Deep foundations

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