Piles have been used by mankind for foundation purposes since prehistoric times. Their behavior, however, is far from completely clear and a substantial volume of research is being carried out on the subject.
Pile Foundation field is continually evolving, with development in technologies, methods of analysis and in design approaches. In fact, the design of piles is a complex matter which, although based on the theoretical concepts of soil mechanics, relies heavily on empiricism. This is an inevitable consequence of the marked variability of behavior of piles, which is partly due to random factors but more significantly to the effects of installation techniques.
Experimental evidence and investigations are covered first, with discussions of the effect of installation techniques on bearing capacity and load settlement response. Monitoring of the installation parameters for CFA piles allows a deeper insight into the influence of installation on pile behavior and suggests the possibility of moving from monitoring to controlling installation.
Experimental evidence Pile groups and piled rafts:
It is claimed that the most valuable experimental evidence for pile group and piled raft design comes from observation of the behavior of full scale structures. Evidence collected refers to the settlement and time settlement behavior of piled foundations, to load sharing between the piles and the raft, to load distribution among the piles; it is interpreted in terms of simple geometric parameters and may form the basis of simple empirical predictions On the contrary, for obvious practical reasons, the ultimate bearing capacity of piled foundations has been studied experimentally on small scale models at 1g or in centrifuges, starting from the pioneering work of Cooke (1986). The same applies to the behavior of pile groups under horizontal loads, both at working load and at failure.
Pile groups and piled rafts analysis
Einstein warns that ‘things should be as simple as possible, but not simpler’. Increasingly complex models for the analysis of the behavior of piled foundations are being developed, and the use of 3D finite element analyses is becoming more common.
Attention should be given not only to the second part of Einstein’s warning, but also to the first.
In fact, available methods for analysis of soil-structure interaction for piled foundations are probably perfectly adequate for engineering purposes, provided they are properly employed, paying due attention to the determination of soil properties and the implementation of the analysis.
Pile Foundation Design
The design of a piled foundation should be aimed (more or less consciously) to satisfying some optimization criterion that is to achieve maximum economy while maintaining satisfactory behavior.
Behavior is assessed by comparing the predicted values of some quantity (e.g. the absolute or differential settlement, the bending moments and shears, the factor of safety against a bearing capacity failure) with threshold values (admissible values), generally given by codes or regulations.
Current design practice is based on the assumption that a piled foundation behaves as a pile group with the cap clear of the ground; the design requisite is to ensure that a proper factor of safety is achieved against failure.
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