Shallow foundations

Introduction

It is termed a shallow foundation, but also a spread foundation or, even, a footing, one that transmits the action from the structure to a load-bearing layer located close to the ground surface.

In functional terms, what distinguishes a shallow foundation from a deep foundation is that, in its interaction with the soil, to balance out the vertical load, the portion of a shallow foundation load not processed through the base is generally negligible and, therefore, ignored in the design process. As for deep foundations, their interaction with the ground along the lateral surface (also called the shaft) is normally taken into account in the design, given its relevance to the equilibration of the vertical load.

In the great majority of building and bridge foundations, the vertical actions are clearly predominant, with the horizontal forces, normally associated with variable or even accidental actions, such as wind or earthquakes, being of modest or moderate value. It is therefore understandable that, normally, the base of shallow foundations is horizontal. Footings with inclined base are sometimes adopted for gravity retaining walls, as will be seen in the next chapter, in view of the large and permanent nature of the horizontal force transmitted to the foundation.

With respect to the ground, the design of a shallow foundation must satisfy the safety requirements relative to both the ultimate and the serviceability limit states.

The most important ultimate limit state consists of the failure of the ground under the footing, due to insufficient bearing capacity relative to vertical loading. This causes a very large vertical displacement that may also induce an ultimate limit state in the structure.

Another very relevant limit state involves excessive foundation settlement. Normally, this is associated with a serviceability limit state, but, under certain circumstances, it may induce an ultimate limit state in structural elements adjacent to that foundation.

The major objective of the present chapter on shallow foundations is to present methodologies to: i) evaluate the bearing capacity for vertical loading; and ii) estimate the settlement. Based on these procedures, it is then possible to define, for a given practical case, the elevation of the footing base and its dimensions in plan. The further design of the foundation as a structural element (namely establishing its height and the area of steel, in the case of reinforced concrete) is outside the scope of this book.

The sizing of shallow foundations has been made for centuries by means of empirical criteria, with the validity restricted to the soils of a given zone or region, adjusted over time on the basis of the successes and failures achieved. These rules were essentially developed for stone masonry structures.

The generalization of frame structures, associated with the use of steel and concrete as the most current structural materials from the middle of the 19th century onwards, has created additional requirements as to what concerns foundation design. In fact, the more frequent construction of taller structures with larger spans has resulted in higher and more concentrated foundation loadings.

The theoretical tools available for shallow foundation design were developed in the 20th century, in parallel with the development of soil mechanics from the pioneering work of Terzaghi in the 1920s. Nevertheless, due the complexity of some of the questions involved in the design process, many of the methods currently applied are still a mix of theoretical, rationally proved solutions, and semi-empirical, or even empirical, experience-based approaches.

In the present chapter, the empirical component has been limited to the indispensable minimum, with emphasis being placed on the discussion of the mechanical phenomena involved in foundation behavior and on design methods fundamentally based on rational arguments. This will allow the reader, should the will or the need arise, to resort, with critical sense, to the perusal of specialized publications, namely foundation manuals, where other methods are presented (Coduto, 2001; Frank, 2003).