If you consider that Frankfurt is the only city in Germany boasting a skyscraper skyline, you might expect the ground under this European metropolis to be ideal for high buildings. But that is not the case; the ground there is predominantly clay.
What is the Frankfurt clay?
Clay Layer Is Counterproductive
Clay has many disadvantages for high-rise construction. Described in technical jargon as “cohesive soils”, clays deform strongly under pressure. This does not happen immediately, but over a long period of time. When clay absorbs water, it is stored in pores in the clay structure. When a building is built on clay, water is slowly displaced from the pores, closing the cavities in a process called consolidation.
In buildings built on layers of clay, these processes can cause great damage. On one hand, the deformation causes severe subsidence, leading to cracks or uneven settling. The bigger problem, however, is that the deformation takes place over such a long period of time. The consequences of settlement cannot be concealed directly when the building is expanded.
Tall Buildings can sink or lean
Consequences of an insufficient foundation for Frankfurt‘s high-rise buildings can be seen in the SGZ-Bank high-rise built in 1972 (since then completely modernized and now called Park Tower). Unexpectedly high settlements and misalignments occurred during construction, and builders spent years counteracting this. Only 10 years after completion did the settlements come to rest. The building has sunk in steps up to 31 centimeters (12 inches) and is now 1: 300 inclined. Even if this does not impair the use of the building, it shows that special emphasis must be placed on high-rise foundations in Frankfurt.
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