When a village slides – and what geophysics has to do with it
The mountain village of Brienz in the canton of Graubünden is sliding ever faster towards the Albula valley – currently at a speed of up to one metre per year. This is problematic. In addition to protecting the inhabitants, important infrastructure such as the train line, high-voltage power line and cantonal road must be safeguarded, as these elements are also sliding.
Given the high rate of movement, the municipality and the canton decided to have the structure and mechanics of the huge sliding mass examined more closely. For this purpose, in addition to collecting data from targeted deep drilling, as much spatial information as possible must be gathered. Questions to be addressed include the horizontal delimitation of the landslide mass, as well as the course and depth of the sliding surface.
The main difficulty is the large thickness of the landslide mass: the geological layers from near the surface to a depth of about 250 metres are of equal interest. Aside from the extraordinary size of the landslide, we are highly experienced with this type of task: the search for different sliding and movement planes, as well as their lateral demarcation from the stable subsoil, belong to one of our core competencies.
In Brienz we were commissioned to carry out geophysical investigations. In total we have recorded five seismic profiles with a total length of 8,500 m and an exploration depth of 250 m. The applied hybrid seismic measurements, consisting of refraction tomography and reflection seismics, are particularly well suited to solving complex geological problems. They provide a high-resolution visualisation of the geological stratigraphy below the surface. In addition, geoelectric investigations provide more precise information about any water appearing in the area of the landslide mass.
All the data gathered should help us to determine whether there are effective and feasible solutions for stopping the landslide.
The results of the geophysical measurements are longitudinal and transverse profiles through the landslide mass, with information on the layer boundaries and their depths: