Recently fiber-optic sensing technologies have been applied for performance monitoring of geotechnical structures such as slopes, foundations, and retaining walls. However, the validity of measured data from soil-embedded optical fibers is strongly influenced by the properties of the interface between the sensing fiber and the soil mass. This paper presents a study of the interfacial properties of an optical fiber embedded in soil with an emphasis on the effect of overburden pressure. Laboratory pullout tests were conducted to investigate the load-deformation characteristics of a 0.9 mm tight-buffered optical fiber embedded in soil. Based on a tri-linear interfacial shear stress-displacement relationship, an analytical model was derived to describe the progressive pullout behavior of an optical fiber from soil matrix. A comparison between the experimental and predicted results verified the effectiveness of the proposed pullout model. The test results are further interpreted and discussed. It is found that the interfacial bond between an optical fiber and soil is prominently enhanced under high overburden pressures. The apparent coefficients of friction of the optical fiber/soil interface decrease as the overburden pressure increases, due to the restrained soil dilation around the optical fiber. Furthermore, to facilitate the analysis of strain measurement, three working states of a soil-embedded sensing fiber were defined in terms of two characteristic displacements.

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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.

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