6 Case study: Skyper piled raft
6.1 General
Skyper is 154 [m] highrise 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 61.
The tower has a basement with three underground floors and 38 stories with an average estimated applied load of 426 [kN/m^{2}]. 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 [m^{2}]. 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), ElMossallamy 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 62 shows a layout of Skyper with the piled raft.


Figure 62 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 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 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.
6.3 FENet
The raft is divided into triangular elements with maximum length of 2.0 [m] as shown in Figure 63. 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/m^{2}]. Consequently, the total effective applied load on the raft including own weight of the raft and piles is assumed to be N = 810 [MN].


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/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}]
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/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. 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/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 64 that consists of 7 soil layers. The total depth under the ground surface is taken to be 56.4 [m].



6.7 Results
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 ElMossallamy 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 69 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 61 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 threedimensional 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 69 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 66 Settlement for elastic piled raft using German recommendations EAPiles for upper values
Table 61 Overview of calculation results of other models after Richter and Lutz (2010)
Method 
BEM 
FEM 
Elast. half space 
Measured 

Average settlement 
S_{kpp} 
[cm] 
4.8 
6.3 
5.07.3 (9.5) 

Max. settlement 
S_{max} 
[cm] 
6.0 
7.5 
 
5.5^{*} 
Bearing factor 
α_{kpp} 
[%] 
71 
82 
5979 

Modulus of subgrade 
k_{s} 
[MN/m^{3}] 
about 2.0 
1.62.8 


Average pile load 
Q_{p} 
[MN] 
12.5 
14.3 
10.313.9 

Min. pile load 
Q_{p,min} 
[MN] 
9.9 
11.6 
8.510.1 

Max. pile load 
Q_{p,max} 
[MN] 
16.1 
17.6 
13.820.5 

Average pile stiffness 
k_{p} 
[MN/m] 
261 
301 
125280 

* 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 pileraft foundations. PhD thesis, Technical University Darmstadt.
[3] DIN 4014: Bohrpfähle Herstellung, Bemessung und Tragverhalten
Ausgabe März 1990
[4] EAPfähle (2007): Empfehlungen des Arbeitskreises "Pfähle" EAPfähle; 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 PfahlPlattengründungen
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. 2956
[7] El Gendy, M./ El Gendy, A. (2018): Analysis of raft and piled raft by Program ELPLA
GEOTEC Software Inc., Calgary AB, Canada.
[8] ElMossallamy, 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 PfahlPlattengründung 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, 477493
[11] Sales, M., Small, J. and Poulos, H. (2010): Compensated piled rafts in clayey soils: behaviour, measurements, and predictions.
Can Geotech. J. 47: 327345.
[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): LastVerformungsverhalten des Messeturmes Frankfurt/ Main
Vorträge der Baugrundtagung 1990 in Karlsruhe, Seite 371380
[14] Vrettos, C. (2012): Simplified analysis of piled rafts with irregular geometry.
Int. Conf. Testing and Design Methods for Deep Foundations, Kanazawa.