Different Analysis Methods of Piled Rafts (original) (raw)

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

Piled raft foundations: design and applications

Géotechnique, 2001

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 emp...

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

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.

A contribution to optimum design of piled rafts

Géotechnique, 1998

The majority of piled raft foundations have been designed ignoring any contribution from the raft or pile cap, although it is well known that the raft plays an important role in the overall performance. In such conventional designs, the overall settlement of the foundation is likely to be very small, owing to the installation of more piles than are necessary. However, from an economical point of view, it is preferable that the foundation is designed in such a way that the average and differential settlements are limited to an acceptable level, but where the load-carrying capacity of the raft is taken into account. In this paper, a framework for a new design concept, in which piles are installed only beneath the central area of a relatively flexible raft to minimize the differential settlement, is presented through the results of an extensive parametric study. The so-called 'hybrid' approach, developed by Clancy, was used in the analyses. The study showed that piled rafts may...

An Experimental Study of the Piled Raft Foundation Subjected to Combined Vertical and Lateral Load

2021

1-g experimental tests have been carried out to investigate the behavior of piled raft foundation subjected to combined vertical and lateral load. The vertical load was applied till a constant settlement of test models and it was followed by the horizontal incremental load. The piled raft models including a single pile, unpiled raft and pile group models had been tested on cohesionless soil. The pile configurations of 2×2 and 3×3 with a constant spacing of 3.5 times diameter of a pile and constant pile length of 25 times diameter of a pile were adopted for the piled raft models in this study. The raftsoil contact was an important factor to reduce the horizontal displacement of the piled raft. There was a significant reduction found in the horizontal displacement of the piled raft foundation due to the presence of the vertical load.At an assumed horizontal load of 100 N, there was about 3.75 to 5.33 times reduction in the horizontal displacement of adopted piled raft models in the pr...

IJERT-A Study on Piled Raft Foundation: State of Art

International Journal of Engineering Research and Technology (IJERT), 2013

https://www.ijert.org/a-study-on-piled-raft-foundation-state-of-art https://www.ijert.org/research/a-study-on-piled-raft-foundation-state-of-art-IJERTV2IS80575.pdf With increasing in urbanisation in last three decades all over the world led to rapid increase in number and height of buildings even on problematic subsoil conditions. Piled raft system proves to be more effective on such problematic subsoil conditions. It takes the high vertical load and used to bring the settlement, differential settlement and tilting of structure within the permissible limit. Piled raft system proves to be cost effective than the conventional pile foundation system. Piled raft foundation accounts for complex soil-structure interaction, which needs interaction between structure engineer and geotechnical engineer for giving most economical and safe design of the system. This paper reveals the performance of piled raft foundation in sandy soil, clayey soil and, layered soil carried through experimental and numerical analysis.

An Approximate Analysis Procedure for Piled Raft subjected to Vertical Loading

IJSRD, 2013

Piled raft is a geotechnical composite construction consisting of three elements raft, piles and soil. Addition of piles in raft strategically improves ultimate load carrying capacity, decreases overall and differential settlement .The design of piled raft is complicated due to complex interaction between rafts, pile and soil. In recent years due to advent of multistory buildings there has been a significant rise in usage of piled raft in India. This paper presents a simplified method for analysis of piled raft with use of software SAFE considering all the interactions. In the scope of this paper, the design procedure for piled rafts is discussed and results are verified with that available from literature.