A school in a fortress is already a unique feature. However, the Gymnasium Zitadelle has an extra element alongside its long history: it stands on springs. The reason for this is a tectonic zone beneath the fortress, which began causing cracks in the building in 1979. This so-called “Rursprung” runs diagonally through the fortress complex. Ultimately, the decision was made to “saw” the building and elevate it on steel springs for long-term stability.
The south wing from the 1960s was torn open, and so-called spring bodies were installed—one of the techniques used to support the sinking section. While the historical parts are largely made of field-fired bricks, this section is made of concrete. This allowed for the sinking part of the building to be cut through and separated from the non-sinking part by a separation joint.
The building was sawed through like a loaf of bread,” explains Thomas Jelen, a civil engineer from Jülich, who is responsible for regular technical monitoring. However, hardly anyone can imagine all the work that had to be done to create two independent building sections. The roof membrane had to be separated while remaining waterproof, sinks and wastewater pipes had to be relocated, escape routes needed extra steps, and lighting was added to compensate for the height difference. Heating pipes had to be designed flexibly, and of course, completely new room finishes and closures were created.
“The separation was not just about cutting the bread; everything had to be meticulously planned from the start to ensure that the building remained structurally sound and its infrastructure intact,” Jelen notes, recalling his predecessor Helmut Stralek, who is now nearing retirement but enjoys sharing his knowledge. Essentially, two buildings were created through the cutting process. You can see this when you stand in front of the building: two different levels are visible.
The stabilization with spring bodies is known as active stabilization, explains Stralek. One side of the building is sinking in a controlled manner, while the other side is supported by spring bodies, Jelen adds. When the spring bodies beneath the supported part are no longer under tension, steel plates are placed underneath to re-tension the springs. The spring bodies compensate for the loss of soil pressure, effectively imitating the counterpressure of the ground.
All of this is monitored electronically via modem, according to Jelen. A lift of 70 millimeters plus a precautionary overshoot of 35 millimeters has been implemented. It is crucial to estimate how much deformation the building can tolerate. “We have secured many buildings,” says Stralek. They adapted the spring bodies from underground mining in the Ruhr area to meet the specific needs of the building, the engineer explains.
He also explained the passive stabilization techniques applied to the historical parts of the building, which require a different approach: deformations are allowed in order to restore the original condition afterward. This includes injecting cracks that have developed in the historic sections of the building. A special “citadel mortar” is used for this injection process. Boreholes, 20 centimeters deep, were drilled into the masonry and fitted with injection nozzles. This “trass lime” is then pressed into the voids. This method is particularly gentle on the structure, which became especially important after 1984 when the citadel was added to the list of historic monuments, and the Rhineland Office for Monument Preservation suddenly had a significant say in the matter. Since then, it has involved civil engineering services under monument protection aspects, which presents a unique challenge, according to Stralek.
The visit to the spring bodies beneath the south wing is impressive. However, it requires a bit of adventurous spirit to climb the narrow stairs for a glimpse of the springs in a room below the south wing. But it’s worth it.
