## 2 Engineering Background
### Field Monitoring Layout  
In view of the complex and changeable characteristics of the composite strata in the \"Keyuan Station-Shenzhen University Station\" section, this paper uses a quasi-distributed fixed-point grating monitoring system based on weak reflectivity Bragg fiber grating (FBG) to monitor the layered settlement of the strata. The quasi-distributed fixed-point grating monitoring system adopts a distributed fixed-point grating fiber optic cable, which is an internal fixed-point design, and can realize spatial discontinuous non-uniform strain segmentation measurement. It has excellent coupling for stratum deformation monitoring and can realize compression and tensile deformation measurement.  
The layout of the field monitoring system is shown in Figure 2. Seven surface monitoring points and three stratum layered settlement monitoring holes are set respectively. The interval between adjacent surface monitoring points is 3.0 m, and the stratum layered settlement monitoring holes are set as follows: hole 1 # and 3 # are 22 m deep, hole 2 # is 20 m deep. The research shows that the monitoring data near the surface is greatly affected by temperature, and the temperature compensation method can effectively reduce the influence of temperature on the deformation of shallow soil. In this field test, a double-ended measurement and testing system composed of 1.0 m fixed-point grating optical cable and temperature compensation optical cable is used. The metal spiral armored optical cable in the three boreholes forms a loop at the bottom of the hole, and the transmission optical cable is connected to the transmission optical cable in one inlet and one outlet respectively to form a complete optical fiber monitoring system. It can not only describe the deformation information of the whole section of the borehole, but also realize the automatic monitoring in the field. The specific monitoring system layout process is as follows: After the completion of the on-site drilling, the sensing cable is placed in it, and a certain proportion of clay ball, fine sand and gravel mixture is backfilled according to the nature of the on-site stratum. During the backfilling process, the sensing cable is kept in a straight and pretensioned state to ensure that the fiber and the surrounding rock of the borehole are well coupled. Finally, the sensing signal is transmitted to the ground through the signal transmission cable, and the portable dense distributed optical fiber demodulator is used to demodulate the strain of the sensing cable. Through the analysis and processing of the strain data, the formation deformation information in the borehole is extracted.  
The wavelength range of the demodulator used in the field test is 1528-1568 m, the wavelength resolution is 1 pm, and the repeatability is \(\pm\)2 pm. Internal 4-channel design, single-channel demodulation rate \(<\) 5 s, single-channel measurement distance up to 20 km, the maximum number of measuring points 2000, all meet the test requirements.  
The initial monitoring began at 21:37 on 15 November 2021. The longitudinal horizontal distance between the shield cutter head and the monitoring section was 30 m, and the shield was progressing at a rate of 4 rings/day (about 1.5 m per ring). The shield cutter head reached the bottom of the monitoring section at 21:30 on 20 November. At 21:25 on 21 November, the shield body was detached entirely (the longitudinal horizontal distance between the cutter head and the monitoring point was 6 m) and moved forward till it was connected.  
Figure 2: Layout of monitoring system. (**a**) Monitoring section layout. (**b**) Optical cable layout.