Foundation Engineering: Burj Dubai vs Emirates Twin Towers

Introduction

Two landmark structures that have redefined the Dubai skyline in recent years are the Burj Dubai, reputedly the world's tallest building upon completion, and the Emirates Twin Towers, consisting of a hotel and office complex. Although located in geographically similar areas, the foundation engineering for the two projects involved different testing protocols and regimens as well as different substrata considerations that required different solutions. This study examines these similarities and differences and presents a review of the relevant peer-reviewed, governmental, and scholarly literature concerning the foundation projects for both major buildings. A summary of the research and important findings is presented in the concluding section.

Background and Overview of Dubai's Construction Boom

Driven by its proven petrochemical reserves, the United Arab Emirates (UAE) emerged in the early 21st century as a burgeoning economic powerhouse in the Arabian Gulf region. Tangible evidence of this growth is the remarkable speed with which numerous major building projects have sprung up in Dubai, where the world's tallest building, the Burj Dubai, joins the Emirates Twin Towers and other world-class structures in redefining the city's skyline and international image. In the construction industry, it is axiomatic that a building is only as good as its foundation, and this principle was taken seriously during the design and development phases of both projects, as described below.

Burj Dubai Foundation Design and Testing

As the tallest building in the world, the Burj Dubai's spiral form is created by three wings arranged around a central core, each of which buttresses the others (Dubai Spire 2008, p. 81). Supporting the massive central core and buttresses is a complex foundation consisting of various components, including a podium, a raft, and bored piles that operate together. According to the architectural engineers tasked with designing the foundation:

"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 foundation system is a piled raft, founded on deep deposits of carbonate soils and rocks. It is founded on a 3.7-meter thick raft supported on bored piles, 1.5 meters in diameter, extending approximately 50 meters below the base of the raft" (Poulos & Bunce 2008, p. 1).

Although official statements about the project remain confidential regarding certain aspects of the building's design, one governmental source describes it as "a city within a city" inspired by "the cultural and historical influences of the GCC [Gulf Cooperative Council]," an influence achieved in the tower's base where "the geometry reflects the six-petal desert flower of the region" (Elsheshtawy 2004, p. 183).

The testing protocols and regimens used for the Burj Dubai foundation are comparable in some ways to those employed at the Emirates Twin Towers site, but differ with respect to the unique siting considerations involved. With regard to the general region, Arabian Gulf geology has been heavily influenced by marine sediment depositions resulting from a series of sea-level shifts during relatively recent geological periods (Poulos & Bunce 2008). The UAE is generally a flat country with few elevations outside the mountainous regions in the northeast, and the Burj Dubai site is located in a relatively low-lying area. The near-surface geology of this region is characterized by heavy deposits from the Quaternary to late Pleistocene ages, including marine sands, mobile Aeolian dune sands, and evaporite deposits (Poulos & Bunce 2008).

Dubai is located near the eastern edge of the geologically stable Arabian Plate but is separated from the unstable Iranian Fold Belt situated north of the Arabian Gulf, placing the Burj Dubai site within a potentially active seismic zone (Poulos & Bunce 2008). Nevertheless, the overall stability assessment was favorable, and extensive testing regimens were followed to identify potential weaknesses and areas requiring special attention. Based on these analyses, engineers placed piles for the tower foundation with a minimum center-to-center spacing of 2.5 times the pile diameter. Tests were then conducted to assess vertical and lateral stability, treating the foundation as a block consisting of the piles and surrounding soil or rock (Poulos & Bunce 2008). According to the project's engineers, "A factor of safety of just less than 2 was assessed for vertical block movement, excluding base resistance of the block, while a factor of safety of greater than 2 was determined for lateral block movement, excluding passive resistance. A factor of safety of approximately 5 was obtained against overturning of the block" (Poulos & Bunce 2008, p. 3).

Given the potential for seismic activity, engineers for the Burj Dubai also conducted tests to determine the likelihood of liquefaction during a seismic event. Testing confirmed that marine-source depositions and sand extended to 3.5 meters below the site's ground level and possessed the potential to liquefy during a seismic event; however, the foundations of the podium and tower structures extend below this level and are not regarded as being at risk (Poulos & Bunce 2008). Because much of the building's infrastructure extends below ground, engineers also considered these factors during the design phase for buried services and the shallow foundation components located within the top 3.5 meters, and appropriate testing was conducted to assess the potential impact of liquefaction (Poulos & Bunce 2008).

The results of these testing protocols confirmed that occasional layers held the potential to liquefy; however, engineers regarded these potentials as well within acceptable limits, particularly given the other foundation elements involved. Poulos and Bunce report that "taking into account the imposed confining stresses at the foundation level of the Tower, this was considered to have a negligible effect on the design of the Tower foundations. The assessed reduction factor to be applied to the soil strength parameters, in most cases, was found to be equal to 1.0 and hence liquefaction would have a minimal effect upon the design of the Podium foundations" (2008, p. 4). Based on computer modeling of the site, the foundation project engineers identified the settlement contours for the tower and podium.

Ongoing analysis of tower raft settlement has been underway since concreting was completed. Stress conditions in the raft have been identified through the placement of strain rosettes at the top and bottom of the raft (Poulos & Bunce 2008). In addition, three pressure cells have been mounted at the bottom of the raft, and five piles have been evaluated for strain to identify the precise load distribution. At the latest monitoring point, it was projected that approximately three-quarters of the dead load would be pressing on the building's foundation; these estimates do not include the effect of the raft, cladding, or live loading, which together account for approximately 20% of the building's total mass (Poulos & Bunce 2006). The projected maximum settlement of the tower foundation has been calculated using a variety of analytical tools, and all projected settlements have fallen within an acceptable range (Poulos & Bunce 2008).

Conclusion

Both the Burj Dubai and the Emirates Twin Towers employ piled raft foundation systems on deep carbonate or calcareous soil deposits, reflecting the shared geological character of the Arabian Gulf region. Their differences, however, are equally instructive. The Burj Dubai required extensive liquefaction risk assessment and block-stability analysis appropriate to a single supertall structure, while the Emirates Twin Towers project demanded evaluation of a broad matrix of load combinations and benefited from exceptionally large-scale pile load testing. In both cases, the foundation engineering teams drew on rigorous site investigation, advanced computer modeling, and comprehensive testing programs to deliver solutions capable of supporting world-class structures in a geologically complex environment. The research underscores the principle that foundation design must be responsive to the specific demands of each project, even when neighboring sites share broadly similar subsoil conditions.