Assessing the Frictional Resistance between Fiber-Optic Sensor Cable and Different Soil Types

Fiber-optic sensing systems embedded in soil are widely used to detect surrounding events via wave propagation through the soil media. Sensor cable performance is likely a function of coupling between the sensor and the surrounding soil. The purpose of this research is to evaluate the friction resistance between the cable sensor and the confining soil under controlled laboratory conditions, as greater resistance can correlate to better coupling. The cable used for this study was a 0.5-in. diameter industry standard, armored, 24-strand fiber-optic cable with a rubber-coated jacket. Gravel, sand, clay, and flowable fill were tested to illustrate geological variability of the soil-sensor characteristic behavior. To assess the pull-out stress, a test apparatus consisting of an acrylic box with 6-in. x 9-in. x 12-in. interior dimensions was designed for this study. Openings on the two 6-in. x 9-in. faces allowed the cable to pass through, 3-in. from the bottom and side edges. Soil moisture contents and densities were selected to match field conditions. The cable was axial strained at a rate of 0.126 in. per minute. The peak strength and residual pull-out strengths were monitored and recorded using a load-cell and a sampling rate of 2 kHz. Results showed the soils had peak resistances between 2.0 and 5.3 psi. The three flowable fill specimens varied greatly, resistances ranging between 30 and 90 psi along the cable.