Abstract: Based on BOTDR technology, Brillouin strain monitoring technology has been applied preliminarily in the field of geotechnical engineering. However, a series of experiments and engineering applications reveal that there are large differences induced by using different types of fiber optic cables, due to the sheath effect. In this study, three kinds of cables were selected to comparably investigate the engineering performance, namely new strain sensing fiber optical cable, suit fiber optical cable and armored fiber optical cable. Based on BOTDR fiber optic strain monitoring technology, a number of experiments, including precise positioning (temperature stimulation), strain measurement range (drift) and temperature measurement, were carried out. As a result, the effect of fiber optic sheath on tunnel monitoring was obtained. Finally, a suitable fiber optic cable type was identified for different engineering applications, and monitoring results show good agreements with the actual deformation and temperature change. Therefore, this study initially solves the problem of fiber selection in tunnel monitoring technology based on BOTDR technology and further provides the theoretical basis for engineering applications.

Key words: tunnel monitoring, fiber selection, sheath effect, BOTDR technology