The geology of Dubai which is mostly desert have created difficulties and challenges throughout Dubai’s transformation from a desert to high-rise skyscrapers. An example of skyscraper in Dubai is the Burj Khalifa also the tallest building in the world at height of 2,717 ft. This research paper will be about desert geology of Dubai and how civil engineers around the world have overcome the challenges to construct the Burj Khalifa.
Dubai is in the eastern geologically stable Arabian plate, thus situated away from seismically active area in Iranian Gulf. If analyzing Dubai from a geotechnical aspect, Dubai shows a distribution of various environment. Due to nature of deposition and hot climate, Dubai’s desert ground contains layered subsurface of various soil that are complex. Subsurface soils range from, medium dense to loosen coarse silty sands that are covered by series of very weak sandstone embedded with very weakly cemented sand. Beneath the cemented sand lays gypsum fine-grained siltstone and moderately weak conglomerate/calcilutite (Hanry G, 22). Although Dubai is a hot and relatively dry place, it hasn’t been always like this before. Two million years ago Dubai was a wet place. The evidence can be spotted 80 kilometers to the east of Dubai in Hajar Mountains. The valleys in Hajar Mountain give evidence that once water had carved it through erosion. The water has even been flowing in flood plains in the areas where the skyscrapers are now built. Gradually as the temperature rose and the rivers became dry. Even after desert started covering the area, not all waters disappeared. To this day waters that didn’t vanish, instead they were infiltrated underground. This information is important because it explains Dubai’s bedrock origin.
The origin of Dubai’s bedrock started 95 million years ago at Hajar Mountains which once were part of the ocean floor. Hajar Mountains that are 80 Kilometers away from Dubai were formed as a result of clash of two continental plates 30 million years ago. These mountains were not formed throughout the whole Dubai, instead heavy rains ripped off the rocks and deposited them to Dubai by rivers and creating a layer of rubble. The rubbles are made up of rounded rocks (gravel) and carbonate minerals deposited between the gravel voids. These carbonates then cemented the rubbles into bedrocks (Hanry G, 22).
Sandy material covers approximately 15-meter depth of Dubai’s subsurface (Harry G). For the reason being that sand is the predominant subsurface soil in Dubai’s ground, it is essential to uncover the mechanical behavior of sands to load.
Degradation and Settlement in Sands
Although sands relatively follow elastic deformation, engineers are more concerned about inelastic settlement part of sands. The cause of inelastic settlement in soils is sand degradation that has reshaped and broken soil grains due to large overhead loads. Voids exist between sand grains. As result of the large loads, grains get crushed and small enough to fit into these voids. When the voids are filled, total mass of sand experiences decrease in volume. Initially sand degradation can be a slow process, but with every sand that gets chipped the load increases on other larger grains and process of chipping speed up. If the load were to be hauled, there won’t be any rebound because sand has been deformed inelastically. The second cause of the settlement is due distortion settlement (larger-area settlements). This settlement is due to side movement of sand under load. When a structure is placed on sand, sands that is under edges of the building experience less confining pressure and thus start moving to side where there is less pressure. This is a big concern for building that are several hundred feet tall as it can lead to cracks in foundation (Samuel,10-50).
Shear Strength of Sands
The main controlling factor of shear strength in sand is its density. The denser the sand the more strength sand gains. Degradation of sand eventually becomes useful improving factor for sands strengths. Degradation helps void spaces to be filled with crushed grains and as a result increase the density. The footing of building itself can be one of other improving factor for sands strength. Alone, great pressure from footing itself can increase strength of underlying sands. Increase in sand’s strength due to pressure increase is true as sand’s strength is directly proportional to the amount of overload pressure. It should be also noted that sand becomes more viscous when it’s saturated with water, thus sand can lose its shear strength by half. As in Dubai, there are many waterbeds under bedrock that can create potential risk for overhead sands to get submerged (Samuel,10-50).
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Foundation Design of the Burj Khalifa
In high rise building such as the Burj Khalifa special type of foundation needs to be designed to withstand the structure against external forces and soft sand. Unlike deep foundation, shallow foundation can’t resist, inclined, lateral, or uplift loads. This type of foundation is called ‘deep foundation’. The most common type of the deep foundation is pile foundation. Pile foundations passes the load from the structure into layers of soil and rock that are at considerable depth. Both soil and rock behave as preventing force due to created friction that act in opposite direction of structural load. This preventing force will hold the foundation and structure fixed. Pile foundations are most desirable when the soil near the surface has low bearing capacity (the capacity of the soil to resist load applied from the foundation). The tasks of piles are:
- Transmit majority of the load from structure along its length to more resistant soil or bed rock.
- Reduce soil settlement (vertical movement of ground by compressive force of structure).
- Prevent a sink to form near site where the ground water table is high. Excavation can weaken strength of soil.
- Increases density of the soil by compressing.
The concrete that is used both in structure and foundation are high performance concretes. These high-performance concretes were only produced for construction of Burch Khalifa. There are 25 different ingredients mixed to create the high-performance concrete that stays solid as its pumped and solidifies very fast. Concrete’s young modules of 43800N/mm2 allows a strength of C80 concrete (Wilson). The 192 piles which were drilled 50 meters deep and 3 meters apart are Reinforced Concrete Piles. These 1.5-meter diameter piles are made from concrete C80 and have a support consisting of a steel cage made up of long bars and steel in the form of individual rings. In each pile was poured with 12,500 cubic meters of concrete (Wilson).
The foundation design of the Burj Khalifa is unlike any other foundations in the world including skyscrapers in New York. This is mainly because of type of soil and material under the foundation. In most skyscrapers in New York a strong ground can be obtained only by drilling 4 or 5 meters to sit your foundation on. In Dubai at least 3 to 4 meters is sand followed by 2 meters of weak sandstones and weak conglomerate. To overcome all these obstacles and design a foundation that the entire weight of the building can be placed on safely, engineers chose a design approach that is based on principle of friction. The way this principle works is the surface of the 192 piles will create friction against soil. Although a bedrock does exist deep down in Dubai’s ground, engineers found friction principle a better option (Hanry G, 22-36).
As explained before, the two methods of piling are: a) putting the piles directly onto bedrock, b) relying on friction force of each pile against soil. Beneath the tower, engineers placed 192 piles to a depth of 50 meters to support a 3.7-meter tick raft of concrete which weighs 500,000 tons. During construction period the whole foundation load is applied halfway through the construction period. For big constructions, this process may take many years. Since engineers were dealing with large amount of structure load on soft sand, engineers let foundation settle down before proceeding further. The total live and dead load were so huge that foundation had settled about 75mm (Wilson).
Overall Body Design
The structural design system used in the Burj Khalifa is called buttress core. The core is the hexagon in the middle of the building. This hexagon core functions to resist the twisting force of the building, but it is still very slender to support such intended height of the Burj Khalifa. To support the rest of structure load buttresses were designed. Buttress in the Burj Khalifa are the three wings that start extending from the core and stretch away from the core. These three wings which has a ‘Y’ shape act as stands for the core and structure. Hexagonal core together with three buttresses rise to 10 story creating a tower base to support enormous weight of the steel and concrete. As the building rise, the buttresses also rise in the shape of Y. To further protect the strength of building, the building gets coated with glistening mirror glass against sun heat corrosion (Wilson). Maximum and minimum pile analysis showed that the maximum loads were more concentrated on the corners of three Y wings with magnitude of 35 MN. Minimum loads (12-13 MN) are more located in center.
Conclusion
Earth might has set a limit on how much civil engineers can accomplish on this planet, but it has not limited them to attain that limit. This was proven once more when the ambitious skyscraper Burj Khalifa make its way out from one hardest geological condition. This is not just a skyscraper; it is the tallest and grandiose of its kind. Ground is one of the leading factors for a building’s stability. Constructing the Burj Khalifa was only possible, when geologist took the first step and studied the ground.
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