Fiber optic sensors for underground structural health monitoring: Survivability of sensors under shotcreting and drill-And-blast impacts

Measurement of structural parameters, such as strain and temperature at thousands of locations, along a single fiber sensor, is achievable by distributed fiber optic sensors. Different from the conventional monitoring systems, this unique feature opens new possibilities for structural health monitoring (SHM) of large underground structures. However, limited research has been conducted on maximizing the possible applications in underground structures such as tunnels and caverns. To develop a SHM scheme for underground structures, different fiber optic sensors (FOSs) are investigated based on the Brillouin Optical Time Domain Reflectometry (BOTDR) technology. The main aim of this research is to develop a BOTDR based underground monitoring system that survives during drill-And-blast of construction. After developing the sensors for the system, implementation issues such as survivability of sensors under shotcreting and drill-And-blast are addressed. A water spray test has been conducted in the laboratory to check the survivability of the sensors against shotcreting pressure. On the basis of these results, the sensors were then installed in a tunnel for trial. The trial test demonstrated that these customized sensors survived the pressure of shotcreting and drill-And-blast impact. The results showed that the distributed fiber optic sensing technique as a powerful monitoring tool has great potential in underground applications.