RIASSUNTO
Abstract
On October 5th, 2010, cracks were found in a gallery 1.80 m high and 1.40 m wide. The gallery is 100 years old and runs parallel to a valley flank. After eliminating several possible causes of cracking, a landslide producing the damages had to be taken into consideration. Monitoring revealed that a landslide was occurring, loading the gallery lining. As stabilization of the slope was not an option for several reasons, it was decided to replace the gallery by a new one deeper inside the slope, which will be ready for operation in 2017. Thus the old gallery has to be kept in operation till then and it was decided to reinforce the old gallery by a heavy reinforced shotcrete lining 10 cm thick. However, it has to be realized that stiffening the support of a tunnel in a landslide attracts the stresses and the tunnel is greatly damaged in the long term. Nevertheless, it may be a positive measure in order to allow a structure to survive a particular span of time.
1 Introduction
On October 5th, 2010, cracks and spalls were found in a 30 m long section of a gallery 100 years old, which had not been observed before (Figure 1). The gallery had been excavated in a tectonically strongly stressed and therefore highly disintegrated, weathered and slightly dipping sandwich of clayey shales, sandstones and marls belonging to the Flysch-zone very close to the thrust by the Northern Calcareous Alps. Geological mapping and reconnaissance drilling revealed that the sandwich described above is covered by 5 m of weathering loam and a rock debris layer 4 m thick. Overburden of the gallery amounts to some 25 m. The excavation had been supported by a timber structure and a U-shaped profile had been cast. The crown had been supported by a cut stone arch (Figure 1).
In order to answer the questions "Will it collapse?" and "What has to be done to keep the gallery stable?" first of all the deformation performance of the structure as well as the causes of cracking had to be found, which were completely unclear when the damages were observed for the first time, though many authors described tunnel damages and their causes (e.g. Mueller 1978, Poisel & Oehreneder 1991, Poisel et al. 1996, Wang 2010).
After eliminating several causes, to which the particular damage pattern (Figure 1) could have been attributed, a landslide deforming the gallery had to be regarded as the most probable cause for the damages (Poisel et al. 1996). Modelling of the system landslide – gallery in a profile along the dip direction of the slope using FLAC (Itasca 2011; Figure 2) showed an identical damage pattern in the gallery support as observed (Figure 1).
The gallery is running parallel to a slope some 110 m high (Figure 3). At the toe of the slope a weir is situated which was modernized in 2009 and 2010. In doing so the reservoir level, which is kept constant during operation, has been increased by 1 m, the tailwater level has been decreased by 1.10 m over a length of ca. 300 m and at the toe of the slope some 20,000 m³ had to be excavated in order to make space for a fish migration device. Additionally in the summer of 2009 210 mm of rain fell in 4 days measured at a nearby weather station, which had occurred three times since the construction of the gallery. Therefore it had to be assumed that most probably both events (modernization of the weir as well as the precipitation event of 2009) reactivated an old, dormant landslide.