Issue 66

W. Frenelus et alii, Frattura ed Integrità Strutturale, 66 (2023) 56-87; DOI: 10.3221/IGF-ESIS.66.04

Continuous effective monitoring of the linings is essential to ensure safe operation throughout the lifetime of the tunnels. Indeed, the selection or the design of any remote sensing technique should be the most suitable or optimal in order to properly satisfy the monitoring objectives. The most common sensors utilized in the monitoring of the primary and secondary lining of tunnels are presented in Tab. 3. Monitoring results are related to the effectiveness of the monitoring system and also to the period over which the monitoring data is processed and interpreted. As already stated, continuous monitoring is extremely important for all parts of deep tunnels. However, it is even more important for lining structures which are majoritary made of fragile materials such as concrete. Thus, even if the monitoring results indicate that the health conditions of the linings are normal with regard to the monitoring period, it remains crucial that monitoring of these be continued. For instance, in the Mixian tunnel, the monitoring results show that the linings behave normally after more than 3 months [70]. Nevertheless, ensuring the safety and stability of this tunnel at all times requires effective continuous monitoring systems. For illustration (Fig. 10), in the Bai Ni-jin No. 3 tunnel where a monitoring program was carried out by 10 encapsulated FBGs, considerable deformation changes were recorded in the secondary lining for 3 consecutive years [14].

Figure 10: Variation of strains monitored by FBG sensors in a section of a secondary tunnel lining (Data from Li et al. [14]. Groundwater leak monitoring Ensuring the continued structural health of deep rock tunnels cannot be done without proper monitoring of groundwater leakage. In fact, over time, groundwater infiltration problems are almost unavoidable during the lifetime of tunnels due to the inevitable reduction in the stiffness of the linings [81]. In operated tunnels, groundwater leaks are mainly found at structural joints in existing cracks. When their flows are very low in their initial states, it is generally difficult to detect them by simple inspection, especially in deep tunnels. At the same time, they can gradually increase the loads in tunnel linings, grow rapidly and be responsible for numerous cracks that seriously affect the structural integrity of tunnels. It has been reported by Sun et al. [82] that the structural and functional constituents of tunnels can be degenerated by the effects of water leakage. Any deterioration has the consequence of reducing the performance and durability of the structures, in particular when it is not treated in real time. As such, early warnings about groundwater leaks are necessary in order to adopt adequate measures to address groundwater infiltration issues as quickly as possible. Indeed, the longevity of the tunnels is considerably threatened by groundwater leaks. This is because the spreading of cracks in the concrete lining is facilitated by groundwater leaks [83]. Many structural health issues such as corrosion, voids, spalling and cracking are mainly due by water ingress in tunnels [84]. Grouting techniques are typically utilized to diminish groundwater leaks in tunnel [66, 85, 86]. However, over time, even if the most adequate grouting techniques are employed, groundwater leakage can appear inside a given deep rock tunnel [66]. For instance, groundwater leakage was one of the major diseases causing liner cracking in Jizhao Tunnel (Guizhou, China), and although adequate treatments were applied, long-term monitoring was adopted mainly in the most damaged sections [87]. Groundwater leakage not only accelerates the deterioration of the lining structures, but also reduces the mechanical properties of the rocks surrounding the tunnels. If it is not detected and addressed in its early stages, it can become more severe and increase the risk of unforeseen partial or complete tunnel failures. Therefore, to continuously ensure safe operation of tunnels, reliable monitoring of groundwater infiltration is strongly required [39]. In fact, groundwater seepage must be detected and diagnosed quickly and accurately [88], owing to its enormous consequences. In addition to

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