When I first visited Dubai in 1994 to assist with the Burj Al Arab hotel project (then referred to as the Chicago Beach Hotel), the tallest building was the World Trade Centre at 184m. The Burj Al Arab hotel took over that mantle, at a little over 300m, but soon after, the taller of the Emirates twin Towers, at 355m, had overtaken the iconic hotel structure.
In 2003, ground investigation work on the Burj Dubai commenced, and by mid-2008, it had become the world’s tallest structure, well over 680m and on its way to a final height anticipated to be in excess of 800m.
In October the Nakheel Tall Tower project was made public, and included a tower that would be in excess of 1000m tall. Even while this was being announced, rumours were emerging that a building of 2400m in height was being contemplated.
The rapid increase in building height has increased the demands on foundation design, and has led to a considerable improvement in both design and construction techniques for bored piles.
As a result of a substantial amount of laboratory research, and full-scale load testing of piles in Dubai, the design values of skin friction and end bearing that have been adopted in Dubai have increased steadily, with a consequent increase in the allowable load capacity and computed stiffness.
Measurements of the settlement of tall buildings during and after construction have demonstrated that these increased pile capacities and stiffness values are justified.
The use of polymer drilling fluids, in contrast to water or bentonite as used previously, has also contributed to improved bored-pile performance. In addition, modern techniques for construction control and testing have become routine.
Overall, it may be stated with considerable confidence that the development of tall towers in Dubai and the consequent demands on foundation designers have led to modern methods of design, construction and testing being adopted at a rate which far exceeds that in almost any other part of the world.
The use of advanced numerical techniques of foundation analysis, employing complex finite element and boundary element methods, considered to be well beyond the state of practice just a few years ago, has now become reasonably routine for tall building projects in Dubai.
Given the remarkable rate of acceptance and adoption of innovative methods in pile design and construction in Dubai, it may be prudent to draw breath and ask the following question: what are the structural and geotechnical limitations to even taller buildings than those presently under consideration?
In partial response to this question, the following answers may be offered:
1. A major constraint to the increased height of super-tall buildings is the strength of the materials from which the superstructure will be constructed. The current upper limit of concrete strengths of 110 to 120 MPa will need to be stretched considerably further, or else a material other than conventional concrete may need to be developed.
2. The dynamic response of super-tall structures under wind and earthquake loadings may reveal unexpected complexities.
While the natural period of super-tall structures is very high, perhaps in excess of 20 seconds, higher modes of vibration may be excited at much lower periods, and so a much more complete series of dynamic analyses must be made to try and ensure that the periods of higher modes of vibration do not coincide with the dominant periods of the dynamic wind and earthquake forces.
3. The demands on the foundation system can become extremely significant. The ground conditions in Dubai involve cemented calcareous sands and rocks that are weak, typically with unconfined compressive strengths less than 5 MPa. In such foundation materials, highly-loaded foundations can stress the ground sufficiently to break the cementation bonds and to, in effect, reduce it to a much more compressible uncemented sand.
If this occurs, the settlements that the foundation system experiences may become extremely large and exceed those acceptable for proper functioning of a building. In such cases, methods will need to be developed to either spread the loads further or else to increase the strength of the soils supporting the piles by some form of improvement process, perhaps involving a process of artificially-induced cementation.
4. Even if the ground can be strengthened adequately, a further constraint is the strength of the foundation materials. Even with the 1000+m Nakheel tower, one of the major foundation constraints is the strength of the concrete from which the foundation barrettes are being constructed.
As with the superstructure, stronger foundation material may be called for, and it may be necessary to re-think the possibility of using steel or another form of metal for the key foundation elements, rather than concrete.
As in the past, engineers will be forced to meet the challenges arising from man’s desire to push the boundaries of human experience and technological development. A combination of computational ability, performance measurement and further geotechnical and structural research will be critical factors in the ability of engineers to answer the challenges posed by the design and construction of super-tall buildings.
In the end, the main inhibitor to ever-increasing building height may not be related to engineering, but to the human condition. People may well increasingly resist the psychological and physical challenges that accompany living so far above the earth’s surface, and having to cope with such fragile systems of access and escape.
Dr Harry Poulos is a senior principal of Coffey Geotechnics.
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