The Geology of the Hill of Summersberg Castle in Gufidaun (South Tyrol, Italy) and Its Geotechnical Challenges in Mitigation Works

Abstract

In 1957 and 1987, parts of the castle hill of Summersberg castle collapsed and pulled sections of the castle down onto the valley bottom and endangered an important access road. Due to this fact, the government of the Autonomous Province of Bolzano decided to ensure the castle hill. As the detailed geological structure of the castle rock and the reasons for its instability are unknown, a engineering geological mapping was investigated. Results suggest, that the castle hill has a shell-like structure: the core consists of a slightly foliated metagabbro. The foliation, folding and alteration increases towards the edge of the hill. The uniaxial compressive strength of samples from the inner region is commonly slightly higher than 100 MPa, while the outer part decreases to approximately 50 MPa. Field mapping indicates, that the fracture network is dominated by the effects of persistent faults spaced at 5–10 m. Most of the faults and associated fractures are very steep and run parallel to the slope. Therefore, we suggest a strong negative control on the natural slope stability. A structure model shows, that the castle hill could be divided into three parts with a different fracture system and with different failure mechanisms. Our findings suggest that not the strength of the rocks, but the kinematic freedom offered by the orientation of the discontinuities is responsible for the main instabilities. On this basis, a mitigation concept for the slope was drafted. It is comprised of steel networks, braced anchors, anchor beams and shotcrete