Ground movement monitoring using an optic Fiber Bragg Grating sensored system

Abstract

The authors have developed a fiber optic ground movement monitoring system using the optic Fiber Bragg Grating (FBG). A series of FBG's are glued to the outside of flexible plastic elements. These flexible elements are connected together to form a single probe. When the flexible element is bent, the FBG would sense the flexural strain as a result of the bending. Twice integration of the strains along the longitudinal axis of the probe yields the distribution of the displacement of the monitoring probe associated with the bending. The FBG sensored monitoring probe can be inserted in an inclinometer casing or directly grouted in a borehole to monitor ground movement. The sensitivity and range of allowable bending of the monitoring probe can be adjusted according to the need in the field. The new system can be used to monitor stability of natural slopes, man-made retaining structures (including darns or dikes), braced cuts, and tunnel excavations, etc. The FBG based monitoring system has been experimented to monitor the displacement distribution of a laterally loaded pile in Yuin-Lin, Taiwan and stability of a Yellow River dike in Honan, China. This paper describes the principles of the FBG sensored monitoring probe system and presents field applications of the new sensor systems.