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.

2- Given 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.

7.4         Loads

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 E_p        =          33234              [MN/m²]

Poisson's ratio v_p         =          0.167               [-]

Unit weight                γ_b         =          23.60               [kN/m³]

 

For piles:

Modulus of elasticity E_p        =          36406              [MN/m²]

Unit weight                γ_b         =          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

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 7-5       Load-settlement relation from pile load test

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 S_v, interaction settlement S_rv and total settlement S_r 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 S_v [mm] using the method "Given Load-Settlement Curve"

 

 

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

 

 

Figure 7-11   Total settlement of piles S_r [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

 

‎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

 

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"

Figure 7-16   Pile load [MN] using the method "Given Load-Settlement Curve"

 

 Table 7-4                 Comparison between various calculated pile loads

   

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

6           Case study: Skyper piled raft

6.1         General

Skyper is 154 [m] high-rise building supported on a piled raft foundation. The tower was one of the tallest three skyscrapers in Frankfurt, Germany when it was completed in 2004, ‎Figure 6-1.

 

The tower has a basement with three underground floors and 38 stories with an average estimated applied load of 426 [kN/m²]. The raft of the Skyper tower has a uniform thickness of 3.5 [m] supported by 46 bored piles with a diameter 1.5 [m]. Piles are arranged under the core structure in 2 rings; external ring has 20 piles, 31 [m] long while the internal ring has 26 piles, 35 [m] in length. The raft has an irregular plan shape with an area of 1900 [m²]. The raft founded on a typical Frankfurt clay at a depth 13.4 [m] below ground surface. The subsoil at the location of the building consists of gravels and sands up to 7.4 [m] below ground surface underlay by layers of Frankfurt clay extending to a depth of 56.4 [m] below ground surface followed by incompressible Frankfurt Limestone layer. The groundwater level is 5 [m] below ground surface.

 

Extensive studies using different calculation methods were carried out by Saglam (2003), El-Mossallamy et al. (2009), Sales et al. (2010), Richter and Lutz (2010), Vrettos, C. (2012), Bohn (2015) to evaluate the Skyper piled raft foundation design

 

Figure 6-1              Skyper [1]

‎Figure 6-2 shows a layout of Skyper with the piled raft.

 

 

L=31[m]

L=35[m]

 

 Figure 6-2              Layout of Skyper with piled raft

 

6.2         Analysis of the piled raft

Using the available data and results of the Skyper piled raft, which have been discussed in details 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.

2- German standard DIN 4014.

3- German recommendations EA-Piles (lower values).

4- German recommendations EA-Piles (upper values).

The foundation system is analyzed as rigid and elastic piled rafts. In which, the raft is considered to be either rigid or elastic plate supported on 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.

6.3         FE-Net

The raft is divided into triangular elements with maximum length of 2.0 [m] as shown in ‎Figure 6-3. Similarly, piles are divided into elements with 2.0 [m] length.

6.4         Loads

The uplift pressure on the raft due to groundwater is considered to be P_w = 160 [kN/m²]. Consequently, the total effective applied load on the raft including own weight of the raft and piles is assumed to be N = 810 [MN].

G1 (L=31[m], D=1.5[m])

G2 (L=35[m], D=1.5[m])

Figure 6-3              Mesh of Skyper piled raft with piles

  

6.5         Pile and raft material

In the analysis, the raft thickness is 3.5 [m]. The piles are considered in the calculation with the corresponding diameter 1.5 [m] and the lengths 31 [m] and 35 [m].  The following values were used as pile and raft material:

For the raft:

Modulus of elasticity E_p        =          34 000             [MN/m²]

Poisson's ratio v_p         =          0.25                 [-]

Unit weight                γ_b         =          0.0                   [kN/m³]

 

For piles:

Modulus of elasticity E_p        =          22 000             [MN/m²]

Unit weight                γ_b         =          0.0                   [kN/m³]

6.6         Soil properties

The average clay properties used in analysis can be described as follows:

 

Modulus of compressibility

Based on the back analysis presented by Amann et al. (1975), the distribution of modulus of compressibility for loading of Frankfurt clay with depth is defined by the following empirical formula:

                                                         (3.1)

while that for reloading is:

                                                         (3.2)

where:

E_s         Modulus of compressibility for loading [MN/m²]

E_so       Initial modulus of compressibility, E_so = 7 [MN/m²]

z           Depth measured from the clay surface, [m]

W_s        Modulus of compressibility for reloading [MN/m²]

 

Undrained cohesion c_u

The undrained cohesion c_u of Frankfurt clay increases with depth from c_u = 100 [kN/m²] to c_u = 400 [kN/m²] in 70 [m] depth under the clay surface according to Sommer/ Katzenbach (1990). To carry out the analyses using German standard and recommendations, an average undrained cohesion of c_u = 200 [kN/m²] is considered.

 

Limit pile load Ql

Russo (1998) suggested a limiting shaft friction not less than 180 [kN/m²] meeting undrained shear strength of 200 [kN/m²]. To carry out the analysis using a hyperbolic function, a limit shaft friction of τ = 180 [kN/m²] is assumed. The limit pile load for pile group 1 is calculated from:

 

                     (2.3)

while that for pile group 2 from:

 

                     (2.4)

 

where:

Q_l        Limit pile load, [MN]

τ           Limit shaft friction, τ = 180 [kN/m²]

D         Pile diameter, [m]

l           Pile length, [m]

 

Poisson’s ratio

Poisson’s ratio of gravels and sands is taken to be ν_s = 0.25 [-].

To carry out the analysis, the subsoil under the raft is considered as indicated in the boring log of ‎Figure 6-4 that consists of 7 soil layers. The total depth under the ground surface is taken to be 56.4 [m].

 

T, Clay

S, Sand

G, Gravel

 

 

Figure 6-4              Boring log

  

6.7         Results

As examples for results of different analyses by ELPLA, ‎Figure 6-5 and ‎Figure 6-6 show the settlement, while ‎Figure 6-7 and ‎Figure 6-8 show the pile load for both rigid and elastic piled rafts using German recommendations EA-Piles for upper values.

6.8         Measurements and other results

The construction of Skyper started in 2003 and finished in the first half of 2004. According to Richter and Lutz (2010), all calculations resulted in a predicted settlement of 5 up to 7.5 [cm] for the tower, while according to El-Mossallamy et al. (2009) the bearing factor of piled raft α_kpp was computed in a range of 60% to 85%. The observed settlement was 5.5 [cm] directly after the completion of the shell only. After Lutz et al. (2006) with α_kpp ≈0.6, the average max. pile forces ranges between 12 to 14 [MN], while min. pile forces ranges between 10 to 11[MN].

 

‎Figure 6-9 compares results of settlement, bearing factor of piled raft and min and max pile loads obtained by ELPLA with the predicted results from the other methods. For more comparison, ‎Table 6-1 shows the other results for another different methods presented by Richter and Lutz  (2010). Based on settlement measurements 4 years after construction, the maximum settlement under the foundation is about 5 to 5.5 [cm]. Using the three-dimensional finite element method, a settlement of 6.3 [cm] was calculated according to Richter and Lutz (2010).

 

6.9         Evaluation

It can be concluded from ‎Figure 6-9 that results obtained from different analyses available in ELPLA can present rapid and acceptable estimation for settlement, bearing factor of the piled raft and pile loads. This case study shows also that analyses available in ELPLA are practical for analyzing large piled raft problems. Because of they are taking less computational time compared with other complicated models using three dimension finite element analyses.

  

Figure 6-5              Settlement for rigid piled raft using German recommendations EA-Piles for upper                values

Figure 6-6              Settlement for elastic piled raft using German recommendations EA-Piles for upper values

 

Figure 6-7              Pile load [MN] for rigid piled raft using German recommendations EA-Piles for upper values

Figure 6-8              Pile load [MN] for elastic piled raft German recommendations EA-Piles for upper values

Figure 6-9              Results obtained from measurements and ELPLA

 

Table 6-1                 Overview of calculation results of other models after Richter and Lutz (2010)

Method			BEM	FEM	Elast. half space	Measured
Average settlement	Skpp	[cm]	4.8	6.3	5.0-7.3 (9.5)
Max. settlement	Smax	[cm]	6.0	7.5	-	5.5*
Bearing factor	αkpp	[%]	71	82	59-79
Modulus of subgrade	ks	[MN/m3]	about 2.0		1.6-2.8
Average pile load	Qp	[MN]	12.5	14.3	10.3-13.9
Min. pile load	Qp,min	[MN]	9.9	11.6	8.5-10.1
Max. pile load	Qp,max	[MN]	16.1	17.6	13.8-20.5
Average pile stiffness	kp	[MN/m]	261	301	125-280

* Directly after the completion of the shell only

  

6.10          References

 

[1]        Amann, P./ Breth, H./ Stroh, D. (1975): Verformungsverhalten des Baugrundes beim Baugrubenaushub und anschließendem Hochhausbau am Beispiel des Frankfurter Ton

Mitteilungen der Versuchsanstalt für Bodenmechanik und Grundbau der Technischen Hochschule Darmstadt, Heft 15

[2]        Bohn, C. (2015): Serviceability and safety in the design of rigid inclusions and combined pile-raft foundations. PhD thesis, Technical University Darmstadt.

[3]       DIN 4014: Bohrpfähle Herstellung, Bemessung und Tragverhalten

Ausgabe März 1990

[4]       EA-Pfähle (2007): Empfehlungen des Arbeitskreises "Pfähle" EA-Pfähle; Arbeitskreis    Pfähle (AK 2,1) der Deutschen Gesellschaft für Geotechnik e.V., 1. Auflage, Ernst &            Sohn, Berlin.

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

Bautechnik 9/06

[6]       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

[7]       El Gendy, M./ El Gendy, A. (2018): Analysis of raft and piled raft by Program ELPLA

GEOTEC Software Inc., Calgary AB, Canada.

[8]        El-Mossallamy, Y., Lutz, B. and Duerrwang, R. (2009): Special aspects related to the behavior of piled raft foundation. Proceedings of the 17th International Conference on Soil Mechanics and Geotechnical Engineering, M. Hamza et al. (Eds.).

[9]       Richter, T and Lutz, B. (2010): Berechnung einer Kombinierten Pfahl-Plattengründung   am Beispiel des Hochhauses „Skyper“ in Frankfurt/Main.

            Bautechnik 87 (2010), Heft 4.

[10]     Russo, G. (1998): Numerical analysis of piled raft

Int. J. Numer. Anal. Meth. Geomech., 22, 477-493

[11]      Sales, M., Small, J. and Poulos, H. (2010): Compensated piled rafts in clayey soils: behaviour, measurements, and predictions.

            Can Geotech. J. 47: 327-345.

[12]      Saglam, N. (2003): Settlement of piled rafts: A critical review of the case histories and calculation methods.

            M.Sc. thesis, The middle east technical university.

[13]      Sommer, H./ Katzenbach, R. (1990): Last-Verformungsverhalten des Messeturmes Frankfurt/ Main

Vorträge der Baugrundtagung 1990 in Karlsruhe, Seite 371-380

[14]      Vrettos, C. (2012): Simplified analysis of piled rafts with irregular geometry.

            Int. Conf. Testing and Design Methods for Deep Foundations, Kanazawa.

5           Case study 5: Westend 1 piled raft

5.1         General

Westend 1 is 208 [m] high skyscraper and standing on a piled raft. The tower lies in Frankfurt city, Germany. It was completed in 1993. The tower was the third tallest skyscraper in Frankfurt and also in Germany until 1993, ‎Figure 5-1.

Using instruments installed inside the foundation of Westend 1, an extensive measuring program was established to monitor the behavior of the building. Because these instruments record raft settlements, raft contact pressures and loads on pile heads and along pile shafts, the building was a good chance to verify many analysis methods for piled raft. Extensive studies were carried out by Poulos et al. (1997) Poulos (2001), Reul and Randolph (2003) and Chaudhary (2010) on analyzing the piled raft by methods of Poulos and Davis (1980), Poulos (1991), Poulos (1994), Ta and Small (1996), Sinha (1996), Franke et al. (1994), Randolph (1983) and Clancy and Randolph (1993). The results were compared together and with those of the measurements.

 

The building has a basement with three underground floors and 51 stories with an average estimated applied pressure of 412 [kN/m²]. The foundation area is about 2900 [m²] founded on Frankfurt clay at a depth of 14·5 [m] under the ground surface. Raft thickness varies from 4·65 [m] at the middle to 3 [m] at the edge. A total of 40 bored piles with equal diameter and length, each 30 [m] length and 1.3 [m] in diameter. Piles are arranged under the raft in 2 rings under the heavy columns of the superstructure. The subsoil at the location of the building consists of gravels and sands up to 8 [m] below the ground surface underlay by layers of Frankfurt clay extending to great depth of more than 100 [m] below the ground surface. The groundwater level lies at 4.75 [m] under the ground surface.

 

Figure 5-1              Westend 1 [1]

‎Figure 5-2 shows a layout of Westend 1 with the piled raft according to Reul and Randolph (2003).

 

Figure 5-2              Layout of Westend 1 with piled raft after Reul and Randolph (2003)

 

5.2         Analysis of the piled raft

Using the available data and results of the Westend 1 piled raft, which have been discussed in details 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.

2- German standard DIN 4014.

3- German recommendations EA-Piles (lower values).

4- German recommendations EA-Piles (upper values).

 

The foundation system is analyzed as rigid and elastic piled rafts. In which, the raft is considered to be either rigid or 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.

5.3         FE-Net

The raft is divided into triangular elements with a maximum length of 2.0 [m] as shown in ‎Figure 5-3. Similarly, piles are divided into elements with 2.0 [m] length.

5.4         Loads

The uplift pressure on the raft due to groundwater is considered to be P_w = 81 [kN/m²]. Consequently, the total effective applied load on the raft including own weight of the raft and piles is assumed to be N = 950 [MN]. The load is defined by a uniform load of 412 [kN/m²] on the entire raft.

 

Figure 5-3              Mesh of Westend 1 piled raft with piles of element length = 2.0 [m]

5.5         Pile and raft material

In the analysis, the raft thickness is assumed to be 4.2 [m]. All piles are equal diameter and length, each 30 [m] length and 1.3 [m] in diameter. The following values were used as pile and raft material:

 

For the raft:

Modulus of elasticity E_p        =          34 000             [MN/m²]

Poisson's ratio v_p         =          0.25                 [-]

Unit weight                γ_b         =          0.0                   [kN/m³]

 For piles:

Modulus of elasticity E_p        =          22 000             [MN/m²]

Unit weight                γ_b         =          0.0                   [kN/m³]

5.6         Soil properties

The average clay properties used in analysis can be described as follows:

 

Modulus of compressibility

Based on the back analysis presented by Amann et al. (1975), the distribution of modulus of compressibility for loading of Frankfurt clay with depth is defined by the following empirical formula:

                                                         (3.1)

while that for reloading is:

                                                         (3.2)

where:

E_s         Modulus of compressibility for loading [MN/m²]

E_so       Initial modulus of compressibility, E_so = 7 [MN/m²]

z           Depth measured from the clay surface, [m]

W_s        Modulus of compressibility for reloading [MN/m²]

 

Undrained cohesion c_u

The undrained cohesion c_u of Frankfurt clay increases with depth from c_u = 100 [kN/m²] to c_u = 400 [kN/m²] in 70 [m] depth under the clay surface according to Sommer/ Katzenbach (1990). To carry out the analyses using German standard and recommendations, an average undrained cohesion of c_u = 200 [kN/m²] is considered.

 

Limit pile load Ql

Russo (1998) suggested a limiting shaft friction not less than 180 [kN/m²] meeting undrained shear strength of 200 [kN/m²]. To carry out the analysis using a hyperbolic function, a limit shaft friction of τ = 180 [kN/m²] is assumed, which gives a limit pile load of Q_l = 22 [MN]. It is calculated from:

 

                     (2.3)

 

where:

Q_l        Limit pile load, [MN]

τ           Limit shaft friction, τ = 180 [kN/m²]

D         Pile diameter, [m]

l           Pile length, [m]

 

Poisson’s ratio

Poisson’s ratio of gravels and sands is taken to be ν_s = 0.25 [-].

 

To carry out the analysis, the subsoil under the raft is considered as indicated in the boring log of ‎Figure 5-4 that consists of 12 soil layers. The total depth under the ground surface is taken to be 108 [m].

 ‎Figure 5-5 to ‎Figure 5-8 show load settlement relations for the different analyses.

 

S, Sand

G, Kies

T, Ton

 

 

Figure 5-4              Boring log

 

 

Figure 5-5              Load-settlement relation (hyperbolic function)

 

Figure 5-6              Load-settlement relation (DIN 4014)

Figure 5-7              Load-settlement relation (EA-Piles, lower values)

 

Figure 5-8              Load-settlement relation (EA-Piles, upper values)

5.7         Results

As examples for results of different analyses by ELPLA, ‎Figure 5-9 and ‎Figure 5-10 show the settlement for both rigid and elastic piled rafts using German recommendations EA-Piles for upper and lower values, while ‎Figure 5-11 and ‎Figure 5-12 show the pile load for both rigid and elastic piled rafts using hyperbolic function.

 5.8         Measurements and other results

The construction of Westend 1 started in 1990 and finished in 1993. According to Lutz et al. (1996) the recorded settlement at the center of the raft 2·5 years after completion of the shell of the building is 12 [cm], while the bearing factor of piled raft from the measured pile loads is α_kpp= 0.49. The measured minimum and maximum pile loads of 9·2 [MN] and 14·9 [MN] respectively are taken according to Franke and Lutz (1994).

‎Figure 5-13 compares results of settlement, bearing factor of piled raft and min and max pile loads obtained by ELPLA with those of measurements. For more comparison, ‎Figure 5-14 shows the other results for the other different methods presented by Reul and Randolph (2003). In which, using the three-dimensional finite element method, a displacement of 10.9 [cm] was obtained.

 

5.9         Evaluation

It can be concluded from ‎Figure 5-13 that results obtained from different analyses available in ELPLA can present rapid and acceptable estimation for settlement, bearing factor of the piled raft and pile loads. This case study shows also that analyses available in ELPLA are practical for analyzing large piled raft problems. Because they are taking less computational time compared with other complicated models using three dimensional finite element analyses.

  

Figure 5-9              Settlement for rigid piled raft using German recommendations EA-Piles for lower                values

           

 

 

 

Figure 5-10          Settlement for elastic piled raft using German recommendations EA-Piles for lower values                       

Figure 5-11          Pile load [MN] for rigid piled raft using hyperbolic function

 

Figure 5-12          Pile load [MN] for elastic piled raft using hyperbolic function

Figure 5-13          Results obtained from measurements and ELPLA

 

 

Figure 5-14          Comparison of different methods and measurements ( Reul and Randolph (2003))

 

5.10          References

[1]       Abate , S. (2009): Analysis and Parametric Study of Piled Raft Foundation Using Finite             Element Based Software.

Msc thesis, Addis Ababa University.

[2]        Amann, P./ Breth, H./ Stroh, D. (1975): Verformungsverhalten des Baugrundes beim Baugrubenaushub und anschließendem Hochhausbau am Beispiel des Frankfurter Ton

Mitteilungen der Versuchsanstalt für Bodenmechanik und Grundbau der Technischen Hochschule Darmstadt, Heft 15.

[3]       Cecilia, B. (2015): Serviceability and safety in the design of rigid inclusions and combined pile-raft foundations.

PhD thesis, Technical University Darmstadt.

[4]       Clancy, P. & Randolph, M. (1993): An approximate analysis procedure for piled raft      foundations.

Int. J. Numer. Anal. Methods Geomech. 17, 849–869.

[5]       Chaudhary, K. (2010): Reconsiders for soil-structure interaction problems with   significant material stiffness contrast.

PhD thesis, National University of Singapore.

[6]       DIN 4014: Bohrpfähle Herstellung, Bemessung und Tragverhalten

Ausgabe März 1990

[7]       EA-Pfähle (2007): Empfehlungen des Arbeitskreises "Pfähle" EA-Pfähle; Arbeitskreis    Pfähle (AK 2,1) der Deutschen Gesellschaft für Geotechnik e.V., 1. Auflage, Ernst &            Sohn, Berlin.

[8]       Franke, E., Lutz, B. & El-Mossallamy, Y. (1994): Measurements and numerical modelling of high rise building foundations on Frankfurt Clay. Proceedings of a conference on vertical and horizontal deformations of foundations and embankments.

ASCE Geotechnical Special Publication No. 40, Vol. 2, pp. 1325–1336.

[9]       Franke, E., Lutz, B. (1994): Pfahl-Platten-Gründungs-Messungen..

Report for the German Research Council (DFG) No. Fr60-1/11.

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

Bautechnik 9/06

[11]     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

[12]     El Gendy, M./ El Gendy, A. (2018): Analysis of raft and piled raft by Program ELPLA

GEOTEC Software Inc., Calgary AB, Canada.

[13]     Lutz, B. / Wittmann, P. / El Mossallamy, Y./ Katzenbach, R. (1996): Die Anwendung von Pfahl-Plattengründungen: Entwurfspraxis, Dimensionierung und Erfahrungen mit Gründungen in überkonsolidierten Tonen auf der Grundlage von Messungen.

Vorträge der Baugrundtagung 1996 in Berlin, pp. 153–164. Essen: DGGT.

[14]     Poulos, H./ Davis, E. (1980): Pile Foundation Analysis and Design

John Wiley & Sons, Inc.

[15]     Poulos, H. (1991): Analysis of piled strip foundations.

Proceedings of the conference on computer methods and advances in geomechanics.

pp. 183–191, Rotterdam: Balkema.

 

[16]     Poulos, H. (1994): An approximate numerical analysis of pile–raft interaction.

Int. J. Numer. Anal. Methods Geomech. 18, 73–92.

[17]     Poulos, H. G., Small, J. C., Ta, L. D., Sinha, J. & Chen, L. (1997): Comparison of some             methods for analysis of piled rafts..

Proc. 14th Int. Conf. Soil Mech. Found. Engng, Hamburg 2, 1119-1124.

[18]     Poulos, H. (2001): Piled raft foundations: design and applications.

Géotechnique 51, No. 2, 95-113

[19]     Randolph, M. (1983): Design of piled raft foundations.

Proceedings of the international symposium on recent developments in laboratory and field tests and analysis of geotechnical problems, Bangkok, pp. 525–537.

[20]      Reul, O./ Randolph, M. (2003): Piled rafts in overconsolidated clay: comparison of in situ measurements and numerical analyses

Géotechnique 53, No. 3, 301-315

[21]     Russo, G. (1998): Numerical analysis of piled raft

Int. J. Numer. Anal. Meth. Geomech., 22, 477-493

[22]      Small , J. (2002): Soil-Structure interaction.

Australian Geomechanics Journal.

[23]      Sommer, H./ Katzenbach, R. (1990): Last-Verformungsverhalten des Messeturmes Frankfurt/ Main

Vorträge der Baugrundtagung 1990 in Karlsruhe, Seite 371-380

[24]      Sinha, J. (1996): Piled raft foundations subjected to swelling and shrinking soils.

PhD thesis, University of Sydney, Australia.

[25]      Ta, L./ Small, J. (1996): Analysis of piled raft systems in layered soils.

Int. J. Numer. Anal. Methods Geomech. 20, 57–72.