Re-assessment Of Foundation Settlements For The Burj .
See discussions, stats, and author profiles for this publication at: Re-assessment of foundation settlements forthe Burj Khalifa, DubaiARTICLE in ACTA GEOTECHNICA · FEBRUARY 2012Impact Factor: 2.49 · DOI: 10.1007/s11440-012-0193-4CITATIONSREADS33354 AUTHORS, INCLUDING:G. RussoHarry G. PoulosUniversity of Naples Federico IIUniversity of Sydney58 PUBLICATIONS 237 CITATIONS199 PUBLICATIONS 3,085 CITATIONSSEE PROFILESEE PROFILEJohn C. SmallUniversity of Sydney95 PUBLICATIONS 1,248 CITATIONSSEE PROFILEAvailable from: Harry G. PoulosRetrieved on: 05 February 2016
Re-assessment of foundation settlements forthe Burj Khalifa, DubaiGianpiero Russo, Vincenzo Abagnara,Harry G. Poulos & John C. SmallActa GeotechnicaISSN 1861-1125Volume 8Number 1Acta Geotech. (2013) 8:3-15DOI 10.1007/s11440-012-0193-41 23
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Author's personal copyActa Geotechnica (2013) 8:3–15DOI 10.1007/s11440-012-0193-4RESEARCH PAPERRe-assessment of foundation settlements for the Burj Khalifa,DubaiGianpiero Russo Vincenzo AbagnaraHarry G. Poulos John C. Small Received: 14 April 2012 / Accepted: 12 October 2012 / Published online: 30 October 2012Ó Springer-Verlag Berlin Heidelberg 2012Abstract This paper deals with the re-assessment offoundation settlements for the Burj Khalifa Tower inDubai. The foundation system for the tower is a piled raft,founded on deep deposits of calcareous rocks. Two computer programs, Geotechnical Analysis of Raft with Piles(GARP) and Non-linear Analysis of Piled Rafts (NAPRA)have been used for the settlement analyses, and the paperoutlines the procedure adopted to re-assess the foundationsettlements based on a careful interpretation of load testson trial piles in which the interaction effects of the pile testset-up are allowed for. The paper then examines theinfluence of a series of factors on the computed settlements. In order to obtain reasonable estimates of differential settlements within the system, it is found desirable toincorporate the effects of the superstructure stiffness whichact to increase the stiffness of the overall foundation system. Values of average and differential settlements for thepiled raft calculated with GARP and NAPRA were foundto be in reasonable agreement with measured data on settlements taken near the end of construction of the tower.Keywords Case history Footings and foundations Full-scale tests Piles Rafts SettlementG. Russo (&) V. AbagnaraUniversity of Naples, Naples, Italye-mail: pierusso@unina.itH. G. Poulos J. C. SmallCoffey Geotechnics, Sydney, AustraliaJ. C. SmallUniversity of Sydney, Sydney, Australia1 IntroductionThe Burj Khalifa in Dubai was officially opened in January2010 and, at a height of 828 m, is currently the world’stallest building. The foundation system is a piled raft, aform of foundation that is being used increasingly to support tall structures where the loads are expected to beexcessively large for a raft alone and where the raft and thepiles are able to transfer load to the soil. The foundationdesign process for this building has been described byPoulos and Bunce [12].An important component of the design of a piled raftfoundation is the detailed assessment of the settlement anddifferential settlement of the foundation system, and theircontrol by optimising the size, location and arrangement ofthe piles, and the raft thickness. Many different methods ofanalysis have been devised in order to predict the behaviour of raft and piled raft foundations [2, 4, 5, 6, 10, 13, 15,17, 19, 20], and these range from simple hand-basedmethods to complex three-dimensional numerical analyses.In this paper, attention is focussed on two methods thatmodel the raft as an elastic plate and the piles as interactingnon-linear springs. The computer codes implementingthese methods are described very briefly and are thenapplied to the Burj Khalifa, which is founded on a piledraft. The evaluation of the ground modulus values isdescribed using a combination of field test and laboratorydata and the results of pile load tests. The method ofinterpreting the pile load test data is discussed, and theimportance of allowing for interaction between the test pileand the surrounding reaction piles in emphasised. The twoprograms are then used to compare the computed settlements with available measurements of foundation settlements, and with the ‘‘Class A’’ predictions made by thefoundation designers and the peer reviewers.123
Author's personal copy4An important objective of the paper is to explore howpile load test data should be used when predicting thesettlement performance of piled and piled raft foundationsystems, and to examine some factors that may have animportant influence on predicted foundation settlements.2 Computer analysesThe settlement analyses used in this paper for the BurjKhalifa have employed two computer programs, Geotechnical Analysis of Raft with Piles (GARP) and Nonlinear Analysis of Piled Rafts (NAPRA), which idealise thepiled raft foundation as a plate supported by non-linearinteracting springs. A very brief description of these programs is given below.2.1 Program GARPThe computer program GARP [18] uses a simplifiedboundary element analysis to compute the behaviour of apiled raft when subjected to applied vertical loading,moment loading, and free-field vertical soil movements.The raft is represented by a thin elastic plate and isdiscretized via the finite element method using eight-nodedelements. The soil is modelled as a layered elastic continuum, and the piles are represented by elastic–plastic orhyperbolic springs, which can interact with each other andwith the raft. Pile–pile interactions are incorporated viainteraction factors [9]. Simplifying approximations areutilised for the raft–pile and pile–raft interactions. Beneaththe raft, limiting values of contact pressure in compressionand tension can be specified so that some allowance can bemade for non-linear raft behaviour. The output of GARPincludes: the settlement at all nodes of the raft; the transverse, longitudinal, and torsional bending moments withineach element of the raft; the contact pressures below theraft; and the vertical loads on each pile. In its present form,GARP can consider vertical and moment loadings, but notlateral loadings or torsion.2.2 Program NAPRAThe computer program NAPRA [13, 15] computes thebehaviour of a raft subjected to any combination of verticaldistributed or concentrated loading and moment loading.The raft is modelled as a two-dimensional elastic bodyusing the thin plate theory, and utilising the finite elementmethod, adopting a four- or nine-noded rectangularelement.The piles and the soil are modelled by means of interacting linear or non-linear springs. It is assumed that theinteraction between the raft and the soil (the piles) is purely123Acta Geotechnica (2013) 8:3–15vertical; accordingly, only the axial stiffness of the springsis required.The soil is assumed to be a layered elastic continuum.The Boussinesq solution for a point load and the closedform solution for a rectangular uniformly loaded area at thesurface of an elastic half space are used to calculate the soildisplacements produced by the contact pressure developedat the interface between the raft and the soil. The layeredcontinuum is solved by means of the Steinbrennerapproximation [3, 13], and as such, invokes the simpleassumption that the stress distribution within an elasticlayer is identical with the Boussinesq distribution for ahomogeneous half space [13].The interaction factor method is used to model pile topile interaction and a preliminary boundary element (BEM)analysis allows calculation of the interaction factorsbetween two piles at various spacings. Interaction betweenaxially loaded piles beneath the raft and the raft elements isaccounted for via pile–soil interaction factors computedwith a preliminary BEM procedure. The reciprocal theoremis used to maintain that the soil–pile interaction factor isequal to the pile–soil interaction factor.A stepwise incremental procedure is used to simulate thenon-linear load-settlement relationship of a single pile, thetotal load to be applied is subdivided into a number ofincrements, and the diagonal terms of the pile–soil flexibility matrix are updated at each step. A computation of thenodal reactions vector is made at each step to check fortensile forces between raft and soil and an iterative procedure is used to make them equal to zero. Basically, thisprocedure releases the compatibility of displacementsbetween the raft and the pile–soil system in the node wheretensile forces were detected, although the overall equilibrium is maintained by a re-distribution of forces. An iterative procedure is needed since after the first run someadditional tensile forces may arise in different nodes. Theoutput of the code is represented by the distribution of thenodal displacements of the raft and the pile–soil system,the load sharing among the piles and the raft, the bendingmoments and the shear in the raft, for each load increment.Abagnara et al. [1] have compared GARP and NAPRAanalyses for a simple case, and have concluded that bothprograms give comparable results, but that some of thesimplifying assumptions employed in each program giverise to differences in detail. For example, the difference inraft settlements may be due to the differences in the detailsof calculation of the soil layer stiffness using the Boussinesq/Steinbrenner approach. The difference in plate element types may also contribute to the differences. For thepiled raft, the differences may arise because of differencesin the methods used to compute the single pile stiffnessvalues, the interaction factors and the pile–raft and raft–pile interactions.
Author's personal copyActa Geotechnica (2013) 8:3–15In this paper, attention will be focussed on analysescarried out with NAPRA, although a comparison will alsobe presented between the GARP and NAPRA analyses.3 Settlement assessment for the Burj Khalifa Tower,Dubai3.1 Foundation layoutThe Burj Khalifa project in Dubai, United Arab Emirates(UAE), comprises a 160 storey high rise tower, with apodium development around the base of the tower,including a 4–6 storey garage. The Burj Khalifa is locatedon a 42,000 m2 site. The tower is founded on a 3.7 m thickraft supported on 194 bored piles, 1.5 m in diameter,extending 47.45 m below the base of the raft; podiumstructures are founded on a 0.65 m thick raft (increased to1 m at column locations) supported on 750 bored piles,0.9 m in diameter, extending 30–35 m below the base ofthe raft. A plan view of the foundation is shown in Fig. 1.5The ground conditions at the site comprise a horizontally stratified subsurface profile which is complex andhighly variable in terms of the strata thickness due to thenature of deposition and the prevalent hot arid climaticconditions. The main strata identified were as follows:1.2.3.4.3.2 Ground investigation and site characterisationThe investigations involved the drilling of 32 boreholes toa maximum depth of about 90 m below ground level and 1borehole to a depth of 140 m under the tower footprint.Standpipe piezometers were installed to measure theground water level which was found to be relatively closeto the ground surface, typically at a level of 2.5 m DMD.The ranges of measured SPT N values are summarised inTable 1. There was a tendency for N values to increasewith depth, beyond an elevation of about -8 m DMD.5.6.Very loose to medium dense silty sand (Marinedeposits).Weak to moderately weak calcarenite, generallyunweathered with fractures close to medium spacedinterbedded with cemented sand. This material isgenerally underlain by very weak to weak sandstonewhich is generally unweathered with fractures close tomedium spaced interbedded with cemented sand.Very weak to weak calcarenite, calcareous sandstone,and sandstone; this formation is slightly to highlyweathered with fractures extremely close to closelyspaced and interbedded with cemented sand. Bands of1–5 m thickness are also present of medium dense tovery dense, cemented sand with sandstone bands andlocally with bands of silt.Very weak to weak gypsiferous sandstone, gypsiferouscalcareous sandstone occasionally gypsiferous siltstone. This material is generally unweathered toslightly weathered with fractures extremely close toclosely spaced and interbedded with cemented sand.The formation is interbedded with dense to very dense,cemented silty sand and occasionally silt with sandstone bands.Very weak to weak calcisiltite, conglomeritic calcisiltite, and calcareous calcisiltite. This material is generally moderately to highly weathered, occasionallyslightly and completely weathered with fracturesextremely close to medium spaced. Calcareous siltstone was encountered in the majority of the deeperboreholes comprising very weak to weak occasionallymoderately weak calcareous siltstone in bands with athickness of 0.5–14.4 m generally slightly to moderately weathered occasionally highly to extremelyweathered.Very weak to weak and occasionally moderately weakcalcareous siltstone, calcareous conglomerate, conglomeritic sandstone, and limestone. This material isTable 1 Summary of measured SPT valuesElevation (m)2.5 to -1-1 to -8Fig. 1 Plan view of the Khalifa Tower foundation systemRange of SPT values0–4050–400-8 to -140–100-14 to -3040–200-30 to -40100–200-40 to -80100–400123
Author's personal copy67.Acta Geotechnica (2013) 8:3–15generally slightly weathered and occasionallyunweathered and moderately weathered to highlyweathered. Occasionally encountered as calcisiltiteinterbedded with bands of siltstone and conglomerate.Very weak to moderately weak claystone interbeddedwith siltstone. This material is generally slightlyweathered with close to medium-spaced fractures.Between -112.2 and -128.2 m occasional bands ofup to 500 mm thick gypsum were encountered. Below-128.2 m the stratum was encountered as weak tomoderately weak siltstone with medium to widelyspaced fractures.shear, and constant normal stiffness (CNS) direct sheartests.Some of the relevant findings from the in situ and laboratory testing are as follows:1.2.Table 2 summarises the stratigraphy adopted for thefoundation settlement analyses.3.3 In situ and laboratory test resultsA comprehensive series of in situ tests was carried out,including pressuremeter tests, down-hole seismic, crosshole seismic, and cross-hole tomography to determinecompression (P) and shear (S) wave velocities through theground profile. The vertical profile of P-wave velocity withdepth gave a useful indication of variations in the nature ofthe strata between the borelogs.Conventional laboratory classification tests (moisturecontent of soil and rock, Atterberg limits, particle sizedistribution, and hydrometer) and laboratory tests fordetermining physical (porosity tests, intact dry density,specific gravity, particle density) and chemical propertieswere carried out. In addition, unconfined compression tests,point load index tests, and drained direct shear tests werecarried out. A considerable amount of more advancedlaboratory testing was undertaken, including stress pathtriaxial tests, resonant column testing for small-strain shearmodulus, undrained cyclic triaxial tests, cyclic simple3.4.5.The cemented materials were generally very weak toweak; unconfined compressive strength (UCS) valuesranged mostly between about 0.1 and 6 MPa, theaverage values for each layer being the ones reportedin Table 2.Values of the Young’s modulus from pressuremetertests (first and second reload cycle) were found to be ingood agreement with values from correlation withshear waves velocities. From calcarenite (0 to -7.5 m)to sandstone (-7.5 to -24 m), Young’s modulus isapproximately constant with depth; at greater depths,the average values decrease in the gypsiferous sandstone (-24 to -28.5 m) then they slightly increase inthe calcisiltite (from -28.5 to -68.5 m) and finallydecrease in the siltstone (from -68.5 to -91 m).Triaxial stress path testing (at strain levels of 0.01 and0.1 %) was found to give results for Young’s modulusthat were in good agreement with pressuremeter andgeophysics testing results.Resonant column testing was found to give lowervalues for the Young’s modulus when compared withvalues from pressuremeter tests, geophysics tests, andtriaxial stress tests.Constant normal stiffness (CNS) tests were carried outon three samples taken from stratum 5a to assess theultimate skin friction values and the potential forcyclic degradation at the pile–soil interface. Thesetests indicated values of peak monotonic shear stressranging from 360 to 558 kPa, with only a littledifference between the peak monotonic and theresidual cyclic shear stress values.Table 2 Stratigraphic model adopted for settlement assessmentUCS qu (MPa)StratumDescriptionLevel at the top ofthe stratum (m DMD)Thickness (m)Adopted level at topof layer (m DMD)12Marine depositsCalcarenite/calcareous sandstone1.15 to 2.96-0.27 to -1.951.85 to 4.32.87 to 10.752.5-1.23aCalcareous sandstone/sandstone-4.13 to -12.0610.5 to 21.43-7.3–-13.513b24Gypsiferous sandstone-21.54 to -26.691.7 to 7.75-2425aCalcisiltite/conglomeritic calcisiltite-27.64 to -31.1539.2 to 46.75-28.51.35bCalcareous siltstone-501.76Calcareous/conglomeritic strata-67.19 to -76.0431 (from 140 m deepBH only)-68.52.57Claystone/siltstone interbeddedwith gypsum layers-98.19Proved to 39.6 mthickness-90–123
Author's personal copyActa Geotechnica (2013) 8:3–153.4 Geotechnical modelThe key parameters for the assessment of the settlementbehaviour of the Khalifa Tower piled raft foundation system are the values of the Young’s modulus of the strata forboth raft and pile behaviour under static loading. In a nonlinear analysis, the values of ultimate skin friction of piles,the ultimate end-bearing resistance of the piles, and theultimate bearing capacity of the raft would also berequired, but in this paper, only linear elastic analyses havebeen undertaken using NAPRA and GARP analyses, having explored the little influence of non-linearity up to themaximum observed load level. Attention has thus beenfocussed on evaluating relevant values of Young’s modulusfor each stratum.As a first step in obtaining these values, the relativestiffness of the various soil layers was assessed consideringvalues of the Young’s modulus from the following data:1.2.3.4.pressuremeter tests (initial loading, first reload, secondreload cycles);geophysics tests (correlation with shear wavevelocities);resonant column tests (Initial, 0.0001, 0.001, 0.01 %strain levels);triaxial stress path tests (0.01 and 0.1 % strain levels);Values of the various Young’s modulus values areplotted in Fig. 2, and although inevitable scatter existsamong the different values, there is a reasonably consistentgeneral pattern of variation with depth.Layer 3b (see Table 2) has arbitrarily been chosen as thereference layer, and for each type of test, values of theYoung’s modulus for a layer i, Ei, have been related to thevalue for layer 3b, E3b. The values of Ei/E3b have then beenaveraged using the following data: reload cycles frompressuremeter testing; seismic data; resonant column dataat a strain level of 0.01 %, and the triaxial stress path tests.Figure 3 shows the different assessed relative stiffnessprofiles so obtained, and Table 3 summarises the averagevalues of relative Young’s m
Re-assessment of foundation settlements for the Burj Khalifa, Dubai . tlements taken near the end of construction of the tower. . raft settlements may be due to the differences in the details
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