Acta Geotechnica (2013) 8:3-15 DOI 10.1007/s11440-012-0193-4 RESEARCH PAPER Re-assessment of foundation settlements for the Burj Khalifa, Dubai Gianpiero Russo Vincenzo Abagnara. Harry G. Poulos John C. Small Received: 14 April 2012/Accepted: 12 October 2012/Published online: 30 October 2012 © Springer-Verlag Berlin Heidelberg 2012 Abstract This paper deals with the re-assessment of foundation settlements for the Burj Khalifa Tower in Dubai. The foundation system for the tower is a piled raft, founded on deep deposits of calcareous rocks. Two com- puter 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 paper outlines the procedure adopted to re-assess the foundation settlements based on a careful interpretation of load tests on trial piles in which the interaction effects of the pile test set-up are allowed for. The paper then examines the influence of a series of factors on the computed settle- ments. In order to obtain reasonable estimates of differ- ential settlements within the system, it is found desirable to incorporate the effects of the superstructure stiffness which act to increase the stiffness of the overall foundation sys- tem. Values of average and differential settlements for the piled raft calculated with GARP and NAPRA were found to be in reasonable agreement with measured data on set- tlements taken near the end of construction of the tower. Keywords Case history Footings and foundations. Full-scale tests Piles Rafts · Settlement . G. Russo (). V. Abagnara University of Naples, Naples, Italy e-mail: pierusso@unina.it H. G. Poulos J. C. Small Coffey Geotechnics, Sydney, Australia J. C. Small University of Sydney, Sydney, Australia 1 Introduction The Burj Khalifa in Dubai was officially opened in January 2010 and, at a height of 828 m, is currently the world's tallest building. The foundation system is a piled raft, a form of foundation that is being used increasingly to sup- port tall structures where the loads are expected to be excessively large for a raft alone and where the raft and the piles are able to transfer load to the soil. The foundation design process for this building has been described by Poulos and Bunce [12]. An important component of the design of a piled raft foundation is the detailed assessment of the settlement and differential settlement of the foundation system, and their control by optimising the size, location and arrangement of the piles, and the raft thickness. Many different methods of analysis have been devised in order to predict the behav- iour of raft and piled raft foundations [2, 4, 5, 6, 10, 13, 15, 17, 19, 20], and these range from simple hand-based methods to complex three-dimensional numerical analyses. In this paper, attention is focussed on two methods that model the raft as an elastic plate and the piles as interacting non-linear springs. The computer codes implementing these methods are described very briefly and are then applied to the Burj Khalifa, which is founded on a piled raft. The evaluation of the ground modulus values is described using a combination of field test and laboratory data and the results of pile load tests. The method of interpreting the pile load test data is discussed, and the importance of allowing for interaction between the test pile and the surrounding reaction piles in emphasised. The two programs are then used to compare the computed settle- ments with available measurements of foundation settle- ments, and with the "Class A" predictions made by the foundation designers and the peer reviewers. Springer 4 An important objective of the paper is to explore how pile load test data should be used when predicting the settlement performance of piled and piled raft foundation systems, and to examine some factors that may have an important influence on predicted foundation settlements. 2 Computer analyses The settlement analyses used in this paper for the Burj Khalifa have employed two computer programs, Geo- technical Analysis of Raft with Piles (GARP) and Non- linear Analysis of Piled Rafts (NAPRA), which idealise the piled raft foundation as a plate supported by non-linear interacting springs. A very brief description of these pro- grams is given below. 2.1 Program GARP The computer program GARP [18] uses a simplified boundary element analysis to compute the behaviour of a piled 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 is discretized via the finite element method using eight-noded elements. The soil is modelled as a layered elastic con- tinuum, and the piles are represented by elastic-plastic or hyperbolic springs, which can interact with each other and with the raft. Pile-pile interactions are incorporated via interaction factors [9]. Simplifying approximations are utilised for the raft-pile and pile-raft interactions. Beneath the raft, limiting values of contact pressure in compression and tension can be specified so that some allowance can be made for non-linear raft behaviour. The output of GARP includes: the settlement at all nodes of the raft; the trans- verse, longitudinal, and torsional bending moments within each element of the raft; the contact pressures below the raft; and the vertical loads on each pile. In its present form, GARP can consider vertical and moment loadings, but not lateral loadings or torsion. 2.2 Program NAPRA The computer program NAPRA [13, 15] computes the behaviour of a raft subjected to any combination of vertical distributed or concentrated loading and moment loading. The raft is modelled as a two-dimensional elastic body using the thin plate theory, and utilising the finite element method, adopting a four- or nine-noded rectangular element. The piles and the soil are modelled by means of inter- acting linear or non-linear springs. It is assumed that the interaction between the raft and the soil (the piles) is purely Springer Acta Geotechnica (2013) 8:3-15 vertical; accordingly, only the axial stiffness of the springs is required. The soil is assumed to be a layered elastic continuum. The Boussinesq solution for a point load and the closed form solution for a rectangular uniformly loaded area at the surface of an elastic half space are used to calculate the soil displacements produced by the contact pressure developed at the interface between the raft and the soil. The layered continuum is solved by means of the Steinbrenner approximation [3, 13], and as such, invokes the simple assumption that the stress distribution within an elastic layer is identical with the Boussinesq distribution for a homogeneous half space [13]. The interaction factor method is used to model pile to pile interaction and a preliminary boundary element (BEM) analysis allows calculation of the interaction factors between two piles at various spacings. Interaction between axially loaded piles beneath the raft and the raft elements is accounted for via pile-soil interaction factors computed with a preliminary BEM procedure. The reciprocal theorem is used to maintain that the soil-pile interaction factor is equal to the pile-soil interaction factor. A stepwise incremental procedure is used to simulate the non-linear load-settlement relationship of a single pile, the total load to be applied is subdivided into a number of increments, and the diagonal terms of the pile-soil flexi- bility matrix are updated at each step. A computation of the nodal reactions vector is made at each step to check for tensile forces between raft and soil and an iterative pro- cedure is used to make them equal to zero. Basically, this procedure releases the compatibility of displacements between the raft and the pile-soil system in the node where tensile forces were detected, although the overall equilib- rium is maintained by a re-distribution of forces. An iter- ative procedure is needed since after the first run some additional tensile forces may arise in different nodes. The output of the code is represented by the distribution of the nodal displacements of the raft and the pile-soil system, the load sharing among the piles and the raft, the bending moments and the shear in the raft, for each load increment. Abagnara et al. [1] have compared GARP and NAPRA analyses for a simple case, and have concluded that both programs give comparable results, but that some of the simplifying assumptions employed in each program give rise to differences in detail. For example, the difference in raft settlements may be due to the differences in the details of calculation of the soil layer stiffness using the Bous- sinesq/Steinbrenner approach. The difference in plate ele- ment types may also contribute to the differences. For the piled raft, the differences may arise because of differences in the methods used to compute the single pile stiffness values, the interaction factors and the pile-raft and raft- pile interactions. Acta Geotechnica (2013) 8:3-15 In this paper, attention will be focussed on analyses carried out with NAPRA, although a comparison will also be presented between the GARP and NAPRA analyses. 3 Settlement assessment for the Burj Khalifa Tower, Dubai 3.1 Foundation layout The Burj Khalifa project in Dubai, United Arab Emirates (UAE), comprises a 160 storey high rise tower, with a podium development around the base of the tower, including a 4-6 storey garage. The Burj Khalifa is located on a 42,000 m² site. The tower is founded on a 3.7 m thick raft supported on 194 bored piles, 1.5 m in diameter, extending 47.45 m below the base of the raft; podium structures are founded on a 0.65 m thick raft (increased to 1 m at column locations) supported on 750 bored piles, 0.9 m in diameter, extending 30-35 m below the base of the raft. A plan view of the foundation is shown in Fig. 1. 3.2 Ground investigation and site characterisation The investigations involved the drilling of 32 boreholes to a maximum depth of about 90 m below ground level and 1 borehole to a depth of 140 m under the tower footprint. Standpipe piezometers were installed to measure the ground water level which was found to be relatively close to the ground surface, typically at a level of 2.5 m DMD. The ranges of measured SPT N values are summarised in Table 1. There was a tendency for N values to increase with depth, beyond an elevation of about -8 m DMD. y axis [m] E 150 100 50 O -50 -100- -150 L -150 -100 wing C 1 -50 podium wing A wing B 1 100 0 x axis [m] Fig. 1 Plan view of the Khalifa Tower foundation system 50 150 200 The ground conditions at the site comprise a horizon- tally stratified subsurface profile which is complex and highly variable in terms of the strata thickness due to the nature of deposition and the prevalent hot arid climatic conditions. The main strata identified were as follows: 1. Very loose to medium dense silty sand (Marine deposits). 2. Weak to moderately weak calcarenite, generally unweathered with fractures close to medium spaced interbedded with cemented sand. This material is generally underlain by very weak to weak sandstone which is generally unweathered with fractures close to medium spaced interbedded with cemented sand. 3. Very weak to weak calcarenite, calcareous sandstone, and sandstone; this formation is slightly to highly weathered with fractures extremely close to closely spaced and interbedded with cemented sand. Bands of 1-5 m thickness are also present of medium dense to very dense, cemented sand with sandstone bands and locally with bands of silt. 4. Very weak to weak gypsiferous sandstone, gypsiferous calcareous sandstone occasionally gypsiferous silt- stone. This material is generally unweathered to slightly weathered with fractures extremely close to closely spaced and interbedded with cemented sand. The formation is interbedded with dense to very dense, cemented silty sand and occasionally silt with sand- stone bands. 5 5. Very weak to weak calcisiltite, conglomeritic calcisil- tite, and calcareous calcisiltite. This material is gen- erally moderately to highly weathered, occasionally slightly and completely weathered with fractures extremely close to medium spaced. Calcareous silt- stone was encountered in the majority of the deeper boreholes comprising very weak to weak occasionally moderately weak calcareous siltstone in bands with a thickness of 0.5-14.4 m generally slightly to moder- ately weathered occasionally highly to extremely weathered. 6. Very weak to weak and occasionally moderately weak calcareous siltstone, calcareous conglomerate, con- glomeritic sandstone, and limestone. This material is Table 1 Summary of measured SPT values Elevation (m) 2.5 to -1 to -8 to 14 -14 to -30 -30 to -40 -40 to 80 1 8 Range of SPT values 0-40 50-400 0-100 40-200 100-200 100-400 Springer 6 generally slightly weathered and occasionally unweathered and moderately weathered to highly weathered. Occasionally encountered as calcisiltite interbedded with bands of siltstone and conglomerate. 7. Very weak to moderately weak claystone interbedded with siltstone. This material is generally slightly weathered with close to medium-spaced fractures. Between 112.2 and -128.2 m occasional bands of up to 500 mm thick gypsum were encountered. Below - 128.2 m the stratum was encountered as weak to moderately weak siltstone with medium to widely spaced fractures. Table 2 summarises the stratigraphy adopted for the foundation settlement analyses. 3.3 In situ and laboratory test results A comprehensive series of in situ tests was carried out, including pressuremeter tests, down-hole seismic, cross- hole seismic, and cross-hole tomography to determine compression (P) and shear (S) wave velocities through the ground profile. The vertical profile of P-wave velocity with depth gave a useful indication of variations in the nature of the strata between the borelogs. Conventional laboratory classification tests (moisture content of soil and rock, Atterberg limits, particle size distribution, and hydrometer) and laboratory tests for determining physical (porosity tests, intact dry density, specific gravity, particle density) and chemical properties were carried out. In addition, unconfined compression tests, point load index tests, and drained direct shear tests were carried out. A considerable amount of more advanced laboratory testing was undertaken, including stress path triaxial tests, resonant column testing for small-strain shear modulus, undrained cyclic triaxial tests, cyclic simple Table 2 Stratigraphic model adopted for settlement assessment Stratum Description 1 2 3a 3b 4 5a 5b 6 7 Marine deposits Calcarenite/calcareous sandstone Calcareous sandstone/sandstone Gypsiferous sandstone Calcisiltite/conglomeritic calcisiltite Calcareous siltstone Calcareous/conglomeritic strata Claystone/siltstone interbedded with gypsum layers Springer 1.15 to 2.96 -0.27 to 1.95 -4.13 to 12.06 -21.54 to -26.69 -27.64 to -31.15 -67.19 to -76.04 -98.19 shear, and constant normal stiffness (CNS) direct shear tests. Some of the relevant findings from the in situ and lab- oratory testing are as follows: Level at the top of the stratum (m DMD) 1. The cemented materials were generally very weak to weak; unconfined compressive strength (UCS) values ranged mostly between about 0.1 and 6 MPa, the average values for each layer being the ones reported in Table 2. 2. 3. 4. Values of the Young's modulus from pressuremeter tests (first and second reload cycle) were found to be in good agreement with values from correlation with shear waves velocities. From calcarenite (0 to -7.5 m) to sandstone (-7.5 to −24 m), Young's modulus is approximately constant with depth; at greater depths, the average values decrease in the gypsiferous sand- stone (-24 to 28.5 m) then they slightly increase in the calcisiltite (from -28.5 to -68.5 m) and finally decrease in the siltstone (from -68.5 to -91 m). Triaxial stress path testing (at strain levels of 0.01 and 0.1%) was found to give results for Young's modulus that were in good agreement with pressuremeter and geophysics testing results. Resonant column testing was found to give lower values for the Young's modulus when compared with values from pressuremeter tests, geophysics tests, and triaxial stress tests. 5. Constant normal stiffness (CNS) tests were carried out on three samples taken from stratum 5a to assess the ultimate skin friction values and the potential for cyclic degradation at the pile-soil interface. These tests indicated values of peak monotonic shear stress ranging from 360 to 558 kPa, with only a little difference between the peak monotonic and the residual cyclic shear stress values. Acta Geotechnica (2013) 8:3–15 Thickness (m) 1.85 to 4.3 2.87 to 10.75 10.5 to 21.43 1.7 to 7.75 39.2 to 46.75 31 (from 140 m deep BH only) Proved to 39.6 m thickness Adopted level at top of layer (m DMD) 2.5 -1.2 -7.3 -13.5 -24 -28.5 -50 -68.5 -90 UCS qu (MPa) 2 1.3 1.7 2.5 T Acta Geotechnica (2013) 8:3-15 3.4 Geotechnical model The key parameters for the assessment of the settlement behaviour of the Khalifa Tower piled raft foundation sys- tem are the values of the Young's modulus of the strata for both raft and pile behaviour under static loading. In a non- linear analysis, the values of ultimate skin friction of piles, the ultimate end-bearing resistance of the piles, and the ultimate bearing capacity of the raft would also be required, but in this paper, only linear elastic analyses have been undertaken using NAPRA and GARP analyses, hav- ing explored the little influence of non-linearity up to the maximum observed load level. Attention has thus been focussed on evaluating relevant values of Young's modulus for each stratum. As a first step in obtaining these values, the relative stiffness of the various soil layers was assessed considering values of the Young's modulus from the following data: 1. pressuremeter tests (initial loading, first reload, second reload cycles); 2. geophysics tests (correlation with shear wave velocities); 3. resonant column tests (Initial, 0.0001, 0.001, 0.01 % strain levels); 4. triaxial stress path tests (0.01 and 0.1 % strain levels); Values of the various Young's modulus values are plotted in Fig. 2, and although inevitable scatter exists among the different values, there is a reasonably consistent general pattern of variation with depth. Layer 3b (see Table 2) has arbitrarily been chosen as the reference layer, and for each type of test, values of the Young's modulus for a layer i, Ei, have been related to the value for layer 3b, E36. The values of E/E36 have then been averaged using the following data: reload cycles from pressuremeter testing; seismic data; resonant column data at a strain level of 0.01 %, and the triaxial stress path tests. Figure 3 shows the different assessed relative stiffness profiles so obtained, and Table 3 summarises the average values of relative Young's modulus that were adopted for the analyses and the interpretation of the pile load test data. The absolute values of Young's modulus for each of the different layers have been then obtained by fitting the load- settlement curves of the single piles obtained from the load tests, and the process of fitting the load-settlement curves to obtain the Young's modulus values is described below. 3.5 Pile load tests A program of pile load testing was undertaken which involved the installation of seven test piles in the podium area near the location of the Khalifa Tower. All the test piles and reaction piles were bored cast in situ and 7 constructed under polymer fluid. A permanent casing, 6 m long, was installed from the top of each pile to just above the highest strain gauge level for all the trial piles tested in compression and tension. Five piles, designated as P1, P2, P3, P4, and P5, were tested in compression; two, P3 and P5, were shaft grouted. Test pile P6 was tested in tension and test pile P7 was laterally loaded. Only the compression load tests on trial piles P1, P2, and P4 have been considered for the present paper. Table 4 summarises the main features of these piles. Figure 4a shows the load test arrangements for piles P1 and P2, which con- sisted of the test pile and six reaction piles, while Fig. 4b shows the set-up for pile P4, which consisted of the test pile and four reaction piles. All the reaction piles were 1.5 m in diameter. Steel load distribution plates were grouted to the top of the test piles and hydraulic jacks were placed between the steel plates and the reaction beams. Steel reaction beams were used to transfer the load from the hydraulic jacks to the installed reaction piles. Macalloy bars were used as reaction anchors to transfer the load from the beams to the reaction piles. Six cycles of loading were applied to trial piles P1 and P2 while nine cycles of loading were applied to trial pile P4, which was the pile designated to be tested cyclically. Four main types of instrumentation were used in the compression test piles: 1. concrete embedment vibrating wire strain gauges, to allow measurement of axial strains at six levels along the pile shafts and hence estimation of the axial load distribution; 2. extensometers, to measure change in length of the piles, and installed at the same levels as the vibrating wire strain gauges to provide back-up information on axial load distribution with depth; 3. displacement transducers at the top of piles, to measure the vertical movement at the pile heads; 4. load cells, to monitor the load applied to the pile via the jacks. 3.6 Back-analysis interpretation of load tests to obtain Young's modulus values The computer program NAPRA was used to carry out the back analyses of compression tests on the three test piles considered. Since a detailed soil profile at each trial pile location was not available, the same geotechnical model was adopted for all three piles. For comparison purposes, the three load tests were back- analysed both taking and not taking into account interac- tion between test piles and reaction piles. It is now well recognised that ignoring interaction between the test pile and the reaction piles can lead to an over-estimation of the pile head stiffness [8, 9, 11, 14]. Springer/n 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11-16, 2008 FOUNDATION DESIGN FOR THE BURJ DUBAI - THE WORLD'S TALLEST BUILDING Harry G. Poulos Coffey Geotechnics Sydney, Australia. ABSTRACT This paper describes the foundation design process adopted for the Burj Dubai, the world's tallest building. The foundation system is a piled raft, founded on deep deposits of carbonate soils and rocks. The paper will outline the geotechnical investigations undertaken, the field and laboratory testing programs, and the design process, and will discuss how various design issues, including cyclic degradation of skin friction due to wind loading, were addressed. The numerical computer analysis that was adopted for the original design together with the check/calibration analyses will be outlined, and then the alternative analysis employed for the peer review process will be described. The paper sets out how the various design issues were addressed, including ultimate capacity, overall stability under wind and seismic loadings, and the settlement and differential settlements. Grahame Bunce Hyder Consulting (UK), Guildford, UK The comprehensive program of pile load testing that was undertaken, which included grouted and non-grouted piles to a maximum load of 64MN, will be presented and “Class A” predictions of the axial load-settlement behaviour will be compared with the measured behavior. The settlements of the towers observed during construction will be compared with those predicted. INTRODUCTION The Burj Dubai project in Dubai comprises the construction of an approximately 160 storey high rise tower, with a podium development around the base of the tower, including a 4-6 storey garage. The client for the project is Emaar, a leading developer based in Dubai. Once completed, the Burj Dubai Tower will be the world's tallest building. It is founded on a 3.7m thick raft supported on bored piles, 1.5 m in diameter, extending approximately 50m below the base of the raft. Figure 1 shows an artist's impression of the completed tower. The site is generally level and site levels are related to Dubai Municipality Datum (DMD). The Architects and Structural Engineers for the project were Skidmore Owings and Merrill LLP (SOM) in Chicago. Hyder Consulting (UK) Ltd (HCL) were appointed geotechnical consultant for the works by Emaar and carried out the design of the foundation system and an independent peer review has been undertaken by Coffey Geosciences (Coffey). This paper describes the foundation design and verification processes, and the results of the pile load testing programs. It also compares the predicted settlements with those measured during construction. Paper No. 1.47 GEOLOGY The geology of the Arabian Gulf area has been substantially influenced by the deposition of marine sediments resulting from a number of changes in sea level during relatively recent geological time. The country is generally relatively low-lying (with the exception of the mountainous regions in the north- east of the country), with near-surface geology dominated by deposits of Quaternary to late Pleistocene age, including mobile Aeolian dune sands, evaporite deposits and marine sands. Dubai is situated towards the eastern edge of the geologically stable Arabian Plate and separated from the unstable Iranian Fold Belt to the north by the Arabian Gulf. The site is therefore considered to be located within a seismically active area. GEOTECHNICAL INVESTIGATION & TESTING PROGRAM The geotechnical investigation was carried out in four phases as follows: Phase 1 (main investigation): 23 boreholes, in situ SPT's, 40 pressuremeter tests in 3 boreholes, installation of 4 standpipe piezometers, laboratory testing, specialist laboratory testing 1 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11-16, 2008 and contamination testing – 1st June to 23rd July 2003; Phase 2 (main investigation): geophysical boreholes with cross-hole and tomography geophysical surveys carried out between 3 new boreholes and 1 existing borehole - 7th to 25th August, 2003; Phase 3: 6 boreholes, in situ SPT's, 20 pressuremeter tests in 2 boreholes, installation of 2 standpipe piezometers and laboratory testing - 16th September to 10th October 2003; Phase 4: 1 borehole, in situ SPT's, cross-hole geophysical testing in 3 boreholes and down-hole geophysical testing in 1 borehole and laboratory testing. The drilling was carried out using cable percussion techniques with follow-on rotary drilling methods to depths between 30m and 140m below ground level. The quality of core recovered in some of the earlier boreholes was somewhat poorer than that recovered in later boreholes, and therefore the defects noted in the earlier rock cores may not have been representative of the actual defects present in the rock mass. Phase 4 of the investigation was targeted to assess the difference in core quality and this indicated that the differences were probably related to the drilling fluid used and the overall quality of drilling. Disturbed and undisturbed samples and split spoon samples were obtained from the boreholes. Undisturbed samples were obtained using double tube core barrels (with Coreliner) and wire line core barrels producing core varying in diameter between 57mm and 108.6mm. Standard Penetration Tests (SPTs) were carried out at various depths in the boreholes and were generally carried out in the overburden soils, in weak rock or soil bands encountered in the rock strata. Pressuremeter testing, using an OYO Elastmeter, was carried out in 5 boreholes between depths of about 4m to 60m below ground level typically below the Tower footprint. The geophysical survey comprised cross-hole seismic survey, cross-hole tomography and down-hole geophysical survey. The main purpose of the geophysical survey was to complement the borehole data and provide a check on the results obtained from borehole drilling, in situ testing and laboratory testing. The cross-hole seismic survey was used to assess compression (P) and shear (S) wave velocities through the ground profile. Cross-hole tomography was used to develop a detailed distribution of P-wave velocity in the form of a vertical seismic profile of P-wave with depth, and highlight any Paper No. 1.47 variations in the nature of the strata between boreholes. Down-hole seismic testing was used to determine shear (S) wave velocities through the ground profile. Fig 1: Impression of Burj Dubai when Complete Laboratory Testing The geotechnical laboratory testing program consisted of two broad classes of test: 1. Conventional tests, including moisture content, Atterberg limits, particle size distribution, specific gravity, unconfined compressive strength, point load index, direct shear tests, and carbonate content tests. 2. Sophisticated tests, including stress path triaxial, resonant column, cyclic undrained triaxial, cyclic simple shear and constant normal stiffness (CNS) direct shear tests. These tests were undertaken by a variety of commercial, research and university laboratories in the UK, Denmark and Australia. 2 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11-16, 2008 GEOTECHNICAL CONDITIONS The ground conditions comprise a horizontally stratified subsurface profile which is complex and highly variable, due to the nature of deposition and the prevalent hot arid climatic conditions. Medium dense to very loose granular silty sands (Marine Deposits) are underlain by successions of very weak to weak sandstone interbedded with very weakly cemented sand, gypsiferous fine grained sandstone/siltstone and weak to moderately weak conglomerate/calcisiltite. Table 1. Summary of Geotechnical Profile and Parameters Strata 1 2 3 4 Paper No. 1.47 Sub- Strata la 1b 2 3a 3b 3c 4 Subsurface Material Medium dense silty Sand Loose to very loose silty Sand Medium dense to very dense Sand/ Silt frequent with sandstone bands Very weak to weak Calcareous Sandstone Very weak to moderately weak -1.20 Calcarenite Very weak to weak Calcareous Sandstone Level at top of stratum (m DMD) Very weak to weak gypsiferous Sandstone/ calcareous Sandstone +2.50 +1.00 -7.30 -13.50 -21.00 -24.00 S (m) Thicknes 1.50 2.20 6.10 6.20 7.50 3.00 Groundwater levels are generally high across the site and excavations were likely to encounter groundwater at approximately +0.0m DMD (approximately 2.5m below ground level). The ground conditions encountered in the investigation were consistent with the available geological information. 4.50 The ground profile and derived geotechnical design parameters assessed from the investigation data are summarized in Table 1. UCS (MPa) 2.0 1.0 1.0 2.0 Undrained Drained Modulus* Modulus* Eu (MPa) E' (MPa) 34.5 11.5 500 50 250 140 140 30 10 400 40 200 110 110 Ult. Comp. Shaft Frictio n (kPa) 350 250 250 250 250 3 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11-16, 2008 5 6 7 5a 5b 6 Paper No. 1.47 Very weak to moderately weak Calcisiltite/ 7 Conglomeritic Calcisiltite Very weak to moderately weak Calcisiltite/ Conglomeritic Calcisiltite Very weak to weak Calcareous/ Conglomerate strata -28.50 -50.00 -68.50 -91.00 Stiffness values from the pressuremeter reload cycle, the specialist tests and the geophysics are presented in Figure 2. There is a fair correlation between the estimated stiffness profiles from the pressuremeter and the specialist testing results at small strain levels. Non-linear stress-strain responses were derived for each strata type using the results from the SPT's, the pressuremeter, the geophysics and the standard and specialist laboratory testing. Best estimate and maximum design curves were generated and the best estimate curves are presented in Figure 3. Weak to moderately weak Claystone/ Siltstone interbedded with gypsum layers * Note that the Eu and E' values relate to the relatively large strain levels in the strata. An assessment of the potential for degradation of the stiffness of the strata under cyclic loading was carried out through a review of the CNS and cyclic triaxial specialist test results, and also using the computer program SHAKE91 (Idriss and Sun, 1992) for potential degradation under earthquake loading. The results indicated that there was a potential for degradation of the mass stiffness of the materials but limited potential for degradation of the pile-soil interface. An allowance for degradation of the mass stiffness of the materials has been incorporated in the derivation of the non-linear curves in Figure 3. 21.50 18.50 22.50 1.3 >46.79 1.7 2.5 1.7 0.00 -10.00 -20.00 -30.00 -40.00 -50.00 -60.00 -70.00 -80 00 -90.00 ON. 1.xx XX 0 310 405 560 405 5000.0 ** 10000.0 O 250 325 450 325 E Value (MPa) 15000.0 20000.0 Strata 3a (Sand) 285 Strata 4 (Gypsiferous Sandstone) Strata 2 (Calcarenite) Strata 5a (Calcisiltite/ conglomeritic Calcisiltite) 325 400 Strata 3b (Sandstone) Strata 3c (Sand) Strata 5b (Calcisiltite/ conglomeritic Calcisiltite) 325 25000.0 -Strata 1 (Sand)] Fig 2: Modulus Values vs Elevatio ◆ Borehole 2 - Pressuremeter Reload 1 Borehole 2- Pressuremeter Reload 2 ◆ Borehole 3- Pressuremeter Reload 1 Borehole 3- Pressuremeter Reload 2 Borehole 4- Pressuremeter Reload 1 Borehole 4- Pressuremeter Reload 2 Borehole 25- Pressuremeter Reload 1 Borehole 25- Pressuremeter Reload 2 ◆ Borehole 28 - Pressuremeter Reload 1 Borehole 28- Pressuremeter Reload 2 * Stress Path Txl at 0.01% strain * Stress Path Txl at 0.1% strain Resonant Column at 0.0001% strain A Resonant Column at 0.001% strain O Resonant Column at 0.01% strain Geophysics E Values (lowest) Adopted Small Strain Design Values 4 3.5 3 2.5 2 1.5 1 0.5 0 0 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11-16, 2008 0.02 Strata 2 Proposed Nonlinear Ground Strata Characteristics 0.04 Strata 3a 0.06 Strata 3b 0.08 Paper No. 1.47 Strain Strata 3c & 4 0.1 0.12 Strata 5a, 5b, 6, Fig 3: Non-linear Stress-strain Curves 0.14 GEOTECHNICAL MODELS AND ANALYSES A number of analyses were used to assess the response of the foundation for the Burj Dubai Tower and Podium. The main design model was developed using a Finite Element (FE) program ABAQUS run by a specialist company KW Ltd, based in the UK. Other models were developed to validate and correlate the results from the ABAQUS model using software programs comprising REPUTE (Geocentrix, 2002), PIGLET (Randolph, 1996) and VDISP (OASYS Geo, 2001). The ABAQUS model comprised a detailed foundation mesh of 500m by 500m by 90m deep. The complete model incorporated a 'far field' coarse mesh of 1500m by 1500m by 300m deep. A summary of the model set up is as follows: Soil Strata: Modeled as Von Mises material (pressure independent), based on non-linear stress-strain curves Tower Piles: Modeled as beam elements connected to the soil strata by pile-soil interaction elements. Class A load-settlement predictions were used to calibrate the elements; Podium Piles: Beam elements fully bonded to the soil strata; Tower and Podium Loadings: Applied as concentrated loadings at the column locations; Tower raft submerged weight: Applied as a uniformly distributed load; Tower Shearing Action: Applied as a body load to the tower raft elements, in a direction to coincide with the appropriate wind action assumed; Building Stiffness Effect: Superstructure shear walls (not interrupted at door openings) were modeled as a series of beam elements overlaid on the tower raft elements. The moment of inertia was modified to simulate the stiffening effect of the tower, as specified by SOM. FOUNDATION DESIGN An assessment of the foundations for the structure was carried out and it was clear that piled foundations would be appropriate for both the Tower and Podium construction. An initial assessment of the pile capacity was carried out using the following design recommendations given by Horvath and Kenney (1979), as presented by Burland and Mitchell (1989): 0.5 Ultimate unit shaft resistance f = 0.25 (qu) where f, is in kPa, and qu= uniaxial compressive strength in MN/m² The adopted ultimate compressive unit shaft friction values for the various site rock strata are tabulated in Table 1. The ultimate unit pile skin friction of a pile loaded in tension was taken as half the ultimate unit shaft resistance of a pile loaded in compression. The assessed pile capacities were provided to SOM and they then supplied details on the layout, number and diameter of the piles. Tower piles were 1.5m diameter and 47.45m long with the tower raft founded at -7.55mDMD. The podium piles were 0.9m diameter and 30m long with the podium raft being founded at -4.85mDMD. The thickness of the raft was 3.7m. Loading was provided by SOM and comprised 8 load cases including four load cases for wind and three for seismic conditions. The initial ABAQUS runs indicated that the strains in the strata were within the initial small strain region of the non- linear stress strain curves developed for the materials. The secant elastic modulus values at small strain levels were therefore adopted for the validation and sensitivity analyses carried out using PIGLET and REPUTE. A non-linear analysis was carried out in VDISP using the non-linear stress strain curves developed for the materials. Linear and non-linear analyses were carried out to obtain predictions for the load distribution in the piles and for the settlement of the raft and podium. The settlements from the FE analysis model and from VDISP have been converted from those for a flexible pile cap to those for a rigid pile cap for comparison with the REPUTE and PIGLET models using the following general equation and are shown in Table 2: Srigid = 1/2 (8 centre + 8 edge) flexible 5/n