A Record-Breaking Journey
On 6th January 2004, Dubai’s sandy soil was dug up. The goal was to build the tallest building in the world. By the end of 2004, the world’s tallest building was Taipei 101. It was located in Taiwan and stood a little over 500 meters tall.
But Dubai had bigger plans. They wanted a building so tall that it would not just be the tallest but also 62% taller than the second tallest. This massive difference would discourage anyone from trying to break its record.
Before Burj Khalifa, the world’s tallest buildings were only 5%–10% taller than their predecessors. At most, they were 19% taller. But this time, they aimed for 829 meters—62% taller than the last record.
Construction took 5.5 years. Finally, on 1st October 2009, the Burj was completed. The world was left astounded.
Resisting the Sandstorm
The Magnificent Burj Khalifa is resisting a strong sandstorm of 100 kilometers per hour.
The Deep and Strong Foundation
A quick tour below the beautiful petals leads to its foundation. This structure has the whopping depth of a 10-story building.
Surprisingly, the foundation of the Burj Khalifa must carry electricity 24/7. Any small issue with the flow of electricity will weaken its foundation. The result could be catastrophic on a heavy, windy day.
The Raft Foundation
Interestingly, the raft of the foundation is as thick as two human beings. Even though the raft looks like a simple structure, constructing it was a massive task. This is mainly because a huge amount of concrete had to be poured in a single volume.
The construction of this raft started with the placement of steel rebars. The next step was pouring in the concrete, which was no easy feat in the 40-degree heat in Dubai. That’s why the engineers executed this work during the nighttime. They also mixed the concrete with ice cubes while pouring it.
The process of concreting the entire raft was done in four separate parts, each lasting 24 hours.
Soil Settlement Challenge
There were many more challenges in front of Burj Khalifa’s chief design engineer, Mr. Bill Baker. First, let’s solve the biggest challenge: soil settlement. This is what would happen if the design calculations for settlement went wrong.
Understanding Soil Settlement
In normal building construction, engineers always find something called hard strata—a hard soil where the building can rest. During the construction phase, the weight of a building increases, and it’s normal for it to settle by a few inches.
Let’s observe the building settlement one more time. During this process, the soil below the foundation gets compressed and settles. Now, the soil can produce a proper reaction force and balance the building’s weight.
However, this settlement should be within a safe limit.
The Weak Soil of Dubai
Now, let’s take a cross-section of Dubai’s soil. It’s just loose sand and weak sedimentary rock. Even after digging 140 meters deep, the engineers failed to find strong, hard strata.
If the Burj Khalifa engineers had constructed a normal raft foundation on this site, it would have settled too much. A catastrophe would have inevitably happened.
The Piled Raft Solution
The chief engineer, Bill Baker, came up with a simple solution for this massive issue: the frictional force of the surrounding soil. Here, Mr. Bill Baker is trying to pierce through the sand using a sharp and thin rod. It’s a common observation that after a certain distance, the rod won’t go down. This is due to the increased frictional force provided by the surrounding sand as the rod goes down.
To generate this frictional force, he added a number of piles below the raft foundation. The depth of these RCC piles is equal to 10 floors of the Burj Khalifa. Now, immerse the foundation inside the soil and take a cross-section of it.
Testing the Foundation
Let’s test this foundation. These piles generate frictional force against the weight of the building. With the help of soil reaction force and additional frictional force, the raft pile foundation reaches settlement much earlier and within safe limits.
When the Burj Khalifa’s construction was complete, it had a settlement of just around five centimeters, which is quite safe.
Constructing the Piles
The next big challenge was constructing these piles with perfection and making the design a reality. For the construction of the piles, they first started with drilling a hole using an auger excavator.
The blades of this device perfectly remove the soil. However, Mr. Baker faced an issue—the groundwater of Dubai.
Preventing Soil Collapse
Due to the heavy machinery around, the borehole would collapse and fill in with salty groundwater. The solution was clear. As the soil was excavated, they simultaneously poured a drilling fluid through the auger shaft.
This created a slurry. The slurry, being denser than water, exerted hydrostatic pressure on the walls of the borehole. This prevented soil collapse.
Reinforcement and Concreting
Once the borehole was ready, the workers placed a temporary hollow steel cylinder to hold the soil intact for concreting. Then, they placed steel reinforcement bars welded into a long cylinder. In normal concreting, laborers must use concrete vibrators to make the concrete compact.
Using these machines was impossible in such deep boreholes. This is why SCC C60, a special kind of concrete that flows like a liquid, was used for the piles. The concrete was poured with the help of a tremie pipe. The construction of the foundation alone took two years.
The entire sequence of foundation construction for the Burj Khalifa is illustrated here .Now, we have achieved a good foundation design to hold up the tallest building in the world in loose Dubai soil.
Resisting Sandstorms
Although this piled raft is standing against the gravitational pull, Dubai’s heavy sandstorms are yet another test.
The piled raft design we developed would fail during a heavy sandstorm.
Strengthening the Tower
Do you have any design suggestions to overcome this issue? If you want to strengthen this tower to prevent it from falling, would you add glue to the center of the base plate or the edges?
When the Burj is glued at the center, it falls due to wind force. When the building is glued at its edge, it stands strong.
Optimizing Pile Placement
In the original Burj Khalifa design, we can apply a similar technique to the pile design. Just increase the number of piles in the wing area. Due to this neat design change, the Burj can withstand wind velocities of up to a whopping 240 kilometers per hour.
Interestingly, to analyze this optimal placement of piles, the engineers performed rigorous pile load tests. These involved applying heavy loads on a temporary test pile and studying the settlement. These tests lasted over six months and occurred at 23 spots on the Burj location.
Why the Foundation Needs Electricity Now: the answer you’ve all been waiting for. Why does the Burj Khalifa Foundation have to carry electricity continuously?
Preventing Corrosion with Electrolysis
If this is not done, the salty water seeping from the Persian Sea will corrode the rebars inside the piles. To overcome this problem, they used the physics behind batteries—electrolysis. They made these rebars cathodes and used titanium mesh as a sacrificial anode.
When DC current from the rectifiers is impressed between them, the electrons deposit on the cathode. This prevents corrosion of the rebars but heavily corrodes the anode metal. After years, the anode must be replaced.
The Risk of Oversupply
One thing is clear—to prevent corrosion completely, we must supply the optimum amount of current flow. However, what if we oversupply? This leads to a phenomenon known as hydrogen embrittlement. This phenomenon makes the bars brittle, and they crack quickly.
This is why the cathodic protection system they developed must be accurate—no oversupply or undersupply of electricity. Of course, any failure of the electricity supply in the cathodic protection region would weaken the foundation over the years.
Conclusion
Burj Khalifa utilizes an advanced cathodic system to prevent corrosion by seawater. Engineers put in a controlled DC current to maintain the rebars in order to keep the building in working condition in the future. However, an ideal balance has to be maintained an insufficient current leads to corrosion while an overcurrent leads to hydrogen embrittlement, causing the building to lose strength. This precise supply is required to maintain Burj Khalifa foundations in working condition in the future.
