Issue 66

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

which is necessary for better processing and better decision making. Adequate SHMs are essential in decision making related to tunnel situations or treatments. Fig. 2 shows specific objectives of SHM applied to tunnel engineering. Sensors Sensing method Range

Resolution (Accuracy) 0.01% gauge length

Main parameters or Measurement

Modulation method

Strain / Temperature Strain / Temperature Strain / Temperature Strain / Temperature Strain / Temperature Temperature

 10,000

Fabry-Perot

Point

Phase

Wavelength

 5000

1 – 2 

℃ /0.1

FBG

Quasi-distributed

Rayleigh (OTDR)

1 /1 m

2000 m

Distributed

Intensity

℃

Rayleigh (OFDR)

℃ 1 /0.1 cm

100 m

Distributed

Frequency

Brillouin (BOTDA)

0.1 /0.3 m

1000 m

Distributed

Frequency

℃

Raman Intensity Table 1: Pertinent information on fiber optic sensors in underground engineering (after Gong et al. [38], with permission from Elsevier). Distributed ℃ 1 8000 m

Figure 2: Relevance of tunnel structural health monitoring

Real-time decision making depends also on the reliability of the sensors used in SHM systems. Likewise, the type of decision is tributary of the severity extent of the health problems detected. In all situations, real-time decision making is essential for safety and economy. One the one hand, if decision outcome is favorable for maintenance, costs can be reduced by acting in real time. Besides, if tunnel traffic needs to be temporarily restricted, life and economy can be saved by avoiding unexpected accidents. On the other hand, if in the worst case, the tunnels have to be permanently closed, life and economy can also be saved by avoiding huge unforeseen accidents. In any case, real-time decisions are of paramount importance and can be made efficiently by adequate SHMs. For deep tunnels, comprehensive structural health monitoring leads to the monitoring of different parts of the tunnel structures with the aim of persistently monitoring their most critical health issues. Fig. 3 shows the critical components or aspects of tunnel structures where proper monitoring techniques are required (EDZ stands for Excavation Damaged Zones).

Figure 3: Structural health monitoring approach in deep rock tunnels

The monitoring data must be processed and interpreted adequately. Data interpretation play a key role in the establishment and adoption of maintenance plans. Therefore, the response from the sensors should be clearly detailed without any interference that could clog them. Representations of the structural disease detected in a given tunnel should be as accurate

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