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Foundation Design for Tall Buildings

Harry G. Poulos1, Dist. MASCE

1Coffey Geotechnics, 8/12 Mars Road, Lane Cove West, NSW Australia 2066; Phone +61 2 9911 1000; Fax +61 2 9911 1001; harry_poulos@coffey.com ABSTRACT: This paper sets out the principles of a limit state design approach to design a pile or piled raft foundation system for tall buildings, and involves three sets of analyses:

1. An overall stability analysis in which the resistances of the foundation components are reduced by the appropriate geotechnical reduction factor and the ultimate limit state (ULS) load combinations are applied.

2. A serviceability analysis, in which the best-estimate (unfactored) values foundation resistances and stiffnesses are employed and the serviceability limit state (SLS) loads are applied.

3. An analysis to obtain foundation loads, moments and shears for structural design of the foundation system.

The importance of appropriate parameter selection and load testing is emphasized. The approach is illustrated via its application to a high-rise building in Korea.

INTRODUCTION

The last two decades have seen a remarkable increase in construction of tall buildings in excess of 150m in height, and an almost exponential rate of growth. A significant number of these buildings have been constructed in the Middle East and Asia, and many more are either planned or already under construction. “Super-tall” buildings in excess of 300m in height are presenting new challenges to engineers, particularly in relation to structural and geotechnical design. Many of the traditional design methods cannot be applied with any confidence since they require extrapolation well beyond the realms of prior experience, and accordingly, structural and geotechnical designers are being forced to utilize more sophisticated methods of analysis and design. In particular, geotechnical engineers involved in the design of foundations for super-tall buildings are increasingly leaving behind empirical methods and are employing state-of-the art methods.

There are a number of characteristics of tall buildings that can have a significant influence on foundation design, including the following:

1. The building weight increases non-linearly with increasing height, and thus the vertical load to be supported by the foundation, can be substantial.

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2. High-rise buildings are often surrounded by low-rise podium structures which are subjected to much smaller loadings. Thus, differential settlements between the high- and low-rise portions need to be controlled.

3. The lateral forces imposed by wind loading, and the consequent moments on the foundation system, can be very high. These moments can impose increased vertical loads on the foundation, especially on the outer piles within the foundation system.

4. The wind-induced lateral loads and moments are cyclic in nature. Thus, consideration needs to be given to the influence of cyclic vertical and lateral loading on the foundation system, as cyclic loading has the potential to degrade foundation capacity and cause increased settlements.

5. Seismic action will induce additional lateral forces in the structure and also induce lateral motions in the ground supporting the structure. Thus, additional lateral forces and moments can be induced in the foundation system via two mechanisms:

a. Inertial forces and moments developed by the lateral excitation of the structure;

b. Kinematic forces and moments induced in the foundation piles by the action of ground movements acting against the piles.

6. The wind-induced and seismically-induced loads are dynamic in nature, and as such, their potential to give rise to resonance within the structure needs to be assessed. The fundamental period of vibration of a very tall structure can be very high, and since conventional dynamic loading sources such as wind and earthquakes have a much lower predominant period, they will generally not excite the structure via the fundamental mode of vibration. However, some of the higher modes of vibration will have significantly lower natural periods and may well be excited by wind or seismic action.

This paper will review some of the challenges that face designers of foundations for very tall buildings, primarily from a geotechnical viewpoint. The process of foundation design and verification will be described for a proposed tall tower in Korea.

TYPICAL HIGH-RISE FOUNDATION SETTLEMENTS

Before discussing details of the foundation process, it may be useful to review the settlement performance of some high-rise buildings in order to gain some appreciation of the settlements that might be expected from two foundation types founded on various deposits. Table 1 summarizes details of the foundation settlements of some tall structures founded on raft or piled raft foundations, based on documented case histories in Hemsley (2000), Katzenbach et al (1998), and from the author’s own experiences. The average foundation width in these cases ranges from about 40m to 100m. The results are presented in terms of the settlement per unit applied pressure, and it can be seen that this value decreases as the stiffness of the founding material increases. Typically, these foundations have settled between 25 and 300mm/MPa. Some of the buildings supported by piled rafts in stiff Frankfurt clay have settled more than 100mm, and despite this apparently excessive settlement, the performance of the structures appears to be quite satisfactory. It may therefore be

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concluded that the tolerable settlement for tall structures can be well in excess of the conventional design values of 50-65mm. A more critical issue for such structures may be overall tilt, and differential settlement between the high-rise and low-rise portions of a project.

Table 1. Examples of Settlement of Tall Structure Foundations

Foundation Type Founding Condition

Location No. of Cases Settlement per Unit Pressure mm/MPa

Raft Stiff clay

Limestone

Houston; Amman; Riyadh

2

2

227-308

25-44

Piled Raft Stiff clay

Dense sand

Weak Rock

Limestone

Frankfurt

Berlin; Niigata

Dubai

Frankfurt

5

2

5

1

218-258

83-130

32-66

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FOUNDATION DESIGN ISSUES

The following issues will generally need to be addressed in the design of foundations for high-rise buildings:

1. Ultimate capacity of the foundation under vertical, lateral and moment loading combinations.

2. The influence of the cyclic nature of wind, earthquakes and wave loadings (if appropriate) on foundation capacity and movements.

3. Overall settlements. 4. Differential settlements, both within the high-rise footprint, and between high-rise

and low-rise areas. 5. Possible effects of externally-imposed ground movements on the foundation

system, for example, movements arising from excavations for pile caps or adjacent facilities.

6. Earthquake effects, including the response of the structure-foundation system to earthquake excitation, and the possibility of liquefaction in the soil surrounding and/or supporting the foundation.

7. Dynamic response of the structure-foundation system to wind-induced (and, if appropriate, wave) forces.

8. Structural design of the foundation system; including the load-sharing among the various component of the system (for example, the piles and the supporting raft), and the distribution of loads within the piles. For this, and most other components

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of design, it is essential that there be close cooperation and interaction between the geotechnical designers and the structural designers.

A general process by which these issues can be considered is set out below.

FOUNDATION DESIGN PROCESS

The process of foundation design is well-established, and generally involves the following aspects:

1. A desk study and a study of the geology and hydrogeology of the area in which the site is located.

2. Site investigation to assess site stratigraphy and variability. 3. In-situ testing to assess appropriate engineering properties of the key strata. 4. Laboratory testing to supplement the in-situ testing and to obtain more

detailed information on the behaviour of the key strata than may be possible with in-situ testing.

5. The formulation of a geotechnical model for the site, incorporating the key strata and their engineering properties. In some cases where ground conditions are variable, a series of models may be necessary to allow proper consideration of the variability.

6. Preliminary assessment of foundation requirements, based upon a combination of experience and relatively simple methods of analysis and design. In this assessment, considerable simplification of both the geotechnical profile(s) and the structural loadings is necessary.

7. Refinement of the design, based on more accurate representations of the structural layout, the applied loadings, and the ground conditions. From this stage and beyond, close interaction with the structural designer is an important component of successful foundation design.

8. Detailed design, in conjunction with the structural designer. As the foundation system is modified, so too are the loads that are computed by the structural designer, and it is generally necessary to iterate towards a compatible set of loads and foundation deformations.

9. In-situ foundation testing at or before this stage is highly desirable, if not essential, in order to demonstrate that the actual foundation behavior is consistent with the design assumptions. This usually takes the form of testing of prototype or near-prototype piles. If the behavior deviates from that expected, then the foundation design may need to be revised to cater for the observed foundation behavior. Such a revision may be either positive (a reduction in foundation requirements) or negative (an increase in foundation requirements). In making this decision, the foundation engineer must be aware that the foundation testing involves only individual elements of the foundation system, and that the piles and the raft within the system interact.

10. Monitoring of the performance of the building during and after construction. At the very least, settlements at a number of locations around the foundation

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should be monitored, and ideally, some of the piles and sections of the raft should also be monitored to measure the sharing of load among the foundation elements. Such monitoring is becoming more accepted as standard practice for high-rise buildings, but not always for more conventional structures. As with any application of the observational method, if the measured behavior departs significantly from the design expectations, then a contingency plan should be implemented to address such departures. It should be pointed out that departures may involve not only settlements and differential settlements that are greater than expected, but also those that are significantly smaller than expected.

DESIGN CRITERIA

Limit State Design Approach – Ultimate State

There is an increasing trend for limit state design principles to be adopted in foundation design, for example, in the Eurocode 7 requirements and those of the Australian Piling Code (1995). In terms of limit state design using a load and resistance factor design approach (LRFD), the design criteria for the ultimate limit state are as follows:

Rs* S* (1) Rg* S* (2)

where Rs* = design structural strength = s. Rus, Rg* = design geotechnical strength = g. Rug, Rus = ultimate structural strength, Rug= ultimate strength (geotechnical capacity), s = structural reduction factor, g = reduction factor for geotechnical strength, and S* = design action effect (factored load combinations).

The above criteria are applied to the entire foundation system, while the structural strength criterion (equation 1) is also applied to each individual pile. It is not considered to be good practice to apply the geotechnical criterion (equation 2) to each individual pile within the group, as this can lead to considerable over-design. Rs* and Rg* can be obtained from the estimated ultimate structural and geotechnical capacities, multiplied by appropriate reduction factors. Values of the structural and geotechnical reduction factors are often specified in national codes or standards. The selection of suitable values of g requires considerable judgment and should take into account a number of factors that may influence the foundation performance. As an example, the Australian Piling Code AS2159-1995 specifies values of g between 0.4 and 0.9, the lower values being associated with greater levels of uncertainty and the higher values being relevant when a significant amount of load testing is carried out.

Load Combinations

The required load combinations for which the structure and foundation system have to be designed will usually be dictated by an appropriate structural loading code. In some cases, a large number of combinations may need to be considered. These

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may include several ultimate limits state combinations, and serviceability combinations incorporating long-term and short-term loadings.

Design for Cyclic Loading

In addition to the normal design criteria, as expressed by equations 1 and 2, it is suggested that an additional criterion be imposed for the whole foundation of a tall building to cope with the effects of repetitive loading from wind and/or wave action, as follows:

       Rgs*  Sc*            (3) 

where Rgs* = design geotechnical shaft capacity, Sc* = maximum amplitude of wind loading, and = a reduction factor.

This criterion attempts to avoid the full mobilization of shaft friction along the piles, thus reducing the risk that cyclic loading will lead to a degradation of shaft capacity. In most cases, it is suggested that can be taken as 0.5, while Sc* can be obtained from computer analyses which give the cyclic component of load on each pile, for various wind loading cases.

Soil-Structure Interaction Issues- Analyses for Structural Foundation Design

When considering soil-structure interaction to obtain foundation actions for structural design (for example, the bending moments in the raft of a piled raft foundation system), the worst response may not occur when the pile and raft capacities are factored downwards (for example, at a pile location where there is not a column, load acting, the negative moment may be larger if the pile capacity is factored up). As a consequence, additional calculations may need to be carried out for geotechnical reduction factors both less than 1 and greater than 1. As an alternative to this duplication of analyses, it would seem reasonable to adopt a reduction factor of unity for the pile and raft resistances, and the factor up the computed moments and shears (for example, by a factor of 1.5) to allow for the geotechnical uncertainties. The structural design of the raft and the piles will also incorporate appropriate reduction factors.

Serviceability Limit State

The design criteria for the serviceability limit state are as follows:

max all (4)

max all (5)

where max = maximum computed settlement of foundation, all = allowable foundation settlement, max = maximum computed local angular distortion and all = allowable angular distortion.

Values of all and all depend on the nature of the structure and the supporting soil. Table 1 sets out some suggested criteria from work reported by Zhang and Ng

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(2006). This table also includes values of intolerable settlements and angular distortions. The figures quoted in Table 2 are for deep foundations, but the authors also consider separately allowable settlements and angular distortions for shallow foundations, different types of structure, different soil types, and different building usage. Criteria specifically for very tall buildings do not appear to have been set, but it should be noted that it may be unrealistic to impose very stringent criteria on very tall buildings on clay deposits, as they may not be achievable. In addition, experience with tall buildings in Frankfurt (see Table 1) suggests that total settlements well in excess of 100mm can be tolerated without any apparent impairment of function. It should also be noted that the allowable angular distortion, and the overall allowable building tilt, reduce with increasing building height, both from a functional and a visual viewpoint.

Table 2. Suggested Serviceability Criteria for Structures (Zhang and Ng, 2006)

Quantity Value Comments

Limiting Tolerable Settlement mm

106 Based on 52 cases of deep foundations.

Observed Intolerable Settlement mm

349 Based on 52 cases of deep foundations.

Limiting Tolerable Angular Distortion rad

1/500 Based on 57 cases of deep foundations.

Limiting Tolerable Angular Distortion rad

1/250 (H<24m)

to

1/1000 (H>100m)

From 2002 Chinese Code

H = building height

Observed Intolerable Angular Distortion rad

1/125 Based on 57 cases of deep foundations.

Dynamic Loading

Issues related to dynamic wind loading are generally dealt with by the structural engineer, with geotechnical input being limited to an assessment of the stiffness and damping characteristics of the foundation system. However, the following general principles of design can be applied to dynamic loadings:

The natural frequency of the foundation system should be greater than that of the structure it supports, to avoid resonance phenomena. The natural frequency depends primarily on the stiffness of the foundation system and its mass, although damping characteristics may also have some influence.

The amplitude of dynamic motions of the structure-foundation system should be within tolerable limits. The amplitude will depend on the stiffness and damping characteristics of both the foundation and the structure. It is of interest to have some idea of the acceptable levels of dynamic motion,

which can be expressed in terms of dynamic amplitude of motion, or velocity or acceleration. Table 3 reproduces guidelines for human perception levels of dynamic motion, expressed in terms of acceleration (Mendis et al, 2007). These are for

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vibration in the low frequency range of 0-1 Hz encountered in tall buildings, and incorporate such factors as the occupant’s expectancy and experience, their activity, body posture and orientation, visual and acoustic cues. They apply to both the translational and rotational motions to which the occupant is subjected. The acceleration levels are a function of the frequency of vibration, and decrease as the frequency increases. For example, allowable vibration levels at a frequency of 1 Hz are typically only 40-50% of those acceptable at a frequency of 0.1 Hz. It is understood that, for a 10 year return period event, with a duration of 10 minutes, American practice typically allows accelerations of between 0.22 and 0.25m2/s for office buildings, reducing to 0.10 to 0.15 m2/s for residential buildings.

Table 3. Human Perception Levels of Dynamic Motion (Mendis et al, 2007)

Level of

Motion

Acceleration m2/s

Effect

1

<0.05

Humans cannot perceive motion

2 0.05 - 0.1

Sensitive people can perceive motion. Objects may move slightly

3 0.1 – 0.25 Most people perceive motion. Level of motion may affect desk work. Long exposure may produce motion sickness.

4 0.25 – 0.4 Desk work difficult or impossible. Ambulation still possible. 5 0.4 – 0.5 People strongly perceive motion, and have difficulty in

walking. Standing people may lose balance. 6 0.5 – 0.6 Most people cannot tolerate motion and are unable to walk

naturally. 7 0.6 – 0.7 People cannot walk or tolerate motion.

8 > 0.85 Objects begin to fall and people may be injured.

Design for Ground Movements

Foundation design has traditionally focused on loads applied by the structure, but significant loads can also be applied to the foundation system because of ground movements. There are many sources of such movements, and the following are some sources that may be relevant to tall buildings:

1. Settlement of the ground due to site filling, reclamation or dewatering. Such effects can persist for many years and may arise from activities that occurred decades ago and perhaps on sites adjacent to the present site of interest. Such vertical ground movements give rise to negative skin friction on the piles within the settling layers.

2. Heave of the ground due to excavation of the site for basement construction. Ground heave can induce tensile forces in piles located within the heaving ground. Excavation can also give rise to lateral ground movements, which can induce additional bending moments and shears in existing piles.

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3. Lateral and vertical movements arising from the installation of piles near already-installed piles. These movements may induce additional axial and lateral forces and bending moment in the existing piles.

4. Dynamic ground motions arising from seismic activity. Such kinematic motions can induce additional moments and shears in the piles, in addition to the inertial forces applied by the structure to the foundation system.

Such ground movements do not reduce the geotechnical ultimate capacity of the piles, but have a two-fold influence:

The foundations are subjected to additional movements which must be considered in relation to the serviceability requirements. Because the action of ground movements on piles is a soil-structure interaction problem, the most straight-forward approach to designing the piles for the additional forces and moments is to compute the best-estimate values, and then apply a factor on these computed values to obtain the design values, as suggested previously for on soil-structure interaction.

DESIGN METHODS AND TOOLS

Once the necessary geological and geotechnical information has been obtained, the design process generally involves three key stages:

1. Preliminary analysis, assessment and design; 2. The main design process 3. Detailed analyses to check for complexities that may not be captured by the

main design process. The methods and tools that are employed in each of these stages need to be appropriate to the stage of design. Some typical design methods, and their mode of use, are set out below.

Preliminary analysis and design

In this stage, use can make use of spreadsheets, MATHCAD sheets or simple hand or computer methods which are based on reliable but simplified methods. It can often be convenient to simplify the proposed foundation system into an equivalent pier and then examine the overall stability and settlement of this pier. For the ultimate limit state, the bearing capacity under vertical loading can be estimated from the classical approach in which the lesser of the following two values is adopted:

1. The sum of the ultimate capacities of the piles plus the net area of the raft (if in contact with the soil);

2. The capacity of the equivalent pier containing the piles and the soil between them, plus the capacity of the portions of the raft outside the equivalent pier.

For assessment of the average foundation settlement under working or serviceability loads, the elastic solutions for the settlement and proportion of base load of a vertically loaded pier (Poulos, 1994) can be used, provided that the geotechnical profile can be simplified to a soil layer overlying a stiffer layer. Figures 1 and 2 reproduce these solutions, from which simplified load-settlement curves for

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an equivalent pier containing different numbers of piles can be estimated, using the procedure described by Poulos and Davis, 1980).

An alternative approach can be adopted, using the “PDR” approach described by Poulos (2002). In this approach, the simplified equations developed by Randolph (1994) can be used to obtain an approximate estimate of the relationship between average settlement and the number of piles, and between the ultimate load capacity and the number of piles. From these relationships, a first estimate can be made of the number of piles, of a particular length and diameter, to satisfy the design requirements.

 

FIG. 1. Settlement of equivalent pier in soil layer (Poulos, 1994).

FIG 2. Proportion of base load for equivalent pier (Poulos, 1994).

Main design evaluation and sensitivity study

For this stage, it may be appropriate to use computer methods for pile and pile-raft analysis such as, DEFPIG (Poulos, 1990), PIGLET (Randolph, 2004), GROUP8 (Ensoft, 2010), REPUTE (Geocentrix, 2006), GARP (Small and Poulos, 2007) and NAPRA (Mandolini et al, 2005). All such programs have some limitations; for example, some assume a rigid cap or raft, some do not allow for contact between the cap/raft and the soil, and some can only consider vertical loading. However, all these programs are capable of allowing for non-linear pile-soil behavior, albeit in an approximate manner, and accordingly, the following procedure may be employed:

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1. For the stability of the foundation system in the ultimate limit state, the ultimate limit state loading combinations are applied and the pile resistances are reduced by a geotechnical reduction factor. The foundation system satisfies the criterion in equation (1) if it does not collapse under any of the imposed load combinations.

2. For the average settlement of the foundation system, the serviceability limit state loadings (or the working loads) are applied to the foundation system. In this analysis, the pile resistances are not factored, but are instead best estimates, while the geotechnical stiffness characteristics employed in the analysis are also best estimates. For long-term loadings, long-term geotechnical and structural parameters are used, whereas, for wind and earthquake loadings, short-term stiffness and strength parameters, and short-term structural stiffness characteristics, are used.

From these analyses, the number and arrangement of piles in the foundation system can be adjusted in order to seek an optimal computed performance.

Detailed design and the final design check

For the final stages of design, once the basic pile configuration has been decided, it may be desirable to use a finite element and finite difference analyses, preferably three-dimensional, such as PLAXIS 3D and FLAC3D to verify that the foundation performance is consistent with that computed from the main design stage, and also to examine the influence of any factors such as the effects of the lateral resistance of the raft and or surrounding walls on the lateral response of the foundation system. Caution should be exercised in using two-dimensional analyses as they can often be misleading and can give settlements, differential settlements and pile loads which are inaccurate, for example, as noted by Prakoso & Kulhawy (2001).

In this stage, every effort should be made to ensure the following requirements are satisfied:

1. The geotechnical model used is appropriate for the ground conditions; 2. The constitutive behavior of the soil layers is consistent with the behavior

of the foundation soils; 3. The geotechnical parameters have been assessed appropriately; 4. Account is taken of the stiffness of the superstructure when computing the

foundation performance. In many cases, the key outputs from the geotechnical analyses are the equivalent

spring stiffnesses of each pile within the foundation system, as well as those of the various portions of the raft. These stiffnesses are for both vertical and lateral responses, and if necessary, torsional responses, of the piles. These characteristics are provided to the structural designer who can then incorporate them into the complete structure-foundation model. In this way, the most realistic estimates may be made of the settlement and differential settlements, and proper account can be taken of the interactions between the structure and the foundation. Clearly, such a process requires close cooperation and understanding between the structural and foundation designers.

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Geotechnical parameter assessment

A key element in undertaking each of the three stages of design is to try and employ geotechnical parameters that are consistent with the method being used. In a preliminary analysis, when there is a paucity of geotechnical data, it may be appropriate to employ parameters based on SPT values However, such parameters would be quite inappropriate in a three dimensional finite element analysis carried out for the detailed design stage (although no doubt this does happen on occasions). Reliable quantitative data on major high-rise projects can generally be derived from in-situ testing, especially cone, pressuremeter and dilatometer tests. While high-level laboratory testing remains feasible, it is often overlooked because of cost, timing and availability problems.

In deriving soil stiffness values to be used for settlement predictions, seismic testing, either via seismic cones or geophysical methods, is becoming increasingly important. Such testing enables the small-strain shear modulus, G0, to be derived from the measured shear wave velocity. This small-strain value may then be used either with a suitable constitutive model which considers the strain-dependency of stiffness, or alternatively to estimate the operative stiffness for the stress or strain levels appropriate to the foundation system. Mayne et al (2009) describe a simple approach which has been used successfully with elastic theory to predict non-linear load-settlement characteristics of single piles. Such an approach may be able to be extended to consider pile groups.

Poulos et al (2001) have developed an approximate approach in which values of the secant Young’s modulus (relative to the small-strain value) can be estimated as a function of the factor of safety against failure An example of such relationships are reproduced in Figure 3 for a clay soil, and for axially loaded piles, laterally loaded piles and shallow foundations. It will be noted that, for a given factor of safety, different values of the secant modulus apply to the different foundation situations, because of the differences in the strain levels induced in the soil.

 

FIG. 3. Secant modulus ratio for various foundation types on clay: Go/su = 500 (Poulos et al, 2001).

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When modeling a foundation system using a soil model that does not incorporate the stress- or strain–dependency of soil stiffness, it is still possible to make approximate allowance for the increase in stiffness with increasing depth below the foundation by using a modulus that increases with depth. From approximate calculations using the Boussinesq theory to compute the distribution of vertical stress with depth below a loaded foundation, it is possible to derive a relationship between the ratio of the modulus to the small strain value, as a function of relative depth and relative stress level. Such a relationship is shown in Figure 4 for a circular foundation, and may be used as a convenient, albeit approximate, means of developing a more realistic ground model for foundation design purposes. When applied to pile groups, the diameter can be taken as the equivalent diameter of the pile group, and the depth is taken from the level of the pile tips. 

 

FIG. 4. Ratio of modulus to small-strain modulus for circular foundation (p=applied pressure, pu = ultimate pressure).

The Role of Pile Testing

Pile testing is an essential component of tall building foundation design. The results of such tests serve several purposes, including:

1. Verification of the design assumptions regarding pile shaft and base capacity;

2. Verification of design assumptions regarding pile head stiffness; 3. Verification of the construction technique and the integrity of the as-

constructed shaft and base. With the increase in required pile capacities as buildings have become taller,

there has been increasing use made of the Osterberg cell test technique (Osterberg, 1989). This test is attractive because it is self-reacting, and with suitable placement of the cells, can load the pile base and pile shaft to failure, unlike most other types of test. Of particular interest is the ability to identify “soft toes” developed during construction and flaws in pile shaft construction if the shaft is suitably instrumented.

Depth/Footing 

Diameter     z/d

Ratio of Modulus to Small Strain …

p/pu=0.2

p/pu=0.333

p/pu=0.5

p/pu=0.8

p/pu=1.0

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APPLICATION TO THE INCHEON TOWER, KOREA

Introduction

A 151 storey super high-rise building project has been under design since 2008, located in reclaimed land constructed on soft marine clay in Songdo, Korea. This building is illustrated in Figure 5 and is described in some detail by Badelow et al (2009); thus, only a brief summary is presented here.

FIG. 5. Incheon 151 Tower (artist’s impression).

Ground Conditions and Geotechnical Model

The Incheon area has extensive sand/mud flats and near shore intertidal areas. The site lies entirely within an area of reclamation, which is likely to comprise approximately 8m of loose sand and sandy silt, constructed over approximately 20m of soft to firm marine silty clay, referred to as the Upper Marine Deposits (UMD). These deposits are underlain by approximately 2m of medium dense to dense silty sand, referred to as the Lower Marine Deposits (LMD), which overlie residual soil and a profile of weathered rock.

The lithological rock units present under the site comprise granite, granodiorite, gneiss (interpreted as possible roof pendant metamorphic rocks) and aplite. The rock materials within about 50 metres from the surface have been affected by weathering which has reduced their strength to a very weak rock or a soil-like material. This depth increases where the bedrock is intersected by closely spaced joints, and sheared and crushed zones that are often related to the existence of the roof pendant sedimentary / metamorphic rocks. The geological structures at the site are complex and comprise geological boundaries, sheared and crushed seams - possibly related to faulting movements, and jointing.

From the available borehole data for the site, inferred contours were developed for the surface of the “soft rock” founding stratum within the tower foundation footprint and it was found that there was a potential variation in level of the top of the soft rock (the pile founding stratum) of up to 40m across the foundation.

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The footprint of the tower was divided into eight zones across the site, these being considered to be representative of the variation of ground conditions and geotechnical models were developed for each zone. Appropriate geotechnical parameters were selected for the various strata based on the available field and laboratory test data, together with experience of similar soils on adjacent sites. One of the critical design issues for the tower foundation was the performance of the soft UMD under lateral and vertical loading, hence careful consideration was given to the selection of parameters for this stratum. Typical parameters adopted for foundation design are presented in Table 4.

Foundation Layout

The foundation comprises a 5.5 m thick concrete mat and piles supporting columns and core walls. The numbers and layout of piles and the pile size were obtained from a series of trial analyses through collaboration between the geotechnical engineer and the structural designer. The pile depth was determined by considering the performance and capacity of piles of various diameters and length. The pile depths required to control settlement of the tower foundation were greater than those required to provide the geotechnical capacity required. The pile design parameters for the weathered/soft rock layer are shown in Table 5 and were estimated on the basis of the pile test results in the adjacent site and the ground investigation data such as pressuremeter tests and rock core strength tests.

The final design employed 172 piles of 2.5m diameter, founded in the “soft rock” stratum, with lengths below the base of the raft varying from about 36m to 66 m, depending on the depth to the desired founding level. The base of the raft was about 14.6m below ground surface level. The pile layout was selected from the various options considered, and is presented in Figure 6.

Table 4. Summary of Geotechnical Parameters

Strata Typical Thickness m

Ev MPa

Eh MPa

fs kPa

fb

MPa

UMD (4 layers) 25.2 7 – 15 5-11 29-48 -

LMD 2.5 30 21 50 -

Weathered Soil 2.0 60 42 75 -

Weathered Rock 13.5 200 140 500 -

Soft Rock (above EL-50m)

10.0

300 210 750 12

Soft Rock (below EL-50m)

36.5 1700 1190 750 12

Ev = Vertical Modulus fs = Ultimate shaft friction Eh = Horizontal Modulus fb = Ultimate end bearing

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FIG. 6. Foundation Layout.

Table 5. Ultimate Resistances for Pile Analysis

Material Ultimate Friction

fs(kPa)

Ultimate End Bearing

fb(MPa)

Weathered Rock

500 5

Soft Rock 750 12

Loadings

The overall loadings used for the foundation design were developed by the structural designer and are summarized in Table 6.  

Table 6. Design Load Components

Load Component Value

Dead Load 5921.4 MN

Live Load 639 MN

Horizontal wind (x-direction) 149 MN

Horizontal wind (y-direction) 115 MN

Earthquake (x-direction) 110 MN

Earthquake (y-direction) 110 MN

Moment (x‐direction)  21600 MNm 

Moment (y‐direction)  12710 MNm 

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Preliminary Assessment

For the preliminary assessment of the foundation performance, a simplified geotechnical model was adopted, with constant layer thicknesses being assumed beneath the building footprint. A constant pile length of 50m was adopted for the calculations.

For the preliminary assessment, only the effects of vertical loading were considered, with emphasis being placed on the load capacity and the settlement under the dead plus live loading. The ultimate axial capacity of a single pile was computed to be 244 MN. Thus, for the single pile failure mode, the computed group capacity under vertical loading was 172*244 = 41968 MN, while the net raft capacity was 5930 MN, giving a total of 47898 MN. For the block failure mode, the total capacity of the block containing the piles, the soil between the piles, and the portions of raft outside the piles was computed to be in excess of 60000 MN, and thus the single pile failure was found to be critical. The overall factor of safety under purely vertical load was therefore 47898/6560.4 = 7.3 which was considered to be more than adequate. In terms of the limit state criterion in equation 2, a reduction factor of 0.65 was used to factor down the foundation capacity, together with load factors on dead load and live load of 1.25 and 1.5 respectively. It was found that the criterion was easily satisfied. The average settlement was computed using the equivalent pier approach and the curves in Figure 1. Under the dead plus live loading, the average settlement was computed to be about 75 mm, which again was considered to be acceptable for preliminary design purposes.

Detailed Assessment

Analysis of overall stability. For the detailed design phase, the ultimate limit state (ULS) combinations of load were input into a series of non-linear pile group analyses using a computer program CLAP (Combined Load Analysis of Piles) developed by Coffey (2007). The pile axial and lateral capacities were reduced by geotechnical reduction factors of 0.65 for axial load, and 0.40 for lateral load). The smaller factors for lateral load reflected the greater degree of uncertainty for lateral response. For the detailed analysis with CLAP, it was possible to take account of differing soil profiles and hence the eight different profiles identified during the ground interpretation process were employed.

In all cases analyzed, the foundation system was found to be stable, i.e. the computed foundation movements were finite, and generally the maximum computed settlement under the ULS loadings was less than 100mm. Thus, the overall stability condition was deemed to be satisfied.

Cyclic stability. From the CLAP analyses, the components of cyclic wind loading were obtained and used to check the cyclic stability criterion in equation 3. Figure 7 plots the ratio for each pile in the group. The largest cyclic load component in any pile was 29.2 MN, and the ratio of this cyclic load to the factored-down pile shaft resistance was found to be 0.43, which was less than the maximum allowable value of 0.5. Thus the cyclic load criterion was satisfied and little or no cyclic degradation of pile capacity should be expected.

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Predicted performance under vertical loading. For the settlement analysis of the foundation system, the computer program GARP (Small and Poulos, 2007) was used as the main analysis tool. The GARP analysis used as the main design tool, and without taking any account of the stiffness of the superstructure, the analysis gave a maximum settlement of 67mm and a maximum differential settlement of 34mm. The maximum angular rotation (not taking into account the stiffness of the superstructure) was found to be 1/780. Both the settlement and angular rotation values were considered to be acceptable.

It can be noted that the very rough average preliminary settlement from the chart in Figure 1 of 75mm was of a similar order to that obtained from the GARP analysis.

FIG. 7. Results of cyclic loading analysis – Load Case 0.75(DL+LL+WL).

Detailed assessment of final foundation design. To provide a check on the GARP analyses, and to examine the effects on foundation performance of the basement wall surrounding the piled raft, analyses were also carried out using the commercially-available program PLAXIS 3-D Foundation. For the purposes of this paper, and subsequent to the execution of the foundation design, the PLAXIS analyses were also used to examine the effects of including the presence of the raft, and two separate cases were analysed:

1. The piles being connected to the raft which is in contact with the underlying soil but not with the surrounding soil above the raft base level (Case 1). This is the usual case considered for a piled raft, where only contact below the raft is taken into account.

2. The piles being connected to the raft, which is in contact with both the underlying soil and the soil surrounding the basement walls of the foundation system (Case 2). This is the actual case that is to be constructed. In this case, account was taken of vertical walls that are 14.6m deep and 1.2m thick.

Plate elements had to be fixed to the bottom of the solid elements of the raft and the pile heads fixed to the plate as this is required in PLAXIS if the pile heads are to rotate with the raft. The sides of the excavation were supported by retaining walls that were modelled in the mesh. The finite element mesh for the problem (Case 2) is shown in Figure 8, and it may be seen that the soil is divided into layers representing the materials of Table 2. Because of the limitations of PLAXIS 3D, the soil profile

Ratio 

Pile Number

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was assumed to be horizontally layered below the foundation footprint with the same profile as that employed in the preliminary assessment. The soil layers were treated as Mohr-Coulomb materials to allow for non-linear effects,

FIG. 8. Finite element mesh showing material layers.

Vertical loading. In the first analysis, vertical loading only was applied to the foundation for both Case 1 and Case 2. As may be expected, when contact between the basement walls and the soil is considered, the deflection of the raft is reduced. This may be seen from the load-deflection plot of Figure 9 where the percentage of load applied to the raft versus the vertical deflection is plotted. The reduction in vertical displacement caused by taking the embedment of the raft into consideration is about 8 mm in this case.

FIG. 9. Load-deflection behaviour at raft centre (vertical loading).

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These values of maximum settlement and maximum differential settlement were somewhat less than those from the GARP analysis (56mm max. settlement and 40mm differential, versus 67mm and 34mm from GARP). This difference may reflect the inherent conservatism in the use of interaction factors within the GARP analysis. Nevertheless, the agreement between the two analyses was considered to be adequate and the comparison indicates that a program such as GARP can be a very useful design tool, particularly when a large number of different cases (pile number and configurations) is to be analysed prior to deciding upon the final layout and number of piles.

Horizontal loading. In order to examine the effects of including the soil above the raft in the analysis, a PLAXIS 3-D analysis was undertaken for lateral loading only for both Case 1 and Case 2 as well as the case where the raft was assumed not to be in contact with the ground. The latter case was modelled in PLAXIS by placing a thin soft layer of soil underneath the raft. The results of the analysis showed that the predicted lateral deformation at working load was less than the conventional case of no raft contact, when the contact or embedment of the foundation was taken into account as shown in Figure 10. This figure shows the lateral deflection at the central point of the raft versus the percentage of lateral load applied to the foundation.

FIG. 10. Load-deflection behaviour of central point of the raft (Horizontal loading).

Deformed meshes in the case of horizontal loading are presented in Figures 11 (Case 1) and 12 (Case 2). Because of the bending of the piles under lateral loading, it is of interest therefore to compare the moments induced into one of the piles in the leading row for each of the cases. Figure 13 shows bending moment distributions for a pile on the leading edge of the raft. It may be seen that, when the raft is in contact with the soil at the sides of the basement above raft level, and/or the raft is in contact with the ground, the bending moments that were calculated via the finite element analysis are lower than from the conventional type of analysis (where the raft is

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assumed to make no contact). However, in this case because of the large number of piles, the effect of the walls on the reduction of pile moment is small.

FIG. 11. Deformed raft and piles under lateral loading (Case 1).

FIG. 12. Deformed raft and piles under lateral loading(Case 2).

The program CLAP, which is a modified version of the computer program DEFPIG (Poulos, 1990), was used as the main design tool for considering the lateral response of the foundation. It is interesting to note that CLAP gave a maximum lateral displacement of 22mm and a maximum pile bending moment of 15.7MNm. These values are comparable to those obtained from PLAXIS 3D and indicate that, for the design of piled rafts with a large number of piles, it is probably adequate to ignore the presence of the cap when computing the lateral response of the foundation and the distribution of bending moment within the piles.

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FIG. 13. Pile moments for horizontal loading, with and without contact.

Pile Load Testing

A total of five pile load tests were planned, four on vertically loaded piles via the Osterberg cell procedure, and one on a laterally loaded pile jacked against one of the vertically loaded test piles. For the vertical pile tests, two levels of O-cells were installed in each pile, one at the pile tip and another at between the weathered rock layer and the soft rock layer. The cell movement and pile head movement were measured by LVWDTs in each of four locations, and the pile strains were recorded by the strain gauges attached to the vertical steel bars. The vertical pile tests were undertaken in early 2010 after which the project was put on hold. One of the tests was found to have construction-related defects and was excluded from consideration. The average and range of results of the remaining three tests are shown in Table 7 for the two main supporting layers. It can be seen that the performance of the test piles exceeded design expectations, and that there could be scope for re-evaluating the foundation pile configuration.

Table 7. Assessed Average Performance of Three Test Piles

Location Parameter Ultimate Design

Value

Average Mobilized

& Range

Soft Rock End Bearing (MPa) 12.0 24.3 (18.9-37.6)

Shaft Friction (kPa) 750 1534 (1326-1994)

Weathered Rock Shaft Friction (kPa) 500 708 (356-1054)

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CONCLUSIONS

This paper has set out an approach for the design of pile foundation systems for high-rise buildings, using a limit state design approach. This approach involves three sets of analyses: an overall stability analysis using factored-down soil and pile resistances, an ultimate limit state analysis using unfactored soil and pile resistances to obtain design structural actions, and a serviceability analysis. In addition, a check can be carried out to assess the ratio of cyclic load amplitude to factored-down pile shaft resistance. It is suggested that if this ratio for a pile is less than about 0.5, there should be a low risk of cyclic degradation of shaft resistance occurring.

The application of the approach has been illustrated via its use for the 600m tall Incheon tower. It has been demonstrated that it is possible to obtain reasonably consistent outputs from the various stages of design, ranging from preliminary analysis using hand calculation tools, through the main design process using appropriate design software, to the detailed analysis and final checking phase using high-level numerical analysis. Via the latter analyses, the effect of considering the embedment of the raft was found to have a relatively modest effect on foundation settlements and pile load lateral response and bending moments.

A finding of practical importance is that for tall buildings supported by piled raft foundations with a large number of piles, a conventional pile group or piled raft analysis may often be adequate, albeit conservative, for estimating the vertical and lateral behaviour of the foundation, and the distributions of pile load and bending moment within the piles in the foundation system.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the significant contributions to the Incheon Tower analyses of Ms. Frances Badelow, Prof. John Small, Dr. Helen Chow and Tristan McWilliam of Coffey Geotechnics, and the cooperation of Dr. Ahmad Abdelrazaq of Samsung Corporation and Mr. S.H. Kim of JinYoung ENC with structural and geotechnical aspects of the Incheon Tower.

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