JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
CIVIL ENGINEERING
RECENT STUDIES ON PILED-RAFT FOUNDATION: STATE OF ART
1V.J.SHARMA, 2 S.A.VASANVALA, 3C.H.SOLANKI
1, 2, 3 Applied Mechanics Department, Sardar Vallabhai National Institute of Technology, Ichchhanath, Surat-7, Gujarat,India.
ABSTRACT : Due to increase of civilization in recent decades the demand of land for construction purpose has increased exponentially. As most of the time site available consists of weak subsoil conditions, piled raft foundation has been preferred on large scale due its efficiency of carrying high vertical loads within the permissible settlement limits and cost effectiveness compared to pile foundation system.This paper presents the brief review of various studies on piled-raft foundation in last decade.The paper also reveals latest modification done in conventional piled-raft as per site conditions and loading criteria.
Keywords: Differential Settlement, Total Settlement, Finite Element Method, Non-Identical Piles, Composite Piled-Raft, Viscohypoplastic
ISSN: 0975 –6744| NOV 10 TO OCT 11 | Volume 1, Issue 2Page 1
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ISSN: 0975 –6744| NOV 10 TO OCT 11 | Volume 1, Issue 2Page 1
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1. INTRODUCTION
In traditional foundation design, it was customary to consider first the use of shallow foundation such as a raft (possibly after some ground-improvement methodology performed). If it wasnot adequate, deep foundation such as a fully piled foundation was used instead. In the former, it was assumed that load of superstructure was transmitted to the underlying ground directlyby the raft. In the latter, the entire design loads were assumed tobe carried by the piles [24].In recent decades, another alternative intermediate between shallow and deep foundation, what is called piled raft foundation or settlement reducing piles foundation, has been recognized by civil engineers. The piled raft concept has been used extensively in Europe and Asia. In this concept, piles are provided to control settlement rather than carry the entire load. Piled raft foundation has been proved to be an economical way to improve the serviceability offoundation performance by reducing settlement to acceptable levels. The favorable application of piled raft occurs when the raft has adequate loading capacities, but the settlement or differential settlement exceed allowable values. Conversely, the unfavorable situations for piled raft include soil profiles containing soft clays near the surface, soft compressible layers at relatively shallow depths and some others [24]. In the unfavorable cases, the raft might not be able to provide significant loading capacity, or long-term settlement of the compressible underlying layers might reduce the contribution of raft to the long-term stiffness of foundation.As we know piled-raft foundation system is applied in weak subsoil conditions where chances of foundation settlement play vital and major role in design of foundation system. This paper presents review of various studies carried out on piled-raft foundation inclayey soil, sandy soil and weathered rocks. Various researchers have worked on piled–raft foundation system in clayey soil. Their work includes study related to bearing behavior, composite system, non-linear FEM analysis, piled-raft system with non-identical piles, etc. The Studies related to sandy soil includes separate analysis of pile and raft, case studies and use of FEM codes. Few works has been carried out on weathered rock which includes paper [3,7,10,and 25].Researcher [7] has carried out work in sand as well as weathered rock.
2 .REVIEW
The review includes various papers published in various journals such as American society of civil engineering, Proceedings of the 17th International Conference on Soil Mechanics and Geotechnical Engineering ,Proceedings of ICE , etc .They have been grouped into major areas as:
- Study of pile raft system in clay
- Study of pile raft system in sand and weathered rock
- Study of pile raft system in clay :
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. The study on design of piled-raft system in clayey soils have been carried by [25, 15, 32, 31, 27, 2819, 22 and 12].The design of piled raft system involves basically bearing capacity of supporting subsoil and permissible allowable total and differential settlements. Poulos H.G. [25] had given outline of simplified method of carrying out a preliminary feasibility study and design of a piled raft system. The analysis is programmed via a spreadsheet program or a mathematical program such a MATHCAD. The approach gives a reasonable estimate of the order of the magnitude of settlement and differential settlement of foundation when applied to case study of piled raft on stiff clay and series of centrifuge tests on reconstituted soil. Liew S. Setal [15] have designed and presented instrumentation results of reinforcement concrete piled raft supporting 2500Ton oil storage tanks on a very soft alluvial clayey soil of about 40m thick in Riau Province of Sumatra, Indonesia. The design concept has been validated with the results of instrumentation Seo Y.Ketal [32] have proposed design charts of piled raft foundationson soft clay. The design charts were first presented for the evaluation of both bearing capacity and total settlement in the raft alone foundation system. Load settlement relationship curves are used to evaluate the ultimate soil bearing capacity. The total settlement is evaluated by applying various traditional safety factors of the uniformly distributed loads. Then, the parametric studies were carried out for the piled raft foundation system. In the numerical analyses, elasto-plastic finite elements models are used to present the foundation response and the design charts, which enable the determination of the raft size and pile length and spacing of piles.Sanctis L.Detal. [31] has proposed a simple design criterion to evaluate the ultimate vertical load of a piled raft as a function of its component capacities, which can be simply evaluated by the conventional bearing capacity theories. A broad parametric studyiscarried out using 3D FEM analysis to define the failure load coefficientsaccounting for the interaction between the raft and the pilegroup at failure. Also a guideline has been given to assess the factor of safety of vertically loaded piled raft.As per Meisam Rabiebi [27] the maximum bending moment in raft increases with increase raft thickness, decrease pile number and decrease in pile length. Central and differential settlement decreases with increase raft thickness and uniform increase in pile length. Further Meisam Rabiei [28] used a computer program ELPLA and found that pile configuration and load distribution are very important and effective in piled raft settlement , maximum moment and pile bearing factor. Three basic pile configurations and three load distribution types were considered. Pile configuration 1 has the pile uniformly distributed under whole raft area. Pile configuration 2 has piles under central area of the raft as well as under the edge of the raft. In pile configuration 3 the piles are placed only in the central area of the raft, the detail is shown in fig.1 .Novak L.J etal [19] carried out analysis of pile-raft foundations with 3D finite-element method.They found following reasons for use of the 3D Finite Element Method (FEM): (1) the problem is so complex that simplified methods cannot model the problem correctly; and (2) codes for the FEM are available,powerful, and capable of being run on the personal computer. Comparisons were made between experimental and analytical results for two piled –raft foundations resting on stiff clay and the FEM has shown to yield excellent results for the cases analyzed.Oh E.Y.Netal. [22] have carried out numerical analysis of un-piled raft and piled raft on two different soil conditions. The analysis was carried out in two case studies with three typical load intensities of serviceability load.The subsoil was modeled as linear elastic materials, raft as beam element and piles as element with shear and normal coupling springs. For case study 1 with sandy soil condition maximum settlement of piled raft depends on piles spacing and number of piles. For case study 2 with clayey subsoil, the settlement of un-piled raft was similar for different raft thickness .Raft thickness was found to have obvious effect on differential settlement.The settlement of piled raft at piled area showed a bowl shape settlement patternand edge of the raft strip showed a downward deviation from the settlement bowl. Lee J H.etal [12] has conducted series of 3D elasto-plastic finite element analyses to investigate the bearing behavior of a square piled raft subjected to vertical loading. In this study, the main characteristic of these analyses was to permit soil slip at the pile–soil interface. Pile positions, pile number, pile length and loading distributions on the raft were varied, and theeffects of pile–soil slip, pile geometries and loading types were examined. Furthermore, the proportion of load sharing of the raft and piles at theultimate state and the relationship between the settlement and overall factor of safety was evaluated. The results show that the use of a limited number of piles, strategically located, might improve both bearingcapacity and the settlement performance of the raft.The work related to piled- raft foundation resting on over-consolidated clay was carried out by [8, 29, 30], which includes numerical study of piled-raft foundation behavior and comparison with field measured results. Reul O.etal [30] carried out comparisons of in-situ measurements and numerical analyses for three piled raft foundations on over-consolidated clay between overall settlement , differential settlement and load carried by piles by back analysis. Three main performance indicators of the piled raft were proposed: the proportion of load carried by the piles,and the maximum settlement and maximum differential settlement, both as a proportion of the corresponding quantity for an unpiled raft foundation. The last indicator suggests that improved layout of the pile support can lead to a reduction both in the maximum differential settlement and in the overall quantity of piles.
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ISSN: 0975 –6744| NOV 10 TO OCT 11 | Volume 1, Issue 2Page 1
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Figure 1. Model configuration and properties used in the analysis
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Garcia F etal [8] had applied viscohypoplastic model to the case history of piled raft foundation- Messeturm tower in Frankfurt Germany with 3D FEM analysis with ABAQUS and the user subroutine UMAT developed by A. Niemunisand the geotechnical group of the Karlsruhe university in Germany. The calculated resultsare compared with the in situ measurementswith the purpose to verify the viscohypoplastic law in a boundary valueproblem. It was found that viscohypoplastic model can predict withgood level of agreement the behavior of a piled raft foundation. Reul O. [29] has carried outnumerical study of bearingbehaviorof piled rafts in overconsolidated clay. It is shown thatthe interaction between piles and rafts is a majorinfluence on bearing capacity of piled-raft foundation.
To mobilize shallow soil in coastal areas to participate in the interaction of piled raft foundation sufficiently, the researchers extended the concept of piled raft to a new type of foundation named composite piled raft. In the system of composite piled raft, the short piles made of flexible materials were used to strengthen the shallow soft soil, while the long piles made of relatively rigid materials were used to reduce the settlements and the cushion
beneath the raft was used to redistribute and adjust the stress ratio of piles to subsoil.Researchers [13, 41, 14, 40] had carried out rigorous analysis on CPRF system .Liang F.Y. etal[13] have carried out numerical analysis of composite piled raft with cushion subjected to vertical load. Influencing factors such as ratio of length to diameter and elastic moduli of piles as well as thickness and elastic modulus of cushion were studied in details. Load-sharing ratios of piles and subsoil as well as foundation settlement were also investigated. Zheng J.J. etal [41] carried out 3D non-linear finite elementmodeling ofcomposite foundation formed by Cement-Flyash-Gravel-lime piles which is a widely used ground improvement technique. A CFG–lime multi-pile composite foundation is a new concept utilizing CFG and lime piles as shown in fig 2. The parameters studied include the length and diameter of piles and the thickness of the cushion. The stress distribution beneath the composite foundation, the influence of the cushion on load-settlement behavior, and the ratio of stresses in the piles to those in the subsoil are also studied. The results show that settlement is much more significantly affected by the length and diameter of the CFG pile rather than that of the lime pile, thus CFG pile acts as a settlement-reducing pile. On the other hand, the load distribution between piles and subsoil is significantly affected by the cushion thickness. Optimization of composite piled raft foundation with varied rigidity of cushion was carried out by Liang F.etal [14 ].The analysis model of piled raft foundation was set up using the fictitious pile method , and Fredholm’s integral equations of the second kind were deduced to solve the problems .By simulating the cushion with wrinkle springs , the effect of cushion was taken into consideration. The method issuitable to analyze the foundation underworking loads and could be applied tosolve problems of different rigidities of piles and cushion. ZHAO M.H.etal [40 ] have carried out settlement calculation for long –short composite piled raft foundation.Based on the shear deformation method,the Mylonakis and gazetas model wasintroduced . Considering the effects of cushion , the flexible factors of interaction were provided.Then the settlement calculation for long-short composite piled raft foundation was developed. The non-linear analyses of piled-raft foundation have been interesting area of study for many researchers which include[16,17, 37, 33, and 4].Maharaj D.K etal [16] worked on non-linear finite element analysis of piled –raft foundation resting on clay soil layers .. In this study the raft and piles are assumed to be linearly elastic and Drucker-Prager criterion hasbeen used to represent soil as elasto-plastic material.Based on three-dimensional non-linear finite element analysis, it was found that the addition of even a small number of piles increases the load-carrying capacity of a raft foundation. The axial load distribution shows that the piles reach their ultimate capacity earlier than the raft. Maharaj D.K. [17 ] continued his work in non-linear finite element analysis of piled raft foundation under the application of uniformly distributed load .Fig 3 shown below gives finite element discretization and boundary conditions applied to pile raft foundation. Load settlement curves of raft and piled raft foundation have been provided for different raft and pile stiffness. Vásquez L.G etal [37] had carried out non-linear three-dimensional finite element analysis to estimate capacity of piled raft.
Figure 2. Sketch of CFG –lime pile composite
foundation
Figure3. Finite element discretization for piled-raft
A windows based general-purpose finite element program (AMPS) was used to carry out the analyses.A suitable nonlinear model is used to model the soil behavior ,and contact interfaces are included to study the interaction between soil and piles and between soil and slab .Two piled-raft system were analyzed. It was found that the contribution of the pile tip in the resistance tends to increase as the deformation increases.Non-linear finite element analysis of piled-raft foundation resting on layers of soil having different stiffness have been done by [33,4,5].Small J.C.etal [33] has developed a method of analysis for piled raft foundation where piles exhibits nonlinear load –deflection behavior. The raft is analysed by finite element methods, while the piles are treated as springs having a variable stiffness,so as to model any non-linear behaviour. The soil is treated as an elastic medium that may consist of layers of soil having different stiffnesses. The method has been incorporated into the computer program GARP (General Analysis of Rafts with Piles). This program is used to analyse a tall building that was constructed on a piled raft foundation, and a comparison is made of the calculated and observed behaviour. Chen J etal [4]has presented a nonlinear analysis method for simulating the interaction of superstructure –pile-raft-soil system in layered soil.The flexibility coefficients of the pile-pile and the pile-soil are deduced , the stiffness matrix of the pile-soil system is established, and interaction equation of superstructure –pile-raft –soil system is formulated. The distribution of the settlement of the foundation ,which was large in the middle and small in the boundary and like a dish, was consistent with the measured data in field.Further using nonlinear analysismethod and program of the interaction of superstructure-pile-raft-soil system in layered soil ,reaction force on pile head and displacement characteristic of raft of the piled raft foundation were analysed when the thickness of the raft, spacing of piles ,length and diameter of piles were changed.Recently it has been found that piled-rafts having non-identical piles shows efficient performance, researchers [36,5,35,6] had worked in thefield of piled-raft with non-identical pile lengths.Tan Y.C etal [36] has given a design approach in which foundation of medium rise building is designed using skin-friction piles of different length with longer piles at the center and progressive shorter pile towards the edge.For detailed design of foundation two cases were considered.Case 1 considers overall settlement behavior of foundation system to predict settlement profile for structural design .Meanwhile case 2 considersinteraction between the pile-soil-structure (foundation raft) of foundation system to determine load distribution and local settlement of piles and also structuraldesign of raft to complement case 1. The foundation system was monitored and the results obtained showed that foundation system adopted performed satisfactory. Chow H.S.W etal [ 5] have used finite layer method for the analysis of piled rafts with piles of different lengths and diameters. The soil is divided into horizontal layers with different material properties and only vertical loads may be applied to the raft. Interactions between raft-soil-pile are computed Comparisons of actual measurements and the results of finite element calculations have shown that the finite layer method can provide reasonably accurate solutions. It is very easy to prepare data for the finite layer analysis as only the pile locations and properties need to be specified, and the piles discretised into linear elements. This method is more efficient than the conventional three-dimensional finite element analysis.Tan Y.C etal [35]discussed methodology of a floating piled raft foundation system consisting of non-identical piles with longer piles in central portion and progressively shorter piles towards edge for medium rise building (5-storey) on soft clay.Two major cases were considered for detail foundation analysis and design.Case 1 of pile-soil-structure interaction was taken in consideration to determine stress distribution, deformation and settlement of raft and piles. Case 2 considers overall settlement behavior of piled raft (immediate and consolidation settlement). Chow H.S.W [6] has done her Phd thesis on behavior of piled rafts supported by non-identical piles examined by use of computer program APRILS based on finite layer and finite element methods.The finite layer method is used for analysis of layered soil system. This method can be applied different shape of loadings and has shown good agreement with the theoretical solutions.The FEM is used for analysis of raft and piles. Full interaction between raft, piles and soil is considered in the analysis.