2010 Volume 126 Issue 10_11 Pages 569-576
When the structures such as rock slopes, underground caverns for the geological disposal of high level radioactive waste and underground power plants are constructed, the potential for displacement and damage in the surrounding rock mass is increased due to stress concentration and creep phenomenon. Therefore, the long-term monitoring of rock stability for construction of rock cavern and slopes is important to maintain stability of rock structures. Recently, development of sensors that use optical fiber have expanded mainly in the field of civil engineering and, in rock and soil mechanics, since several parameters such as temperature, strain, pressure, pH etc. can be obtained by using backscattered light in optical fibers. Additionally, some important advantages using optical fiber are its high resistance to the electric insulation failure, long-term endurance and ability to transmit data over long distances. This paper describes development of a multiple-type displacement sensor using FBG, which has potential for the long-term durability and high accuracy. Laboratory tests were carried out to investigate the relational expression computed from the relationship of the variation in the Bragg wavelength, temperature and displacement. Accuracy of the prototype developed in this research is better than 0.5% or 1/100 mm. The in situ tests involving long-term monitoring using boreholes were conducted to confirm the workability and applicability of the prototype. From the results of the in situ tests, workability is equivalent to the extensometers usually used for long-term monitoring, and the displacement computed from the variation of Bragg wavelength is almost equal to the artificially induced deformation.
