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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 89
Edited by: M. Papadrakakis and B.H.V. Topping
Paper 182

Three-Dimensional Finite-Element Analysis of Large Piled Rafts: A Proposed Simplified Modelling

A.A. Hemada1, T.T. Abdel-Fattah1 and A.Y. Akl2

1Department of Soil Mechanics, Housing and Building National Research Centre, Cairo, Egypt
2Department of Structural Engineering, Cairo University, Egypt

Full Bibliographic Reference for this paper
A.A. Hemada, T.T. Abdel-Fattah, A.Y. Akl, "Three-Dimensional Finite-Element Analysis of Large Piled Rafts: A Proposed Simplified Modelling", in M. Papadrakakis, B.H.V. Topping, (Editors), "Proceedings of the Sixth International Conference on Engineering Computational Technology", Civil-Comp Press, Stirlingshire, UK, Paper 182, 2008. doi:10.4203/ccp.89.182
Keywords: piled raft, finite elements, elasto-plastic analysis, interface, lateral loads, simplified modelling.

The problem of detailed three dimensional finite element modelling of piled raft foundation is a significant one, that may be either beyond the capabilities of personal computers or it may require a large CPU time. This paper suggests two practical techniques to overcome the above drawbacks.

The first proposed technique is based on the employment of three dimensional interface elements for modelling of far field bottom soil layer. The depth of the soil mesh under pile tips is selected, such that no part of the soil contributing to foundation settlements is neglected. It is evident from previous studies [1], as well as from the present analyses, that far field zones of the finite element mesh behave linear elastically and consequently it can be modelled using elastic three dimensional interface elements. The equations used to calculate the interface stiffness are presented. The minimum depth of soil below pile tips that should be modelled using finite elements, is investigated.

The second proposed technique is a simplified approach for handling of lateral loads due to wind and earthquakes on symmetrical piled rafts. As a result of symmetry, under vertical loading, only a part of the foundation is modelled. On the other hand, complete analysis dictates considering the problem under lateral loading due to wind or earthquakes. This requires considering the full model which may exceed the capabilities of personal computers. The proposed method requires two stages of analysis. The first stage is a detailed three dimensional non linear finite element analysis of the reduced model under vertical loads in order to evaluate the following; the portion of the total vertical working load transferred by each of the raft and piles to the soil and the corresponding average settlement of the foundation. The second stage is a finite element analysis under lateral loads performed on the raft, with both soil and piles modelled using interface elements with the secant stiffness estimated from the first stage.

To verify the use of the proposed simplified approaches, three dimensional test problems have been solved and the results are compared to the corresponding detailed three dimensional finite element models, as well as back analysis of a well monitored piled raft foundation [2]. Good agreement between both methods and available observations was observed, which proves the accuracy and efficiency of the proposed techniques. It is recommended that when using the first proposed technique, for piled rafts with floating pile groups, then the part of the finite element mesh under pile tip level that needs to be modelled using finite elements is not less than 15% of the total depth contributing to foundation settlement.

H.G. Poulos, J.C. Small, L.D. Ta, J. Sinha and L. Chen, "Comparison of Some Methods for Analysis of Piled Rafts", Proceedings of 14th ICSMFE, Hamburg, vol. 1, 69-72, 1997.
H. Sommer, P. Wittmann and P. Ripper, "Piled raft foundation of a tall building in Frankfurt clay", Proceedings of 11th ICSMFE, San Fransisco, Vol. 4, 2253-2257, 1985.

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