Remerciez-le!

Remerciez @Admin pour avoir partagé cet document gratuitement, de la manière la plus simple, en partageant sur les réseaux sociaux.

655 Equivalent pier theory for piled raft design Équivalence de la théorie de l

655 Equivalent pier theory for piled raft design Équivalence de la théorie de la jetée pour la conception de radeau empilé Balakumar V. Simplex Infrastructures Limited, Chennai, Tamil Nadu, India. Huang M. Arup Geotechnics, Brisbane, Australia. Oh E., Balasubramaniam A.S. GriffithUniversity Gold Coast Campus, Brisbane, Australia. 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 leur éradication, a entraîné un 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. KEYWORDS: Piled raft, Pier, Peat. 1. INTRODUCTION The design of foundation system for structures that cannot tolerate settlements, the aspect of balancing the performance and cost, had always been a challenge for the foundation designers. Due to the complexity involved in the soil structure interaction analyses, required for an optimum design, designers have so far been resorting to the traditionally designed pile foundations system permitting very small limiting settlements. Even though this approach produces a safe design, the economics of the design becomes questionable. 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 Keeping the above objective in mind researchers like Burland (1995) and subsequently Polous (2001) had brought out the use of piles with the raft to reduce the settlement of the raft.This had led the advent of the combined piled raft foundation system, which provides a skilful geotechnical concept to design the foundation for structures which are sensitive to large settlements. The piled raft analyses is a three dimensional interaction problems, wherein, the load transfer mechanism is a complicated interaction process by which the load is shared The interactive process between the Various procedures based on observational study (Katzenbach etal.,2000a) small scale model studies such as centrifuge models (Horikoshi 1995) 1g model studies (Balakumar,2008) and the resulting interactive process with the numerical modeling (Clancy 1993; Russo,1998;) supported by the development of new geotechnical computational facilities (Polous and Small 2007) has led to the the piled raft foundation system being extensively used to support tall and heavily loaded structures in a successful manner permitting larger settlements close to the permissible value (Polous,2008;., Yamashita et al .,2010). 2. DESIGN PROCESS The satisfactory performances of piled raft largely depend upon the performance of the pile group of piled raft in providing the initial stiffness and then allow the raft to have a higher capacity by functioning as settlement reducer. Hence after ascertaining the feasibility of the piled raft to support the structure, a preliminary analyses has to be done to finalize the computational details of the constituent elements. Primarily the number, length of the piles, the load shared by the pile group are the essential parameters in addition to the properties of the supporting soil layers. In the case of the piled raft the pile group capacity and the overall capacity of the piled raft play an important role. The second stage of analyses has to produce these data in a reliable manner such that when used in the final analyses, the analyses will produce a design which need not be subjected to any iteration process. This requirement makes the procedure to be more realistic and simple enough such that the computational efforts are minimum and economical. Even though the existing methods can provide a design approach, these involve a very detailed computational efforts, not really warranted for the second stage of design, from the commercial design organisation point of view.. Therefore it is essential to have a relatively simple design procedure so that the second stage of work can give adequate but reasonably accurate data for the final analyses. 656 Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013 3. SELECTION OF DESIGN PROCESS Among the various methods studied, it was considered that the equivalent pier concept was found to be more suitable. The applicability of the equivalent pier theory to pled raft analyses has been established by Horikosh (1995) But the study was restricted only to a small pile group placed in the center of the raft , placed on a over consolidated clay layer. Although the study has produced very important and useful data, the applicability needs to be validated with other available results from a general soil profile. In this particular study the results of two such cases one from the observational study conducted on an instrumented piled raft supporting a 12 storeyed building and the other from the parametric study conducted independently are reanalyzed using equivalent pier concept. In this particular case the ratio Le/L namely the ratio of the pier length to the pile works out to unity and hence the equivalent length of the pier is taken to be the same as that of the pile. Once the piles are replaced by a pier then the solution for the single pile can be applied to estimate the load settlement characteristics, and the load sharing response; the load shared by the pier becomes the load shared by the pile group. With this idealisation it is possible to run the analyses as an axisymmetric two dimensional problem. 4. VALIDATION In order to establish the applicability of the equivalent pier theory two cases were considered for which published results are available. The models were selected, one from a parametric study carried out analytically and the other model was from an observational study carried out on the behaviour of piled raft supporting a 12 storied structure. 4.1 Validation based on numerical study Extensive parametric studies have been carried out in Griffith university Gold Coast campus and the results had been published by Oh etal.,( 2008 ).These studies had been based on the general soil profile compiled from the number of geotechnical investigation data collected. A 9 pile group (3x3) with 5d spacing has been considered The spacing of the piles considered is 5d (d – diameter of the pile). The d/t ratio is taken as unity and accordingly the raft thickness and the pile diameter have been taken as 800mm. The general soil profile comprises of 13m thick medium dense to dense sand layer, followed by 3m thick highly compressible organic layer termed as peat. This layer is followed by dense sand and hard clay. The Es values of various layers have been taken based on the N- values from the standard correlations. The equivalent pier modulus is taken from the expression, Eeq = Es +(Ep-Es)At/Ag (1) Where Eeq is the equivalent pier modulus, Es is the elastic modulus of the soil, Ep is the elastic modulus of the pile, At is total cross sectional area of the pile , and Ag is the plan area of the pile group. The pier considered along with the parameters is presented in Figure 1. 4.2 The observational study As a part of an extensive research programme, a 12 storeyed commercial cum residential apartment was designed and supported on piled raft (Balakumar and Ilamparuthy ) was instrumented and monitored .The piled raft system comprised of 93piles of 600mm diameter and 14M deep from the bottom of the raft. The raft thickness was 600mm so that the d/t ratio was maintained as unity. The layout of piles and other pertinent data are given in earlier publications .A two pile groups with a tributary raft diameter of 6m was converted into an equivalent pier and was loaded in small increments till the settlement reached 100mm. The pier was resting in a medium dense to dense sand. The details of the pier,and uploads/Management/ equivalent-pier-theory-for-piled-raft-design.pdf