the design of high-rise building foundations · pdf filethe design of high-rise building...

Geotec Hanoi 2011 October ISBN 978-604-82-000-8

ID No./ pp. 12

The Design of High-Rise Building Foundations

Harry G. PoulosCoffey Geotechnics, Sydney, Australia E-mail: [email protected]

Keywords: foundation; high-rise; limit state design; Korea; Middle East; piled raft; piles; settlement; stability;ultimate capacity.

ABSTRACT: This paper sets out the principles of design for a pile or piled raft foundation system for tallbuildings via a limit state design approach. This approach 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 separate ULS analysis is carried out in which the ULS load combinations are applied but in which the

unfactored resistances of the foundation components are employed. The consequent computed foundationactions (i.e. pile forces and, if appropriate, raft moments and shears) are then multiplied by a structural actionfactor to obtain the values for structural design.

3. A serviceability analysis, in which the best-estimate (unfactored) values foundation resistances and stiffnessesare employed and the serviceability limit state (SLS) loads are applied.The approach is illustrated via its application to a simple hypothetical case, and then to high-rise buildings in the

Middle East and Korea.

1. INTRODUCTION

High rise buildings are usually founded on some formof piled foundation which is subjected to a combinationof vertical, lateral and overturning forces. Conventionalmethods of assessing foundation stability may not beadequate when designing such foundations becausethey tend to focus primarily on foundation resistanceunder vertical loading. This paper sets out a limit statedesign approach for tall building foundation systems,with attention being focused on piled and piled raftfoundation systems, which are the predominant typescurrently used. Some of the characteristics of piled raftsare outlined, and then the principles of the designapproach are set out. Examples of the application of thisapproach are described for a hypothetical problerm, andthen for two high-rise buildings, one in Doha Qatar, andanother on reclaimed land in Incheon South Korea.

2. PILED RAFT FOUNDATION SYSTEMS

Piled raft foundations utilize piled support for control ofsettlements with piles providing most of the stiffness atserviceability loads, and the raft element providingadditional capacity at ultimate loading. The beneficialcharacteristics of piled rafts have been described byRandolph (1994), Poulos (2001) and Phung (2010),among others.

A geotechnical assessment for design of such afoundation system therefore needs to consider not onlythe capacity of the raft and pile elements, but theircombined capacity and their interaction under

serviceability loading. This section sets out some of thebasic design issues to be considered in piled raft design,and some of the characteristics of behaviour that arefound from numerical analyses.

The most effective application of piled rafts occurswhen the raft can provide adequate load capacity, butthe settlement and/or differential settlements of the raftalone exceed the allowable values. Poulos (2001) hasexamined a number of idealized soil profiles, and foundthat the following situations may be favourable: Soil profiles consisting of relatively stiff clays Soil profiles consisting of relatively dense sands.

In both circumstances, the raft can provide asignificant proportion of the required load capacity andalso contribute to the foundation stiffness, especiallyafter the pile capacity has been fully mobilized.

Conversely, there are some situations which are lessfavourable, including: Soil profiles containing soft clays near the surface Soil profiles containing loose sands near the surface Soil profiles which contain soft compressible layers

at relatively shallow depths Soil profiles which are likely to undergo

consolidation settlements Soil profiles which are likely to undergo swelling

movements due to external causes.In the first two cases, the raft may not be able to

provide significant load capacity and stiffness, while inthe third case, long-term settlement of the compressibleunderlying layers may reduce the contribution of theraft to the long-term stiffness of the foundation. The

mailto:[email protected]


ID No./ pp. 12

latter two cases should be treated with caution, as theground movements can affect adversely the foundationperformance.

It has also been found that the performance of a piledraft foundation can be optimized by selecting suitablelocations for the piles below the raft. In general, thepiles should be concentrated in the most heavily loadedareas, while the number of piles can be reduced, or eveneliminated, in less heavily loaded areas (Horikoshi andRandolph, 1998).

3. DESIGN REQUIREMENTS

3.1 Design Issues

The following issues usually need to be addressed in thedesign 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) onfoundation capacity and movements.

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

footprint, and between high-rise and low-rise areas.5. Structural design of the foundation system; including

the load-sharing among the various components ofthe system (for example, the piles and the supportingraft), and the distribution of loads within the piles.For this, and most other components of design, it isessential that there be close cooperation andinteraction between the geotechnical designers andthe structural designers.

6. Possible effects of externally-imposed groundmovements on the foundation system, for example,movements arising from excavations for pile caps oradjacent facilities.

7. Earthquake effects, including the response of thestructure-foundation system to earthquake excitation,and the possibility of liquefaction in the soilsurrounding and/or supporting the foundation.

8. Dynamic response of the structure-foundation systemto wind-induced (and, if appropriate, wave) forces.In this paper, attention will be concentrated on the

first five design issues.

3.2 Design Requirements

In limit state format (for example, as per the AustralianPiling Code AS2159-1995), the design criteria for theultimate limit state may be expressed as follows:

Rs* S* (1)

Rg* S* (2)

where Rs* = design structural strength = s. Rus

Rg* = design geotechnical strength = g.Rug

Rus = ultimate structural strengthRug = ultimate geotechnical capacitys = structural reduction factorg = geotechnical reduction factorS* = design action effect (the factored loadcombination).

The above criteria in equations 1 and 2 are appliedto the entire foundation system, while the structuralstrength criterion (equation 1) is also applied to eachindividual pile. However, it is not considered to be goodpractice to apply the geotechnical criterion (equation 2)to each individual pile within the group, as this can leadto considerable over-design (Poulos, 1999).

The geotechnical strength criterion will be satisfiedif the foundation system with the reduced strengthsdoes not collapse under the ultimate limit state designaction effects (factored-up load combinations).

Rs* and Rg* can be obtained from the estimatedultimate structural and geotechnical capacities,multiplied by appropriate reduction factors. Values ofthe structural and geotechnical reduction factors areoften specified in national codes or standards. Theselection of suitable values of g requires judgment andtakes into account a number of factors that mayinfluence the foundation performance.

3.2 Load CombinationsThe required load combinations for which the structureand foundation system have to be designed will usuallybe dictated by an appropriate structural loading code. Insome cases, a large number of combinations may needto be considered. For example, for the Emirates Projectin Dubai (Poulos and Davids, 2005), a total of 18 loadcombinations was analyzed for each tower, these being1 loading set for the ultimate dead and live loadingonly, 4 groups of 4 loading sets for variouscombinations of dead, live and wind loading for theultimate limit state, and 1 set for the long-termserviceability limit state (dead plus live loading).

3.3 Design for Cyclic LoadingIn addition to the normal design criteria, as expressedby equations 1 and 2, Poulos and Davids (2005) havesuggested that an additional criterion be imposed for thewhole foundation of a tall building to cater for theeffects of repetitive loading from wind action, asfollows:

Rgs* Sc* (3)

where Rgs* = design geotechnical shaft capacitySc*= half amplitude of cyclic axial wind-

induced load= a factor assessed from geotechnical

laboratory testing.


ID No./ pp. 12

This criterion attempts to avoid the full mobilizationof shaft friction along the piles, thus reducing the riskthat cyclic loading will lead to a degradation of shaftcapacity. For the Emirates project in Dubai,wasselected as 0.5, based on laboratory tests. Sc* can beobtained from computer analyses which give the cycliccomponent of load on each pile, for various windloading cases.

3.4 Soil-Structure Interaction Issues

For structural design of the foundation system, soil-structure interaction needs to be considered for thegeotechnical ultimate limit state (for example, thebending moments in the raft of a piled raft foundationsystem). It should be recognized that the worst responsemay not occur when the pile and raft capacities arefactored downwards. As a consequence, additionalcalculations need to be carried out for geotechnicalreduction factors both less than 1 and greater than 1. Asan alternative to this duplication of analyses, it wouldseem reasonable to adopt a reduction factor of unity forthe pile and raft resistances, and then factor up thecomputed moments and shears (for example, by a factorof 1.5) to allow for the geotechnical uncertainties. Thestructural design of the raft and the piles will alsoincorporate appropriate reduction factors.

3.5 Serviceability Limit State

The design criteria for the serviceability limit state areas follows:

max all (4)

max all (5)

where max = maximum computed settlement offoundation

all = allowable foundation settlement, max = maximum computed local angular

distortionall = allowable angular distortion.

Values of all and all depend on the nature of thestructure and the supporting soil.

Some suggested criteria reported by Zhang and Ng(2006) are shown in Table 1. This table also includesvalues of intolerable settlements and angulardistortions. Criteria specifically for very tall buildingsdo not appear to have been set, but it should be notedthat it may be unrealistic to impose very stringentcriteria on very tall buildings on clay deposits, as theymay not be achievable. Experience with tall buildings inFrankfurt Germany suggests that total settlements wellin excess of 100mm can be tolerated without anyapparent impairment of function.

Table 1. Serviceability Criteria for Structures (Zhang andNg, 2006)

Quantity ValueLimiting Tolerable Settlement,mm

106

Observed IntolerableSettlement, mm 349

Limiting Tolerable AngularDistortion, rad

1/250 (H<24m)1/330 (24<H<60m)1/500 (60<H<100m)1/1000 (H>100m)

Observed Intolerable AngularDistortion , rad 1/125

3.6 Design Tools and Analysis Requirements

In order to undertake the above design analyses, it isalmost essential to use a computer program. Such aprogram should ideally have the following abilities:1. For overall stability, the program should be able to

consider:a. Non-homogeneous and layered soil profilesb. Non-linearity of pile and, if appropriate, raft

behaviour;c. Geotechnical and structural failure of the piles

(and the raft)d. Vertical, lateral and moment loading (in both

lateral directions), including torsion;e. Piles having different characteristics within the

same group.2. For serviceability analysis, the above

characteristics are also desirable, and in addition,the program should have the ability to consider:a. Pile-pile interaction, and if appropriate, raft-pile

and pile-raft interaction;b. Flexibility of the raft or pile cap;c. Some means by which the stiffness of the

supported structure can be taken into account.There do not appear to be any commercially

available software packages that have all of the abovedesirable characteristics, other than three-dimensionalfinite element packages such as PLAXIS 3D orABAQUS, or the finite difference program FLAC3D.The programs REPUTE, PIGLET and DEFPIG havesome of the requirements, but fall short of a number ofcritical aspects, particularly in their inability to includeraft-soil contact and raft flexibility.

The author has developed the pile group analysispackages that, between them, provide most of thefeatures listed above. The programs include PIGSwhich analyzes the settlement and load distributionwithin a group of piles subjected to axial and momentloading, CLAP, that computes the distributions of axialand lateral deflections, rotations, axial and lateral loads,and moments at the top of a group of piles subjected toa combination of vertical load, lateral loads andmoments in each horizontal direction, and a torsionalload, and GARP, which analyses the behaviour of apiled raft subjected to vertical and moment loading.


ID No./ pp. 12

Details of this program are given by Small and Poulos(2007).

4 A SIMPLE EXAMPLE

As a simple example of the application of the suggestedapproach, the case shown in Figure 1 has beenanalysed. In this case, a 4m square raft, 1m thick, isconsidered (this would normally be one portion of alarger foundation system). For convenience, only asingle set of loadings has been considered for theultimate and serviceability states, as follows:

Ultimate:Vertical = 12 MNHorizontal (in the x-direction) = ±1 MNMoment (acting in the x-direction) = ±7 MNm.Serviceability:Vertical = 9MN

The loads and moment are assumed to be applied viaa central column 0.8m square. It is assumed that piles0.4 m in diameter and 20 m long will be driven into astiff clay layer, with the following properties:

Undrained shear strength su = 150 kPa Drained Young’s modulus (for vertical loading)

= 45 MPa Undrained Young’s modulus for lateral loading

= 31.5 MPa Ultimate skin friction = 60 kPa Ultimate end bearing pressure = 1 MPa Ultimate raft bearing pressure = 0.9 MPa. Ultimate lateral pressure = Nc su where Nc =

2+2z/d ≤ 9, z = depth below surface and d = pilediameter.

The ultimate geotechnical strength in compression is1.64 MN and 1.52 MN in tension, while the ultimatestructural strength of the piles is 3.8MN in compressionand 2.5 MN in tension.

Two cases have been considered: 9 piles in a 3x3 square configuration, at a centre-to-

centre spacing of 1.6m in each direction; 16 piles in a 4x4 square configuration, at a centre

to-centre spacing of 1.2m in each direction.

4.1 Overall StabilityFor the overall stability assessment, the programDEFPIG has been used, and a geotechnical reductionfactor of 0.65 has been applied to both the vertical andlateral resistances. The raft is assumed to be in contactwith the underlying soil.

Table 2 summarizes the results of the analyses. Whilethe computed settlements, lateral movements androtations are not meaningful, they do at least provide anindication of the proximity to overall collapse of thefoundation system. It can be seen that both the 9-pile

and 16-pile systems satisfy the ultimate limit statecriterion in that they do not collapse when theresistances are factored down and the ultimate limitstate loadings are applied. It is also clear that the 16-pilegroup is further from failure than the 9-pile group, asthe computed movements and rotation in that case aremuch smaller.

Figure 1. Simple Example Analyzed

Table 2. Summary of Computed Behaviour under Ultimatelimit State Conditions

Quantity 9-Pile Group 16-Pile GroupMaximum settlement ,mm

32 11

Lateral movement, mm 10 4Rotation, rad 1/217 1/833Stable Overall? YES YES

4.2 Cyclic Loading Assessment

DEFPIG has been used to compute the cycliccomponent of vertical load in the piles due to the lateraland moment loadings. In this case, the pile resistanceswithin the analysis are unfactored. Table 3 summarizesthe results, and computes the ratio of the cyclic axialload to the design ultimate shaft capacity of each pile(in this case, 0.99MN, being the ultimate value of 1.52MN multiplied by a geotechnical reduction factor of0.65, assuming the skin friction is the same in bothcompression and uplift. It can be seen that the 16 pilegroup satisfies the cyclic loading criterion but the 9 pilegroup does not.


ID No./ pp. 12

Table 3. Summary of Cyclic Loading AssessmentQuantity 9-Pile Group 16-Pile GroupMaximum cyclic axialload Sc* MN 0.59 0.47

Design GeotechnicalShaft Capacity Rgs*MN

0.99 0.99

Ratio = Sc*/ Rgs* 0.60 0.48Criterion (eq. 3)satisfied? ( ≤0.5)

NO YES

4.3 Serviceability Analysis

The program GARP has been used to compute thedistributions of settlement, pile load, bending momentand shear within the foundation system under theserviceability loading. The results are summarized inTable 4, and reveal that there is relatively littledifference between the settlement and differentialsettlement for the two groups, and relatively littledifference in group performance.

Table 4 Serviceability Analysis ResultsQuantity 9-Pile Group 16-Pile GroupMax. Settlement mm 8.3 6.3Min. Settlement mm 7.1 5.0Max. Angular Rotationrad

1/4760 1/4600

Max. Raft MomentMNm/m

1.44 1.53

Max. Raft Shear MN/m 1.21 1.30Max. Pile Load MN 0.72 0.51

4.4 Analysis for Structural DesignFor this analysis, the ultimate limit state loadings havebeen applied and the pile and raft resistances have notbeen factored down. The key objective here is to obtainthe maximum pile loads and pile head moments so thatthe components of the system can be designed to satisfythe structural requirements. Ideally, a program such asGARP would be used, but GARP does not have thecapability of considering lateral loadings. Hence, theprogram DEFPIG has been used again. Table 5summarizes the key outputs from the DEFPIG analyses,namely the pile head loads and moments. Also shownin this table are the corresponding values that would beobtained from an analysis in which the pile resistanceswere factored down by a geotechnical reduction factorof 0.65. The following points can be noted:a. Larger moments and axial loads are computed for

the 9-pile group, as would be expected.b. When the pile resistances are factored down,

unrealistically large bending moments are computed,while the axial loads are equal to the factored-downdesign resistances, i.e. the axial loads are artificiallylimited to these values.

c. When the pile resistances are factored down, a largerproportion of the load is computed to be carried bythe raft. This in turn would imply that the raft actionsare larger than if g is taken as 1.0.On the basis of these results, it would seem

preferable to obtain the design actions for the piles byfactoring up (for example, by 1/g) the values computedby applying the ultimate limit state loads and notfactoring down the pile resistances.

Table 5. Summary of Analyses for Structural Actions

4.5 SummaryThis simple example illustrates the process followed toassess the suitability of a foundation system to supportultimate load combinations and to perform satisfactorilyunder serviceability loadings. Table 3 summarizes thecomputed performance in relation to the various designcriteria considered. In this case, it appears that a criticalfactor is the cyclic wind loading. Were it not for thiscriterion, a 9-pile group would be satisfactory, but dueto the large proportion of the shaft resistance that ismobilized by the cyclic wind loading, a 16-pile group isrequired.

5. APPLICATION TO HIGH-RISE TOWER INQATAR

5.1 IntroductionA high-rise tower is currently under construction inDoha Qatar. The tower will be in excess of 400 m talland will have 74 storeys and three basement levels. It isfounded on a pile-supported raft, with piles extending40 to 50m below the base of the raft. A low-rise podiumarea is to be located adjacent to the tower. As part of thefoundation design process, the author undertook a peerreview of the geotechnical aspects of the project and thefoundation design. The process adopted to assess thefoundation design is described below.

Quantity 9-Pile Group 16-Pile Group

g = 1.0 g

=0.65g =1.0

g =0.65

Maximum AxialPile Head LoadMN

1.52 1.06 1.43 1.06

MaximumLateral Load atPile Head MN

0.117 0.118 0.076 0.076

Maximum PileHead MomentMNm

0.035 0.057 0.023 0.023

% Vertical LoadCarried by Raft

24 35 12 19


ID No./ pp. 12

5.2 Geotechnical Model

A total of 23 boreholes were drilled at the site, to depthsof up to 120m. The in-situ testing consisted of thefollowing:

a. SPT tests in upper superficial deposits and at somelower levels where the rock was weak and corerecovery was poor.

b. Geophysical investigations, including cross-holetomographic imaging, downhole seismic surveys, a750 point microgravity survey and a 6-line resistivitysurvey.

c. 53 pressuremeter tests within four of the boreholesbeneath the tower, to measure strength anddeformation characteristics of the various strata.

d. 53 packer tests within seven boreholes, to measurepermeability within the various strata.

e. 6 standpipes to monitor the groundwater levels.An extensive program of laboratory testing was

undertaken, both conventional and specialized. Theconventional tests included particle size distribution,unconfined compressive strength, point load strength,and carbonate content tests. The specialized testsincluded the following: Stress path triaxial tests, to measure deformation

properties of the strata. Resonant column tests, to measure the small-strain

modulus values of the rock core samples. Cyclic undrained triaxial tests, to assess the effects of

cyclic loading on the strength and stiffness of rockcore samples.

Constant normal stiffness direct shear tests, tomeasure the pile-soil skin friction and the effects ofcyclic loading.Finally, a program of pile load testing was

undertaken, consisting of four compression tests onpiles of various length (3 with 1.5m diameter and onewith 0.9m diameter) and two tension tests on piles

about 26m long, one 0.9m in diameter and the other0.75m diameter.

On the basis of the above information, ageotechnical model was progressively developed for thesite, The site was quite uniform laterally, and so only asingle model was necessary. Table 6 summarises thefinal model adopted for the foundation designverification process. The modulus and skin frictionparameters were influenced heavily by the results of thepile load tests. It will be noted that the strata generallybecome weaker with increasing depth, and no reliableend bearing stratum was found within an acceptabledepth. For the raft, an ultimate bearing pressure of 2.1MPa was assessed.

5.3 Foundation Details and Layout

The foundation design concept involved the use of apiled raft for the high-rise tower and piles for thepodium areas. The basement levels varied below thetower, and were between 15.6 and 21.6m belowexisting ground level, which was typically at a reducedlevel (RL) of +2m QNHD (Qatar National HeightDatum). The raft beneath the tower was typically 2.5mthick and the piles were 1.5m in diameter, with lengthsof 12m, 22m or 32m below the raft, depending on theirlocation. The slab for the podium was to be 1m thickand was not to be joined to the tower slab. Figure 2shows the foundation layout for the tower.

There is a total of 232 piles, with 40 12m long pilesbeneath the wings, 163 piles 22m long below the mainfoundation area, and 29 piles 32 m long below the liftpits.

Table 7 summarizes the loads which were adoptedfor the design verification. Various combinations offactored loads were used as per the Australian LoadingCode AS 1170-2000.

Table 6. Geotechnical Model Adopted for Verification of Tower Foundation DesignMaterial RL at top of

Stratum2 (mQNHD)

Thicknessm

TypicalUCS3

MPa

Young’sModulus

MPa(ShortTerm)

Young’sModulus

MPa (Long-Term)

UltimateSkin

Friction fs1

kPa

Ultimate EndBearing fb

MPa

Limestone -5 15 15 1650 1500 560 15

Transition Zone -20 3 4 720 600 675 12

Shale -23 3 4 720 600 525 4.6

Chalk –1 -26 20 0.6 315 150 400 4.8

Chalk – 2 -46 66 0.2 315 150 250 3.4

Umm ErRadhuma

-112 >25 2 1100 1000 - -

1 For compression loading. Values for tension were reduced from these values.2 The raft base level varied between from 15.6 to 21.6 m below existing ground level (deeper levels below lift pits).3 UCS = uniaxial compressive strength


ID No./ pp. 12

Figure 2 Foundation Layout for Tower

Table 7. Summary of Loads on Foundation SystemLoad Type Vertical

Load MNHorizontalLoad MN

MomentLoadingMNm

Dead (G) 3069 0 0

Live (Q) –assumed to

be 10% of G

307 0 0

Wind in x-direction Wux

0 62 11200

Wind in y-direction Wuy

0 82.3 15430

5.4 Overall Stability Assessment

The program DAMPIG, a development of the DEFPIGprogram, was used for the assessment of the overallstability of the tower foundation system. This programallowed for the modification of the capacity andstiffness of selected piles within the system so that thethree different lengths of pile (12, 22 and 32m) could beconsidered. The computed compressive capacities ofthe three pile lengths were 42.9MN, 60.9MN and68.9MN respectively, while the corresponding axialpile head stiffness values were 4098, 4293 and3822MN/m. The lower stiffness for the longer pilereflected the influence of the weaker Chalk layer atdepth.

A geotechnical reduction factor of 0.65 was appliedto the pile and raft capacities, given that a significantamount of in-situ and pile load testing had been carriedout.

For all load combinations considered, the foundationgroup was found to be stable. This conclusion wasconsistent with the foundation designer’s assessment.


In the original peer review assessment, the check forcyclic loading was not carried out because the pile load

testing and the laboratory constant normal stiffnessdirect shear tests indicated that no cyclic degradation ofskin friction should occur under the anticipatedcombinations of mean and cyclic shear stresses imposedon the pile shaft. However, a check was madesubsequently for a load combination of 0.9 times deadload plus or minus the wind loads. Typical results forloading in the x-direction are shown in Figure 3. In thiscase, a small number of piles have a ratio of cyclic axialload to factored-down shaft resistance close to 0.5, butnot above, and thus the foundation system would beassessed to be satisfactory from the cyclic loadingviewpoint.

CYCLIC LOAD CHECK

0

0.1

0.2

0.3

0.4

0.5

0.6

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 113 120 127 134 141 148 155 162 169 176 183 190 197 204 211 218 225 232

Pile Number

Cyc

lic L

oad

/ Fac

tore

d Sh

aft R

esis

tanc

e

Figure 3 Summary of Cyclic Load Assessment for WindLoading in the x-Direction

5.6 Assessment of Settlement Under ServiceabilityLoading

Three sets of settlement analyses were carried out:1. Preliminary calculations to assess the effects of pile

length, using a simplified axi-symmetric foundationmodel with the program PLAXIS;

2. Preliminary calculations with the DEFPIG program,to obtain alternative assessments of averagesettlement;

3. Detailed analyses with the program GARP, takinginto account the actual pile configuration, thedetailed distributions of loading, and the stiffness ofthe raft.In all cases, the geotechnical model in Table 8 was

employed.The two preliminary analyses gave comparable

average final settlements under full dead plus liveloadings, being within the range 140 to 160mm for therange of pile lengths actually used. The PLAXISanalysis also indicated that there would be little benefitin having piles longer than about 35 to 40m, becausemuch of the settlement was derived from the lowerweak limestone layer well below a feasible pile tiplevel.

The finite element model for the GARP analysis isshown in Figure 4, where the concentrated anddistributed vertical loadings are also shown. Figure 5shows the computed contours of final settlement and


ID No./ pp. 12

indicated that the maximum long-term settlementoccurred near the centre of the tower and was about 230mm. This value was somewhat larger than the value of184mm computed by the original designer using a non-linear three-dimensional finite element analysis with theprogram LUSAS. The minimum settlement of about44mm occurs at the edge of the foundation footprint.The analysis indicated that the raft carried about 23% ofthe applied load under serviceability conditions, due tothe stiff limestone on which the raft was founded.

Figure 4 GARP Model Used for Tower Analysis

Figure 5 Contours of Computed Long-Term Settlement ofTower

The maximum computed angular rotation was of theorder of 1/220 and occurred near the south-east junctionbetween the tower and the podium. A similar value wasobtained by the original designer. This value wasconsiderably greater than the target value of 1/400, butit was considered that such a value should not lead tostructural distress as a substantial proportion of thesettlement and differential settlement would occurduring construction and before the architectural finisheswere applied.

5.7 Analyses for Pile Head Stiffness Values

Values of the axial stiffness of each pile within thefoundation were required for input into the structuralanalysis of the complete structure-foundation system,The computer program PIGS was used to obtain thesestiffness values for the piles when subjected to bothlong-term and short-term loading. PIGS is a FORTRANprogram that calculates the settlement of single pilesand pile groups subjected to vertical loading. It allowsfor non-linear pile response via an assumed hyperbolicload-settlement behaviour for each pile, and theinteraction factor method is employed to considerinteraction among piles within a group. The group canbe subjected to a series of vertical and moment loadingsif the group has a rigid cap, or alternatively, each pile inthe group can be subjected to a specified vertical load.The program computes the distribution of settlementand load within the group.

The following simplifying assumptions were made: Each pile was subjected to an axial load equal to the

average long-term load of 12.07 MN. The effects of the raft being in contact with the soil

were not taken into account.The computed pile head stiffness values ranged

between 71 MN/m and 635 MN/m, with the smallervalues being for piles towards the centre of the tower,and the larger values being towards the edges of thefoundation footprint. These values are far less thanthose for a single pile reported in Section 5.4. Thisdifference, together with the considerable variation ofthe computed stiffness values, demonstrates the vitalimportance of considering pile-pile interaction withinthe foundation system for tall buildings.

5.8 Analyses for Structural Design

For this structure, only GARP analyses for the deadplus live loading case were undertaken to check themore complete three-dimensional analyses carried outby the designer. The maximum pile axial force fromGARP was found to be 38.5 MN, which was larger thanthe value of about 33 MN obtained by the designer. Themaximum moments in the raft were found to be similarin magnitude, 15.8 MNm/m from GARP, and 16.3MNm/m from the three dimensional analysis. It wastherefore concluded that the latter analysis was asuitable basis for computing the pile loads andobtaining the raft moments. For the final design, thestructural analysis of the complete structure-foundationsystem was carried out using the three dimensionalanalysis together with the computed values of stiffnessfor each of the piles. The design pile loads, and raftmoments and shears, were obtained from this analysis.


ID No./ pp. 12

6. APPLICATION TO INCHEON TOWER,KOREA

6.1 Introduction

Currently, a 151 storey super highrise building projectis under design, located in reclaimed land constructedon soft marine clay in Songdo, Korea. The foundationsystem considered comprises 172 No. 2.5m diameterbored piles, socketed into the soft rock layer andconnected to a 5.5m thick raft. This building isillustrated in Figure 6 and is described in detail byBadelow et al (2009); thus, only a brief summary ispresented here.

6.2 Ground Conditions and Geotechnical ModelThe Incheon area has extensive sand/mud flats and nearshore intertidal areas. The site lies entirely within anarea of reclamation, which is likely to compriseapproximately 8m of loose sand and sandy silt,constructed over approximately 20m of soft to firmmarine silty clay, referred to as the Upper MarineDeposits (UMD). These deposits are underlain byapproximately 2m of medium dense to dense silty sand,referred to as the Lower Marine Deposits (LMD),which overlie residual soil and a profile of weatheredrock. The natural surface is at an elevation of aboutEL0.

The lithological rock units present under the sitecomprise granite, granodiorite, gneiss (interpreted aspossible roof pendant metamorphic rocks) and aplite.The rock materials within about 50 metres from thesurface have been affected by weathering which hasreduced their strength to a very weak rock or a soil-likematerial. This depth increases where the bedrock isintersected by closely spaced joints, and sheared andcrushed zones that are often related to the existence ofthe roof pendant sedimentary / metamorphic rocks. Thegeological structures at the site are complex andcomprise geological boundaries, sheared and crushedseams - possibly related to faulting movements, andjointing.

Figure 6. Incheon 151 Tower (artist’s impression)

From the available borehole data for the site,inferred contours were developed for the surface of the

“soft rock” founding stratum within the towerfoundation footprint. These are reproduced in Figure 7.It can be seen that there is a potential variation in levelof the top of the soft rock (the pile founding stratum) ofup to 40m across the foundation.

The footprint of the tower was divided into eightzones which were considered to be representative of thevariation of ground conditions and geotechnical modelswere developed for each zone. Appropriategeotechnical parameters were selected for the variousstrata based on the available field and laboratory testdata, together with experience of similar soils onadjacent sites. One of the critical design issues for thetower foundation was the performance of the soft UMDunder lateral and vertical loading, hence carefulconsideration was given to the selection of parametersfor this stratum. Typical parameters adopted for thefoundation design are presented by Badelow et al(2009).

6.3 Foundation Layout

The foundation comprises a mat and piles supportingcolumns and core walls. The numbers and layout ofpiles and the pile size were obtained from a series oftrial analyses through collaboration between thegeotechnical engineer and the structural designer. Thepile depth was determined by the geotechnical engineer,considering the performance and capacity of piles. Thepile layout was selected from the various optionsconsidered, and is presented in Figure 8.

Figure 7 Inferred Contours of Top of Soft Rock – IncheonTower


ID No./ pp. 12

Figure 8. Foundation Layout

6.4 LoadingsTypical loads acting on the tower were as follows: Vertical dead plus live load: Pz(DL+LL) = 6622MN Horizontal wind loads:

Px(WL) = 146MN Py(WL) = 112MN Horizontal earthquake loads: Px(E) = 105MN Py(E) = 105MN Wind load moments:

Mx(WL) = 12578MN-m My(WL) = 21173MN-m Wind torsional load: Mz(WL) = 1957MN-m

The vertical loads(DL+LL) and overturningmoments (Mx, My) were represented as vertical loadcomponents at column and core locations. The loadcombinations, as provided by the structural designerwere adopted throughout the geotechnical analysis, and24 wind load combinations were considered.

6.5 Assessment of Pile CapacitiesThe geotechnical capacities of piles were determined bythe shaft friction and end bearing capacities of pile, andthe required pile length was generally assessed based onthese geotechnical capacities to provide the requiredload capacity. For a large pile group founding in weakrock, the overall settlement behavior of the pile groupcould control the required pile lengths rather than theoverall geotechnical capacity. In this case, the soft rocklayer was considered to be a more appropriate foundingstratum than the overlying weathered rock, in particularthe soft rock below Elevation EL-50m. This is becausethis stratum provides a more uniform stiffness andtherefore is likely to result in a more consistentsettlement behavior of the foundation. The basic guidelines to establish the pile founding depth were: Minimum socket length in soft rock = 2 diameters; Minimum toe level = EL-50m.

The pile depths required to control settlement of thetower foundation were greater than those required toprovide the geotechnical capacity required. The piledesign parameters for the weathered/soft rock layer areshown in Table 8 and were estimated on the basis of the

pile test results in the adjacent site and the groundinvestigation data such as pressuremeter tests and rockcore strength tests.

Table 8. Ultimate Capacities for Pile Analysis

Material Ultimate Frictionfs(kPa)

Ultimate EndBearingfb(MPa)

Weathered Rock 500 5Soft Rock 750 12

Pile load tests prior to commencement of the mainpiling works have been planned. Based on theinterpreted findings of these tests, the pile capacitieswill be verified and the pile design confirmed.

6.6 Overall Stability

The ultimate limit state (ULS) combinations of loadwere input into a series of CLAP analyses with the pileaxial and lateral capacities reduced by geotechnicalreduction factors (0.65 for axial load, 0.40 for lateralload). The smaller factors for lateral load reflected thegreater degree of uncertainty for lateral response. In allcases analyzed, the foundation system was found to bestable, i.e. the computed foundation movements werefinite, and generally the maximum computed settlementunder the ULS loadings was less than 100mm.


Table 9 summarizes the results of the cyclic loadingassessment while Figure 9 shows the assessed factorfor each pile within the foundation system. Theassessment indicates that the criterion in equation 3 issatisfied, and that degradation of shaft capacity due tocyclic loading in unlikely to occur.

Table 9. Summary of Cyclic Loading AssessmentQuantity Value

Maximum Cyclic Axial Load Sc* (MN) 74.0Maximum Ratio = Sc*/Rgs* 0.43Cyclic Loading Criterion Satisfied? Yes

Figure 9. Results of cyclic loading analysis – LoadCase 0.75(DL+LL+WL)


ID No./ pp. 12

6.8 Assessment of Vertical Pile Behavior

The vertical pile head stiffness values for each of the172 foundation piles under serviceability loading (DL +LL) were assessed using the computer program CLAP.CLAP was used to assess the geotechnical capacities,interaction factors and stiffness values for each piletype under serviceability loading for input into thegroup assessment. The various pile types included thevarying pile founding levels, which were dependent onthe level of the top of the Soft Rock. The computedaxial pile head stiffness values ranged between a highof 1300MN/m and a low of 600MN/m, with an averagevalue of 850MN/m. The pile stiffness values for eachindividual pile, together with the computed interactionfactors, were then used in the program GARP tocompute the magnitude and distribution of settlement.

6.9 SettlementThe maximum settlements predicted ranged from about50mm (using the estimated upper bound modulus) toabout 75mm (with the estimated lower bound modulus),with differential settlements between the centre andedge of the slab ranging from about 30mm to 50mm.For the tower alone, an elastic analysis using an“equivalent block” approach via a 2-D axi-symmetricfinite element analysis gave maximum and minimumsettlement values of 43mm and 18mm, respectively,which are in good agreement with those predicted byone of the project reviewers using an equivalent blockhand calculation approach (43mm and 19mmrespectively for the tower alone).The predictedsettlements using the various analysis methods werereasonably consistent, giving a level of confidence tothe settlement values predicted. Figure 10 shows atypical set of settlement contours derived from theGARP analyses, while Figure 11 show the computeddistribution of pile loads.The analysis indicated that, under serviceabilityconditions, the raft would carry only about 2% of theapplied load, due to the fact that it rested on therelatively soft marine deposits.

Figure 10. Computed settlement contours below andaround Incheon Tower

Figure 11 Computed distribution of axial pile loads forIncheon tower

6.10 Assessment of Lateral Pile BehaviorOne of the critical design issues for the Towerfoundation was the performance of the pile group underlateral loading. The numerical modeling packages usedin the analyses comprised the following: 3D finite element computer program PLAXIS 3D

Foundation; The program CLAP.

PLAXIS 3D provided an assessment of the overalllateral stiffness of the foundation. The program CLAPwas used to assess the lateral stiffness provided by thepile group assuming that the raft is not in contact withthe underlying soil, and then a separate calculation wascarried out to assess the lateral stiffness of the raft andbasement. Table 10 presents typical values of computedlateral stiffness for the piled mat foundation obtainedfrom the analyses.

Table 10. Summary of Lateral Stiffness of Pile Group andRaft

Horizl.Load(MN)

Dirn. PileGroupDispl.(mm)

LateralPile

Stiffness(MN/m)

LateralRaft

Stiffness(MN/m)

TotalLateral

Stiffness(MN/m)

149 X 17 8760 198 8958

115 Y 14 8210 225 8435

7. CONCLUSIONS

This paper has set out an approach for the design of pilefoundation systems for high-rise buildings, using a limit


ID No./ pp. 12

state design approach. This approach involves three setsof analyses:

i. An overall stability analysis in which theresistances of the foundation components arereduced by the appropriate geotechnical reductionfactor and the ultimate limit state (ULS) loadcombinations are applied. The requirements of thecode will be satisfied if the foundation system doesnot collapse under any of the sets of ULS loadings.

ii. A serviceability analysis, in which the best-estimate(unfactored) values foundation resistances andstiffnesses are employed and the serviceability limitstate (SLS) loads are applied. The design will besatisfactory if the computed deflections androtations are within the specified allowable limits.

iii. For structural design of the raft and the piles, theresults of the above ULS analysis are notconsidered to be relevant because the loads that canbe sustained by the piles are artificially reduced bythe geotechnical reduction factor. The most rationalapproach appears to be to carry out a separate ULSanalysis in which the ULS load combinations areapplied but in which the unfactored resistances ofthe foundation components are employed. Theconsequent computed foundation actions (i.e. pileforces and, if appropriate, raft moments and shears)are then multiplied by a structural action factor toobtain the values for structural design.

iv. In addition, a check can be carried out to assess theratio of cyclic load amplitude to factored-down pileshaft resistance. It is suggested that if this ratio for apile is less than about 0.5, there should be a lowrisk of cyclic degradation of shaft resistanceoccurring.

v. For high-rise towers in Qatar and Korea, theprocess has been used to assess overall stability,foundation settlements and pile head stiffnessvalues.

.8. ACKNOWLEDGEMENTSThe author gratefully acknowledges the contributions ofFrances Badelow, Tristan McWilliam, Helen Chow andPatrick Wong in relation to the analyses for the towerdescribed in the paper. S.H. Kim, Ahmad Abdelrazaqand their teams in Korea had a major involvement inthe Incheon Tower foundation design. Professor JohnSmall has been pivotal in developing the GARPprogram and implementing it in a user-friendly form.

9. REFERENCESBadelow, F., Kim, S.H., Poulos, H.G. and Abdelrazaq,

A. (2009). “Foundation Design for a Tall Tower in aReclamation Area”. 7th Int. Conf. Tall Buildings,Hong Kong.

Horikoshi, K. and Randolph, M.F. (1998). “AContribution to the Optimum Design of Piled Rafts”.Geotechnique, 48(2): 301-317.

Phung, Duc Long (2010). Piled Raft – A Cost-EffectiveFoundation Method for High-rises. Geot. Eng. Jnl.SEAGS & AGSSEA, Vol. 41, No.3, September 2010.

Poulos, H.G. (1999). The design of piles with particularreference to the Australian Piling Code. AustralianGeomechanics, 34(4): 25-39.

Poulos, H.G. (2001). Piled Rafts – Design andApplications. Geotechnique, 51(2): 95-113.

Poulos, H.G. and Davids, A.J. (2005). FoundationDesign for the Emirates Twin Towers, Dubai. Can.Geot. Jnl., 42: 716-730.

Randolph, M.F. (1994). Design Methods for PileGroups and Piled Rafts. Proc. XIII Int. Conf. SoilMechs. & Found. Eng., New Delhi, Vol.5, 61-82.

Randolph, M.F. and Wroth, C.P. (1978). Analysis ofdeformation of vertically loaded piles. Jnl. Geot.Eng. Div., ASCE, 104(GT12): 1465-1488.

Sinha, J. and Poulos, H.G. (1999). Piled Raft Systemsand Free Standing Pile Groups in Expansive Soils.Proc. 8th Australia-New Zealand Conf.Geomechanics, Hobart, Aust. Geomechs. Society, 1:207-212.

Small, J.C. and Poulos, H.G. (2007). A method ofanalysis of piled rafts. Proc. 10th Australia NewZealand Conf. on Geomechanics, Brisbane, 1: 550-555.

Zhang, L. and Ng, A.M.Y.(2006). Limiting tolerablesettlement and angular distortion for buildingfoundations. Geotech. Special Publication No. 170,Probabilistic Applications in GeotechnicalEngineering, ASCE (on CD Rom).

the design of high-rise building foundations · pdf filethe design of high-rise building...

Documents