INVESTIGATING THE BEHAVIOR OF PILED RAFT FOUNDATION IN CORE WALL FOUNDATION OF HIGH-RISE BUILDINGS

Core structure is an indispensable part of high buildings. Normally, the foundation of the core structure has a raft of larger size than the other foundations in the same project; therefore, the foundation of the core structure can be viewed as a small piled raft foundation. Currently, when calculating the piled foundation of the core, it is mostly assumed that the piles system will bear all the project loads. But this calculation method is not suitable for the actual constructions as well as does not make full use of the bearing capacity of the structure and the ground, leading to using more materials and causing more waste. Core structure aims to increase both stiffness and horizontal load capacity in high-rise buildings, so the moment inside the core transmitted to the foundation is very large. One of the shortcomings of the Plaxis 3D Foundation software is its inability to declare the moment affecting on the foundation due to the loads in this program just includes distributed loads, line loads and point loads in the geometry model. Consequently, when using Plaxis 3D Foundation software to calculate the core wall foundation of high-rise buildings, this moment is converted into an equivalent pair of moments. The research shows that when the core foundation of a high-rise building is placed on soft soil, the pile carries 96%, most of the load transmitted to the foundation. However, when the foundation is placed on hard soil, the soil surrounding the raft will bear about 10% of the load transmitted to the foundation. If this matter is skipped, there will be a large error in calculation and design.


INTRODUCTION
Piled raft foundation is used widely in the structure of high-rise buildings because of its capability to bear load, and the piles and raft settlement are improved significantly compared to the conventional piled foundation. Baziar et al. showed the benefits of using piled raft foundation and application of piled raft foundation in hard clay [1]. In addition, there are some other examples about using piled raft foundation for different soil structures, which are found in Hemsley's books [2]. The efficiency of piled raft foundation is almost presented by Randolph, in which, they are rafts which can bear the above load themselves, and piles are added to increase the total loading capacity and make settlement, deformation between raft-piles in allowable limit [3]. Beckett, Bowels stated the same idea that the piles must be designed in such a way that most of their bearing capacity is utilized to transfer the loads from superstructures, through weak, compressible strata or water onto stronger, more compact, less compressible and stiffer soil or rock at depth [4,5]. Poulos presented the strip on springs method to analyze the settlement of piled raft foundation [6]. Part of the raft is treated as a trip and the piles are treated as springs. This substitution was made for the four interactive components (raft-raft, pile-pile, raft-pile, pile-raft). The impact of external factors was analyzed and calculated by computer in order to define the soil settlement of each part. These numbers on settlement and the series were then analyzed in order to identify the settlement, the moment due to the load impacted on the series and the settle-ment of the soil outside the series of the raft. This method was performed via a computer program called GASP (Geotechnical Analysis of Strip with Piles) and it demonstrated that the settlement was similar to the previous analysis methods. However, this method has some limitations, especially it does not consider the torque in raft and the settlement at a point where the two series intersect the point of analysis. Poulos used the differential finite method called plate on springs. This analysis was done via GARP program [7]. This program allows users to classify the geological cross section and the effect of the pile on the final load capacity for both compressive and tensile cases, to develop the slip resistance under the raft and to demonstrate soil settlement area in the foundation system. This approximate method is similar to how the GASP program works for piles range. A later version of GARP replaced the differential finite method for rafts with a finite element method (FEM) and used an innovative approach to develop limited load capacity in piles. [20,25] described a similar approach, in which different interactions are obtained from elastic theory and nonlinear behavior of the piles is considered through the hypothesis of the hyperbolic curve of the load -settlement of the single pile. The pile-pile interaction applies only to elastic components of pile settlement, while the nonlinear pile settlement component is calculated from the load inside the pile [8,9]. The finite element method (FEM) combined with boundary elements was developed by Hain and Lee [10]. This method considers finite elements for rafts and boundary elements for pile groups. In this method, the elastic method in the continuous environment Mindlin is used to solve the interaction of the piles. The method of Hain and Lee has the advantage of using the model's symmetrical quarter angle so that it can be used to analyze the piled raft foundation of more than 200 piles. It also allows changes in geometry, pile hardness and raft thickness. As a result, the FEM method is more effective than the previous method for analyzing the piled raft foundation. The 2D numerical analysis method has been examined by Desai, Hewitt and Gue (1994), Prakoso and Kulhawy (2001), Seo (2006) [11][12][13][14]. In these analyses, FLAC software is used to model piled raft foundations. The foundations are modeled as plain strain or axial symmetry. In both cases, an approximate calculation is made, especially for the pile, so that the soil settlement and raft size can be determined. However, being similar to spring-based plate analysis, this analysis still fails to determine torque in rafts. Many authors have performed 3-dimensional FEM for piled raft foundation system to study the behavior of load -settlement, the interaction between the rafts and piles with surrounding ground as well as study the load-bearing capacity of raft and pile group in piled raft foundation structure [15][16][17][18][19][20][21]. But although many such investigations were conducted with remarkable success, the limitations of this traditional approach cannot be disregards and maybe are unacceptable in modern practice [4]. Furthermore, there was little change in the number of researches on the effect of piled raft foundation design on highrise building considering soil -pile -structure interaction. This paper applies the finite element method (FEM) using Plaxis 3D Foundation software [15] to study the behavior of piled raft foundation in the core wall foundation of high-rise buildings. Authors also compare the total applied load on piles when being calculated by Safe and Plaxis 3D Foundation. As a results, the load distribution on piled raft foundation of high-rise buildings placed on soft soil and hard soil is determined.

Plaxis 3D Foundation
The Plaxis 3D Foundation program is a three-dimensional finite element program, developed by Plaxis, a company located in the Netherlands. It is used to analyze the deformation and stability for foundation structures, including off-shore foundations [22]. The Plaxis 3D interface compose two sub-programs as Input and Output. At Input, by setting material data sets, properties of soil and other elements (boreholes, embedded piles, plates, etc.) are assigned. The basic soil elements of the 3D finite element mesh are the 10-node tetrahedral elements [23].

Parameters
The piled foundation has a scaffold size of 2.1*2.1m, 1m thick, placed on four 9m-long 300x300mm piles of reinforced concrete with load P=1000 kN and moment M=100 kN.m. Soil parameters are summarized in Table 1. Internal friction angle φ degree 8 31

Parameters
Dilation angle ψ degree 0 1 The plan and section views are shown in Fig. 1, which the length unit is 'mm'.

Calculate the applied load on the top of the pile by analytical method
Weight of the raft: Calculate the applied load on the top of the pile using Plaxis 3D Foundation software [22] Concentrated force is referred to as an evenly distributed force acting on the foundation. q = N tt /(L*B) = 1000/(2.1*2.1) = 227kN/m 2 The load impact on the foundation without a moment is shown in Fig. 2. Moment is converted into two random pair of moments F as shown in Fig Calculation model and results obtained from Plaxis 3D Foundation software are shown in Fig. 5(a,b).
In the model, three distinct elements are illustrated such as embedded piles for the piles, floor element for the raft and volume element for the soil. At the bottom of the model and in the two horizontal directions, the boundary is fullly rigid. Results calculated by Plaxis are summarized in Table 2.

Comparison of two results obtained from analytical method and Plaxis 3D Foundation
Results calculated by analytical method and Plaxis are different and the difference is 7.2% for piles P1, P2 and 8% for piles P3, P4, respectively (Table 3). This difference is resulted from the fact that the analytical method does not consider the influence of ground surrounding the foundation and piles.

Calculation parameters
The piled raft has the dimension of 8.2*22.6*2.5m and is located in the soil layer No.3. The used piles are barrette piles with the size of 1000*2800 mm, length 48 m and the end of the pile is plugged into the soil layer No. 7. Soil parameters are summarized in Table 4. The groundwater level is at -4.5 m and the bottom of the raft is at -17.35 m. The constitutive model used in Plaxis is a linear elastic plastic model with a Mohr-Coulomb (M-C) plasticity criterion.
In which, k v and k h are vertical permeability and horizontal permeability respectively. From the Extended 3D Analysis of Building Systems (ETABS) model results calculated by authors, the three most dangerous internal force pairs to calculate the core wall foundation are shown in Table 5 [24]. The layout of the building's core wall foundation is illustrated in Fig. 6, which the unit of length is 'mm'.

Calculation results
The calculation results of the core wall foundation of Ly Chinh Thang project by Plaxis 3D Foundation software are summarized in Table 6.   Fig. 9 [24].

Figure 9: Calculation model of core wall foundation by Safe
The calculation results of the applied load on the pile by Safe software are summarized in

Calculation parameters
The pile raft has dimensions of 16.6*10.6*2.3m and is located in the soil layer 1a. The used pile is a bored pile with diameter D = 1m, length 52m and the tip of the pile is plugged into soil layer 7.
Based on the geological survey profile, the parameters of the ground soil are shown in Table 8. The groundwater level is at the elevation of -0.4 m and the base of the raft is at -6.3 m. Table 9 summarizes the most dangerous internal force pairs to calculate the foundation for core wall.

DISCUSSION
It can be easily found that the load distribution coefficient in the 3 load cases of the two above calculation projects is almost unchanged.
In the apartment -office building Sacomreal -Hung Vuong, the raft foundation is placed on soft soil (silty clay) with E = 1,460 kN/m 2 . Looking at the table 10, we can see that the load distribution coefficient αpr is approximately equal to 96%. It means that the piles carry the majority of the load and the raft does not have considerable effect. This result stays the same idea with the conventional design approach for the piled raft foundations presented by Randolph (1994) [3]. Ly Chinh Thang office building project is located on a hard soil layer with E=30,000 kN/m 2 . The load distribution coefficient αpr is approximately 90%. This means that the soil around the raft shares partly a portion of about 10% of the total load transmitted to the foundation. It is noticeable that the type of soil beneath the raft affects significantly the behavior of piled rafts. Comparing with the pile and raft load share in Sacomreal -Hung Vuong project, we can see obviously that the relative percentage of total load carried by piles and raft rises considerably in Ly Chinh Thang project which placed in medium sand. Therefore, piles and raft load share is the main distinctive characteristic that diversifies this type of foundation from other type of piled foundations' design [23]. From Figs. 10, 11 16, 17 it can be seen that the total applied load on piles when being calculated by Safe is always greater than the load calculated by Plaxis 3D Foundation. Distribution force applied on each pile when being calculated by Plaxis and Safe are different because Safe does not consider the influence of the ground around the raft foundation. On the other hand, in Plaxis, special types of elements like plate elements are used to model structural behavior of the piled raft foundation. Plate elements are the 6-node triangles with six degrees of freedom per node (three translational and three rotational). In this study, embedded piles, which are modeled like beam elements are used for the model of piles. This usage offers two main benefits. First, the interaction with the continuum as the skin resistance and the foot resistance is considered. Second, the embedded piles can be placed in any direction within the subsoil without any alteration of the mesh. This is achieved by crossing through a 10-node tetrahedral soil element while creating virtual nodes. Although embedded piles do not have a "real" volume and a "real" interface, they have a virtual elastic zone which is created by assigning an equivalent pile diameter within the material data set of the embedded piles. This virtual elastic zone sets aside the plastic behavior of the soil within the zone and approaches to the actual volume pile behavior [22]. Moreover, because of the "virtual" volume and interface, it is impossible to evaluate the effect of strength reduction factor (Rinter).
Rinter is taken as rigid (1.0) with the assumption that the interface does not affect to the strength in the surrounding soil [23]. Moreover, in Plaxis 3D Foundation, the model has to be divided into elements which compose the "finite element mesh". As a result, the settlements and stresses are expected to have major fluctuations by changing the mesh as very coarseness, coarseness, medium, fine or very fine.

CONCLUSIONS
The contribution of raft to the total bearing capacity of piled raft foundation in the core wall foundation of high-rise buildings depends on the geotechnical characteristics of the soil beneath the raft. Indeed, while in soft soil, the piles bear mostly the load transmitted to the foundation (approximately 96%), this percentage only reaches around 90% when the construction is located on hard soil. It means that the role of the raft in sharing load increases considerably (about 6%) in case of the high-rise building is located on hard soil. If the core wall foundation of a high-rise building is placed on a hard soil, the load-bearing capacity of the ground surrounding the pile and the raft should be taken into account to choose the optimal design plan for the core foundation.