Trostburg Castle in South Tyrol: Engineering Geological Investigations on the Castle Hill

Abstract

Constructed in the 13th century, Trostburg Castle is situated on an impressive castle hill above the village Waidbruck (Ital. Ponte Gardena) in South Tyrol (Italy), about 17 km northeast of Bozen (Ital. Bolzano). The walls of Trostburg Castle are crossed by many cracks. There are various possible causes for these damages. On the one hand, there are geogenic causes, such as the curved surface of the castle hill (whaleback), composed of the volcanic Trostburg formation, and the low rock mass strength due to the adverse orientation of some joint sets. On the other hand there are anthropogenic influences. The castle was extended several times leading to a potential overload of the foundation walls. During the Second World War, Trostburg Castle was damaged, half-ruined and exposed to weathering for a long time. Probably a combination of various geogenic and anthropogenic causes led to the castle’s recent damages. The geological conditions of the castle hill were investigated with a special focus on the petrography and the tectonic joints. The result of the work was a large-scale (1:2,500) geological map as well as detailed outcrop descriptions, focusing on petrographic and structural analyses, e.g. by thin sections and pole plot diagrams. A thin section microscopy analysis of different tuff and ignimbrite layers of the alternating Trostburg formation demonstrated that there is no sign of fluvial relocation of the grains after falling out from an eruption cloud. Therefore the tuffs of the Trostburg formation can be described as actual ash fall. As another result of this study besides the potential overload of the foundation, the steeply dipping joints parallel to the slope are one of the most important causes for many cracks in the walls of Trostburg Castle, making it necessary to monitor the joints as well as the cracks at least periodically.