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 51.
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/m^{2}]. The foundation area is about 2900 [m^{2}] 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 52 shows a layout of Westend 1 with the piled raft according to Reul and Randolph (2003).


Figure 52 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 loadsettlement relations of piles, El Gendy et al. (2006) and El Gendy (2007):
1 Hyperbolic function.
2 German standard DIN 4014.
3 German recommendations EAPiles (lower values).
4 German recommendations EAPiles (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 loadsettlement relations of piles. In which, results of other analytical solutions and measurements are compared with those obtained by ELPLA.
5.3 FENet
The raft is divided into triangular elements with a maximum length of 2.0 [m] as shown in Figure 53. 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/m^{2}]. 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/m^{2}] on the entire raft.
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/m^{2}]
Poisson's ratio v_{p} = 0.25 []
Unit weight γ_{b} = 0.0 [kN/m^{3}]
For piles:
Modulus of elasticity E_{p} = 22 000 [MN/m^{2}]
Unit weight γ_{b} = 0.0 [kN/m^{3}]
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/m^{2}]
E_{so} Initial modulus of compressibility, E_{so }= 7 [MN/m^{2}]
z Depth measured from the clay surface, [m]
W_{s} Modulus of compressibility for reloading [MN/m^{2}]
Undrained cohesion c_{u}
The undrained cohesion c_{u} of Frankfurt clay increases with depth from c_{u }= 100 [kN/m^{2}] to c_{u }= 400 [kN/m^{2}] 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/m^{2}] is considered.
Limit pile load Ql
Russo (1998) suggested a limiting shaft friction not less than 180 [kN/m^{2}] meeting undrained shear strength of 200 [kN/m^{2}]. To carry out the analysis using a hyperbolic function, a limit shaft friction of τ = 180 [kN/m^{2}] 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/m^{2}]
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 54 that consists of 12 soil layers. The total depth under the ground surface is taken to be 108 [m].
Figure 55 to Figure 58 show load settlement relations for the different analyses.



5.7 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 513 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 514 shows the other results for the other different methods presented by Reul and Randolph (2003). In which, using the threedimensional finite element method, a displacement of 10.9 [cm] was obtained.
5.9 Evaluation
It can be concluded from Figure 513 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.
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 pileraft 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 soilstructure 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] EAPfähle (2007): Empfehlungen des Arbeitskreises "Pfähle" EAPfähle; Arbeitskreis Pfähle (AK 2,1) der Deutschen Gesellschaft für Geotechnik e.V., 1. Auflage, Ernst & Sohn, Berlin.
[8] Franke, E., Lutz, B. & ElMossallamy, 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): PfahlPlattenGründungsMessungen..
Report for the German Research Council (DFG) No. Fr601/11.
[10] El Gendy, M./ Hanisch, J./ Kany, M. (2006): Empirische nichtlineare Berechnung von Kombinierten PfahlPlattengründungen
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. 2956
[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 PfahlPlattengründungen: 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, 11191124.
[18] Poulos, H. (2001): Piled raft foundations: design and applications.
Géotechnique 51, No. 2, 95113
[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, 301315
[21] Russo, G. (1998): Numerical analysis of piled raft
Int. J. Numer. Anal. Meth. Geomech., 22, 477493
[22] Small , J. (2002): SoilStructure interaction.
Australian Geomechanics Journal.
[23] Sommer, H./ Katzenbach, R. (1990): LastVerformungsverhalten des Messeturmes Frankfurt/ Main
Vorträge der Baugrundtagung 1990 in Karlsruhe, Seite 371380
[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.