Some aspects of research and practice for foundations design

After giving the general context of foundation design in France, the present paper summarizes the main features of the methods and rules used for designing foundations: shallow foundations, axially loaded piles and transversely loaded piles. These methods are established mostly with the use of results of Ménard pressuremeter tests (MPM). They are included, in particular, in the Code for civil engineering works of the Ministry of public works, called ‘Fascicule 62 – V’ (MELT, 1993) The original experimental research programmes are mentioned, and the present transition for adapting them to Eurocode 7 on ‘Geotechnical design’ is described. POVZETEK: V prvem delu prispevka so predstavljeni splošni principi projektiranja temeljenja v Franciji. V nadaljevanju so povzete glavne metode in na ela za projektiranje plitvih temeljev ter osno in pre no obremenjenjih pilotov, ki ve inoma temeljijo na rezultatih meritev z Ménardovim presiometrom (MPM). Te metode so sestavni del Predpisov za inženirske konstrukcije t.i. ‘Fascicule 62 – V’ (MELT, 1993), ki jih izdaja Ministrstvo za javna dela. Omenjeni so tudi originalni ekperimentalni raziskovalni programi ter njihova implementacija v Evrokodu 7 o geotehni nem projektiranju. INTRODUCTION AND HISTORICAL BACKGROUND Since the early 1970’s foundations are designed in France using extensively the results of in situ tests, namely cone penetration tests (CPT) and Ménard pressuremeter tests (MPM). This is due, in particular, to the difficulty of taking so-called ‘intact’ samples in most of the grounds... This paper focuses mainly on the use of MPM test results, but some aspects are also valid for CPT results, or are of wider application, such as the predictions of foundation movements for the design of structures. Details on the use of CPTs can be found in the comprehensive papers by Frank and Magnan (1996) and Bustamante and Frank (1999). The Ménard pressuremeter is a specific form of prebored pressuremeters (note that sometimes it can be driven inside a slotted tube) It was invented and developed by Ménard (1955) who also established the first corresponding rules for the design of foundations: settlement of foundations (Ménard and Rousseau, 1962), behaviour of deep foundations under transverse loading (Ménard, 1962-1969), and axial bearing capacity of foundations – whether shallow or deep (Ménard, 1963). As a consequence of the growing interest in the use of MPM for soil characterisation, in the subsequent decade Ménard proposed some further developments of these design directives. The evolution of the rules for the design of shallow and deep foundations was included in a general document published by Ménard (1975). The document dealt with the analysis of available methods of shallow and deep foundation design (bearing capacity (qu), skin friction (qs) for piles and foundation settlements). Roger Frank Some aspects of research and practice for foundation design in France 26 It is clear that the immense advantage of the Ménard pressuremeter test (MPM) is that it provides the geotechnical engineer with both a failure parameter (the limit pressure pl) and a deformation parameter (the pressuremeter modulus EM). It enables him to tackle with the same test the problems of bearing capacity of foundations (using pl), as well as the problems of displacements of foundations (using EM), i.e. the problems of deformation of the structures to be carried. The development of the use of MPM for foundation design was, nevertheless, very often limited by the fact that it needed a new approach, outside the conventional and classical framework of soil mechanics (which had been developed mainly with the use of laboratory tests, like the triaxial test and the oedometer test) see, for instance, the recent paper by Gambin and Frank (2009). Indeed, the rules for the design of foundations from MPM are essentially of ‘direct’ type, i.e. they use direct correlations between the measured parameter (pl or EM) and the ‘design’ parameter (bearing capacity, settlement or transversal displacement). They do not require to determine first a ‘basic’ soil parameter (parameters of shearing resistance or oedometer modulus) to enter, subsequently, into the classical bearing capacity formulae or oedometer or elastic formulae for the settlement.