Analysis and Design of Piled Raft Foundation Chanraksmey Roth; Pongpipat Anantanasakul

Abstract: A number of high rise buildings are built on pile raft foundation system which is difficult to understand its behavior. Soil-structure interaction of piled raft foundation involves pile-soil interaction, pile-soil-pile interaction, raft-soil interaction, and pile-raft interaction. Hence, proper methods to analyze and design are needed. This study aims to evaluation design methods for this particular piled raft foundation. Furthermore, it offers special features of each methods so that useful pieces of information which has drawn from this study is able to help geotechnical engineers in selecting appropriate design method. A series of case studies in Dubai, South Korea are to be discussed. A discussion of the case studies concerning simplified method, finite element method is made and compare to field measurement. Result and conclusion????IntroductionPiled-raft foundation is a practical and cost-effective foundation solution for high-rise buildings as it offers a great bearing capacity and decreases settlement. The Piled raft foundation is a new concept of designing foundation which total load from superstructure is partially shared by the raft through contact with soil the rest of the load is shared by piles through skin friction and toe bearing resistance. The conventional Piled-raft founation design ignores contribution of raft, then all loads are transferred to soil through piles. If raft, on the other hand, carries the total loads coming from the superstructure, the thickness of the raft will be relatively thick which increases total cost of foundation. In addition, the excessive settelement occurs. In such a condition, the Piled Raft foundation should be employed, so that, ultimate load capacity, the settlement and differtial settlement performance are improved. Also, the thickness of the raft decreases. The important criterion governing the design of pile raft is the relative proportion of load caried by raft and piles aL and the effect of the additional pile support on foundation settlement as (Y.M. EI- Mossallamy 2009). To ensure continuity of the soil bearing resistance to the raft, the safty factor of the piles is reduced than the normal safety factor for conventional pile design so that the settlement of the piles are big enough to have consistent contact between soil and raft. Nevertheless, the magnitude of settlement must be smaller than allowable settlement (K. Amornfa 2012).

Previous StudiesPoulos (1980) introduced a simplified method to analyze and design the piled raft foundation. The concept of analysis is similar to analysis of freestanding pile groups except that a single pile is attached circular cap resting on the soil. The analysis for Load-settlement curve to failure almost assembles in principle to the analysis for large-diameter pile in Poulos 1980 Section 5.4.Poulos (2001) discussed a series of available methods to analyze the piled raft behavior as well as their capacities and limitation. The simplified methods were recommended to use for preliminary design or for verify purpose whereas three dimensional analyses, mainly FLAC 3-D is the most powerful numerical methods among methods discussed herein. M. Tariq (2007) investigated the effectiveness of a group of 1072 piles for controlling settlement of 76m in diameter of raft in weak rock. 2D axi-symmetrical and 3D Finite Element Method are employed. The results showed that 2D axi-symmetrical FEM model is able to simulate the complex 3D problem with a good accuracy. Poulos (2009) described a pile raft design process for high-rise buildings including 3 case studies in Dubai, The Emirates Twin Towers, The Burj Dubai, and the Nakheel Tall Tower. A numerical computer analysis, Geotechnical Analysis of Raft with Piles (GARP) was used. For the Emirate Project, the settlement would be as well-predicted as the pile load test was not achieved. However, the measurement during construction was approximately 25% of the prediction because of conservative assumption made in deriving the pile settlement interaction factors. For the Burj Dubai, the maximum settlement predicted is larger than observation. Also the performance of piled raft foundation of this project exceeded expectation. For the Nakheel Tall Tower, the foundation pare was designed economically. The maximum settlements obtained from the GARP analysis was 95mm which was very close to Golder using a full three dimensional finite element analysis with the Plaxis 3D Foundation program. Andr (2011) compared FEM analysis of piled raft foundation in 2D (strain strain condition) and 3D by employing Plaxis 2D and 3D Foundation. it was found that two dimensional model (2D) was overestimate the settlement about 30%, raft bending moment about 30% and the pile force about 10%. Also, the result demonstrated that a special attention should be paid while using plane strain models as inaccuracies happened such as computing differential settlement or the bending moment in the longer direction. Author suggested that as the problem goes more complex, the 3D FEM should be carried out for detail design. Amornfa (2012) discussed the current practice on foundation design of high-rise buildings in Bangkok,Thailand. The conventional method of analysis (the combined stress equation and the plate on springs analysis) and 3D FEM were compared. It found that despite the neglect of pile-pile and raft-pile interaction, the plate on pile springs method offer relatively large dissimilar results if compared to 3D FEM.Hussein (2013) stated that piled raft foundation offers a good load capacity and decrease settlement in cost-saving manner if compared with the traditional foundation design. the percentage ratio of bearing loads between the pile and the raft was presented. Plaxis 3D Foundation is used to modeling problem and compared with experimental work of the same problem with Program Ansys. Results showed that the load carrying by piles decreased as the spacing to diameter ratio increased.

Simplified MethodSimplified methods are usually used in the preliminary or feasibility studies of most geotechnical project. In the practical point of view, simplified method is strongly recommended for assisting designers or planner to quickly compute the number piles, piles length, bearing capacity of the piled raft and its settlement (Y. M. El-Mossallamy 2009). In this report, Poulos-Davis-Randolph (PDR) Method is briefly described. Poulos-Davis-Randolph (PDR) MethodThis method is employed to assess vertical bearing capacity of a pile raft foundation using simple approaches, the ultimate load capacity is normally taken the smallest value of the following two values: the sumation of the ultimate capacities of the all piles and the raft. the ultimate capacities of a block containing the piles and the raft, plus that of the portion of the raft outside the periphery of the piles.1. The stiffness of piled raft foundation can be estimated as follows:Kpr=(Kp+Kr(1-acp))/(1- acp2Kr/Kp)where Kpr= stiffness of piled raftKp= stiffness of piled raftKr= stiffness of the raft aloneacp= raft-pile interation factor2. The proportion of the total applied load carried by the raft is Pr/Pt=Kr(1- acp)/(Kp+Kr(1- acp))=X where Pr= Load carried by the raft Pt= Total applied load3. The raft-pile interaction factor acp can be estimated as follows:acp=1-ln(rc/r0)/zwhere rc= average radius of pile capro= radius of pile4. The total applied load, P1=Pup(1-X)Where Pup= Ultimate load capacity of the piles in the groupX= proportion of load carried by the piles By using this method, the stiffness of the raft is able to be computed manually from elastic theory. Also the stiffnesss of the single pile is calculated from the closed form approximate solution of Randolph and Wroth (1978) while the group settlement ratio (used for computing the pile group stiffness) is approximated by Rs=n0.5, where n = the number of piles.LimitationFor the very stiff or very flexible pile raft, the settlement of the pile-raft system is needed, the elastic-based analysis is likely to be adequate.This method is not suitabe for raft containing bending moment.Influence of Pile Arrangement: It was found that the settlment of a system depends primarily on the number of piles in the group, and not on the arrangement of the piles.

Burlands ApproachBurland (1995) introduced a method that piles are designed to act as settlement reducer and to develop their full bearing capacity at the design load. Burland offered simplified process to design piled raft foundation as following: 1. Estimate the total long term load settlement relationship for raft without piles 2. Assess an acceptable design settlement Sd, which should include a margin of safety.3. P1 is the load carried by the raft corresponding to Sd.4. The load excess P0-P1 is assumed to be carried by settlement-reducing piles. The raft resistance of these piles will be fully mobilized and therefore no factor of safety is applied but about 0.9 should be applied to the conservative best estimate of ultimate shaft capacity, Psu.5. If the piles are located below columns which carry a load in excess of Psu, the piled raft may be analyzed as a raft on which reduced coloumn loads act. At such columns, the reduced load Qr is: Qr=Q-0.9Psu6. The process for estimating the settlement of the piled raft is not explicitly set out by Burland. However, it would appear reasonabe to adopt the approximate approach of Randolph (1994) in which: Spr=Sr*Kr/KprWhere Spr= settlement of piled raftSr=settlemtn of raft without piles subjected to the total applied loadingKr= stiffness of raftKpr=stiffness of piled raft

4

Finite Element MethodPLAXIS 3D Foundation ProgramThis program is one the most widely used tools to analyze the complex problem in field of geotechnical engineering. Plaxis 3D Foundation is a high-performance and computer oriented finite element method software which was created by Vermeer and Brinkgreve (1995). This program offered a special feature that pile is able to embedded in soils and is assumed to be slender beam element. Also, most of interactions of piled raft foundation are taken into account. Moreover, Plaxis 3D Foundation is capable to perform the following tasks:-model and analyze piled raft in arbitrary shape -Apply operating loads or other design performance conditions-Study physical responses, such as stress levels, distributions, or the impact of electromagnetic fields.FLAC 3D FLAC 3D, a commercially available computer program, provided a complete 3D analysis of piled raft foundation system. The special feature of this program is to eliminate some approximate assumptions that simplified method used to be assumed. Non-linear analysis is also performed by FLAC 3D, but a big amount of time will need to be consumed for analyzed the piled raft foundation. On the other hand, FLAC 3D gives a good result in terms of load-settlement relation.The Essential of Pile-Raft Interaction A number of analysts of pile raft foundation used structural analysis programs to represent a plate and the piles as springs. Then the spring stiffness of the piles was computed for a single isolated pile and ignored the effect raft-pile and pile-raft interaction. In a result, the stiffness of the foundation was too large and settlement was too low.Application to Practical DesignIt is recommended to follow 3-stage design process for piled raft:1. The simple PDR method can be employed to roughly compute the number of piles needed to satisfy the overall bearing capacity and settlement needed. 2. A simple approach can be employed to assess the maximum column load which the raft can bear without a pile. This will provide a means of assessing under which columns piles are required, for a particular raft thickness, and the requirement for such piles.3. Employing numerical analyses such as Plaxis 3D Foundation or FLAC 3D can offer a detailed analysis for final design. Also it provides detailed estimates of settlement and differential settlement under various loading combinations, and also details of the raft and pile behavior for structural design.Case StudiesCase Study 1: The Emirates Project The Emirates Project is a twin tower in Dubai which is a 8th tallest building in the world for the office tower and 17th tallest building in the world for the Hotel Tower. The foundations of these two towers were large diameter of piles with raft. The main strata at Emirates site was mostly found in sand. The major design parameters for the foundation system were the ultimate skin friction of the piles, the ultimate end bearing resistance of the piles, the ultimate bearing capacity of the raft, and the Youngs modulus of the soils for both the raft and the pile behavior under static loading. Moreover, the GARP (Geotechnical Analysis of Raft with Piles) (Poulos 1994) was employed as analysis method. Comparison between predicted and measured test pile behavior was carried out. A fair agreement of settlement prediction and measurement of pile was revealed as shown in Fig

Case Study 2: Storage TankThe Storage tank is in circular shape with 76m in diameter and raft thickness is 9m. 2D axi-symmetrical and 3D finite element methods were adopted. Type of construction: Storage Tank Shape: Circular Diameter=76m Raft depth=9m Number of piles=1072m Piles depth=12.7m

Figure showed the 3D modelling of piled raft.Figure showed that the different of settelment in 2D axi-symmetrical and 3D finite element models is less than 10%. It indicated that 2D axi-symmetrical models was in acceptable range. FigurePoulos (2001) compared load settlement from various methods, it was obvious that PDR method agreed well with FLAC3-D whereas FLAC2-D over-predicted the settlements. This indicated that PDR is useful for simple pile raft foundation.

A case of a piled raft foundation with 25 bored piled and the raft thickness is 1m. the width of the raft is 55m. three analysis methods were adapted in this case study: Plate on Pile Springs Method, Plate on Both pile and Soil Springs, and 3D FEM (Plaxis 3D Foundation program). Figure Figure showed that the settlement from the three methods were largely different. 3D FEM provided the highest settlement among the three. This indicated that Plate on Pile Springs Method and Plate on Both pile and Soil Springs underestimated the settlement despite these two methods ignored pile-raft interaction and serveral approximate assumption. In this case, it is recommended to employed 3D FEM for the detail design or last verification of the design.

Case studyA pile raft foundation consisted of 16m in width and piles as following:-Piled raft A: 49 piles (2m in piles spacing)-Piled raft B: 16 piles (4m in piles spacing)-Piled raft C: 9 piles (6m in piles spacing)All the result showed the same trend that plane strain analysis in Plaxis 2D underestimated the differential settlement.

ResultsConclusion

17