scispace - formally typeset
Search or ask a question
Book ChapterDOI

Effect of Piles on the Design of Raft Foundation

01 Jan 2020-pp 57-80
TL;DR: In this paper, the authors have studied the effect of varying pile, raft and soil parameters on the settlement response and load-sharing behavior of the raft and piles in a combined raft, soil and pile foundation system.
Abstract: The combined raft, soil and pile foundation system has reached a high level of familiarity and is now being used to support a large number of structures. When the bearing capacity of the raft foundation is acceptable but the settlement value exceeds the permissible limit, raft-soil system is reinforced with piles to reduce settlement. Here, piles act as settlement reducers to the raft-soil foundation system. Different researchers have studied the effect of varying pile, raft and soil parameters on the settlement response and load-sharing behaviour of the raft and piles in a combined raft, soil and pile foundation system but less attention has been paid to the stress response behaviour of the raft. As this aspect is considered to be important from economics of raft design, a study has been performed to understand the stress response of the raft on the introduction of piles. The stress response of the raft for varying load conditions, Ec/Es ratios, thicknesses of raft and diameters of pile have been studied by performing numerical analyses on the foundation system supporting a moderately loaded 12-storeyed real-time commercial structure located in Chennai, India. An attempt has been made to study the applicability of Equivalent Pier concept. The above analyses were performed for two different layouts of piles. The behaviour of the stress and settlement responses of the raft reinforced with piles was compared with unpiled raft. Staad-Pro V8i and Ansys 16.0 have been used for the study and this paper presents observations and discussions from the study.
References
More filters
Journal ArticleDOI
TL;DR: In this paper, the authors discuss the philosophy of using piles as settlement reducers and the conditions under which such an approach may be successful and some of the characteristics of piled raft behavior are described.
Abstract: In situations where a raft foundation alone does not satisfy the design requirements, it may be possible to enhance the performance of the raft by the addition of piles. The use of a limited number of piles, strategically located, may improve both the ultimate load capacity and the settlement and differential settlement performance of the raft. This paper discusses the philosophy of using piles as settlement reducers and the conditions under which such an approach may be successful. Some of the characteristics of piled raft behaviour are described. The design process for a piled raft can be considered as a three-stage process. The first is a preliminary stage in which the effects of the number of piles on load capacity and settlement are assessed via an approximate analysis. The second is a more detailed examination to assess where piles are required and to obtain some indication of the piling requirements. The third is a detailed design phase in which a more refined analysis is employed to confirm the op...

379 citations

Journal ArticleDOI
TL;DR: In this article, an approximate method of numerical analysis of piled-raft foundations is presented, in which the raft is modelled as a thin plate and the piles as interacting springs of appropriate stiffness.
Abstract: This paper presents an approximate method of numerical analysis of piled–raft foundations in which the raft is modelled as a thin plate and the piles as interacting springs of appropriate stiffness. Allowance is made for the development of limiting pressures below the raft and of the ultimate axial load capacity of the piles. Comparisons between this analysis and existing solutions verify that, despite the approximations involved, the analysis can provide solutions of adequate accuracy for the settlement and pile load distribution within a piled raft. Comparisons are also made with the results of a series of centrifuge tests and with measurements of the performance of a full-scale piled raft. In both cases, the analysis predicts very well the settlement and proportion of load carried by the piles.

151 citations

Journal ArticleDOI
TL;DR: In this article, the authors present the results of model tests on unpiled rafts, free-standing piles and piled rafts of various sizes, which are used as a framework to which field measurements carried out by the Building Research Establishment and settlement observations are used.
Abstract: To design piled raft foundations on stiff clays with precision and economy it is necessary to know how the structural loads are transferred to the supporting soil by the components of the foundation and to understand the factors affecting the overall and differential settlements under working conditions. In cases where piles are essential from the viewpoint of stability it is important to know the distribution of pile loads and whether the raft contributes significantly to the total support. However, where a raft alone can provide adequate bearing capacity but may settle excessively the designer will not wish to include more piles than are necessary to control the settlements. This Paper attempts to bring together information on some of these aspects of design. It presents the results of model tests on unpiled rafts, free-standing piles and piled rafts of various sizes. These are used as a framework to which field measurements carried out by the Building Research Establishment and settlement observations ...

123 citations

01 Jan 2013
TL;DR: In this article, a simple analytical procedure and the effectiveness of the pressuremeter tests in predicting the piled raft behavior, so that the results can be used in the detailed design are presented. But, their accuracy depends upon the accuracy of the evaluation of in-situ parameters like Es.
Abstract: The design of piled raft foundation involves two stages namely a preliminary stage and the final stage. The preliminary design stage involves the identification of the essential parameters namely the number of piles, their diameter and the length along with Es value of the subsoil strata for an optimum design which can produce the required settlement reduction. The existing design methods, whose accuracy depends upon the accuracy of the evaluation of in-situ parameters like Es although can produce satisfactory results, the computational efforts involved and the time does not justify the use of them for the preliminary analyses which will involve repetitions. This paper outlines a simple analytical procedure and the effectiveness of the pressuremeter tests in predicting the piled raft behaviour, so that the results can be used in the detailed design. RÉSUMÉ : La conception d’une fondation avec radiers sur pieuxcomporte deux étapes : une étape préliminaire et une étape finale. L’étape préliminaire comprend l’identification des paramètres fondamentaux, à savoir le nombre de piliers, leur diamètre et leur longueur ainsi que la valeur Es de la strate souterraine pour une conception optimale capable de créer la réduction d’affaissements sollicitée. Les méthodes de conception existantes, dont l’exactitude dépend de celle des paramètres in situ tels qu’Es, bien que passibles de donner des résultats satisfaisants, ne justifient pas leur emploi pour les analyses préliminaires sujettes à répétitions, en raison des efforts de calcul et de temps qu’elles impliquent. Cet article expose une procédure analytique simple et l’efficacité des essais pressiométriques dans la prévision du comportement du radier sur pieux, de manière à permettre l’utilisation des résultats pour la conception détaillée.

4 citations

01 Jan 2013
TL;DR: In this paper, the authors present a simple design process in the form of equivalent pier approach by establishing its applicability by applying it to two cases and demonstrate the effect of deep compressible deposit sandwiched between two dense layers.
Abstract: The objective of generating an economical and safe foundation system reducing the settlement rather than eliminating has led to the change in the design philosophy recognising the fact that most structures can tolerate a certain amount of settlement The piled raft analyses and design is three dimensional interaction problems, wherein, the applied load is transferred by a complicated interaction process between the piles and the raft.. The necessity to have a relatively simple design procedure so that the preliminary design can give adequate but reasonably accurate data for the final analyses is explained. The paper presents such a simple design process in the form of equivalent pier approach by establishing its applicability by applying it to two cases.Also the study has brought out the effect of deep compressible deposit sandwiched between two dense layers. RÉSUMÉ : L’objectif de créer un système de fondation économique et fiable, par la réduction des affaissements plutôt que de leuréradication, a entraînéun changement dans la philosophie de conception, admettant le fait que la plupart des structures sont capables de tolérer un certain nombre d’affaissements. Les analyses et la conception du radier sur pieux comportent des problèmes d’interaction tri dimensionnels, où la charge appliquée est transférée par un processus d’interaction complexe entre les pieux et le pilier. Il est expliqué la nécessité de disposer d’une procédure de conception relativement simple, de manière à ce que la conception préliminaire puisse apporter des données appropriées suffisamment précises pour les analyses finales. L’article présente un processus de conception aussi simple, sous forme d’approche de pilier équivalent, en prouvant sa faisabilité par son application dans deux cas. L’étude a également révélé les effets du dépôt fortement compressible coincé entre deux couches épaisses.

3 citations