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Combined Pile-Raft Foundation, Safety Concept

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Design Chart of Piled Raft Foundations on Soft Clay

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Carsten Ahner1, Dmitri Sukhov1

SUMMARY There are no standards and no design rules for Combined Piled-Raft Foundation available up to now. The investigation of this problem is just at the beginning. The common reliability approach for the elaboration of a safety concept for Combined Piled-Raft Foundation is proposed and future tasks are set.



The problem of design of Combined Piled-Raft Foundation have become more and more important in the last years, when some skyscrapers were built in Germany. Most of them located in Frankfurt Main, where the over soil layer consists of settlement active clay. The rocky Frankfurt limestone is located at a depth of 44 metres. The Combined Piled-Raft Foundation could be a good economic decision for heavy high buildings, because both the bearing capacity of the slab and the


Institut für Massivbau und Baustofftechnologie i. Gr., Universität Leipzig


bearing capacity of the piles will be fully used. Since 1984 the following projects have been built or designed: • Messetorhaus (Fair Gate House), Frankfurt/M. • Messeturm (Fair Tower), Frankfurt/M. • American Express High Rise Building, Frankfurt/M. • Deutsche Postreklame, Frankfurt/M. • Landeskreditbank Baden-Württemberg • Südwestdeutsche Landesbank, Stuttgart • High Rise Building Port of Singapur Authority • High Rise Building of Trade Centre Landsberger Allee, Berlin • Westendstr. 1, Frankfurt/M. • Castor and Pollux, Frankfurt/Main.

Many projects already built or designed show an absolute necessity for elaboration of a common design concept, which could be used by all specialists of civil engineering. The Combined Piled-Raft Foundation acts as a composite construction consisting of the three bearing elements: piles, slab and subsoil. In comparison with the conventional foundation design the Combined Piled-Raft Foundation exhibits a total new dimension for the subsoil-structure interaction because of the new design philosophy, to use the piles up to their ultimate bearing capacity regarding the soil-pile interaction. This leads to an extreme economic foundation with rather low settlements, if the stiffness of the soil increases with depth.


Actually no standards and no definite design strategies are available for the design and the computation of the Combined Piled-Raft Foundation, so additional research based on measurements, model tests, and numerical computer simulations is necessary. It is certain that in the technical and economical sense the new foundation technology of Combined Piled-Raft Foundation is just at the beginning of an interesting development. Some investigations for the design of Combined Piled-Raft Foundation have been published [1, 2, 3, 4, 5, 6, 7, 8, 9]. These reports can be considered as a basis for further research.



The purpose of the current research is to investigate the safety concept for different parts of Combined Piled-Raft Foundation. The reliability of Combined Piled-Raft Foundation should be the same as for normal piled or raft foundation. There is long-standing experience for the design of piled and raft foundations. On the basis of this experience the safety and design philosophy should be investigated. With the help of analysis of current rules for design of piled and raft foundation and with the help of the reliability theory, the different elements for a global safety factor (mean values and deviations of actions and resistance, safety of system, load’s redistribution, criteria of failure) have to be defined for limit states (ULS and SLS). The global safety factor can be


represented by means of the reliability index. This safety level needs to be accepted by the public. With the analysis of piled and raft foundations and aspects of the interaction, their synthesis should be obtained. The available design methods can be investigated for a new safety and design concept, which should not contradict the current one. The new concept for Combined Piled-Raft Foundation should ensure the same reliability level as the existing foundation types and describe the transformation from piled foundation through Combined Piled-Raft Foundation to pure raft foundation. If in the future more knowledge of Combined Piled-Raft Foundation is available, they should be included in the codes.




Design Concept with the Global Safety Factor

The German code DIN 1054 „Permissible Loads on Foundation Soils“ shows how strong foundation soil could be loaded in the case of piled and raft foundations. The code uses the global safety factor γ which depends on the limit state, the occurrence probability of the load and the type of foundation. The γ-factors vary from 1.05 to 2.0. The upper fractile values of the load are multiplied by global safety factor. These design values of the load are compared with the expected resistance (see Figure 3.1.1).


fR (R) fR (R), fS (S) f S (S)


S q R q = γ∗ S q


R, S

Fig. 3.1.1: Global Safety Factor γ where: R S Sq Rq f resistance action or load (expected action) upper fractile value of the load lower fractile value of the resistance probability density function



Safety Concept for the Combined Piled-Raft Foundation

The Combined Piled-Raft Foundation is a complicated system with special properties. The simplified bearing capacity of Combined Piled-Raft Foundation is shown in Figure 3.2.1.

β x σZ
f S (S) (γ - 1) x S q fR (R)







mR, Piles

mR, Slab



mR, Piles

mR, Slab

Fig. 3.2.1: Bearing Capacity of Combined Piled-Raft Foundation


Here, one can see the mean load settlement curves for: • the sum of the piles, • the slab, • the Combined Piled-Raft Foundation (combination of the piles and the slab). Combined Piled-Raft Foundation shall be designed for expected value of the settlement slimit,ULS for ULS and for slimit,SLS for SLS. Normally, the characteristic value of the load Sq is defined as an upper fractile value of the load distribution. This one is connected with the lower fractile value of resistance Rq by means of global safety factor γ (see Figure 3.1.1). The distance between the mean value of the resistance (mR) and the mean value of the load (mS) can be defined as β x σZ , where β is the reliability index and σZ is the total standard deviation obtained from the variations of load and resistance. A possible safety concept for Combined Piled-Raft Foundation could be described as following. As input data the following parameters are used: • Sq • σS upper fractile value of the load

• the load - settlement curves for piles and slab standard deviation of the load standard deviation of the piles resistance standard deviation of the slab resistance reliability index • σR, Piles • σR, Slab • β -


Mean value of the resistance is calculated with mS and σZ:
2 2 2 m R = mS + β ∗ σ Z = mS + β ∗ (σ S + σ R , Slab + σ R , Piles )

The distribution of the bearing capacity mR on mR, Slab and mR, Piles corresponds to load-settlement curves and defines the bearing capacity of the piles and slab and their design. The design of piles and slab is finally derived from these input data.


Investigation of Resistance

The exact values of load-settlement curves are to be obtained by tests, depending on the foundation dimensions and soil type. Because of variation of soil properties, the corresponding load-settlement curves vary too. This latter variation depends on the settlement. For the case of one pile it is possible to write:

mR , Pile ( s) = mR , Friction ( s) + mR , Pressure ( s)

σ R , Pile ( s) = σ R , Friction ( s) 2 + σ R , Pressure ( s) 2

mR , Friction ( s) = U ∗ t ∗ mτ ( s) mR , Pressure ( s) = A∗ m p ( s)

σ R , Friction ( s) = U ∗ t ∗σ τ ( s) σ R , Pressure ( s) = A∗ σ p ( s)



mR , Pile ( s)


settlement mean value of pile bearing capacity standard deviation of pile bearing capacity mean value of the part of pile bearing capacity (skin friction)

σ R , Pile ( s)
mR , Friction ( s)

σ R , Friction ( s)
mR , Pressure ( s)


standard deviation of the part of pile bearing capacity (skin friction)


mean value of the part of pile bearing capacity (point pressure)

σ R , Pressure ( s)
mτ ( s)


standard deviation of the part of pile bearing capacity (point pressure)


mean value of skin friction standard deviation of skin friction mean value of point pressure standard deviation of point pressure circumference of pile shaft the length of the pile the area of the pile tip

σ τ ( s)
m p ( s)

σ p ( s)


Generally, the mean value and standard deviation for each settlement depend on both mean value and standard deviation of pile skin friction τ and point pressure p, and on ratio pile skin´s surface / pile tip´s surface.



Determination of Partial Safety Factors for CBRF

Accordingly to [10, 11], it is possible to consider the partial safety factor of the resistance γR as independent of the partial safety factor of the load γS , if values of sensitivity factors αS, αR are given. For normal distribution:

γ S ∗ Sq ≤

Rq γR

~ ~ mS + α S ∗ β ∗σ S ≤ mR + α R ∗ β ∗σ R
• • whereby

~ α S = 0,7 ~ α R = -0,8

(see [11, 12]),

mR = mR , Pile + mR ,Slab

σ R = σ R , Pile 2 + σ R ,Slab 2
If σ R , Slab > σ R , Pile then (see [10, 11]):

σ R = σ R ,Slab + 0,4∗ σ R , Pile

~ ~ γ S ∗ Sq ≤ mR ,Slab ∗ (1 + α R ∗ β ∗VR ,Slab ) + mR , Pile ∗ (1 + 0,4∗α R ∗ β ∗VR , Pile )
γ S ∗ Sq ≤ Rd , Slab + Rd , Pile γ S ∗ Sq ≤
Rq , Slab

γ R , Slab


Rq , Pile

γ R , Pile



Rd , Slab Rd , Pile Rq , Slab Rq , Pile -

design value of slab bearing capacity design value of pile bearing capacity characteristic value of slab bearing capacity characteristic value of pile bearing capacity partial safety factor for slab bearing capacity partial safety factor for pile bearing capacity

γ R , Slab γ R , Pile -

The estimation α R,Slab = 1 and α R,Pile = 0.4 are very rough. In future research the real sensitivity factors αR should be obtained by Level II Methods of the reliability analysis (e. g., FORM - First Order Reliability Method) for possible ratios R Slab / R Pile . The given proposals correspond only to the normal distribution of S and R. Because of the great influence of the type of distribution on partial safety factors, the adjustment of the lognormal distribution of R should be considered. Accordingly to ENV 1991, Part 1 [11] reliability index β = 3.83 is used for design life equals 50 years.



Because of the plenty foundations projects which apply Combined Piled-Raft Foundations in resent times, the development of an accepted design and safety concept for this type of foundation is extremely necessary. Proposals for a


solution to this problem should be elaborated with the use of additional data of the variation of soil properties.

4 [1]

LITERATURE Rolf Katzenbach; „Zur technisch-wirtschaftlichen Bedeutung der Kombinierten Pfahl-Plattengründung, dargestellt am Beispiel schwerer Hochhäuser“, Bautechnik 70 (1993), Vol. 3, pp. 161-170, Ernst und Sohn


R. Katzenbach, H. Quick, U. Arslan; „Commerzbank-Hochhaus Frankfurt am Main: Kostenoptimierte und setzungsarme Gründung“, Bauingenieur 71 (1996), pp. 345-354, Springer-Verlag 1


Prof. Dr.-Ing. Sommer; „Kombinierte Pfahl-Plattengründung eines Hochhauses im Ton“, Mitteilungen des Grundbauinstitut Prof. Dr.-Ing. H. Sommer und Partner GmbH, Vol. 1, Dezember 1987

[4] [5]

T. Voß; „Messungen an einer Pfahl-Plattengründung in weichem Fels.“, KTB, Bauingenieur 64 (1989), pp. 207-208 M. Thaher; „Pfahlplattengründungen“, Geotechnik Sonderausgabe 1992: Praxisbezogene Anwendung der Zentrifugen-Modelltechnik im Grundbau, Tunnel- und Schachtbau und Umwelttechnik, Deutsche Gesellschaft für Erd- und Grundbau, Essen (1992)


M. Thaher, H. L. Jessberger; „The behaviour of pile-raft foundations, investigated in centrifuge model tests.“, Centrifuge 91, Balkema (1991), pp. 101-106



R. W. Cooke; „Piled Raft Foundations on stiff clays-A contribution to design philosophy“, Geotechnique (UK), Vol. 36, No. 2, Mar. 1986, pp. 169-203


J. Hooper; „A observation on the behaviour of a Piled Raft Foundation on London clay.“, Proceedings, Instit Civil Engr, part 2 (UK), Vol. 55, Dec. 1973, pp. 855-877


P. Clancy, M. F. Randolph; „An approximate analysis procedure for Piled Raft Foundations.“, International J. Num. & Anal. Methods in Geomechn (UK), Vol. 17, No. 12, Dec. 1993, pp. 849-869

[10] [11] [12]

„General Principles on the Specifications of Safety Requirements for Structures“, 1981, Beuth Verlag GmbH, Berlin ENV 1991 „Basis of Design and Actions on Structures“, Part 1: „Basis of Design“, August 1994 ISO 2394 „General Principles on Reliability for Structures“, May 1994


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