The Danger Of Using Pile Caps With Connecting Raft Foundations In The Case Of High Rising Towers
The danger of using pile caps with connecting raft foundations in the case of high rising towers of more than 20 floors.
One of the important projects which has already constructed, after the completion of its concrete structure, many cracks appeared on it, confirming that differential settlement has been happened in this tower.
The tower has 44 floors
It is founded on pile caps foundations thickness from 3 meters to 1.5 metres.
connected with connecting raft with a thickness of 40 to 50 cm.
The length of the piles ranges from 13 to 17 meters, with diameters from 60 to 120 cm.
An examination and visual inspection of all parts of the tower and the types of cracks was carried out, and the result was that the external sector of the tower and the surrounding area of the core had been subjected to differential settlement that caused these cracks. It was also found that even the connecting raft had cracks.
The work was divided into two teams:
1- A structural team to review the structural design of the tower.
they found that there were problems with the structural design of the connecting raft in this way, as it was proven that it needed more thickness and reinforcement to act as a connecting section for this type of foundation.
2- A geotechnical team to review soil reports, create a complete simulation model of the tower’s foundations, observe and evaluate the results.
In fact, the result was that the core area will settle to a significant value due to the increased distance between the columns. Naturally, there was a pull of the loads to the more rigid area, and the result was an expected drop of about 13 cm for the core area.
In this case, it is impossible for the raft connecting the pile caps to resist this large difference in the expected settlement due to the large stiffness difference between it and the pile caps.
There was also a miscalculation of the design parameters for the soil.
suggested solutions:
Using micropiles to strengthen the foundations and stop the increasing subsidence.
Increasing the thickness of the connecting raft from 40 cm to 1.2 metres.
Injecting areas where cracks occurred under the connecting raft.
Conclusion:
The pile-type foundations may save the amount of concrete and the number of piles, but sufficient caution must be taken regarding them in the case of towers with great height.
If used, it is preferable to attach them together by a suitable structural element.
In the case of rocky soil, it is preferable to use its design in their submerged state and not dry.
Maintenance of facilities, especially foundations, is a very expensive work. It is best to conduct a complete evaluation of the efficiency of the design and reports before starting the work by professional engineers.
Engineer Mustafa Magdy
