Hazardous cliff face in the midst of a residential area
A 40-metre cliff face is all that remains of a former quarry in suburban Lucerne. The face is shifting and poses a threat to adjacent residential buildings, which even had to be temporarily evacuated. How to secure this unstable face for the long term?
Expertise involvedGeotechnical engineering
Allgemeine Baugenossenschaft Luzern
A highly sensitive approach
A local geology office has been monitoring the cliff face with measuring instruments and periodic controls since 1991. The deformations have increased in recent years – to such an extent that an automatic alarm system was installed at the beginning of 2015 and a concept for long-term stabilisation was commissioned. Working together with geologists, the geotechnical engineers examined the sandstone face in great detail. While the rocks themselves proved to be stable, the face was gradually folding under its own weight and was therefore bulging in places. This process needed to be stopped. Stabilisation measures, however, involve intervention – how would the face react if anchors were inserted and tightened, for example? This tension could threaten the fragile balance.
Ultimately, the geotechnical experts recommended using nails to stabilise the rocks. This approach does not involve filling any hollow spaces, meaning that drainage is not affected. In order to secure particularly disturbed areas, vertical and horizontal concrete elements are used together with the nails, enabling the face to be stabilised for a further 100 years. Complete stabilisation cannot be guaranteed with such a structure, however, which is why monitoring will continue to be necessary.
Option study, planning project, invitation to tender, construction documents, site management.