Roger Frank

Elasto-plastic analysis of the interface behaviour between granular media and structure

Abstract It is well known that interfaces usually play a major role in the definition of the mechanical behaviour of engineering structures having interactions with the soil. In this paper the general framework of an elasto-plastic constitutive model developed on purpose for describing the interface behaviour is presented. The model is based on a Mohr–Coulomb failure criterion, including deviatoric hardening/softening, phase transformation state (compaction and dilatancy) and ultimate state. The choice of the constitutive parameters and their identification is first discussed. The predictions of the model are then presented and compared with available experimental data from various interface tests between sand and metal plates. The results of the numerical analyses emphasise the key role played by the volumetric behaviour of the interface (compaction and dilatancy), linked in some cases with the change in the normal stress acting on the structure surface and, consequently, controlling the shear resistance at the interface.

Verification of ultimate limit states in geotechnical design by Eurocode 7 in France and Germany

ABSTRACT This paper describes the three design approaches (DA 1, DA 2 and DA 3) offered by Eurocode 7 on “Geotechnical design” for verifying ultimate limit states in persistent or transient design situations (i.e. under fundamental combinations). They are applied and compared in the case of a strip footing under eccentric and inclined loading. Both in France and in Germany, DA 2 has been selected for most geotechnical structures, though with some differences between the two countries for a limited number of cases. The principles and the choices made for the selection of the design approach, as well as of the values for the partial factors of safety, are explained for the two countries.

Development of Axial Pile Load Transfer Curves Based on Instrumented Load Tests

The load transfer method is an efficient and practical tool for the analysis of single piles and combined systems such as pile-raft foundations and rigid inclusions. The existing transfer curves rely either on the measurement of the pressuremeter modulus, which is not widespread at the international level, or they have been developed only for specific pile and ground types. In order to develop new transfer curves valid for all pile and ground types without the need of pressuremeter tests, a database of 50 instrumented pile load tests was examined. The existing curves were first compared to the measurements at the pile shaft and at the tip. Then the parameters of the most appropriate curve types, the cubic root and the hyperbolic curve, were calibrated in order to best match the measurements. A single set of parameters for each proposed curve type is suitable enough for most pile and ground types. The good agreement of the proposed curves was confirmed by applying them to assess the overall load-settlement curve of 72 other pile load tests.

Etude en petites déformations de l'interaction entre une fondation superficielle et une conduite enterrée

ABSTRACT This paper presents the finite element modelling of the interaction between a shallow foundation and a buried pipe. An elastoplastic model with a linear elastic part and Mohr-Coulomb criterion is compared to a non-linear elastic hyperbolic model. It is shown that the elastoplastic model does not allow to represent properly the behaviour of both the foundation and the pipe. Indeed, the choice of a modulus adapted to the prediction of the settlement of the foundation leads to an important underestimation of the stiffness of the soil around the pipe and, consequently, to an overestimation of the stresses in the pipe. On the contrary, the non-elastic model is capable of representing both the high stiffness around the pipe and the significant decrease in stiffness directly below the foundation. The use of nonlinear elasticity is thus interesting, provided it is possible to determine the variation of the modulus with strain and if the loading in monotonous.

Some aspects of research and practice for pile design in France

This paper summarises some aspects of the methods used in France for designing pile foundations under axial and transverse loadings. These methods mostly use the results of Ménard pressuremeter (MPM) tests and concern the determination of the bearing capacity, as well as the prediction of axial and transverse displacements. The prediction of the bearing capacity from CPT results is also given. After mentioning the general context of foundation design in France, the details of these methods are described and some of their experimental background is explained. These methods are now included in the French standard for pile design (published by AFNOR, Justification of geotechnical work—National application standards for the implementation of Eurocode 7—Deep foundations, 206, 2012), fully compatible with Eurocode 7 on ‘Geotechnical design’.

Elasto-plastic Modelling of the Behaviour of Granular Soil-structure Interfaces

It is well known that interfaces usually play a major role in the definition of the mechanical behaviour of engineering structures having interactions with the soil. In this paper the general framework of an elasto-plastic constitutive model developed on purpose for describing the interface behaviour is presented. The model is based on a Mohr-Coulomb failure criterion, including deviatoric hardening/softening, phase transformation state (compaction and dilatancy) and critical state. The choice of the constitutive parameters and their identification is first discussed. The predictions of the model are then presented and compared with available experimental data from various interface tests between sand and metal plates. The results of the numerical analyses emphasise the key role played by the volumetric behaviour of the interface (compaction and dilatancy), linked in some cases with the change in the normal stress acting on the structure surface and, consequently, controlling the shear resistance at the interface.

Projet national FOREVER

ABSTRACT Micropiles group and network concept, developed by F. Lizzi, considers soil and micropiles in a complex interaction. Experimental tests, with vertically or laterally loads, were performed on groups and elementary networks of micropiles, as part of the French national research program FOREVER. These tests were carried out in calibration chambers, in a centrifuge and in full scale. The main object of these tests was to study the influence of different arrangements on the efficiency coefficient. The different tested parameters were the micropile spacing, their number, their installation method, and their inclination. The analysis of the whole experimental program results leads to the following conclusions. For a positive group or network efficiency a large number of micropiles is required. The optimal spacing is about 2 or 3 times the micropile diameter. The network arrangement is, a priori, less favourable than the group one for vertical loading; it is the contrary for horizontal loading. Load transfer method (p-y and t-z curves) is then used in a three dimensional analysis of the experimental results of the elementary 4 micropiles (double easels) network. The loading curves are well reproduced, given some adjustments and this study concluded on the importance of the axial friction on the behaviour of the networks.

Comportement de pieux d'essais instrumentés sous charge horizontale

This paper describes the behaviour of two test piles (one bored and postgrouted and one simply bored, both 31.7 m long and 0.75 m in diameter) subjected to horizontal loads. These full-scale pile tests were carried out for the actual design of the pile foundation of a pier of the Evripos cable-stayed bridge. This bridge will link the Euboea Island to mainland Greece. The two piles have already been subjected to bearing capacity tests under axial loadings. The inclinometer measurements, taken during the present tests, yielded, in particular, the deformed shape of the piles as well as the bending moments. Conclusions could be drawn for the final design of the pile foundation with respect to horizontal loadings. Furthermore, various calculation methods using p–y reaction curves for cohesionless soils have been checked: the Menard pressuremeter method, the method of the American Petroleum Institute recommendations, and the Standard penetration test method of Christoulas. These pile tests show that simple meas...