Now that the towers have been fully dismantled and only the cores remain, the plinth is also being further dismantled. The structure of the underground floors (-1 to -5) is being retained. At the same time the new support structure for ZIN is being developed. Because the new building connects the two towers with a central volume, the existing foundations are inadequate. Therefore, Open Minds, the construction consortium of CFE, BCP, Van Laere and VMA, has already installed 99 additional pile foundations that form the basis by groups of a foundation footing for the new support columns of ZIN. This creates new supports for the unified building, in addition to the existing foundations.

Pile foundations are deep foundations that transfer the weight of the building to the more stable (and deeper-lying) layers of the substrate by means of piles. This involves an ingenious process for which Open Minds hired a specialised company. They work with two machines that each drill approximately one pile foundation per day from floor -5. Due to the limited space on the underground floors, this involves a unique procedure. The pile foundations, hollow metal pipes with a diameter of approximately 50 cm and a final length of 36 metres, are delivered in sections 6 of metres long and welded together underground.

Wondering how this works in practice? We explain the process in five steps, from pile foundation to foundation footing, with the help of a few photos.

STEP 1 – PREPARATIONS

Stability engineers determine and mark the exact locations of the pile foundations, at which the existing floor slab is locally broken up. The hollow metal pipes, delivered in sections of 6m long on floor -2, are readied and brought by crane through holes in the floors to the lower floors. The two drilling machines that can work in the limited clearances are located there. 34 7626

STEP 2 – DRILLING

The lower part of a pile foundation has a special endpiece (see photo). This drill head ‘screws’ a path through the substrate with a rotary motion and displaces the soil sideways. When the first part is adequately deep, a new section of metal pipe is installed. For this the machine makes good use of the holes in the floors above. The drill head that provides the downward rotary motion is ultimately left behind in the ground when the pile is located at the right depth.

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STEP 3 – WELDING

A specialised welder stands ready to securely interconnect the various parts. He ensures that the pile foundation ultimately forms one unit by welding the various parts together seamlessly. When he is finished, the machine drills the pile foundation further into the substrate. This process is repeated until the foundation is located approximately 35 metres deep. There, a sufficiently strong substrate is reached.

STEP 4 – THE FOUNDATION FOOTING

Until all the pile foundations of a foundation footing have been drilled, these hollow pipes are closed with a plastic cap to prevent dirt getting into the piles. The various pile foundations are then filled with concrete and the ground around the piles is excavated to a depth of 2 metres. This ‘hole’ forms the basis for the foundation footing that interconnects the various piles. The pile foundations are then cut to the right height, a reinforcement mesh is placed in the pit and this too is concreted. In this way the various pile foundations together form the basis of one foundation footing for the future support columns.

STEP 5 – CLEANUP

At the end of every workday, the workmen clean the machines thoroughly. Due to the drilling and the use of concrete the machines become very dirty. Ultimately, drilling one pile foundation takes approximately one workday per machine. Because two machines handle this work, this amounts to approximately 50 days of work. The specialised company completed this ingenious process at the beginning of March 2021.