Issue 66

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

   2 B eff n

(9)

Here eff n represents the refractive index of the fiber. A linear relationship is used to represent the offset of the reflected Bragg wavelength (   B ) as follows [31]:





        1 B e B p T     

(10)

Here   B stands for Bragg wavelength offset which is induced by the strain (  ) and temperature change (  T ); e p is effective photo-elastic coefficient of the FBG;  is the thermal expansion constant of grating;  represents thermo elastic constant of the fiber. For typical values,    1.2 / pm and   ℃ 13 / pm for an FBG deploying 1550 nm as central wavelength [38]. Relevant requirements and design The critical requirement of the new FBG sensor is identified. Indeed, as noted by Kerrouche et al. [140], the critical requirement of the sensor network must be determined in order to properly design a monitoring system. The new FBG should be of high resistance which can resist difficult conditions of the aforementioned deep soft rock tunnel. Concretely, it will be a FBG semi-coated with a kind of material capable of absorbing of water and humidity, as shown in Fig. 19.

Figure 19: Illustration of the semi-coated FGB sensor

The rapid reaction of FBG will allow it to convert the absorption of water and humidity into strains. Indeed, on the basis its adsorption ability, polyimide is selected to coat the FBG. Temperature variation will be assessed by using the uncovered part of the FBG. Such a consideration is made because in deep rock engineering, temperature fluctuation cannot be overlooked. The FGB will be able to control the variation in deformation due to the increase in permeability or the early appearance of groundwater leaks. In order to better evaluate the performance of the sensor, it is suggested that strains and temperature be evaluated separately. Thereby, with regard to Equation (10), the wavelength shift related only to strain is determined as below:









  1 B



p

(11)

e

 B

Similarly, the wavelength shift related only to temperature is estimated as follows:









     B B 

T

(12)

The FBG will be applied to control the early onset of groundwater into the Weilai tunnel. Consequently, strains will be detected in the secondary lining of the tunnel, and real-time decisions will help for continuously maintaining the structural integrity of the tunnel. Since the surrounding rock conditions are complicated, the sensitivity of the new FBG sensor will be amplified accordingly. Ordinarily, to adequately improve the sensitivity of the FBG sensor, an amplification factor q can be considered and is determined as below [141]:



  FBG q

(13)

76