Application of distributed fiber optic sensing technique in land subsidence monitoring

Distributed fiber optic sensing (DFOS), a newly developed structure health monitoring technique, has been proved to be a very suitable and useful technique for the monitoring and the early warning of structural engineering. Its application in geotechnical engineering, especially land subsidence resulted by groundwater withdraw, was limited by the complex characteristics of geotechnical materials in the field. In this paper, Brillouin optical time domain reflectometer (BOTDR) and fiber Bragg grating (FBG) techniques were used to monitor the deformation of soil layers and pore water pressure for nearly 2 years in a 200-m borehole with four different types of optical fibers planted directly in the borehole. The result demonstrated that cables with better sheath protection and higher tension strength are more suitable for field monitoring. The main compaction occurs at two thick aquitards which are adjacent to the pumping confined aquifer. The deformation of soil layers shows the conformance with the change of groundwater level over time and the deformation of sand layer had instantaneity while that of clay layer had hysteresis. Rebound amount caused by the rise of ground water level was small, and the re-compression rate significantly decreases after rebounding. This paper demonstrates that the DFOS technique is a very advanced monitoring method for the investigation on the mechanism of land subsidence and the evaluation of soil compression deformation potential.