Aquisgrani terrae motus factus est (part 1): The Aachen cathedral (Germany) built on weak ground?

Abstract

Charlemagne (Charles the Great) placed the centre of his kingdom during Carolingian times in Aachen, Germany (Aquisgrana). This medieval period and his regency are associated with a period of extensive constructions in Aachen, including the Carolingian part of the Aachen cathedral. The cathedral was the first building in Germany to join the UNESCO World Heritage list. The foundations and construction ground of the cathedral were excavated during recent archaeological and geotechnical investigations. The cathedral is founded on pillars and piles in Devonian blue limestone and shale, covered by Pleistocene loess sediments. Remains of Roman buildings, such as thermae, are situated below this part of the cathedral. The pillars are connected by up to 5 m deep foundation walls and are up to 2.4 m thick. Even though information about the construction site is important for the assessment of structural health and remediation measures, it is often rare for old historical buildings. Documentations from construction time concentrate on the construction progress and financial aspects. Detailed descriptions on the foundation ground and techniques are usually not preserved. This information is of relevance for assessment of earthquake related structural damages and site effects on construction. Investigations in the course of remediation measures or archaeological surveys fill this information gap. They provide insights in the ancient foundation techniques and can be used for identifying earthquake related phenomena in the construction site. Furthermore, a direct insight into potential seismic effects associated with the ground and building response on seismic shaking becomes possible. In the case of the Aachen cathedral, typical geotechnical investigation methods including drilling and laboratory tests were applied together with GPR measurements to evaluate the construction ground and evaluate potential seismological effects.