Piles and Pile Foundations

Piles have been used by the mankind for foundation purposes since prehistoric times, but in the last few decades the development in equipments and installation techniques and the pressure towards constructing in areas with poor subsoil properties, have led to a spectacular progress of the piling industry. According to Van Impe (2003) bored and CFA piles account for 50% of the world pile market, while the remaining is covered by driven (42%) and displacement screw piles (6%). Different proportions may be found locally; for instance displacement screw piles are about 60% of the total installed yearly in Belgium while bored and CFA piles reach more than 90% in Italy. Since the behaviour of piles depends markedly on the effects of the installation technique, their design is a complex matter which, although based on the concepts of soil mechanics, inevitably relies heavily on empiricism. The regional design practice in different countries develops along different paths under the push of the local market. In an international seminar on the design of axially loaded piles (De Cock, Legrand, 1997), organized with the aim of reviewing the practice in European countries, it has been confirmed that the common approach to the design of piles is essentially based on semi-empirical rules, sometimes calibrated against purposely performed load tests. It appears that there is room for an improvement of the present practice. The aim of this book is twofold. First, it reports an overview of the present design practice for pile foundations, trying to summarize some of the findings of recent research and to evaluate the above practice in the light of this research. Second, it is shown that the deeper insight of piled foundations behaviour achieved in the last decades may be used for a more rational and economic design of piled rafts. Any engineering problem can be tackled following one or the other of two rather different approaches. They can be illustrated using, as an analogy, two different ways of producing pieces of furniture. One is the modern industrial way, exemplified by the products of organizations like IKEA. The industrial process allows the recourse to excellent designers, advanced techniques and good materials; the result is a variety of objects, widely available on the market at relatively cheap prices. They are rather successful especially among the young generations. You can get there valuable items, provided you are able to choose the right one among the many suggestions. The second way is that of the expert cabinet maker who restores ancient pieces of furniture in his workshop and reproduces them. He uses materials as alcohol, French polish, isinglass and rosewood veneer. The simplicity of tools is compensated by experience and ability; he can produce authentic masterpieces, but in a limited number and hence not readily available. IKEA may be assimilated to a modern and efficient engineering organisation, and its industrial processes to the finite element codes that make our professional life so simpler and allow us to produce reports with impressive multicoloured diagrams. On the other side, the expert craftsman is similar to the old fashioned engineer, who still uses pencil and graph paper, superintends personally site and laboratory investigations and seldom ventures using a spreadsheet, preferring his trustworthy pocket calculator. In principle, any of the two methods may be used successfully. A proper use of the simple craftsman’s methods requires a deep insight into the mechanics of the phenomena and a good deal of experience, to make an intelligent use of simple models and empiricism. This appears to be a significant shortcoming, since experience is not readily available to everyone. On the other side, the sophisticated modern computer codes appear (and are widely believed to be) rather objective, as if the role of insight and experience was vanishing. It may be so, but anyone trying to apply FEM to practical design problems soon discovers how sensitive are the results to apparently minor modelling or computational details or to small variations of some esoteric parameter in a constitutive equation. “A difficulty of advanced numerical analysis” claim Vaughan, Kovacevic and Potts (2004) “is that the knowledge and skill required to perform numerical analyses is substantially greater than for the simple methods of approximate analysis to which we have become accustomed. This is at a time when there is a shortage of trained engineers in general and of geotechnical engineers in particular “. The present day practice of foundation design is suited for simple models and tools such as design charts, well tempered empiricism and so on. These tools are summarized in Part II and III of this book. The sound engineering judgement, which is needed for a successful use of these simple methods, is not necessarily based only on personal experience. First of all, it must be firmly grounded in Mechanics. Terzaghi claims that “theory is the language by means of which lessons of experience can be clearly expressed” ; Peck that “theory and calculations are not substitute for judgement, but are the basis for sounder judgement”. And Immanuel Kant authoritatively states: “There is nothing more practical than a good theory “. Accordingly, a simple mechanistic framework of classical soil mechanics is reported at the beginning of this book, and recalled throughout as a support to the judgement. Furthermore, each equation presented is accompanied by its derivation, or at least an indication of how it is obtained. Second, quite a lot of experience has been collected on piled foundations, and made available through papers, reports, books. A critical scrutiny of such evidence is another important factor in the formation of a sound engineering judgement. For each argument, a discussion of the available experimental evidence and of the main conclusions that can be drawn from it is thus attempted. Third, following the practice of the best restaurants, the menu presented is ample and varied but for each item there are “the suggestions of the chef”. A common design assumption for pile foundations is that of neglecting the contribution of the cap, and assuming that the whole load is transmitted to the soil through the piles. Most codes and regulations suggest or prescribe this approach. The assumption may be reasonable for small pile groups, but in the case of medium and large piled rafts it is unduly conservative. In fact, a proper consideration of the contribution of the raft allows significant economy. Furthermore, present design is capacity based, in the sense that a prescribed coefficient of safety against a bearing capacity failure has to be warranted. Again, most codes and regulations suggest or prescribe this approach. Alternative design requirements could be based on a limiting value of the absolute or differential settlement. Adopting such an alternative approach, an improvement in design practice and substantial economy can be achieved for medium and large piled rafts.