February 2021

• The use of Fiber Optics for Monitoring the Ground as well as Support Elements in Underground Excavations by Nicholas Vlachopoulos, Ph.D., CD, PEng, PE_Gr, Professor at Department of Civil Engineering at Royal Military College of Canada

Professor Nicholas Vlachopoulos discusses distributed optical strain-sensing technique as a solution for measuring the strain distribution along ground support members used in tunneling and mining works.

Geotechnical stability is a major concern for the long-term safety and integrity of underground infrastructures such as tunnels, railway stations, mine shafts and hydraulic power chambers. An effective geotechnical monitoring system is able to provide adequate warning to underground personnel prior to any unexpected major geotechnical failure.

The conventional monitoring techniques such as extensometers, stress meters, and convergence stations which have been typically applied in geotechnical projects have low precision in deep strata and also needs excessive manual operations. In order to overcome these defects, optical fiber sensing technology was proposed which by using this technique a continuous strain profile will be provided and also a better insight into the true behavior of support elements will be achieved.

The technique employs a Rayleigh optical frequency domain reflectometry technology, which measures strain at a spatial resolution of 0.65 mm along the length of a standard optical fiber. A robust laboratory investigation of such optically instrumented support members demonstrated the capability of the technique to capture the expected in situ support behavior in the form of coaxial, lateral, and shear loading arrangements as would be anticipated in the field. Moreover, the microscale data obtained by this optical sensing technique are shown to provide unprecedented insight into the local/micro-scale geomechanistic complexities associated with the bearing capacity of ground support members, especially when compared with data obtained by discrete strain-sensing technologies.

For further studies:

• Vlachopoulos, N., & Forbes, B. The Development and In-Situ use of Fiber Optic Continuous Strain Monitoring for Tunnel Support Elements.
• Gong, H., Kizil, M. S., Chen, Z., Amanzadeh, M., Yang, B., & Aminossadati, S. M. (2019). Advances in fibre optic based geotechnical monitoring systems for underground excavations. International Journal of Mining Science and Technology, 29(2), 229-238.
• Tang, B., & Cheng, H. (2018). Application of distributed optical fiber sensing technology in surrounding rock deformation control of TBM-excavated coal mine roadway. Journal of Sensors, 2018.