# Structural Health Monitoring of a Road Tunnel Intersecting a Large and Active Landslide
## 2 Study Area
### The Tunnel  
The valley road formerly crossing the Passo della Morte gorge was tortuous and threaded by rock falls, avalanches and shallow landslides. Therefore, in 1992, a double-lane tunnel that would cut off these hazards was designed. The tunnel, called San Lorenzo, started to be constructed in 1994 following the New Austrian Tunnelling Method (NATM) and is 2213 m long with both entrances at about 730 m a.s.l. and a maximum rock cover of approximately 370 m.  
During construction, significant water inrush was recorded between 470 m to 730 m from the eastern entrance and around 300 m from the same portal. The water discharge between 470 m and 730 m occurs where the tunnel cuts through the highly fissured dolomitic rock mass. At 300 m from the eastern tunnel entrance, however, the tunnel is crossing the block slide slip surface, which acts as a preferential groundwater path within the clay-shales. Moreover, near the entrance the superficial landslide had been reactivated (Figure 2).  
During a rainy period between October and November 1996 (680 mm in 10 days), the tunnel was flooded due to water inrush in the same areas. Concurrently, both landslides had been deforming significantly. Due to these instabilities, the tunnel was severely damaged and closed. However, since no alternative track is available, the tunnel and the landslides need to coexist.  
Figure 3: Displacement recorded through periodic inclinometric surveys at the I16 borehole location (**left**); once the slip surface was identified (58 m below ground surface), in-place inclinometers were installed to monitor the displacement evolution (**right**).  
In 2005-2008 the tunnel was rebuilt in the first 370 m from the eastern entry. Tests had been performed on the chemical characteristics of the groundwater and a high content of sulphate was detected. This is due to the presence of gypsum-rich rock units in the area, inducing an extremely aggressive environment for standard concrete. Pozzolanic cement (CEM IV\(-\)EN 197 norm) had been used to guarantee resistance to sulphate attacks and ensure the durability of the structure.  
The new design of this section of the tunnel had prescribed the use of independent segments of 12 m to allow deformations due to the slow but unremitting movements of the block slide to be dissipated. Concurrently, a drainage tunnel had been built below the road tunnel to collect the water within the rock mass in order to reduce the landslides' displacements. The drainage tunnel is T-shaped, reaching the road tunnel at 320 m from the eastern entrance and elongating under the superficial landslide, draining in the direction of the Tagliamento River.  
The adopted technical solutions, with independent segments, have been effective as almost 10 years later the road tunnel is still in use. However, the drainage tunnel has been significantly reducing the displacements of the superficial landslide but not those of the block slide. In fact, the discharge from the block slide sector is scarce since the drainage tunnel is located in the clay-shales domain below the slip surface. For this reason, the strain induced on the structure by the movements of the block slide is causing the formation of some cracks in the concrete lining, where the slip surface of the block slide is located. Over the years, some small pieces of concrete have spalled and the fall of little slabs is posing a significant hazard for traffic.