# Structural Health Monitoring of a Road Tunnel Intersecting a Large and Active Landslide
## 5 Discussion  
The monitoring system was able to detect and follow three different types of dynamic within the San Lorenzo tunnel. The cumulated rainfall/snowmelt influences the evolution of the cracks on the tunnel lining, with larger activity in the rainiest year. However, the brittle nature of the concrete does not allow monitoring to predict precisely when widening will occur and, more importantly, when spalling would endanger traffic. Nevertheless, since the activity of the landslide is correlated with the meteoric events and in particular with the accumulation of water in the highly fissured dolomitic rock mass, it is advisable to extend the drainage tunnel reaching the dolomitic rock. Draining this rock mass would have an effect on the slip surface of the block slide, which is the only region along the tunnel where water is present within the clay-shales domain. The slip surface serves as a preferential flow path from the dolomitic rock aquifer and the Tagliamento River. Reducing the storing capacity could reduce the lubrication associated with softening and variation of the pore pressure regime along the block slide slip surface, thus lowering displacement and damage along the tunnel. The overall dynamic of the sensors shows that some fissure are widening while others are closing. The widening cracks are all located in segment 28 because on the left segment 28 is constrained to the stable slope and on the right it is loaded by the movements of the landslide. This induces a bending moment in the segment that opens the cracks. The distribution and direction of the cracks are compatible with those described by [PERSON] [20] in Case IV, with a similar relative geometry of slope instability phenomenon and tunnel. The displacement recorded by the crackmeters deployed between the lining segments allows us to delineate the interaction between the landslide and the tunnel. While the sensors in the left part of segment 28 (F17 and F24) show negligible displacement, the crackmeters in the right part close to segment 29 are widening with 1.2 mm of cumulative displacement for F18 (located downhill) and 6.3 mm for F23 (located uphill). The sensors on the other side of segment 27 measure positive displacements of 4.88 mm (F19, downhill) and 3.17 mm (F22, uphill). These data suggest that while  
Figure 11: Rotation of the two-axis clinometers plotted with the water discharge inside the drainage system within the road tunnel—the graph has been produced directly through the web-based platform. segment 28 is constrained on the left and works as a cantilever under the loading induced by the landslide displacement, segment 27 takes advantage of the design choice of independent segments and undergoes roto-translation (Figure 12). The slip surface of the landslide, which is about 10 m wide, is therefore localised in the area between segment 27 and 28. Direction of the cracks in segment 28 suggests that load is applied directly to such segment. The diagonal cracks of the adjacent segments are due to the shear stress distribution, as described by [PERSON] et al. [21]. The behaviour of clinometer C04 is characterised by the inversion of the vector of projected direction that was initially directed mildly downhill, subsequently rotated in two phases (May-June 2015 and May-July 2016), and is currently directed uphill (Figure 13). These results are linked to the two-phase dynamic of the interaction between the tunnel and landslide (compression and dissipation). The compression phases (April 2014-May 2015, August 2015-July 2016 and September 2016-April 2017) consist of a loading of the tunnel due to the landslide. In these phases, rotation entity and velocity are smaller and the vectors indicating the direction of the clinometers converge towards the centre of the tunnel. The dissipation phases have a more impulsive nature, through the liberation of the energy accumulated by the soil and the structure. This phenomenon corresponds to the sudden downhill rotation of C04 and the contemporary contraction of F15 (Figure 13).