# 7           Case study 7: Burj Khalifa piled raft

## 7.1         General

Burj Khalifa is a 163-storey skyscraper in Dubai, United Arab Emirates. The total height of the building is 829.8 [m], with a podium development at its base, including a 4 to 6-storey garage.  With a total height of 829.8 [m] and a roof height (excluding antenna) of 828 [m], Burj Khalifa has been the tallest structure and building in the world since its topping out in late 2008, Figure 7-1. Burj Khalifa is located on a 42 000 [m2] site. The tower is founded on a 3.7 [m] thick raft supported on 192 bored piles, 1.5 [m] in diameter, extending 47.45 [m] below the base of the raft; podium structures are founded on a 0.65 [m] thick raft (increased to 1 [m] at column locations) supported on 750 bored piles, 0.9 [m] in diameter, extending 30–35 [m] below the base of the raft. The tower raft consists of three wings each is 50 [m] long and 25 [m] wide forming an area of 3305 [m2]. Figure 7-2 shows an isometric view of Burj Khalifa Tower foundation system and a plan for pile locations.

Extensive studies using different calculation methods were carried out by Poulos and Bunce (2008), Badelow & Poulos (2016) and Russo etc. al. (2013).

Figure 7-1       Burj Khalifa [1]

Figure 7-2       Burj Khalifa Tower Foundation system

## 7.2         Analysis of the piled raft

Using the available data and results of the Burj Khalifa piled raft, which have been discussed in detail in the previous references, the nonlinear analyses of piled raft in ELPLA are evaluated and verified using the following load-settlement relations of piles, El Gendy et al. (2006) and El Gendy (2007):

1- Hyperbolic Function for Load-Settlement Curve.

The foundation system is analyzed as an elastic piled raft in which the raft is considered as an elastic plate supported on equal rigid piles.

A series of comparisons are carried out to evaluate the nonlinear analyses of piled raft for load-settlement relations of piles. In which, results of other analytical solutions and measurements are compared with those obtained by ELPLA.

## 7.3         FE-Net

The raft is divided into triangular elements with a maximum length of 2.0 [m] as shown in Figure 7-3. Piles are divided into five elements with 9.49 [m] length.

Only long-term conditions have been considered, and for most of the early analyses, an average load per pile of 23.21 [MN] has been used (this is a representative of the design dead and live loads) and has been applied as an uniformly distributed load on the tower raft of about 1250 [kPa].

Figure 7-3       Mesh of Burj Khalifa piled raft with piles of element length = 2.0 [m]

## 7.5         Pile and raft material

The raft is 3.7 [m] thick and was poured utilizing C50 (cube strength) self-consolidating concrete. The Tower raft is supported by 192 bored cast-in-place piles. The C60 self-consolidating concrete piles are 1.5 [m] in diameter and 47.45 [m] long.

The following values were used as pile and raft material:

For the raft:

Modulus of elasticity Ep        =          33234              [MN/m2]

Poisson's ratio vp         =          0.167               [-]

Unit weight                γb         =          23.60               [kN/m3]

For piles:

Modulus of elasticity Ep        =          36406              [MN/m2]

Unit weight                γb         =          23.60               [kN/m3]

## 7.6         Soil properties

The ground conditions comprise a horizontally stratified subsurface profile which is complex and highly variable, due to the nature of deposition and the prevalent hot arid climatic conditions. Medium dense to very loose granular silty sands (Marine Deposits) are underlain by successions of very weak to weak sandstone interbedded with very weakly cemented sand, gypsiferous fine grained sandstone/siltstone and weak to moderately weak conglomerate/calcisiltite.

Groundwater levels are generally high across the site and excavations were likely to encounter groundwater at approximately 2.5 [m] below ground level.

The drilling was carried out using cable percussion techniques with follow-on rotary drilling methods to depths between 30 [m] and 140 [m] below ground level.

The ground profile and derived geotechnical design parameters assessed from the investigation data are summarized in Table 7-1.

Table 7-1                 Summary of Geotechnical Profile and Parameters

 Strata Sub-Strata Subsurface Material Level at top of stratum     [m DMD] Thickness       H [m] UCS       qs [MPa] Undrained Modulus     Eu [MPa] Ult. Comp. Shaft Frict. fs [kPa] 1 1a Medium dense silty Sand +2.50 1.50 - 34.5 - 1b Loose to very loose silty Sand +1.00 2.20 - 11.5 - 2 2 Very weak to moderately weak Calcarenite -1.20 6.10 2.0 500 350 3 3a Medium dense to very dense Sand/ Silt with frequent sandstone bands -7.30 6.20 - 50 250 3b Very weak to weak Calcareous Sandstone -13.50 7.50 1.0 250 250 3c Very weak to weak Calcareous Sandstone -21.00 3.00 1.0 140 250 4 4 Very weak to weak gypsiferous Sandstone/ calcareous Sandstone -24.00 4.50 2.0 140 250 5 5a Very weak to moderately weak Calcisiltite/ Conglomeritic Calcisiltite -28.50 21.50 1.30 310 285 5b Very weak to moderately weak Calcisiltite/ Conglomeritic Calcisiltite -50.00 18.50 1.70 405 325 6 6 Very weak to weak Calcareous/ Conglomerate strata -68.50 22.50 2.50 560 400 7 7 Weak to moderately weak Claystone/ Siltstone -91.00 >46.79 1.70 405 325

To carry out the analysis, the subsoil under the raft is considered as indicated in the boring log of Figure 7-4 that consists of 12 soil layers. The total depth under the ground surface is taken to be 140 [m].

Figure 7-4       Boring log

Figure 7-5 to Figure 7-6 show load-settlement relations for the different analyses.

Figure 7-6       Load-settlement relation according to a hyperbolic function

## 7.7         Results

As examples for results of different analyses by ELPLA, Figure 7-8 and Figure 7-7 show the settlement for elastic piled raft of Burj Khalifa using methods: "Hyperbolic Function for Load-Settlement Curve" and "Given Load-Settlement Curve from pile-load test", respectively. Besides, Figure 7-9, Figure 7-10 and Figure 7-11 show self-settlement Sv, interaction settlement Srv and total settlement Sr of piles using the method "Given Load-Settlement Curve from pile-load test".

Figure 7-7       Settlement using the method "Hyperbolic Function for Load-Settlement Curve"

Figure 7-8       Settlement using the method "Given Load-Settlement Curve"

Figure 7-9       Self settlement of piles Sv [mm] using the method "Given Load-Settlement Curve"

Figure 7-10   Interaction settlement of piles Srv [mm] using the method "Given Load-Settlement Curve"

Figure 7-11   Total settlement of piles Sr [mm] using the method "Given Load-Settlement Curve"

## 7.8         Measurements and other results

### 7.8.1        Measured settlement

The construction of Burj Khalifa began on 6 January 2004, with the exterior of the structure completed on 1 October 2009. According to Badelow & Poulos (2016) the settlement of the tower raft was monitored from completion of concreting till 18 February 2008. The recorded maximum settlement at 18 February 2008 was 43 [mm] under nearly 80 % of the building load.

A comparison is presented between the measured settlement on 18 February 2008 under 80% of the total load and that computed by ELPLA using Method: "Given Load-Settlement Curve". Figure 7-12 shows a comparison between measured settlement (Feb. 2008) and computed settlement under 80 % of the total load at a cross section of the Wing c, while 0 shows a comparison between extreme values of measured settlement and that calculated for the same case.

Figure 7-12   Measured settlement (Feb. 2008) and computed settlement under 80 % of total load

Table 7-2                 Comparison between measured settlement at February 2008 and that calculated by ELPLA under 80 % of the total load

 Method Smax. [mm] Smin. [mm] SDiff. [mm] Measured (18 February 2008) 43 29 14 ELPLA – Method: "Given Load-Settlement Curve" 48 24 24

Figure 7-13 shows contours of measured settlement [mm] at February 2008 and that calculated by ELPLA under 80 % of the total load using method "Given Load-Settlement Curve"

Figure 7-13   Contours of measured settlement [mm] at February 2008 and that calculated by ELPLA under 80 % of the total load using method "Given Load-Settlement Curve"

The above comparison of the piled raft under 80 % of the total load illustrates that the maximum and minimum results of ELPLA are in good agreement with the measured settlement with difference not exceed 1 [cm]. The measured differential settlement is considerably smaller than that computed because the building stiffness is not considered in ELPLA analysis in this case, which would reduce the differential settlement.

### 7.8.2        Calculated final settlement

Several analyses were used to assess the response of the foundation for the Burj Khalifa Tower and Podium. The main design model was developed using a Finite Element (FE) program ABAQUS run by a specialist company KW Ltd, based in the UK. Other models were developed to validate and correlate the results from the ABAQUS model using other software programs. The design values of settlement were presented by Poulos and Bunce (2008).

Russo etc. al. (2013) deals with the re-assessment of foundation settlements for the Burj Khalifa Tower in Dubai. Re-assessment was carried out using the computer program Non-linear Analysis of Piled Rafts NAPRA with neglecting the structure stiffness effect on raft settlement.

A comparison is presented between the computed settlement in other references and the computed settlement by ELPLA using different Nonlinear analysis methods. The comparison is presented as a cross section at Wing c and tables as in Figure 7-14 and Table 7-3, respectively.

The comparison shows that the results of two methods in ELPLA are in good agreement with the calculated results of Russo etc. al. (2013). The second method (Load-Settlement relation as a Hyperbolic Function for Load-Settlement Curve) results are closer to the design results presented by Poulos and Bunce (2008).

Figure 7-14   Final settlement for elastic piled raft using different analysis models

Table 7-3                 Comparison between various calculated settlement profiles

 Method Smax. [mm] Smin. [mm] SDiff. [mm] Design Values (Poulos and Bunce 2008) 78 60 18 Russo etc. al. (2013) 58 24 34 ELPLA – Given Load-Settlement Curve 58 29 29 ELPLA – Hyperbolic Function for Load-Settlement Curve 79 47 32

### 7.8.3        Calculated final pile loads

The maximum and minimum pile loads were obtained from the three-dimensional finite element analysis for all loading combinations by Poulos and Bunce (2008). The maximum loads were at the corners of the three “wings” and were of the order of 35 [MN], while the minimum loads were within the center of the group and were of the order of 12-13 [MN].

Figure 7-15 and Figure 7-16 show pile loads obtained by ELPLA using method: "Hyperbolic Function for Load-Settlement Curve" and method "Given Load-Settlement Curve from pile-load test", while Table 7-4 compares results of max and min pile loads obtained by ELPLA with those of Poulos and Bunce (2008).

Figure 7-15   Pile load [MN] using the method "Hyperbolic Function for Load-Settlement Curve"

Table 7-4                 Comparison between various calculated pile loads

 Method Pmax. [MN] Pmin. [MN] FEA (Poulos and Bunce 2008) 35 12-13 ELPLA – Given Load-Settlement Curve 38 11 ELPLA – Hyperbolic Function for Load-Settlement Curve 21 13

## 7.9         Evaluation

It can be concluded that results obtained from different analyses available in ELPLA can present rapid and acceptable estimation for settlement and pile loads. This case study shows also that analyses available in ELPLA are practical for analyzing large piled raft problems considering less computational time compared with other complicated models using three- dimensional finite element analyses.

## 7.10          References

[1]        El Gendy, M. / Hanisch, J./ Kany, M. (2006): Empirische nichtlineare Berechnung            von Kombinierten Pfahl-Plattengründungen.

Bautechnik 9/06

[2]        El Gendy, M. (2007): Formulation of a composed coefficient technique for analyzing      large piled raft. Scientific Bulletin, Faculty of Engineering, Ain Shams University, Cairo, Egypt. Vol. 42, No. 1, March 2007, pp. 29-56

[3]        El Gendy, M./ El Gendy, A. (2018): Analysis of raft and piled raft by Program ELPLA GEOTEC Software Inc., Calgary AB, Canada.

[4]        Poulos, H. / Bunce, G. (2008): Foundation Design for the Burj Khalifa, Dubai – the World's Tallest Building.

6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11-16, 2008.

[5]        Russo, G./ Abagnara, V./ Poulos, H. & Small, J. (2013): Re-assessment of foundation settlements for the Burj Khalifa, Dubai. Acta Geotechnica (2013) 8:3–15.

[6]        Badelow, F./ Poulos, H. (2016): Geotechnical foundation design for some of the world’s tallest buildings.

The 15th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering