Prof. Dipl.-Ing. H. Quick Ingenieure und Geologen GmbH provides geotechnical engineering services e. g. for high-rise buildings in Germany and abroad.

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1 Prof. Dipl.-Ing. Hubert Quick City Hall of Dubai, March, 7 th, 2005 o Prof. Dipl.-Ing. H. Quick Ingenieure und Geologen GmbH provides geotechnical engineering services e. g. for high-rise buildings in Germany and abroad. o The geotechnical design procedure for high-rise buildings consists of several phases from the feasibility study and the concept / realisation of the soil investigation to the soil report and finally the final geotechnical design.

2 For a safe design a high quality soil report with the following specifications is indispensable. The serviceability of construction has to be evaluated regarding the uniform settlement of the construction, the differential settlement, the tilting as well as the deflection ratio.

3 o The raft foundation is characterised by the following specifications: the loads are transferred by raft into the ground the raft covers the entire area of the structure In case of expected large differential settlements the raft must be separated (joints), this can cause difficult construction phases. In general the stress distribution (bearing pressure) of a raft foundation is described by Boussinesq. The ultimate limit state (ULS) and for the serviceability limit state (SLS) for a raft foundation must be proved with the following calculations:

4 o With increasing height of buildings respectively increasing loads the depicted raft foundation is not suitable or even not sufficient to transfer the loads properly into the ground. Therefore a pile foundation is often used. The main function of a pile foundation is to transfer the loads into the ground with an adequate factor of safety. The load is transferred by end bearing and the skin friction of the piles into the ground. The ultimate limit state (ULS) and for the serviceability limit state (SLS) for a pile foundation must be proved with the following calculations:

5 o The Commerzbank is situated in Frankfurt, Germany within the banking district. The building reaches a height of 299 m. The building is founded on a pile foundation. The building was constructed directly next to an existing high-rise building. The existing building reaches a height of 103 m and is founded on a raft. A total of 111 telescopic piles with diameter of 1.8 m within the first 20 m beneath the raft, followed by a diameter of 1.5 m were installed. All piles were constructed with a jet grouted shaft as well as jet grouting 10 m underneath the piles in the cavernous limestone. Up to a depth of 5 m beneath the surface fillings were encountered underlying by quaternary sands and gravels in depth of 10 m. Underneath the ground layers of the Hydrobien (Frankfurt clay) is found up to a depth of 35 to 40 m. The ground layers of the Inflaten (Frankfurt limestone) and Certithien (marl) were encountered beneath the Hydrobien. The Frankfurt clay consists of clay interbedded with sand and limestone. The thickness of the Inflaten layer is approx. 25 m. The Inflaten consists of limestone, sand, silt and marl. Two groundwater level were found in the project site: An unconfined groundwater level in the quaternary sands and gravels as well as a confined groundwater level circulating in the sands and limestone of the Hydrobien.

6 The calculations were carried out in consideration of the symmetry of the building. A sixth part of the building was modelled by Finite- and Infinite-Elements. The extent of the model is 200 m x 180 m x 120 m. Infinite elements were used to model the ground outside the tower area. The used material law for the finite elements is a elastic-plastic stress-strain behaviour with the yield conditions of Drucker-Prager with cap. An elastic material law was implemented for the structural elements (piles and raft). The maximum settlements of the building add up to 2.1 cm. The minimum settlements were encountered with 1.5 cm. This leads to a tilting of smaller than 1/2000. These results cause no negative effect on the serviceability of the building.

7 The monitoring shows that the 111 piles of the Commerzbank carry approx. 96 % of the total load of the building. This indicates that not all loads are transferred by the piles into the ground.

8 o In addition to the presented and often used raft or pile foundation, the combined pile-raft-foundation is nowadays often used to transfer the loads into the ground. In comparison to a pile foundation, the combined pile-raft-foundation both the piles and the raft transfer the loads into ground. The loads are transferred by skin friction and end bearing as well as contact pressures of the raft foundation (bearing pressure). The piles are used up to their ultimate bearing capacity (load level) which is higher than the permissible design value for a comparable single pile The combined pile-raft-foundation represents a complex foundation system, which requires a qualified understanding of the soil-structure interactions. The task for the geotechnical engineer is to evaluate the load distribution between the piles and the raft. The distribution of the total load between the raft and the piles is described by the coefficient of CPRF (α CPRF ).

9 The ultimate limit state (ULS) and for the serviceability limit state (SLS) for a combined pile-raft-foundation must be proved with the following calculations:

10 The proof of the ultimate limit state of the combined pile-raft-foundation can be carried out by means of the Finite-Element-Method with the following calculation steps:

11 o The Drehscheibe is situated in Frankfurt, Germany. The new high-rise building Drehscheibe is currently under construction. The building area of approx m² consists of a 77 m high rise tower with 19 floors which is connected to surrounding low buildings with max. 6 floors like shown on the site plan. Below the whole area an underground parking with 3 basement floors on a continuous foundation slab is currently under construction. The raft of the whole building complex had to be designed continuously without any joints because of the groundwater situation. The eccentric loading of the building complex with a raft foundation causes large differential settlements and angular displacements. Thus a raft foundation was not advisable. It was decided to carry out a combined pile-raft-foundation (CPRF) underneath the high-rise building. The surrounding low buildings are constructed by means of a raft foundation. The design of the CPRF consists of a foundation raft with a thickness of 2.5 m and an area of approx. 900 m². The 31 foundation piles with a diameter of 1.2 m and length between 10.5 and 16.5 m are installed underneath the high loaded core section of the high-rise building. The explored ground consists of fills, sand and gravel underlying by tertiary sandy, silty clay (congerien and prososthenien layer). Below this sequence fossil limestone (Landschneckenmergel) and hydrobien layer was found.

12 The reef structures of the fossil limestone consist of an alternating sequence of layered facies of clay and silt (marl) as well as vertical extending columns (fossil reef structures). The experience in the local area gained from former projects like the construction of subway tunnels showed diameters of up to 3 m for these reef structures. The reef structures are described as cavernous and compressible and arose of former coral reefs. The groundwater conditions are characterized by two groundwater levels. One unconfined groundwater table in the Quaternary fills and one confined table in the tertiary fossil limestone. The two levels show approximately the same pressure head of 4 m beneath the surface. The calculations were carried out by means of the Finite-Element-Method. The chosen numerical model represents the realistic geometry of the foundation raft of the high-rise building and a strip of 20 m width of the raft of the bordering low rise buildings, the positioning of the piles as well as the pile length. For the modelling of the soil behaviour an elastic-plastic constitutive law with isotropic hardening was implemented. The used material law for the ground elements is a elastic-plastic stress-strain behaviour (Mohr-Coulomb). An elastic material law was implemented for the structural elements (piles and raft).

13 The maximum settlements of the building add up to 3.0 cm. The minimum settlements were encountered with 2.2 cm. This leads to a tilting of smaller than 1/1000. These results cause no negative effect on the serviceability of the building. The results of the calculation shows that the 31 piles of the Drehscheibe carry approx. 64 % of the total load of the building (α CPRF = 0.64).

14 The Skyper is situated in Frankfurt, Germany. The high-rise building Skyper has just been constructed and consists of a 153 m high tower which is connected to surrounding low buildings. Below the whole area an underground parking with 3 floors on a continuous foundation slab was planned. The eccentric loading of the building complex with a raft foundation causes large differential settlements and angular displacements and negative interactions with the existing neighbouring Villa as well as other neighbouring structures. Thus a raft foundation was not advisable. It was decided to carry out a combined pile-raft-foundation (CPRF). The surrounding low buildings are constructed by means of a raft foundation. Up to a depth of 3 to 6 m beneath the surface fillings were encountered underlying by quaternary sands and gravels in depth of 10 m. Underneath the ground layers of the Hydrobien (Frankfurt clay) is found up to a depth of 50 to 60 m. The ground layers of the Inflaten (limestone) and Certithien (marl) were encountered beneath the Hydrobien. The Frankfurt clay consists of clay interbedded with sand and limestone. The dark grey clay was encountered as stiff to very stiff. The limestone within the Hydrobien layer are encountered with a thickness of a few decimetre to 2.0 m. Two groundwater level were found in the project site: An unconfined groundwater level in the quaternary sands and gravels as well as a confined groundwater level. The unconfined groundwater level was found 3 to 4 m beneath the surface. The confined level s pressure head shows approximately the pressure head.

15 For the modelling of the soil behaviour an elastic-plastic constitutive law with isotropic hardening was implemented. The used material law for the ground elements is a elastic-plastic stress-strain behaviour (Mohr-Coulomb). An elastic material law was implemented for the structural elements (piles and raft). The maximum settlements of the building add up to 5.5 cm. The minimum settlements were encountered with 3.0 cm. This leads to a tilting of smaller than 1/2000. These results cause no negative effect on the serviceability of the building. The results of the calculation shows that the 46 piles of the Skpyer carry approx. 63 % of the total load of the building (αcprf = 0.63). The result of the numerical calculations shows a rather good agreement with the measurement. The measured settlements are still increasing due to consolidation of the clay.

16 The Treptowers is situated in Berlin, Germany directly next to the river Spree. The building reaches a height of 121 m. The building is founded on a combined pile-raft-foundation. The bordering low-rise building with 10 floors is founded on the raft. A total of 54 piles with a diameter of 88 cm were installed. All piles were constructed with a jet grouted shaft. The length of the piles vary from 12.5 m to 16 m. The bottom of the raft is up to 8 m beneath the surface. The ground and groundwater conditions were explored by boreholes up to a depth of 40 m. Up to a depth of 4m beneath the surface fillings and organic soils were encountered underlying by loose to medium dense sands of the Pleistocene. These sands were found up to depth of approx. 19 m. Dense sands were encountered in depth of > 19 m. Locally a layer of boulder clay with a thickness of 1.5 m was found. The groundwater level is approx. 3 to 4 m beneath the surface.

17 The calculations were carried out in consideration of the symmetry of the building. The used material law is an elastic-ideal-plastic stress-strain behaviour with the yield conditions of Mohr-Coulomb. The jet-grouting of the pile shaft is modelled by an expansion of the pile elements. An elastic material law was implemented for the structural elements (piles and raft). The maximum settlements of the building add up to 7.3 cm. The minimum settlements were encountered with 5.0 cm. This leads to a tilting of smaller than 1/2000. The mean value of settlement is 6.3 cm. These results cause no negative effect on the serviceability of the building. The monitoring shows that the 54 piles of the Treptowers carry approx. 55 % of the total load of the building. The α CPRF factor, the summation of the piles loads divided by the total load, amount therefore to 0.55 (α CPRF = 0.62). The factor α CPRF of the numerical calculation is 0.62.

18 The Jumeirah Lake Tower is situated in Dubai, U.A.E. A first study of different foundation systems has been evaluated for: pile foundation combined pile-raft-foundation The ground and groundwater conditions were explored by boreholes up to a depth of 75 m. Up to a depth of 8 m beneath the surface dense sand was encountered underlying by weak sandstone, which reaches a depth of 24 m beneath ground surface. In depth of > 24 m weak siltstone was encountered. The groundwater level is approx. 2 m beneath the surface.

19 The maximum settlements of the building on piles add up to 4.5 cm. The maximum settlements of the building on CPRF add up to 6.9 cm. o The presentation on foundation design shows customary foundation systems for high-rise buildings such as raft and pile foundation. In addition the foundation system of combined pile-raft-foundation is introduced. For all three foundation systems the required proof of the ultimate limit and serviceability limit state has been presented. However, the choice of the adequate system is often not depending on the proof of the ultimate limit state (e.g. base failure, etc.). A raft foundation suits often the according required factors of safety. But the serviceability of the high-rise building and / or neighbouring structures can often not be guaranteed by a raft foundation. For this, pile foundations have been constructed to reduce the settlements and to satisfy the ultimate limit state and what is more the serviceability. The calculaiton of the pile foundation is an idealisation, where only the piles carry the loads of the structure. This does not reflect the realistic behaviour. Where piles are primarily used to reduce settlements (satisfy the serviceability) and where an adequate factor of safety against failure (e.g. base failure) is provided, the combined pile-raft-foundation (CPRF) has been put forward in the past. In this case the loads of the structure are transferred into the ground by piles and raft. This approach is able to reflect the realistic soil-structure-

20 behaviour. Calculations can be carried out with numerical methods, such as the Finite-Element-Method. This successful design and construction has been verified by many structures including many high-rise buildings and bridges.