Risk Management and Risk Communication in Geotechnical

Risk Management and Risk Communication in Geotechnical Engineering by Independent Peer

Review and Special Technical Solutions Rolf KATZENBACH a, Steffen LEPPLA a and Matthias SEIP b

a Technische Universität Darmstadt, Institute and Laboratory of Geotechnics,Germany b Ingenieursozietät Professor Dr.-Ing. Katzenbach GmbH, Germany

Abstract. For all types of construction works the aspects safety, serviceability and sustainability have to be focused during the planning phase, the design phase, the construction phase and in the service phase. Particularly this has to be considered for complex deep foundation systems like the Combined Pile-Raft Foundation (CPRF). The basis for a successful realisation of a project is an adequate soil investigation and a high-level design. For risk mitigation an independent peer review and special technical solutions are necessary. In geotechnical engineering Eurocode 7 (EC 7) defines the scope of necessary measures in relation to the complexity of the project. The complexity of the project is described by the Geotechnical Category GC 1 to GC 3. For the Geotechnical Category GC 3, this is the category for projects with a very high complexity, for risk mitigation comprehensive measures are necessary. The paper explains risk management and risk communication by several examples of realized CPRFs. All examples belong to the Geotechnical Category GC 3.

Keywords. Independent peer review, observational method, Combined Pile-Raft Foundation, in-situ load test

1. Introduction

Due to the complicated soil-structure interaction of complex foundation systems like the Combined Pile-Raft Foundation (CPRF) a sufficient risk management and risk communication is necessary for a successful realization of large and challenging projects. Figure 1. Construction works of piles close to an underground metro station in Frankfurt am Main, Germany.

Especially when constructions take place in inner cities the soil-structure interaction and the effects on existing buildings and underground structures have to be focused (Katzenbach & Kurze 2013, Katzenbach et al. 2014). Figure 1 gives an impression from engineering practice.

Because of the distinctive soil-structure interaction of CPRFs these hybrid foundation systems belong to the Geotechnical Category GC 3, which is the category for construction projects with a very high complexity. For a safe and optimized design and construction of CPRFs the following main aspects have to be considered: � qualified experts for planning, design and

construction � interaction between architects, structural

engineers and geotechnical engineers � adequate soil investigation with core

drillings, laboratory tests and field tests � design of complex deep foundation systems

using the Finite-Element-Method (FEM) in combination with enhanced in-situ load tests for calibrating the soil parameters used in the numerical simulations

Geotechnical Safety and Risk VT. Schweckendiek et al. (Eds.)

© 2015 The authors and IOS Press.This article is published online with Open Access by IOS Press and distributed under the terms

of the Creative Commons Attribution Non-Commercial License.doi:10.3233/978-1-61499-580-7-76

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� quality assurance by an independent peer review (4-eye-principle) combined with the observational method and a contingency action plan if necessary

� independent supervision on the construction site

2. Quality Control Management

For a sufficient risk management and risk communication a quality control management based on the 4-eye-principle is necessary. This principle is a process of an independent peer review as shown in Figure 2. It consists of 3 levels.

Figure 2. Independent peer review process (4-eye-principle). The investor, the experts for planning,

design and the construction company belong to the first level. Planning and design are done according to the requirements of the investor and all relevant documents to obtain the building permission are prepared considering all relevant laws, standards, regulations etc. The building authorities are the second level and are

responsible for the building permission which is given to the investor. The third level consists of the publicly certified experts. They are appointed by the building authorities but work as independent experts. They are responsible for the technical supervision of the planning, design and the construction. The independent peer review by publicly certified experts for geotechnical engineering checks that all information including the results of the soil investigation, the laboratory and field tests and the boundary conditions defined for the geotechnical design are complete and correct.

3. Observational Method

The observational method is part of the risk management and the risk communication. For projects with difficult boundary conditions it is necessary to apply the observational method to review the design during the construction time and, if necessary, during the service time. The observational method is always a combination of the common geotechnical investigations before and during the construction phase and the geodetic survey together with the theoretical modelling and a plan of contingency actions (CEN 2004). Figure 3 shows the principle of the observational method. According to EC 7 the observational method has to be applied to all projects categorized into the Geotechnical Category GC 3, which is the category for projects with a very high complexity.

Figure 3. Principle of the observational method.

R. Katzenbach et al. / Risk Management and Risk Communication in Geotechnical Engineering 77

Only monitoring to ensure the stability and the serviceability of the structure is not sufficient and according to the standardization not permitted for this purpose (Katzenbach et al. 2010). Overall the observational method is an institutionalized controlling instrument to verify the soil and rock mechanical modelling.

The identification of all potential failure mechanisms is essential for defining the monitoring program. The program has to be designed in that way that all these mechanisms can be observed. The measurements need to be of an adequate accuracy to allow the identification of critical tendencies. The required accuracy as well as the boundary values need to be identified within the design phase and before the development of the monitoring program. Contingency actions need to be planned before the construction works start depending on the ductility of the bearing structure. The observational method must not be seen as a potential alternative for a comprehensive soil investigation campaign. A comprehensive soil investigation campaign is in any way of essential importance. Additionally the observational method is a tool of quality assurance and allows the verification of the parameters and calculations applied in the design phase. The observational method helps to achieve an economic and safe construction.

4. In-situ Pile Load Tests

An important technical solution for risk management is an in-situ pile load test. Soil parameters are determined by project- and site-related soil investigations with core drillings and laboratory tests. Those tests are important and essential for the initial definition of soil mechanical parameters of the soil layers, but usually not sufficient for an entire and realistic capture of the complex conditions, caused by the interaction of the subsoil and the construction. (Katzenbach 2005).

In order to reliably determine the ultimate bearing capacity of deep foundation elements, load tests need to be carried out. For those tests often very high counter weights or strong anchor systems are necessary. By using the Osterberg method high loads can be reached without

installing anchors or counter weights. Hydraulic jacks (O-Cells) induce the load in the deep foundation elements using the pile itself partly as abutment. Such pile load tests allow the testing of different soil layers and the determination of the skin friction and the base resistance separately with only one pile by a multi-level test pile with e.g. two jack-levels. The results of the field tests allow a calibration of the numerical simulations. Figure 4 shows the principle scheme of load tests on deep foundation elements.

Figure 4. Principle scheme of pile load tests.

5. Combined Pile-Raft Foundation (CPRF)

The CPRF is a hybrid foundation system that combines the effects of a foundation raft and deep foundation elements like piles and barrettes (Randolph 1994, Poulos 2001). CPRFs are always categorized into the Geotechnical Category GC 3. The bearing capacity and the deformation behaviour are affected by the interactions between the deep foundation elements, the foundation raft and the subsoil. For an optimised and safe design of a CPRF the calculation method has to consider these interactions (Hanisch et al. 2002, ISSMGE 2013).

Due toe the stiffness of the foundation raft the total load of the building Ftot,k is transferred into the subsoil via contact pressure under the �� like piles. The total resistance Rtot,k(s) of the CPRF consists of the resistance of the raft Rraft,k�� Rpile,k,j(s) as explained in Equation (1). The resistance Rpile,k,j(s) of a single pile “j” consists of

R. Katzenbach et al. / Risk Management and Risk Communication in Geotechnical Engineering78

the skin friction qs,k,j(s,z) and the base resistance qb,k,j(s), as shown in Equation (2) to (4).

� � � � � �, , , ,1�

� ��m

tot k pile k j raft kj

R s R s R s (1)

� � � � � �, , , , , ,� �pile k j b k j s k jR s R s R s (2)

� � � �, , , ,,� � � �s k j s k j

R s q s z D dz (3)

� � � �2

, , , , 4�

� �b k j q k jDR s q s (4)

Figure 5 shows the soil-structure interaction of a CPRF. The bearing capacity and the load-settlement behaviour are affected by the interactions between the different elements of the hybrid foundation system CPRF.

Figure 5. Soil-structure interaction of a CPRF. The distribution of the total building load

Ftot,k between the different bearing structures of a �� !CPRF which defines the ratio between the amount of load carried by the p�� pile,k,j(s) and the activated total resistance Rtot,k(s) of the CPRF as shown in Equation (5). The activated total

resistance Rtot,k(s) is equal to the total load of the building Ftot,k.

� �, ,

,

� � pile k jCPRF

tot k

R sR

� ( )

A CPRF coefficient of zero describes a raft foundation without any deep foundation element, a CPRF coefficient of one represents a classic pile group, neglecting the existence of a raft.

Compared to a classic spread or pile foundation the objectives and advantages of a CPRF are a reduction of settlements and differential settlements, an increase of the bearing capacity, a decrease of the bending load and finally a minimisation of the costs (Katzenbach 2005). Further information about the design and the load-settlement behavior are given in Randolph (1983 & 1993), Cooke (1986), Poulos et al. (1997), Russo & Viggiani (1998) and Horikoshi & Randolph (1998) and Reul (2000).

6. First CPRF in Frankfurt Clay

6.1. Project Overview

The high-rise building “Messeturm” in Frankfurt am Main, Germany, is 256.5 m high and is the first high-rise building on a CPRF in Frankfurt Clay (Figure 6). The foundation raft is 58.8 m x 58.8 m wide with a maximum thickness of 6 m in the center and a thickness of 3 m at the edges. The base of the foundation raft is about 11 m to 14 m below the ground surface. The raft is combined with 64 bored piles with a diameter of 1.3 m and a length of 30.9 m in the center ring and 26.9 m at the edges (Figure 7). The total building load, including 30 % of the live loads, is about 1,855 MN. The complex settlement behavior is related to the load-deformation behavior of the foundation system itself and the time-dependent load-deformation behavior of the Frankfurt Clay. Therefore a monitoring program was applied. The maximum measured settlement where about 13 cm in the centre of the CPRF and about 8 cm to 9 cm at the edges of the CPRF (Reul 2000).

5


Figure 6. Messeturm in Frankfurt am Main, Germany.

Figure 7. Alignment of the CPRF.

6.2. Design of the CPRF

The CPRF was calculated with the FEM. Therefore a section of the foundation was modelled using the symmetry of the plan view (Figure 8). The settlements of a pure raft foundation were calculated to 32.5 cm. The

calculated settlements of the CPRF are nearly equal to the in-situ measured values mentioned in Figure 9. The CPRF coefficient is about !CPRF = 0.43 (Reul 2000).

Figure 8. FE-Model of the CPRF.

Figure 9. Measured and calculated settlements. A pure pile foundation would have required

316 piles with 30 m length. In comparison to the realised CPRF with 64 piles and an average length of about 30 m a pure pile foundation


would have required more resources, e.g. concrete and energy, more time and would have been approximately 5.9 Million US$ more expensive.

7. CPRF in Very Soft Soil


A more than 75 m high-rise building in settlement sensitive soil is constructed at the coastline of West Africa. The high-rise building has up to 16 storeys. The foundation system is designed as a CPRF. The annex buildings are up to 60 m high and include apartments and parking levels. All structures have one basement level. The whole structure has a total load of more than 700 MN. Due to its complexity the project is categorized into the Geotechnical Category GC 3 of EC 7.

The soil investigation was carried out to a depth of 80 m below the surface. At the surface clayey sands have been detected. Until a depth of 33 m below the surface is an alternating sequence of medium dense and dense sand layers. Down to the investigation depth follows an alternating sequence of medium dense and dense sand layers and clay and silt layers with low to high plasticity. The groundwater level is close to the surface.

Figure 10. Setup of the pile load test.

7.2. Optimization of the CPRF

For the determination of the bearing capacity, the load-settlement behavior and the internal forces of the CPRF 3-dimensional simulations using the FEM have been necessary. The simulations considered the non-linear behavior of the soil and have been calibrated by back analysis of laboratory and in-situ load tests.

For the in-situ load test O-Cells have been used. The test pile had 3 parts: the upper pile segment 1, the middle pile segment 2 between the upper and the lower O-Cell and the lower pile segment 3 (Figure 10).

For the determination of the base resistance and the skin friction the pile segments were activated differently. For determination of the skin friction and the base resistance of pile segment 3 only the lower O-Cell was activated using the segment 2 as abutment. For determination of the skin friction of pile segment 2 the upper O-Cell was activated and the lower O-Cell was released. Pile segment 1 was the abutment for this test phase. For determination of the skin friction of pile segment 1 the upper O-Cell was loaded and the lower O-Cell was fixed. The pile segments 2 and 3 were used as abutments. Figure 11 shows the mesh of the FEM simulations and the principle arrangement of the pile load test equipment with the 3 pile segments and the upper and lower O-Cells.

Figure 11. FE-Model for back analysis.


The results of the back analysis by FEM simulations are the basis for the adjustment of the estimated soil parameters and were used to verify the developed, simplified stratigraphy for the analysis of the whole foundation system. The results of the pile load test in-situ and of the back analysis are drawn in Figure 12. The comparison of the results shows a good accordance.

Figure 12. Results of in-situ load test and back analysis. The length, the diameter and the number of

piles of the whole CPRF were optimised by the FEM simulations. Figure 13 shows the final CPRF design. The CPRF coefficient is !CPRF = 0.8. During the construction phase and for the first years of service time the loads of the piles, the stresses under the raft and the deformation behavior of the CPRF are measured by a monitoring program according to the requirements of the observational method.

Figure 13. Final CPRF.

8. CPRF in the Vicinity to Existing Structures

8.1. Project overview

South-west of the center of Frankfurt am Main, Germany, a new building complex is realized on a construction site with 21,000 m2. An overview of the project area is given in Figure 14.

Figure 14. Project overview with building Panorama in the south-west.

The new building complex is situated nearby

the historic monastery in the east, the river Main in the south, a metro tunnel in the west and a street and metro tunnel in the north. The building complex consists of 6 parts, 4 of them are high-rise buildings. Figure 15 show the cross section of the project area.

Figure 15. Cross section of Figure 14.


Regarding the complex construction process in several steps and the soil-structure interaction between existing buildings, new buildings, the metro and street tunnels and an existing sewer line the project is classified into the Geotechnical Category GC 3 according to EC 7.

The soil conditions can be summarized as follows: � fillings with a thickness of 2 m to 10 m � quaternary sand and gravel down to 11 m

under surface with a thickness of 1m to 9 m � tertiary Frankfurt Clay, consisting of

alternating layers of clay, limestone and sand, down to 35 m under surface

� Frankfurt Limestone Typically for Frankfurt am Main the upper

groundwater level is in the quaternary sand and gravel layer between 4 m and 6.5 m below the surface. The lower groundwater level is in the tertiary limestone and sand layers within the Frankfurt Clay. The lower groundwater level is normally confined.

8.2. Design of the Foundation System

Depending on the loads of the building complex the foundation system varies in relation to the loads: � spread foundations � classic pile foundations � micropiles � Combined Pile-Raft Foundation (CPRF)

For the optimization of the deep foundation elements, two pile load tests were carried out on the construction site. In addition to the determination of the characteristic skin friction and base resistance the results of the pile load tests were used to re-calculate the distribution of the stiffness of the soil model. This calibration was done by numerical back analysis using the FEM. For example the test setup of test pile TP 1 is shown in Figure 16. TP 1 ends in the Frankfurt Clay. TP 2 ends in the Frankfurt Limestone. The load on the piles was given by O-Cells. The test piles consist of the upper, the middle and the lower pile segments, separated by the upper and the lower O-Cells. The test pile was equipped with measurement devices:

� strain gauges in the upper, the middle and the lower pile segment

� stack extensometer from the surface to the pile top, to the upper O-Cell, to the lower O-Cell and to the pile toe

� extensometer between the O-Cells � displacement transducer inside the O-Cells

Figure 16. Setup of pile load test TP 1. Using the back analysis of the pile load test

the developed numerical soil model was calibrated. For example Figure 17 shows on the right the measured displacements of the pile load test in-situ and the calculated displacements of the back analysis in one of the test phases.

Figure 17. Measurement and back analysis of pile load test TP 1.

The soil mechanical parameters detected during the soil investigation were updated according to the results of the back analysis. This analysis is the basis for the following 2-


dimensional and 3-dimensional numerical simulations.

For the determination of the complex soil-structure interaction between existing buildings, new buildings, the metro and street tunnels and the sewer line, several 2-dimensional and 3-dimensional, non-linear numerical analysis using FEM have been carried out. For example the numerical model of the south-west of the project area with the building Panorama is shown in Figure 18.

Figure 18. Numerical model of the area of the building Panorama, view from the north-east.

The foundation system of this part of the

project area combines a CPRF and a classic pile foundation. It has to be guaranteed that the existing sewer line does not get any load from the new buildings. For the reduction of the deformation of the sewer line the retaining structure does not get any vertical loads from the superstructure. For the settlement relevant loads the predicted settlements of the building is up to a maximum of 3 cm. The predicted settlements of the sewer line are smaller than 1.5 cm. The piles of the CPRF have a diameter of 1.5 m and a length between 14 m and 20 m. The CPRF coefficient is !CPRF = 0.6. The piles of the classic pile foundation have a diameter of 1.5 m and a length between 20 m and 24 m.

In the area of the metro tunnel in addition a 2-dimensional numerical analysis has been carried out in order to get more detailed information about the deformations, the variation of stresses during the different construction phases and the influence on the sealing system of the metro tunnel caused by the excavation and the construction of the retaining system.

The soil mechanical parameters have been varied for the study of the sensitivity of the metro tunnel. According to the results of the numerical simulations and the experiences made in the region of the project area, horizontal displacements of the metro tunnel of less than 1 cm have been expected. Differential displacements at the joints of the tunnel blocks have been expected in the range of only a few millimeters. Based on the 2-dimensional and 3-dimensional numerical analysis no limitation of the serviceability and of course no damages have been expected. The measured displacements of the tunnel blocks were less than 0.5 cm and the maximum differential displacement at the joints of the tunnel blocks was 0.1 cm.

9. Complex Foundation System on Fault Zone


In Darmstadt, a city 25 km south of Frankfurt am Main, Germany, the conference center Darmstadtium was realized directly over the Rhine Valley Fault. The challenging construction was finished in 2007. Figure 19 gives an impression of the whole structure.

Figure 19. Conference center Darmstadtium, Germany.

The results of the soil investigation in the

planning stages showed that under the building the Rhine Valley Fault divides the subsoil in two parts (Figure 20 and 21). In the northern and western parts of the project area, the soil consists of the sediments of the Rhine valley. In the eastern and southern parts of the project area, the soil consists of rock, the so called Granodiorit.


Figure 20. Excavation of the Darmstadtium.

9.2. Design of the Foundation System

Up to now the tectonic processes along the Rhine Valley Fault did not fade out. In the west of the Rhine Valley Fault the surface of Darmstadt sinks up to 0.5 mm per year. The foundation system and the superstructure had to be dimensioned for this tectonic deformation. In the rock a raft foundation and in the area of the Rhine Valley a CPRF was constructed. At the transition of the stiff rock to the sediments a soil replacement was carried out (Figure 21).

Figure 21. Cross section of the Darmstadtium. Additionally to the combined foundation

system and the soil replacement the superstructure is constructed with joints to allow differential deformations.

10. CPRF on Discontinuous Soil Conditions


During the independent peer review process for a new high-rise building in Frankfurt am Main, Germany, the publicly certified expert for

structural engineering informed the building authorities to involve a publicly certified expert for geotechnics. The high-rise building has 38 floors above the surface and 3 sublevels. The total floor space is 57,450 m2. The total height of the building is 136 m. Figure 22 shows the realized high-rise building.

Figure 22. View on the finalized high-rise building. In an early planning stage it was estimated

that the soil and groundwater conditions are common for the Frankfurt subsoil: � fillings to a depth of 10 m followed by

quaternary sand and gravel to variable depth � beneath the quaternary sand and gravel is the

Frankfurt Clay consisting of stiff to semi hard clay with thin limestone layers and sand layers

� the basis is the Frankfurt Limestone � first groundwater level is a few meters

below the surface in the quaternary sand and gravel

� the limestone and sand layers in the Frankfurt Clay have confined groundwater Due to this estimation the foundation system

was planned as a CPRF reaching the Frankfurt Clay.


10.2. Design of the CPRF

For the foundation a CPRF was planned as shown in Figure 23 a). During the review process additional soil investigations were carried out. The results indicated that there is a discontinuity of several meters in the intersection between the settlement active Frankfurt Clay and the rocky Frankfurt Limestone. The top of the Frankfurt Limestone is not as deep as estimated so the piles of the CPRF would have been partly in the limestone (Figure 23 b)).

In this case the piles would have had a much higher resistance due to the very stiff Frankfurt Limestone. Damages would occur in the foundation raft and in the superstructure because the piles founded in the rock would get a higher load than the other piles founded in Frankfurt Clay. Differential settlements would also occur.

On basis of the report of the publicly certified expert for geotechnics the design of the foundation system was adapted and the piles of the CPRF were shortened (Figure 23 c)). The different stages of the foundation design are summarized in Figure 23: � top: first planning stage � middle: after additional soil investigations � bottom: realized foundation system after the

report of the publicly certified expert for geotechnics

The whole construction was realized without any damages.

Figure 23. CPRF in different planning stages.

11. Conclusions

For a safe and sustainable construction of CPRFs it is necessary to consider the soil-structure interaction during the design and the construction phase. Due to this fact CPRFs have to be categorized into the Geotechnical Category GC 3, which is the category with the highest factor of complexity according to EC 7. The independent peer review process requires a publicly certified expert for geotechnics and the application of the observational method to achieve a comprehensive risk management and risk communication.

For the optimization of the CPRF in-situ load tests on the construction site are veru utile. The results give the possibility for a calibration of the numerical models by back analysis. Only by these numerical simulations it is possible to investigate the load-deformation behavior of the complex soil-structure interaction of CPRFs.


Because of the continuously changing load situation, especially on soils with a time-dependent deformation behavior, the main aspect of risk mitigation in all design an construction phases is the adequate consideration of the soil-structure interaction (Russo & Viggiani 1998, Wörner & Pfeiffer 1998).

References

CEN Eurpean Committee of Standardization (2004). Eurococde 7: Geotechnical design - Part 1: General rules.

Cooke, R.W. (1986). Piled raft foundations on stiff clays - a contribution to design philosophy, Géotechnique 36, No. 2, 169-203.

Hanisch, J., Katzenbach, R., König, G. (2002). Kombinierte Pfahl-Plattengründungen, Ernst & Sohn Verlag, Berlin, Germany.

Horikoshi, K., Randolph, M.F. (1998). A contribution to optimal design of piled rafts, Géotechnique 48, No. 3, 301-317.

ISSMGE International Society of Soil Mechanics and Geotechnical Engineering (2013). Combined Pile-Raft Foundation Guideline, Ed. Katzenbach, R., Choudhury, D., Darmstadt, Germany.

Katzenbach, R., (2005). Optimized design of high-rise building foundations in settlement-sensitive soils, International Geotechnical Conference of Soil-Structure Interaction, 39-46, 26-28 May, St. Petersburg, Russia.

Katzenbach, R., Bachmann, G., Leppla, S., Ramm, H. (2010). Chances and limitations of the observational method in geotechnical monitoring, 13 p., 2-4 June, Bratislava, Slovakia.

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Poulos, H.G., Small, J.C., Ta, L.D., Simha, J., Chen, L. (1997). Comparison of some methods for analysing of piled rafts, 14th International Conference on Soil Mechanics and Foundation Engineering, Vol. 2, 1119-1124, 6-12 September, Hamburg, Germany.

Poulos, H.G. (2001). Piled-raft foundation: design and applications, Géotechnique, 51(2), 95-113.

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Randolph, M.F., Clancy, P. (1993). Efficient design of piled rafts, 5th International Conference on Deep Foundations on Bored and Auger Piles, 119-130, 1-4 June, Ghent, Belgium.

Randolph, M.F. (1994). Design method for pile groups and piled rafts, 13th International Conference on Soil Mechanics and Foundation Engineering, Vol. 5, 61-82, 5-10 January, New Delhi, India.

Reul, O. (2000). In-situ Messungen und numerische Studien zum Tragverhalten der Kombinierten Pfahl-Plattengründung, Mitteilungen des Institutes und der Versuchsanstalt für Geotechnik der Technischen Universität Darmstadt, Heft 53.

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Risk Management and Risk Communication in Geotechnical