# Structural Health Monitoring of a Road Tunnel Intersecting a Large and Active Landslide
## 2 Study Area
### The Landslide  
The block slide develops between 970 m and 640 m a.s.l.; on the eastern side, its boundary coincides with a small stream called Rio Verde (Figure 1). The dynamic of the landslide is determined by the presence of rigid dolomitic rock masses over more plastic clay-rich units and it is closely controlled by the structural setting of the area. Likely, in its first stages of evolution, the landslide moved in an eastern direction as a sackung and then subsequently evolved towards the south, heading to the valley bottom. The scarp of the first movement phase coincides with the SE-NW fault connected with the Alpine overthrusts of the area.  
The Passo della Morte block slide has been investigated for more than a decade using several techniques. Numerous boreholes had been drilled in the area, and bore\(-\)core analysis had been carried out along with geological surveys, allowing the stratigraphy of the slope to be described.  
Figure 1: Study area with cross sections A\(-\)A\({}^{\prime}\) and B\(-\)B\({}^{\prime}\) for the geological model; in the photograph on the right the eastern entrance of the tunnel is indicated by a white arrow. GPS: Global Positioning System; PM: name of the GPS benchmark.  
The investigation also involved a secondary, shallow earth slide located at the eastern entrance of the tunnel (Figure 2).  
Geomorphological evidence coupled with Ground-Based SAR [17] supported the assessment of the contour of the area subject to landslide. Then a long series of GNSS (Global Navigation Satellite System) surveys followed the displacement rate of some selected superficial points of interest. From December 2002 to July 2016, the GNSS benchmarks located on the block slide PM15 and PM13 measured a horizontal displacement of 215 mm (1.5 cm/year in average) and 277 mm (almost 2 cm/year in average), respectively. Their corresponding direction can be observed in Figure 1.  
Periodic inclinometric surveys had allowed the location of the slip surface to be detected and then in-place inclinometers were deployed to monitor in real time the displacements at depth (Figure 3). The displacement pattern of the block slide at depth is influenced by extreme rainfall events or rapid snow-melt.  
Figure 2: Geological section of the block slide.  
Piezometers, also working in real-time mode, were coupled with inclinometers to monitor the variation of groundwater head within the slope. The small variation of the water level recorded in the piezometers associated with inclinometer I16 is not correlated with the change of the displacement pattern of the landslide. This is due to the fact that the borehole is located in the medium-low part of the block slide, where low permeability clay-shales are present and therefore the oscillation of data recorded by the instrument is lower than 1 m. The effect of extreme rainfall and snowmelt events on the mobility of the landslide is therefore linked to the saturation of the highly fractured dolomitic rock mass located behind and above the landslide body.  
Despite this significant amount of data, it was necessary to install instruments that would continuously monitor the tunnel to better understand the non-linear interaction between it and the slow-moving landslide, and determine the damage prognosis.