An Analytical Approach for Piled-Raft Foundation Design Based on Equivalent Pier and Raft Analyses by Using 2D Finite Element Method (original) (raw)

Analysis and Parametric Study of Piled Raft Foundation Using Finite Element Based Software

2009

The design of group piles depends on either the group or single pile capacity of piles. In conventional design method of such foundations, the stiffness of the pile cap is barely taken into account. Such design becomes too conservative if the pile cap is in contact with the ground. Because the pile cap contributes in transferring load to the ground and distributing load over the piles. The design method that considers the contribution of the pile cap and interaction between the different elements of group piles is called piled raft foundation. The concept of piled raft foundation leads to economical design. In this paper, analysis and parametric study of piled raft foundation has been conducted. The study is performed using powerful finite element based software, PLAXIS. A number of parameters were selected from the elements of the piled raft system. According to their effect on the response of piled raft system, some are taken to be constant while others are varied. Among the varie...

Analysis of Piled Raft Foundation Using Finite Element Method

2013

1* Raft foundation covers the entire area of the structure, transmitting the entire structural load and reduces differential settlements whereas piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. In recent years, a piled-raft foundation which is a composite structure consisting pile and raft has been proved to be an appropriate alternative instead of conventional pile or mat foundations. In this paper, analysis of piled raft foundation has been carried out by using finite element software ANSYS. For understanding the behavior of piled raft foundation, parametric studies has been carried out in medium sand by varying pile diameters and pile lengths in different combinations. It has been found out that Pile diameter has significant influence on the ultimate capacity of piled raft foundation whereas the pile length has not of much significance. It is conclude...

PERFORMANCE OF PILED RAFT WITH VARYING PILE LENGTH

A piled raft foundation is fairly a new concept in which the total load coming from the superstructure is partly shared by the raft through contact with soil and the remaining load is shared by piles through skin friction. Due to the three dimensional nature of the load transfer, piled-raft foundations are regarded as very complex systems involving many interaction factors such as pile-to-pile, pile-to-raft, raft-to-soil and pile-to-soil. The economy of the foundation system for heavily loaded and " settlement sensitive " structures like tall slender buildings and storage tanks depends upon the method adopted to reduce the settlements to the permissible level rather than eliminating it completely. As a matter of fact when the serviceability requirements are satisfied from the point of view of permissible settlement there is no need to eliminate the settlement completely. The combined piled raft system has proved to be an ideal foundation system to satisfy the above requirement under certain favorable circumstances, namely when the bearing capacity is not a problem but settlement would be beyond the permissible requirements. Although the combined piledraft system was developed with over consolidated clay in mind, its applicability in sand also becomes important as the permissible settlement for the foundation is less than that of foundation resting on clay. Therefore the applicability of piled raft to support moderately loaded buildings and storage tanks on sand and predominantly sandy soils gains importance, further more understanding of load sharing between piles and raft is very much important for the piled raft in sand particularly when the piles are driven because the driving of piles improves the state of compaction of the sand. As the piled raft foundation system transfers the load through a complicated interaction process the effect of the parameters associated with the constituent elements, namely the piles, raft and the soil on the settlement reduction and load sharing behavior becomes very important and needs a detailed study. Although a number of published literatures are available on the effect of various parameters, they all have piles of equal lengths. In case of plaza like structures wherein the raft thickness as well as the pile length can be varied depending upon the capacity requirements, it becomes necessary to understand the effect of variation in pile length on settlement reduction and load sharing behavior of piled raft. The present work is based on the results of small scale 1g model tests conducted on piled raft models with varying configurations of varying pile length. A square raft of 150mm with 3×3 configuration and the pile

Different Analysis Methods of Piled Rafts

Piled raft foundation provides an economical foundation compared to conventional pile foundations. Piled are provided, where the performance of the raft alone does not satisfy the design requirement. Under these circumstances, the addition of minimum number of piles may increase the ultimate load carrying capacity, minimizing the average and/or differential settlement and optimize the thickness of the raft. Some of the available methods for analysis of piled raft behavior presented by different researcher are reviewed and their capabilities and limitations are discussed.

Study on Behaviour of Piled Raft Foundation by Numerical Modelling

2012

A foundation must carry weight of a superstructure and transfer it to underlying subsoil layers without any excessive settlement. It is common practice to first consider a shallow foundation system for any structure. A raft foundation is one such system which covers the entire plan area of the structure. When the raft foundation has adequate bearing capacity but the settlement is not within allowable limit, a group of piles is added under the raft to reduce the settlement. The behaviour of the piled raft foundation system is influenced by various factors such as raft thickness, pile length, pile spacing and number of piles, which must be considered for an economical and effective design. A numerical analysis has been carried out by using geotechnical finite element software, PLAXIS 2-D, to investigate the influence of the above various factors. The aim is to optimally utilize the load-carrying capacity of both the raft and the pile group.

Study on Parametric Analysis of Piled Raft Foundation System Using Finite Element Approach

Journal of Advanced College of Engineering and Management

To accommodate the shear requirement and settlement requirement of high rise construction, the concept of piled-raft foundation has been developed. This research deals with successive analysis of parameters of piled-raft foundation system using PLAXIS-2D as a FEM tool. Plain strain analysis of piled raft foundation system has been conducted out by successive fixing up of parameters. For the analysis two cases has been studied for piled-raft lying on silty soil deposit and on clayey deposit with respect to uniform static loading from superstructure. The result of successive variation of parameters showed that variation has limiting effect on stress and displacement behavior. The analysis is also performed for raft of different relative stiffness and pile of different relative compressibility and load sharing between plain strain pile and raft has been analyzed.

A Critical and Comparative Study On the 2D and 3D Analyses of Raft and Piled Raft Foundations

The piled raft foundation has gained a very high level of acceptance as a foundation system whenever settlement alone governs the design However the professional designers need to be provided with a clear procedure as to when the simpler analytical procedures has to be adopted and at what stage detailed three dimensional analyses need to be taken up This paper tries to provide an answer after explaining the probable failure mechanism of the piled raft with analytical modeling using plane strain and axisymetric idealization with Ansys , Plaxis 2D and Plaxis 3D. The exponential growth in the infrastructure development has forced the designers to accept any ground condition and have to face the challenge of designing a suitable foundation system which will satisfy the safety and economy irrespective of the ground conditions. 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. Such a foundation system would satisfy the safety and the serviceability requirements effectively but may not satisfy the economic requirements both from cost and time point of view.. In many cases raft was found to satisfy the bearing capacity requirements but the control of settlement would not possible. In these cases the presence of the raft and its capability to transfer the load to the soil was completely ignored and the piles were designed as though they would take the entire structural load. Keeping the above objective in mind researchers like Burland (1995) Burland etal (1997), 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 settlements. The concept of piled raft foundation system recognizes the fact that any structure has a certain amount of permissible settlement and the economy of the foundation design depends upon reducing the settlement to the permissible level rather than eliminating it in total In the combined piled raft system the addition of piles enhances to the stiffness of the entire system in the initial stages, and at higher loads provides the additional capacity for the raft to take a higher load at any given settlement compared to the unpiled raft (plain raft). Piled raft foundation system transfers the load by means of a complicated three dimensional interaction among the constituent elements namely the pile, raft, and the soil. Unlike the traditionally designed pile group wherein the interaction is only between piles and the soil, in the case of piled raft there are four interactions namely raft and the soil, pile and the soil, raft and the pile and pile to pile. Further in the case of piled raft the pile group alone is not intended to ensure the safety of the system but it is the combined system of raft, pile and the soil ensures the safety of the structure. Hence in designing the piled raft it is not the pile capacity alone to be considered but the combined capacity of the whole system has to be considered at any given settlement reduction. Therefore the analyses become complicated. 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. and FEA codes like Defpig,Napra,and HyPr etc have enhanced the interaction process between the observational methods (Katzenbach etal., 2000 a; Balakumar and Ilamparuthy, 2007), small scale model studies (Horikoshi1995; Balakumar etal.,2005) and numerical and analytical simulations leading to the improvement of the design process. Consequent to this number of tall and heavily loaded structures have been supported on piled raft and the performance of some of these piled rafts have been monitored and the results may be used to refine the design in the future. The main problem the designers were facing was that many of the traditional methods of analyses could not be applied since they require a high level of extrapolation and approximations which were beyond the comprehension of past experience. As Russo (1998) has pointed out to move from the traditional capacity based design to settlement based design method the analyses must be capable of taking into account properly the soil structure interaction within the foundation system is needed. Finite element analyses are one method which is by far well developed and found to be more suitable to analyze the piled raft problem. However it has been found that in order to reduce the computational efforts under many circumstances the rigors of the method has to be diluted by some approximations and simplified assumptions. While a number of simplified have been developed to analyze the large pile groups, no such method appears to be available for the analyses of piled raft. The design of piled raft has got three stages as pointed out by Poulos (2008) in many of his publications. They are : Since there are three stages of design ,it becomes necessary that appropriate analyses methods have to be adopted so there will not be