# FBG-Based Monitoring of Geohazards: Current Status and Trends
## 4 Current Status of Applying FBG Systems for Geohazard Monitoring
### Land Subsidence Monitoring  
Due to rapid growths of population and industry demand in developing countries, large amount of groundwater is being consumed in urban areas. Due to the compaction of susceptible aquifer systems which comprise aquifers and aquitards, serious land subsidence is induced. In the past few decades, ground-based (e.g., GPS, and Tripod LiDAR) and remotely sensed (e.g., InSAR and LiDAR) geodetic techniques have been employed to measure the movements of land surface [74]. On the contrary, borehole extensometers are utilized to measure vertical displacements. In Japan, fully-distributed FOS systems have been introduced to monitor formation compaction accompanying the exploitation of natural gases in boreholes [75]. [PERSON] et al. initiated a study on applying FBG and BOTDR techniques to land subsidence monitoring in Suzhou, China [56]. There were three aquifers in the study site, namely Af1, Af2 and Af3. The fully distributed strain sensing cables were vertically installed in boreholes for displacement monitoring. The displacements were calculated based on the axial strain measurements of the cables. An FBG mini-osmometer was installed in the borehole for pore water pressure monitoring. As shown in Figure 10, the change of pore water pressure had a close relationship with the displacement of Af3. It is seen that the pore water pressure decreased during period A and increased during period B, then re-decreased during period C and re-increased during period D. This was attributed to the larger consumption of groundwater in the summer than in the winter.  
Groundwater exploitation, along with coal mining, can also cause deformations of overlying rock strata and fracture of shaft linings, which seriously affect the safe production of coal mine. In this aspect, experimental observations and theoretical calculations indicated that the FBG-based monitoring system was more sensitive to tensile strain than compressive tensile strain for rock layers under high loads [76]. Lately, [PERSON] et al. installed FBG sensors into a deep borehole to continuously measure the deformation of rock strata and shaft linings [33]. The monitoring data could help determine the health condition of shaft linings, thus ensuring the safety of coal mining.  
Besides groundwater exploitation, construction activities also induce ground settlement. With the advances of highway transportation, widening or reconstruction of road embarkments has been encountered from time to time. Differential settlements become serious if the fill materials of the new embankment are not well compacted. [PERSON] et al. employed the quasi-distributed FBG strain  
Figure 10: Monitoring results of displacement and pore water pressure during the field test [56]. Af: Aquifer.  
sensors to monitor the pavement behavior after embankment widening [77; 78]. In their full-scale tests, 120 FBG sensors were embedded in the asphalt concrete pavement in the longitudinal and transverse directions. Then differential settlements were applied using a subsidence simulation system consisting of 138 hydraulic jacks. The monitoring results clearly show that the geogrid reinforcement in the embankment enhanced the structural integrity of the pavement. The strains of the base pavement layer were considerably larger than those of the upper layers.  
In large cities all over the world, underground metro tunneling and excavation have resulted in significant ground settlements, which may endanger adjacent buildings and geo-structures. [PERSON] et al. introduced a first attempt to use a series of FBG sensors to measure the tunnel deformation during overhead excavation [79]. The FBG-PVC tube sensors with a total length of 84 m were installed in a shield tunnel of Shanghai Metro Line 2. The deformation profiles were calculated using the theory of bending beam, which agreed well with the Hydrostatic Level readings and finite element analysis results (Figure 11).