Estimating tunnel wall displacements using a simple sensor based on a Brillouin optical time domain reflectometer apparatus

A strain monitoring system based on a PVC tube instrumented with optical fiber connected to a Brillouin Optical Time Domain Reflectometer (BOTDR) apparatus is proposed to monitor rock mass movement in an underground mine. The optical fiber is glued along four lines of the tube surface which are rotated 90° one from each other to capture in-plane and out-of-plane tube bending displacements. A laboratory experimental program is undertaken to validate the proposed sensor as a monitoring tool. In the laboratory, PVC tubes are subjected to controlled displacements on 4 points that represent the attachment locations of the sensor to the rock mass to simulate possible response in the rock mass during mining activities. The longitudinal strain recorded by the optical fiber compare well with the ones provided by the traditional electrical resistance strain gauges. Considering the sensor tube as a one-dimensional linear beam-type element, a back-analysis algorithm is implemented to estimate displacements on the surface of the rock mass using the recorded longitudinal strains. The proposed sensor is installed in a tunnel mine and preliminary strain measurements of this field trial are reported. The conclusions of this study suggest that the proposed sensor can be regarded as a promising and safe tool for tunnel monitoring.