## 4 Application and Analysis of FBG Strain Detection Piles in Slope Deformation Monitoring  
According to the numerical analysis results and engineering geological conditions, in this chapter, the monitoring area is selected to be the foot of the slope near the highway, with strain detection piles to monitor the slope's deep deformation. Array sensors implanted in concrete piles can acquire strain information at different depths as the slope deforms, thus reflecting the internal stress change characteristics of the slope. The axial force of the strain detection pile can be expressed as:  
\[Q_{i}=\varepsilon_{i}\cdot E_{i}\cdot A_{i} \tag{3}\]  
where \(Q_{i}\) is the axial force at section i of the pile. \(\varepsilon_{i}\), \(E_{i}\), and \(A_{i}\) are the strain value, cross-sectional area, and modulus of elasticity at section i, respectively. The sensors used in this paper are FBG strain sensors, the reflected light wavelength of FBG varies with the change in temperature and strain, and the relationship [32] between them can be expressed as follows:  
\[\frac{\Delta\lambda}{\lambda}=\left(1-P_{eff}\right)\Delta\varepsilon+(\alpha+ \xi)\Delta\mathrm{T} \tag{4}\]  
where \(\lambda\) is the initial Bragg wavelength, \(\Delta\lambda\) is the variation in Bragg wavelength, \(P_{eff}\) is the effective photo-elastic coefficient, \(\Delta\varepsilon\) is the applied strain, \(\Delta\mathrm{T}\) is the temperature change, \(\alpha\) and \(\xi\) are the thermal expansion coefficient and thermal-elastic coefficient of the fiber core. Firstly, five deformation observation boreholes (IN 1\(\sim\)IN 5) were arranged in turn at the foot of the slope of K77 + 120\(\sim\)K77 + 215 section of the secondary highway from Yuanyang to Luchun, with a depth of 30 m and a diameter of 127 mm, and the interval was about 10 m, The deployment diagram of FBG strain piles is shown in Figure 11. The FBG strain sensors are tied with wire rope and embedded in the borehole in order at 2 m intervals, and three FBG temperature sensors are installed in the upper, middle, and lower positions of the borehole for temperature compensation at different depth positions. The boreholes are then backfilled with cement mortar. After the cement mortar is consolidated, the piles for detecting slope-deep deformation are formed. The strain detection piles implanted in the slope can deform synchronously with the rock and soil mass, and the longitudinal strain change of the pile during the deep deformation is obtained through the FBG strain sensors buried in the piles, reflecting the characteristics of the slope stress changes. The working principle of FBG strain detection piles to monitor the deep deformation of the slope is shown in Figure 12.  
Figure 11: Deployment diagram of FBG strain piles for deep deformation monitoring. In practical applications, FBG strain sensors with different central wavelength values were used because the same wavelength will cause the signals received by the demodulator to overlap, and the specific position of the sensor cannot be identified from the wavelength value received by the demodulator. The wavelength interval of the FBG sensor used for slope monitoring in this paper is more than 3 nm. The fiber Bragg grating strain sensor had a strain monitoring error of less than 10 \(\upmu\)e, a measurement range of \(-\)1000 to 1000 \(\upmu\)e, and a resolution of 0.7 \(\upmu\)e. The FBG monitoring system was completed on 27 January 2014, and initial strain data were collected on 22 February 2014. Then, the strain monitoring data were collected after 20 days (14 March), 40 days (3 April), and 80 days (13 May). The variation in the longitudinal strain of the detection pile IN 1\(\sim\)IN 5 is shown in Figure 13. Positive strain indicates that the strain sensor is under tension; a negative strain indicates that the strain sensor is under compression. FBG field monitoring mainly focuses on the deep deformation of the slope.