Foundation Comparison: Burj Khalifa vs. Taipei 101
This report aims at assessing the foundation of the world's tallest and second tallest skyscrapers; the Burj Khalifa Tower in Dubai and Taipei 101 in Taipei, Taiwan. Currently, Burj Khalifa is the world's tallest building taking over the title from Taipei 101 topping out at a height of 828 metres. Khalifa is built on silty sand and sandstones while Taipei 101, with a height of 509 metres stands on two tectonic faults with silty sand and clay soils. Both Burj Khalifa and Taipei 101 are built on deep type foundations but with almost-similar soil conditions. Due to the weak and collapse-prone soils these building are built on, various site investigation techniques were initiated prior to the construction to determine each site's soil conditions and build foundation likely to hold the structures firmly. In this regard, this report highlights the rationale behind these foundations and gives a side-by-side comparison of the two foundations.
Taipei 101 in Taipei, Taiwan
Located in the Hsinyi district of Taipei, Taiwan, this skyscraper clinched the world's tallest building title in 2003 from PETRONAS Towers. The structure achieves both strength and flexibility due to the use of high-performance steel construction. Located in an area with constant earthquakes and other natural disasters, the structure had to be built upon a strong foundation. Thus, it is the perfect example that a strong foundation and advanced technology can make a building resistant to typhoons and earthquakes.
Taipei is a coastal city with weak soil conditions thus; structures built in the city tend to sink. The area has alternating silty clay and silty sand deposits. In addition, the alluvium layer is present at an average depth of 45.5 meters while the groundwater table fluctuates at an elevation of 2 meters below the ground surface (C.Y Lee & Partners, 2004).
Moreover, the area is situated on top of two tectonic fault lines and experiences typhoons every summer, with winds reaching 201 kilometres per hour (Binder, 2008). Besides, the large potential earthquakes generate shear forces which may tear buildings apart. Dealing with these natural phenomena required architectural designs that could withstand the impacts associated with them.
The design of the structure's foundation was initiated following numerous full-scale pile trial installations as well as comprehensive instrumented pile load tests. In addition, t-z curve for each sub-surface stratum was assessed and the result used in predicting pile load-settlement behaviour for the specific soil stratification of each pile; pile length was estimated based on expected loads during service.
In this regard, several pile loading tests were conducted in addition to drilling more than 128 boreholes for sampling. Moreover, prior to the foundation's construction, numerous high quality laboratory and field tests were conducted to determine the physical and mechanical properties of soil strata (Lin & Woo, 2007). Besides, sophisticated instrumentation systems were used in monitoring the ground responses and structural performances during the excavation process.
The Foundation Design
To curb potential collapse and damage of the building, the foundation was reinforced by 380 piles driven 80 meters into the ground, extending as far as 30 meters into the bedrock. Each pile is 1.5 meters in diameter and can bear a load of approximately 1,000 metric tons to 1,320 metric tons. In designing the foundation, the piles were topped by a 3 meters thick foundation slab at the edges and 5 meters thick under the largest of columns.
As a precaution, pile group effects such as bearing capacity reduction and settlement increase, were evaluated during the foundation design. In the same line, possible creep behaviours of piles embedded into the bedrock were analysed using results from the pile load tests while the basement, mat, piles as well as retaining diaphragm walls were modelled into one integral system for structural design of the foundation (Yu, 2011).
Furthermore, for piles supporting the main tower, measures of bottom cleaning and post-grouting were used to increase the pile bottom sediments which improving end bearing capacity. As a result, both conventional static and STATNAMIC dynamic loading tests were carried out to validate the bearing capacities as well as behaviours of production piles. Finally, the results from the proof load tests achieved the design requirements well as compared with the simulation using pile ultimate load test results.
The Burj Khalifa Towers, Dubai
The world's tallest building was unveiled in 2010 comprising of over 160 floors with a podium around the base; a representation of technology at its best following a combination of various technological and structural innovations (Bianchi & Critchlow, 2010). Following its completion, the superstructure is an illustration of an efficient and robust structure ever witnessed in the history of modern history of skyscrapers. The tower is built on 3.7 meters thick raft supported by bored piles of 1.5 meters diameter extending to about 50 meters below the base of the raft.
The site where the tower is situated is constantly changed by the deposition of marine sediments bringing about unprecedented fluctuation in the area's sea level during recent geological times. Moreover, Dubai is a low-lying state located with its near-surface geology dominated by deposits of Quaternary to late Pleistocene age, including mobile Aeolian dune sands, evaporite deposits and marine sands. In addition, the soil and rock conditions in the site are relatively loose to medium dense sands overlying weak to very weak sandstone and siltstone with interbeds of gypsiferous and carbonate cemented layers which are also very weak.
Furthermore, Dubai is on 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. This required the construction of a structure capable of withstanding the constant tremors within the area.
Prior to the construction of the foundation, several geotechnical assessments comprising of 33 boreholes were drilled utilizing several techniques. Additionally, SPT sampling and double-tube rock coring as well as 60 pressure-meter tests were performed in addition to cross-hole seismic surveys for both P. And S-wave (Bunce & Poulos, 2008). The SPT sampling were conducted in the overburden soils, in weak rock as well as soil bands encountered in the rock strata. In line with this, static load testing was conducted on 7 test piles before the construction and, 8 production piles were as well tested. The other significant test was a single lateral load test that was performed.
The Foundation Design
The Tower's foundation is made up of a pile supported raft having a solid reinforced concrete raft of 3.7 meters which was poured using C50 grade self-consolidating concrete. The foundation's raft was constructed in four separate pours; three wings and the centre core with each raft pour conducted for over a 24-hour period. Additionally, reinforcement was typically at 300mm spacing in the raft, and arranged in a manner that every 10lh bar in each direction was omitted; this resulted in a series of pour enhancement strips throughout the raft at which 600 mm x 600 mm openings at regular intervals facilitated access and concrete placement as shown below.
Moreover, the Burj Tower's raft is supported by 194 bored cast-in-place piles. The piles each have diameters of 1.5 meter and approximately 43 meters long with a design capacity of 3,000 tons each. In this regard, the tower's pile load test supported more than 6,000 tons and the C60 grade self-consolidating concrete was placed by the tremie method utilizing polymer slurry. The skyscraper's friction piles are supported in the naturally cemented calcisiltite conglomeritic calcisiltite fomiations developing an ultimate pile skin friction of 250 to 350 kPa (Subramanian, 2010). During the construction, the rebar cage was placed in the piles with attention paid to orient the rebar cage in a way the raft bottom rebar could be threaded through the numerous pile rebar cages without interruption, which greatly simplified the raft construction.
The building's foundation was designed in a Y-shape to reduce the impact of wind forces present in the Arabian Gulf and to make the structure simple and easier to construct. The "buttressed" foundation ensures each wing's perimeter columns support the other via a six-sided central core; the result was a stiff building both laterally and vertically (Lee, Kuchma, Baker, & Novak, 2008). The central core gives the structure the required torsional resistance to the corridor walls extending from the core at the end of each wing.
During the construction of Burj Khalifa, the engineers encountered several challenges which they addressed to minimize collapse and damage of the structure. The first problem was the presence of chlorinated groundwater in the proposed site. This was solved by ensuring the piles' concrete mix was a 60 MPa based on triple blend with 25% fly ash, 7% silica fume, and water-to-cement ratio of 0.32 (Baker & Pawlikowski, 2009). Besides, there was installation of waterproofing systems as well as cathodic protection system utilizing titanium mesh to minimize any detrimental effects form corrosive chemicals in…