Foundations of Frankfurt High-Rise Buildings
The tall buildings of Frankfurt face special challenges due to the city’s deformation-sensitive ground. This means that good foundations play a particularly important role for skyscrapers there. Let’s take a look at the evolution of the various foundation types.
All high-rise buildings built in Frankfurt until 1980 have slab foundations. A slab foundation is nothing more than a thick reinforced concrete slab that transfers forces evenly to the subsurface. To get a little idea of the dimensions of such a plate, the Silver Tower‘s slab is 3.5 meters (11 feet) thick.
Due to the unfavorable properties of slab foundations, extreme settlement occurs in the Frankfurt clay. In the first generation of high-rises, builders had to accept this. Settlement joints have separated the high-rise buildings from the adjacent base development in order to avoid cracks between building parts. It has not always been easy to cope with this, with settlements ranging from 20 to 30 centimeters (8 to 12 inches).
Surface foundations are particularly problematic if the load cannot be introduced centrally onto the floor slab. In this case slabs begin to tilt, causing the building to tilt accordingly. Often, however, centralized load introduction cannot be avoided due to asymmetrical property geometries. Engineers had to devise special solutions for the first generation of Frankfurt high-rises. In the Silver Tower, 22 pressure cushions were installed under the base plate. These 5 x 5 meter (16 x 16-foot) rubber cushions could be individually inflated with water to counteract tilting of the base plate. After the building’s completion, the cushions were filled with mortar and fixed in place.
Due to the inevitability of large subsidence, builders have since moved away from surface foundations. Among other reasons, settlement of slab foundations may impact skyscrapers or other construction on adjacent land.
In pile foundations, the building’s load is introduced into the ground via concrete piles driven into the earth. In this way, extremely low subsidence can be achieved by resting the building on lower-lying, load-bearing layers of soil. This type of foundation, which is very widespread in high-rise buildings worldwide, is rare in Frankfurt because the clay layer, which has little load-bearing capacity, is up to 100 meters (328 feet) thick, requiring piles of enormous length. In most cases this is not economical.
The Commerzbank Tower is an exception: here, the clay layer measures only approx. 40 meters (131 feet). It was therefore possible to transfer the load of the 180,000-ton building via 111 drilled piles into the load-bearing Frankfurt rock layer (in German: “Frankfurter Kalke“). This procedure limited settlement to 3 centimeters (1 inch).
Combined Pile and Slab Foundation
Combined pile and slab foundations (in German: Kombinierte Pfahl-Plattengründungen – KPP) are the most widespread among Frankfurt’s high-rise buildings. High-rise loads are introduced into the ground through a floor slab and piles. The decisive difference to pile foundations is that the piles do not need to reach a stable soil layer. The load is transferred to the ground via four effects:
- Spreading of loads over the floor slab
- Introduction of loads via peak pressure at the pile end and skin friction
- The base plate increases pressure on the pile jacket, thus increasing pile friction
- The dense structure of the piles increases the arithmetical load that each individual pile can take, making the process more economical than pure pile foundations.
The Messe Torhaus was the first building in Germany to use a combined pile and slab foundation.
Soil Under Downtown Frankfurt
The soil below the inner city of Frankfurt is actually not suitable for building tall and tall there. The reason for this lies in the rock layers that are there. The most disruptive is the top layer of the so-called Frankfurt clay.
This is because clay has properties that are disadvantageous for high-rise construction: these floors deform strongly under pressure. One or the other high-rise has sunk several dozen centimeters due to its foundation type. Therefore, combined pile and slab foundations are used today in the construction of high-rise buildings in Frankfurt.