the advantages and the drawbacks during computation of each model is given at the end

Modelling and calibration for cyclic soil-structure interface behaviour

The structural performance of many geotechnical systems (e.g. axially-loaded pile foundations), depends on the shearing resistance at the soil interface, which may govern the load bearing capacity of the foundation. Experimental investigations have shown that this interaction is mainly localised within a narrow shear band next to the structure. Under cyclic loading, a contraction of the soil at the interface may arise (net volume loss), possibly leading to a stress relaxation and thus to a reduction of the load bearing capacity (the so-called friction fatigue). Based on the constitutive similarities between soil continua and interfaces, we propose here the adaption of a Generalized Plasticity model for sandy soils for the numerical analysis of interface problems. In this contribution, the results of an experimental campaign for the parameter calibration of the constitutive model are presented. The tests have been conducted with a ring shear device involving different normal stresses, roughness of the steel plates as well as cyclic loading. The new modelling approach shows promising results and has the additional practical advantage that the interface zone and the soil continuum can both be described with the same constitutive model in general boundary value problems

A contact problem aplication for the local behaviour of soil pile interaction

In geotechnical engineering, the main parameter for the performance of structures such as reinforced walls or deep foundations is often the shaft bearing capacity. In numerical analysis, important advancements have been made on studying the behavior of the soil and the retaining structures separately. The performance of many geotechnical foundation systems depends on the shear behavior at the soil structure interface. For deep foundations, the main component that affects friction is the horizontal earth pressure. When a pile is getting axially loaded, the soil grain network at the interface, starts to move and rearrange. In conditions of axial cyclic loading a contractive behavior of soil can generally be observed as in [1] and [2]. This can be explained by the progressive densification and relaxation of the soil under cyclic shear at the soil pile interface, as well as the local refinement of the grain distribution by grain breakage and rearrangements. As the soil contracts and decreases in volume, the normal stress around the pile surface decreases and the soil pile friction degrades. This can lead to failure of the whole geotechnical foundation system. The purpose of the work presented in this paper is to analyze locally (at the element level) the contact behavior of a soil-pile contact problem. Therefore, a 2D shear test is modeled using the Finite Element Method. The formulation of a 4 nodded zero-thickness interface element of Beer [3] is chosen with a linear interpolation function. Four constitutive contact models adapted for contact problems have been implemented. The simple Mohr-Coulomb [4] and Clough and Duncan [5] models were chosen initially, due to the ease of implementation and few number of parameters needed. After, more complicated models in the framework of elasto-plasticity such as: Lashkari [6] and Mortara [7] were implemented for the first time into the finite element code of the shear test problem. They include other phenomena such as: relative density of soil, the stress level and sand dilatancy. From the results the relation between shear displacement and shear stress has been deduced. Finally, a discussion of the advantages and the drawbacks during computation of each model is given at the end

An enhanced interface model for friction fatigue problems of axially loaded piles

The shaft bearing capacity often plays a dominant role for the overall structural behaviour of axially loaded piles in offshore deep foundations. Under cyclic loading, a narrow zone of soil at the pile-soil interface is subject to cyclic shearing solicitations. Thereby, the soil may densify and lead to a decrease of confining stress around the pile due to micro-phenomena such as particle crushing, migration and rearrangement. This reduction of radial stress has a direct impact on the shaft capacity, potentially leading in extreme cases to pile failure. An adequate interface model is needed in order to model this behaviour numerically. Different authors have proposed models that take typical interface phenomena in account such as densification, grain breakage, normal pressure effect and roughness. However, as the models become more complex, a great number of material parameters need to be defined and calibrated. This paper proposes the adoption and transformation of an existing soil bulk model (Pastor- Zienkiewicz) into an interface model. To calibrate the new interface model, the results of an experimental campaign with the ring shear device under cyclic loading conditions are here presented. The constitutive model shows a good capability to reproduce typical features of sand behaviour such as cyclic compaction and dilatancy, which in saturated partially-drained conditions may lead to liquefaction and cyclic mobility phenomena