Article | REF: C248 V3

Deep foundations

Authors: Roger FRANK, Fahd CUIRA, Sébastien BURLON

Publication date: December 10, 2019

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ABSTRACT

This paper presents the geotechnical methods for designing deep foundations, in particular those introduced in the French practice by the limit state approach advocated by Eurocode 7 on « Geotechnical design » and the French application standard for deep foundations NF P 94-262. This standard has modified, in particular, the rules for assessing the bearing capacity of deep foundations, in comparison with the former codes of practice (Fascicule 62-Titre V of CCTG and DTU 13.2). After describing the various types of deep foundations and summarizing the limit state load combinations, this paper deals with the pressuremeter and penetrometer methods for assessing the bearing capacity and for modelling the behaviour of piles under transverse loadings. Other aspects are also dealt with: assessment of the settlement of deep foundations, of the negative friction (downdrag) and lateral thrusts and of group effects. Finally, the requirements of the standard NF P 94-262 are described.

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AUTHORS

  • Roger FRANK: Honorary Professor - École nationale des ponts et chaussées

  • Fahd CUIRA: Scientific Director - Terrasol (Setec Group)

  • Sébastien BURLON: Research Director - Terrasol (Setec Group) - This article is an update of the previous edition published in 1995.

 INTRODUCTION

There are two main ways of transmitting building loads to the underlying soil layers: by surface foundation and by deep foundation. In this article, the word "foundation" is taken to mean the construction element (usually reinforced concrete or steel). Under certain conditions, it can also mean the soil layers themselves (on or through which the construction is to be "founded").

Superficial foundations are, by definition, foundations that rest on the ground or are only slightly embedded in it. These include footings, inverts, etc. .

When the soil near the surface does not have sufficient mechanical properties to support loads via surface foundations, or its resistance is too low, or the anticipated settlements are detrimental to the construction, deep or semi-deep foundations are used. Deep foundations (mainly pile foundations) are those that enable the loads due to the construction they support to be transferred to layers located from the surface down to a depth varying from a few meters to several tens of meters. When calculating the load-bearing capacity of piles, it is therefore necessary to consider, in addition to the resistance of the soil beneath the base, the resistance of the soil on the side walls, i.e. the "axial" friction along the shaft of the piles.

Barrettes are load-bearing cast walls which, although different in shape and requiring a specific technique, are generally comparable in design and behavior to bored piles.

Between the two extremes of shallow and deep foundations, we distinguish between semi-submerged foundations whose base is relatively close to the surface, but for which axial friction cannot be neglected: these include short shafts and piles, shallow bars and most caissons. There is no specific calculation method for this category of foundations, which constitute special cases only. The methods used for shallow or deep foundations will have to be adapted according to each case. In particular, the method of execution or installation, similar to that of a shallow foundation or a deep foundation, will provide guidance.

This article describes the most common methods for calculating deep foundations, including those recommended by the French standard for the application of Eurocode 7 (AFNOR, 2012). For further information, particularly on numerical models and soil-structure interaction, please refer to

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KEYWORDS

standards   |   geotechnical engineering   |   foundations   |   settlements


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