Dear reader here we are publishing a post about foundation criteria for a mega structure, Burj Khalifa. Due to heavy loading from superstructure, in skyscraper like Burj Khalifa, deep foundations were chosen. Now piles are the most common and popular deep foundation option in the current constriction practice. We know that these piles, as a pile group, under a wall or column are embedded by rigid member, most commonly known as pile cap. In contrast to pile cap, we are preferring raft supported on piles for deriving some advantages. The advantages of this type of foundation will be discussed in the post. Here we will learn some information about foundation system of Burj Khalifa.
We know that skyscrapers are subjected to vertical loading including gravity and lateral loading with its subsequent overturning effect. In this situation instead of cap, raft connecting piles provides sufficient load resistance and stiffness as well. So, our discussion was about Burj Khalifa that was founded over raft, supported on 194 piles. These piles were of 1.5 metre diameter bored cast-in-situ piles. The tip levels of piles were reached 164 ft deep with a pile length of 140 feet.
Reinforcement cage are placing before casting. This was only half of pile length of Burj Khalifa
The desired capacity of each pile for this depth was 3000 Tonnes. The desired pile size was loaded to 6000 tonnes in pile load test. Besides the 1.5 m dia pile, different diameter piles were tested with different increment over working loads. The pile load test program also included grouted piles to test. We have published many post regarding pile load test and bored piles. You can visit this for more information.
Pile layout drawing of Pile Supported Raft Foundation of Burj Khalifa
The foundation system (piled raft) was rested on rocks and deep deposition of carbonate rocks. We will discuss about geotechnical investigation for this project in the next post. The 3D foundation analysis was performed with the help of different software and interconnected the predicted results among the software.
From the pile load test it was found that under working load the geotechnical failure was supposed to occur when factor of safety reaches more than 3. This provides a sufficient safety against failure; specially when supported raft will provided further resistance.
The raft construction over 194 piles was difficult especially with placing and fixing bottom rebar of raft between the numerous bars from rebar cages of piles. To suppress these difficulties the rebar cages for bored piles were placed in such a way that they do not interfere much with the rebar of raft on bottom layer.
The foundation system was designed following approach of designing pile supported raft foundation where most loads were assumed to be carried by piles with considering some contribution of raft above them. The aim of this type of foundation was to limit settlements by piles that provide most of stiffness at service loads and component raft above were supposed to provide addition capacity under ultimate loading.
The raft was 12 feet thick and poured in four phases. These are three wings and cores at center. Each poring took over a period of 24 hour.
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