Effects of Cable Sheath on Deformation Coordination between the Sensing Fiber and Sand

Distributed fiber optic sensing has been used for monitoring land subsidence, and accurate measurement of soil deformation relies on the coordination between the optical fiber and soil, also known as the deformation coordination between the optical fiber and soil (DCf–s). To investigate this issue, three types of optical cables with different sheath structures were embedded in triaxial specimens made of standard sand. The fiber strain was measured by optical frequency domain reflectometry. The axial deformation of the specimen was measured by a linear variable displacement transducer sensor. Each specimen was subjected to 12 loading and unloading cycles under varying axial stresses, confining pressures, and dry or wet conditions during the triaxial tests. Results shows that the ϕ3 mm loose-tube optical cable has poor DCf–s with the strain not exceeding 15 μϵ. Under a confining pressure of 200 kPa, 400 kPa, and 500 kPa, the DCf–s coefficients between the ϕ3 mm tight-buffered optical cable and wet sand during loading are 0.266, 0.366, and 0.496, respectively. The DCf–s coefficients of the ϕ5 mm tight-buffered optical cable under identical conditions are 0.186, 0.226, and 0.249. Overall, the ϕ3 mm tight-buffered optical cable exhibits superior DCf–s compared with both the ϕ5 mm tight-buffered optical cable and ϕ3 mm loose-tube optical cable, indicating that the sheath structure is a crucial factor in determining the DCf–s. In addition, the DCf–s is also influenced by several other factors, including the number of loading–unloading cycles and the confining pressure in the soil.