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Pile foundations: experimental investigations, analysis and design

ICSMGE Plenary session E

Piles have been used by mankind for foundation purposes since prehistoric times. Their behaviour, however, is far from completely clear and a substantial volume of research is being carried out on the subject.

The fi eld 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 behaviour of piles, which is partly due to random factors but more signifi cantly to the effects of installation techniques.

A number of comprehensive and authoritative reports on pile foundations have been issued in recent years (Randolph: 1994, 2003; Poulos et al: 2001; Viggiani: 2001; Mandolini: 2003; and Poulos: 2003);

accordingly, this state of the art report does not attempt a complete coverage of the subject, but rather addresses selected topics believed to be timely and relevant.

As with all complex things, the design process should include a real and an imaginary part. The real part can be considered as the available experimental evidence, while the imaginary one (or, perhaps better, the imaginative one) covers methods of analysis and design. These two aspects are discussed separately.

Single pile

Experimental evidence and investigations are covered fi rst, 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 behaviour and suggests the possibility of moving from monitoring to controlling installation.

The results of a static vertical load test on a single pile, in terms of load settlement response, are shown to be markedly influenced by the test setup (eg kentledge, anchor piles, Osterberg cell). Methods for predicting the bearing capacity and displacements under vertical and horizontal load are briefiy reviewed and commented upon.

Pile groups and piled rafts: experimental evidence

It is claimed that the most valuable experimental evidence for pile group and piled raft design comes from observation of the behaviour of full scale structures. Evidence collected refers to the settlement and time settlement behaviour 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 behaviour 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 behaviour 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.

Some particular aspects of behaviour, such as the influence of unloading reloading cycles, long term creep, and the influence of adjacent foundations deserve further study. The situation appears less satisfactory for the case of horizontal loading, where the use of essentially empirical approaches is still widespread.

Design

The design of a piled foundation should be aimed (more or less consciously) to satisfying some optimisation criterion, that is to achieve maximum economy while maintaining satisfactory behaviour.

Behaviour is assessed by comparing the predicted values of some quantity (eg 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.

However, in many cases the piles are adopted to control the absolute or differential settlement; the usual design approach appears thus irrational and over-conservative. In small piled rafts the raft may be made, and usually is, rather stiff so that differential settlement is not a major problem. The requisite for an optimum design may be the limitation of the mean settlement, obtained by adopting long piles uniformly spread below the raft. Criteria are given for the evaluation of the bearing capacity taking in due account the contribution of the raft resting on the soil.

In large piled rafts B/L is usually above unity, and the flexural stiffness of the raft cannot be anything but rather small. The design requisite is the limitation of differential settlement, obtained by the use of piles concentrated in definite locations.

For instance, for uniform load the piles have to be concentrated in the central area of the raft. The factor of safety is rarely a major problem. A number of examples show substantial savings in the number of piles may be obtained both for small and large rafts.

As a general rule, the effectiveness of a pile, in terms of both stiffness and failure load, is reduced by the proximity of other piles.

Such a negative interaction may be mitigated using a few widely spaced piles; furthermore wider spacing allows the raft to transmit a larger portion of the external load directly to the soil, both under working conditions and at failure.

It is a pity that restrictions by codes and regulations hinder a free selection of the most appropriate design approach, forcing the adoption of a capacity-based design whatever the actual design requirement is. The regulations act thus as a restraint rather than a stimulus.

References

Cooke RW (1986). Piled raft foundations on stiff clays: a contribution to design philosophy.

GÚotechnique, 36, No 2, 169-203 Mandolini A (2003). Design of piled rafts foundations: practice and development. Deep Foundations on Bored and Auger Piles, Millpress, Rotterdam, 59-80 Poulos HG (2003). Deep foundations - Can further research assist practice- Proc IV Seminar on deep foundations on bored and auger piles, Millpress, Rotterdam, 45-55 Poulos HG, Carter JP and Small JC (2001).

Foundations and retaining structures - Research and practice. Proc XV Int Conf Soil Mechanics and Foundation Engineering, Istanbul, 4, 25272606 Randolph MF (1994). Design methods for pile groups and piled rafts. Proc XIII Int Conf Soil Mechanics and Foundation Engineering, New Delhi, 5, 61-82 Randolph MF (2003). Science and empiricism in pile foundations design. The 43rd Rankine Lecture. GÚotechnique, 53, No 10, 847-875 Viggiani C (2001). Analysis and design of piled foundations. 1st Arrigo Croce Lecture. Rivista Italiana di Geotecnica, No 1, 47-75 Alessandro Mandolini is associate professor at the civil engineering department, University of Napoli Federico II.

He is a member of ISSMGE committees ITC18 Deep Foun dations, ERTC3 Piles, and ERTC10 Evaluation of Eurocode 7. Mandolini is author or coauthor of over 60 papers on soil mechanics and geotechnical engineering. A paper coauthored with Carlo Viggiani, published in GÚotechnique in 1997, was awarded the Bishop Gold Medal for geotechnical research.

Gianpiero Russo is associate professor in geotechnical engineering department of the University of Napoli Federico II. He is author or co-author of more than 40 papers in the field of soil mechanics and geotechnical engineering.

Carlo Viggiani is professor of foundation engineering at the University of Napoli Federico II, where he is head of the department of geotechnical engineering and dean of the Civil Engineering School.

Viggiani has been chairman of the European Committee for Geotechnical Aspects of Earthquake Engineering, member of the International Committee for the Safeguard of the Leaning Tower of Pisa and member of the scientific committee for the suspension bridge over the Messina Straits.

He is chairman of ISSMGE TC19, Preservation of monuments and historic sites, and member of the committee of monitoring and surveillance of the Tower of Pisa.

Viggiani is author or coauthor of four books and over 150 papers on different topics of soil mechanics and geotechnical engineering.

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