Quantitative Evaluation of Optical Fiber/Soil Interfacial Behavior and Its Implications for Sensing Fiber Selection

An adequate understanding of the interface between optical fibers and geomaterials is a prerequisite for applying distributed optical fiber sensor systems to strain monitoring in geoengineering. This contribution reports a quantitative investigation of the fiber/soil interfacial behavior regarding the influence of fiber types and normal pressures. A simplified model describing the progressive failure of a fiber/soil interface was briefly illustrated. Results of a series of pullout tests on three different soil-embedded optical fibers under various normal pressures were interpreted by this model, through which the fiber/soil interfacial behaviors were quantified. The results showed that the mechanical properties of the three fiber/soil interfaces were similar. Optical microscopic images indicated that the soil particles and the fibers were merely loosely contacted. This led to the formation of a fiber/soil interface susceptible to the normal pressure: 1) the interfacial bond was tightened and 2) the deformation measurement range was widened under high normal pressures. Moreover, the criterion for selecting a strain sensing fiber for geoengineering applications was discussed in terms of interfacial bond, deformation measurement range, ratio of peak to residual interfacial shear strength, Young´s modulus of fiber, and so forth. An assessment of the three fibers reveals that, for ground deformation measurement, each fiber has its own advantages as well as limitations. In field or laboratory applications, a combination of different types of fibers may obtain the best measurement results.